Dithiafulvenyl-substituted triazolobenzothiadiazines as a new family of multifunctional chromophores

Abstract

This work investigates a new class of organic π-chromophores which contain an electron-donating 1,4-dithiafulvenyl group in conjugation with an electron-withdrawing 8H-benzo[e][1,2,4]triazolo[3,4-b][1,3,4]thiadiazin-8-one core in their molecular structures. The synthesis of these compounds used a phosphite-promoted olefination reaction as a key step, through which 1,3-dithiole-2-thione was reacted with corresponding benzotriazolothiadiazinone counterparts to form a flat polycyclic π-framework, allowing electron push–pull effects to occur. The resulting donor–acceptor chromophores were found to exhibit significant intramolecular charge-transfer (ICT) properties, giving strong absorption in the visible region of the spectrum. The triazolyl unit in these molecules can be protonated by strong acids to show enhanced ICT effects, while electrochemical analysis revealed that these compounds possess amphoteric behavior with tunable band gaps (ca. 1.3–1.4 eV) and are potentially useful organic semiconductors. We also demonstrated that this type of chromophore can be readily functionalized on the surface of TiO₂ nanoparticles without losing absorption performance. Finally, the dithiafulvenyl group incorporated in the molecular structure was found to enhance antibacterial activity, rendering the chromophores potential candidates for antibacterial/antimicrobial coatings.

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Introduction

Organic dyes and pigments constitute a vital class of industrial products with extensive applications across numerous scientific and engineering disciplines.^1–4 In materials science, these compounds serve as active components in various optoelectronic devices, including chemical sensors,^5–7 dye-sensitized solar cells (DSSCs),^8–10 optical data storage systems,¹¹ photocatalytic assemblies,^12–14 organic light-emitting diodes (OLEDs),^15–17 and field-effect transistors (FETs).^18,19 In the biomedical field, functional organic dyes enable advanced diagnostic and theranostic techniques, such as photodynamic therapy,²⁰ photothermal therapy,^21,22 and imaging-guided phototherapy.^23,24 A prevalent design strategy for such functional dyes is based on the electron donor–acceptor (push–pull) concept. This approach involves integrating electron-donating (D) and electron-accepting (A) groups within a molecular framework via π-conjugated bridges.^25–28 This molecular architecture provides exceptional tunability of electronic and optical properties, facilitating the development of chromophores and fluorophores that absorb and/or emit in the visible to near-infrared (Vis–NIR) spectral region. Consequently, push–pull organic dyes exhibiting pronounced intramolecular charge-transfer (ICT) characteristics are increasingly prominent in contemporary research on organic optoelectronic materials and devices.^29–33

1,4-Dithiafulvene (DTF) is a five-membered heterocycle that functions as a versatile organic π-donor for constructing advanced optoelectronic materials.^34–37 A notable feature of DTF is its proaromatic character, which imparts aromatic stabilization to its oxidized state (Fig. 1A). This property underpins the excellent redox activity observed in numerous DTF-containing π-systems.³⁴ When conjugated with a π-acceptor, DTF can form an effective push–pull molecular system, enhancing π-electron delocalization across the donor–acceptor framework. Fig. 1B illustrates representative push–pull chromophores featuring a DTF–π–A motif. In these structures, a para-quinodimethane (p-QDM) unit serves as a non-aromatic π-bridge linking the donor and acceptor groups. The resonance scheme in Fig. 1B reveals the origin of the favored π-delocalization; that is, one of the π-delocalized resonance contributors exhibits double aromaticity.

Fig. 1 (A) Single-electron transfer of DTF. (B) Representative push–pull systems with a DTF donor in conjugation with an electron-accepting group through a p-quinodimethane (p-QDM) linkage. (C) Our designed push–pull motif where a DTF group is conjugated with an aza-quinone methide group. (D) Our target DTF-BTTD chromophores and their retrosynthesis.

In the literature, ketone, cyanoimine, malononitrile, and barbituric-type units have been employed as π-acceptors conjugated with DTF to generate push–pull organic chromophores.^38–41 In contrast, imines (Schiff bases) remain relatively unexplored in this context, despite being a versatile class of molecular building blocks. Imines are readily accessible via simple condensation reactions and can be easily modified through interactions with acids or metal ions. They impart structural, chemical, and photochemical functionalities valuable for diverse molecular materials, including functional ligands, receptors, catalysts, molecular cages, and covalent organic frameworks (COFs).^42–47 In organic dyes, the imine unit has frequently been utilized for π-extension and to enhance electronic communication.^48–51

In this work, we developed a new class of push–pull organic chromophores based on DTF (as donor) and imine-containing heterocycles (as acceptors). Fig. 1C illustrates our designed π-conjugated framework, which integrates a DTF group and an imine group via a quinoid π-bridge.⁵² To ensure stability for material applications–addressing the susceptibility of simple imines to nucleophilic attack (e.g., hydrolysis) and facile E/Z isomerization–we embedded the imine (C=N) bond within a π-heterocycle. A survey of the literature led us to an interesting yet underdeveloped heterocyclic system: 8H-benzo[e][1,2,4]triazolo[3,4-b][1,3,4]thiadiazin-8-one (herein abbreviated as BTTD). This heterocyclic scaffold can be conveniently prepared via a high-yielding condensation approach. First reported in the early 1990s,^53,54 the BTTD scaffold was not recognized as useful for materials science until a recent machine learning (ML)-aided synthetic study by Jensen et al.⁵⁵ In that work, 44 BTTD derivatives were predicted by ML generative algorithms as unprecedented dye-like molecules and subsequently synthesized using an automated design-make-test-analyze (DMTA) platform.

Particularly attractive to us is the potential to extend the BTTD scaffold into a DTF-BTTD system containing the desired DTF–imine push–pull motif. As shown in Fig. 1D, the DTF-BTTD system can be synthesized via an olefination reaction between BTTD and 1,3-dithiole-2-thione (DTT).⁵⁶ This molecular architecture represents a significant advancement in chromophore design, offering a versatile platform that integrates multiple functionalities. First, the interplay between the DTT donor and the BTTD acceptor promotes strong intramolecular charge transfer (ICT), enhancing absorptivity in the visible to near-infrared (Vis-NIR) region. Second, the versatile reactivity and inherent pharmacological properties of the BTTD motif provide opportunities for flexible tunability, ligand performance, and specific biological activities. These combined properties are advantageous for advanced optoelectronic and biomedical applications. We herein report a comprehensive study of a series of DTF-BTTD derivatives as new multifunctional organic chromophores, encompassing their synthesis, chemical properties, visible-light absorption, interactions with acids and metal oxide nanoparticles, and biological activities.

Results and discussion

Synthesis of DTF-BTTD chromophores

Our synthesis of DTF-BTTD compounds commenced with the preparation of a series of 4-amino-4H-1,2,4-triazole-3-thiol derivatives (1a–h, Scheme 1) according to reported procedures.^53,54 In these structures, the 5-position of the triazole ring bears a phenyl or substituted phenyl group. Compounds 1a–h were condensed with 1,4-benzoquinone in ethanol to afford the corresponding BTTD derivatives 2a–h in good yields (80–95%). Subsequent olefination of compounds 2a–h with 1,3-dithiole-2-thione (3)^57,58 in the presence of excess triethyl phosphite (P(OEt)₃) at 90 °C yielded the target DTF-BTTD compounds 4a–h in moderate to good yields (62–74%). Overall, this synthetic route is concise, high-yielding, and readily scalable.

Scheme 1 Synthesis of DTF-BTTD chromophores 4a–h.

During some of the olefination reactions described above, an unexpected byproduct was observed. Careful silica gel column chromatography enabled the isolation of these byproducts in pure form, and their structures were identified as DTF-BTTD derivatives 5 (Scheme 2). These compounds arise from a substitution reaction that attaches an additional 1,3-dithiole group to the benzo unit of the DTF-BTTD framework. The formation of this byproduct was more prominent when reactant 3 was not thoroughly purified after its synthesis. Given that the preparation of 3 involves zinc salts as precursors,⁵⁷ we hypothesized that residual zinc(II) in the olefination reaction promoted a subsequent substitution step. To test this hypothesis, we conducted olefination reactions between 2a–e and 3 in the presence of varying amounts of zinc iodide and water. The optimal conditions for producing the dithiole-substituted DTF-BTTD compounds were achieved using zinc(II) iodide (10 mol%) and water (ca. 200 equiv.). Under these conditions, we successfully synthesized dithiole-substituted DTF-BTTD derivatives 5a–e as the major products in satisfactory yields (56–63%, Scheme 2).

Scheme 2 Synthesis of dithiole-substituted DTF-BTTD derivatives 5a–e.

Scheme 3 outlines a proposed mechanism for the substitution reaction on the DTF-BTTD substrate. In this pathway, 1,3-dithiole-2-thione (3) first reacts with P(OEt)₃ in the presence of ZnI₂ and water to generate a dithiolium intermediate (IM-1). Within the DTF-BTTD structure, the carbon atom at the β-position relative to the DTF group is electron-rich and nucleophilic due to resonance effects. This activated β-carbon attacks the electrophilic dithiolium cation IM-1 in a straightforward addition step, forming intermediate IM-2. A final, facile deprotonation then affords the dithiole-substituted product 5. This unexpected synthetic outcome can be attributed to the pronounced intramolecular charge transfer (ICT) between the DTF and BTTD units, which significantly enhances the nucleophilicity of the benzo ring in the BTTD moiety.

Scheme 3 Proposed mechanism for the formation of dithiole-substituted DTF-BTTDs.

X-ray structural properties of DTF-BTTD derivatives

The molecular structures of DTF-BTTD derivatives 4a–h and 5a–e were confirmed by NMR, IR, and MS analyses (see the supplementary information (SI) for details). All these compounds exhibit good solubility in chlorinated solvents (e.g., CH₂Cl₂ and CHCl₃) and dipolar aprotic organic solvents (e.g., acetone, acetonitrile, DMF, and DMSO). During purification, single crystals suitable for X-ray diffraction analysis were obtained for several derivatives. Single-crystal X-ray diffraction (SCXRD) analyses were subsequently performed to unambiguously determine their molecular structures and elucidate their packing arrangements in the solid state.

Fig. 2 shows the molecular structures of 4a–c, 4h, and 5c, with selected geometric parameters provided in Table 1 for comparative analysis. The structure of 4a (Fig. 2A) reveals a nearly coplanar arrangement between the DTF ring and the BTTD moiety, a geometry that facilitates π-electron delocalization across the entire conjugated framework. In contrast, the phenyl group attached to the triazole unit is rotated out of the BTTD plane by a torsion angle of 16.9°. For other DTF-BTTD derivatives bearing different substituents on the phenyl ring, the core DTF-BTTD structure shows minimal variation compared to 4a (Table 1), indicating that substituent effects on the molecular geometry are negligible. The torsion angles between the phenyl rings and their respective BTTD units, however, vary among these compounds. Notably, compound 4h features an intramolecular hydrogen bond between the ortho-hydroxy group and the N3 atom of the triazole ring, with an N3⋯H1 distance of 1.86 Å and an N3⋯H1–O1 angle of 146.1° (see Fig. 2E for atomic labeling). This interaction enforces coplanarity between the phenyl ring and the BTTD unit in 4h.

Fig. 2 ORTEP drawings (at 50% ellipsoid probability) of the molecular structures of (A) 4a, (B) 4b, (C) [4b + H⁺][CF₃COO⁻], (D) 4c, (E) 4h, and (F) 5c.

Table 1 Selected bond lengths (in Å) and torsion angles (φ, in deg) for 4a, 4b, [4b + H⁺], 4c, 4h, 5c, and 6

Entry	4a	4b	[4b + H^+]	4c	4h	5c	6
a φ is the torsion angle across N2–C9–C10–C11.
C1–C2	1.39	1.40	1.39	1.39	1.39	1.40	1.39
C2–C3	1.43	1.43	1.42	1.44	1.43	1.43	1.43
C3–C4	1.35	1.35	1.38	1.36	1.35	1.35	1.34
C4–C5	1.44	1.44	1.42	1.44	1.44	1.45	1.44
C5–C6	1.45	1.45	1.44	1.45	1.45	1.46	1.42
C6–C7	1.36	1.36	1.38	1.37	1.36	1.36	1.37
C7–C2	1.44	1.43	1.41	1.43	1.43	1.42	1.43
C5–N1	1.32	1.31	1.33	1.32	1.32	1.31	1.31
C6–S1	1.77	1.76	1.77	1.77	1.77	1.77	1.77
φ^a	16.9	29.1	3.6	31.9	10.3	7.2	10.5

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Single crystals were also obtained upon protonation of compound 4b with trifluoroacetic acid (TFA). Fig. 2C shows the molecular structure of the resulting [4b + H]⁺ salt, confirming protonation at the N3 position of the triazole unit. The triazolium cation co-crystallizes with a trifluoroacetate counterion and a neutral TFA molecule. This structure clearly demonstrates that the N3 site of 4b is more basic than other nitrogen atoms in the BTTD framework, rendering it more susceptible to interaction with acidic species. Following protonation, the anisole group of 4b adopts a more coplanar orientation relative to the BTTD unit compared to the neutral molecule. This conformation facilitates π-electron delocalization between the anisole (donor) and triazolium (acceptor) rings.

The X-ray structure of the dithiole-substituted derivative 5c is shown in Fig. 2F. Its π-conjugated backbone exhibits a geometry very similar to that of the other DTF-BTTD systems (Table 1). The dithiole ring attached at the C4 position experiences minimal steric clash with neighboring groups and consequently has little influence on the conformation or π-conjugation of the DTF-BTTD core. In the solid state, however, the dithiole group facilitates intermolecular N⋯S interactions, promoting a slipped face-to-face stacking arrangement in the crystal structure of 5c.

During attempts to grow single crystals of compound 4d by antisolvent vapor diffusion using ethanol, X-ray analysis of the resulting crystals surprisingly revealed the structure of a new compound, 6 (Fig. 3A). In 6, an ethoxy group is bonded to the benzo moiety of BTTD at the C7 position. The formation of 6 under these crystallization conditions can be rationalized by the mechanism outlined in Fig. 3B. We propose that the electron-withdrawing trifluoromethyl (CF₃) group in 4d, combined with trace acids in solution, enhances the electrophilicity of the substrate. Ethanol then acts as a nucleophile, attacking 4d via a conjugate addition. The observed regioselectivity is attributed to favorable hydrogen bonding between the ethanol proton and a sulfur atom in the thiadiazine unit. This addition yields intermediate IM-3, which is subsequently oxidized by ambient oxygen to afford the ethoxy-substituted product 6. Similar to 5c, the π-conjugated backbone of 6 shows minimal geometric deviation from other DTF-BTTD structures (Table 1).

Fig. 3 (A) ORTEP drawing (at 50% ellipsoid probability) of the molecular structure of compound 6. (B) Proposed mechanism for the formation of 6.

Crystal packing properties of DTF-BTTD derivatives

X-ray analysis reveals that all DTF-BTTD derivatives possess a planar π-conjugated skeleton and extended flat π-surfaces, structural features that favor intermolecular π-stacking in the solid state. The detailed π-stacking motifs observed in their single-crystal structures are summarized in Fig. 4. These compounds generally pack with either parallel or antiparallel molecular orientations. As shown in Fig. 4A, molecules of 4a adopt an ordered parallel arrangement with a significant tilt angle of approximately 45°. Adjacent molecules exhibit a close C⋯C contact of 3.37 Å, indicative of efficient π-stacking. In contrast, compound 4b crystallizes in an antiparallel arrangement (Fig. 4B), where neighboring molecules engage through intermolecular C⋯S contacts at a distance of 3.41 Å. Interestingly, protonation of 4b with TFA switches the π-stacking mode to a parallel orientation (Fig. 4C). Similar to 4a, the [4b + H]⁺ cations pack in parallel with a tilt angle of 45°, exhibiting intimate intermolecular C⋯C contacts of 3.38 Å.

Fig. 4 Various π-stacking modes observed in the crystal structures of (A) 4a, (B) 4b, (C) [4b + H⁺][CF₃COO⁻], (D) 4c, (E) 4h, (F) 5c, and (G) 6. Selected interatomic distances are indicated in Å and hydrogen atoms are omitted for clarity.

The tolyl-substituted derivative 4c also packs in an antiparallel fashion, but with a relatively small tilt angle of approximately 20° (Fig. 4D). Its molecules form a slipped π-stack, characterized by intermolecular C⋯S contacts at a distance of 3.50 Å. In contrast, compound 4h adopts an antiparallel packing mode with a very small tilt angle (Fig. 4E). The intermolecular separation between adjacent molecules ranges from 3.7 to 3.8 Å. This distance is slightly greater than the van der Waals diameter of carbon, indicating a relatively loose face-to-face π-stacking interaction.

Compound 5c exhibits antiparallel π-stacking with a tilt angle of approximately 32° (Fig. 4F). The attachment of a dithiole ring to the edge of the planar π-framework does not disrupt face-to-face π-stacking. Instead, the dithiole ring facilitates the ordered packing by engaging in an intermolecular N⋯S contact (3.15 Å) between one of its sulfur atoms and a triazolyl nitrogen atom on an adjacent molecule. Furthermore, within the planar π-framework, a sulfur atom of the thiadiazine unit interacts with a benzo carbon of a neighboring molecule at a distance of 4.37 Å.

In contrast to the other derivatives, the crystal structure of 6 lacks the extended π-stacks described above. Instead, its unit cell contains a discrete π-dimer, where a pair of molecules adopt a parallel, face-to-face arrangement (Fig. 4G). Within this dimer, adjacent molecules exhibit a close F⋯F contact (2.83 Å) and an intimate C⋯S interaction (3.45 Å). The distinct packing behavior of 6 likely stems from the electron-withdrawing effect of its CF₃ substituent. Collectively, the diverse π-stacking motifs observed in these single-crystal structures demonstrate that the phenyl substituent on the DTF-BTTD π-framework serves as a handle for the bottom-up control of crystal packing. A deeper understanding of these substituent effects is valuable for the crystal engineering of DTF-BTTD-based organic electronic materials.

Electronic absorption properties of DTF-BTTD chromophores

The electronic absorption properties of the synthesized DTF-BTTD chromophores 4a–h were investigated using UV-Vis spectroscopy and time-dependent density functional theory (TD-DFT) calculations. Their UV-Vis absorption spectra in CH₂Cl₂ at room temperature are shown in Fig. 5A and detailed absorption data are summarized in Table S1, SI. All compounds exhibit a similar broad absorption band between 450 and 650 nm. The strong absorbance in this visible region accounts for the purple color of their solutions. It is noteworthy that the molar absorptivities (ε) of our DTF-BTTD chromophores 4a–h exceed 10⁶ M⁻¹ cm⁻¹ in terms of their low-energy absorption bands. For context, this value significantly surpasses those of typical Ru-based DSSC sensitizers such as N719 dye (ε ≈ 1.42 × 10⁴ M⁻¹ cm⁻¹ at 534 nm)⁵⁹ and conventional triphenylamine-based organic push–pull dyes (2–5 × 10⁴ M⁻¹ cm⁻¹),⁶⁰ and is greater than the established high-absorptivity dye classes including squaraines (1–3 × 10⁵ M⁻¹ cm⁻¹)⁶¹ and cyanines (1–2.5 × 10⁵ M⁻¹ cm⁻¹).⁶² The combination of broad visible-region coverage (450–650 nm) and exceptionally high absorptivity makes the DTF-BTTD scaffold a promising candidate for light-harvesting applications requiring thin active layers.

Fig. 5 (A) Normalized UV-Vis absorption spectra of DTF-BTTDs 4a–h measured in CH₂Cl₂ at room temperature. (B) TD-DFT simulated UV-Vis spectrum of 4c calculated at CAM-B3LYP/6-311+G(2d,p)//M06-2X/6-31+G(d,p) level of theory. (C) Plots of the frontier molecular orbitals (isovalue = 0.03 au) of 4c that contribute to the low-energy absorption bands in its UV-Vis spectrum.

To understand the origin of this low-energy band, TD-DFT calculations were performed on compound 4c as a representative model. The simulated spectrum (in vacuo) predicts the lowest-energy absorption at 451 nm, arising primarily from a HOMO → LUMO transition (98% contribution) (Fig. 5B). As shown in Fig. 5C, both the HOMO and LUMO of 4c are π-orbitals delocalized across the DTF and BTTD moieties, with negligible contribution from the tolyl substituent. The near-identical experimental low-energy bands for 4a–h confirm that the appended phenyl group has little influence on these frontier orbitals. According to the TD-DFT calculation, the second lowest-energy absorption for 4c appears at 361 nm and originates mainly from a H−1 → LUMO transition (85% contribution). The H−1 orbital is localized predominantly on the triazole and tolyl groups (Fig. 5C). This indicates that the appended phenyl group in 4a–h can effectively modulate absorption in the higher-energy spectral region (ca. 300–400 nm). Overall, the TD-DFT simulated spectrum of 4c agrees well with the experimental data in terms of the relative positions and intensities of the major absorption bands.

Our DFT calculations indicate that the HOMO → LUMO transition has strong intramolecular charge-transfer (ICT) character, suggesting that the DTF-BTTD compounds should exhibit solvatochromism.^63,64 To investigate this, the UV-Vis absorption spectra of compound 4c were measured in various organic solvents (Fig. 6A). The results confirm that the low-energy absorption maximum is solvent-dependent.

Fig. 6 (A) UV-Vis absorption spectra of 4c measured in different organic solvents. (B) Correlation plots of maximum absorption energy (E_abs) against the solvent scale E_T(30).

To quantify the solvatochromic effect, the transition energies (E_abs) were correlated with the E_T(30) solvent polarity scale.⁶⁴ The E_T(30) scale was chosen for its broad parameter set and sensitivity to both specific and non-specific solvent interactions. As shown in Fig. 6B, two distinct linear correlations are observed. The first correlation includes hexane, diethyl ether, ethyl acetate, acetonitrile, isopropanol, and ethanol. The second includes toluene, THF, chloroform, DMF, and DMSO. The result for methanol is an outlier and does not fit either trend. In general, more polar solvents induce a greater redshift in the absorption maximum. The origin of the two linear correlations is not yet clear and requires further study; however, their existence suggests two distinct solvation modes for 4c in different organic media. Given that one group consists primarily of oxygenated solvents, specific interactions such as hydrogen bonding or O⋯N and O⋯S contacts may contribute significantly to the solvatochromic shifts, in addition to general solvent polarity effects.

X-ray analysis confirmed that the DTF-BTTD structure can be protonated by TFA at the triazolyl unit to form a stable salt. We therefore anticipated that the DTF-BTTD system would exhibit acidochromism in solution. To investigate this, UV-Vis titrations of 4c with TFA were performed (Fig. 7).

Fig. 7 UV-Vis spectral analysis monitoring the titration of 4c (5.08 × 10⁻⁵ M in CH₂Cl₂) with TFA: (A) addition of TFA from 0.0 to 43.1 mole equiv., and (B) addition of TFA from 47.9 to 234.7 mole equiv. Arrows indicate the trend of spectral change with increasing TFA addition. Inset: Photographic image of the solutions of 4c with varying amounts of TFA.

The titration revealed two distinct stages of spectral change in the long-wavelength region. In the first stage (0.0 to 43.1 molar equivalents of TFA), the low-energy absorption band at 549 nm gradually shifted to 578 nm, with a slight increase in intensity (Fig. 7A). In the second stage (up to 234.7 molar equivalents), this band continued to redshift, and a shoulder at 634 nm gradually emerged (Fig. 7B). The enhancement, rather than attenuation, of absorbance upon protonation demonstrates excellent acid tolerance. The two distinct spectral stages suggest that 4c undergoes consecutive protonation to form mono- and diprotonated species. Based on the X-ray structure of [4b + H⁺][CF₃COO⁻], the first protonation likely occurs at the N3 position of the triazolyl unit. The site of the second protonation remains unclear. Overall, protonation of the DTF-BTTD framework strengthens the donor–acceptor interaction between the DTF and BTTD units, resulting in a redshifted and intensified absorption in the visible region.

To probe the solution-phase properties of protonated DTF-BTTD, a ¹H NMR titration of 4c was performed in CDCl₃ (Fig. 8). During the titration, two benzo protons of the BTTD unit (Hb and Hc) shifted significantly downfield, while the third benzo proton (Ha) remained largely unaffected. This result confirms that protonation occurs predominantly at the triazolyl group. Protonation at this site significantly reduces the electron density at the carbon atoms bearing Hb and Hc through conjugation, explaining their pronounced downfield shifts. Protonation at the thiadiazine ring can be ruled out, as this would be expected to cause a more substantial shift in the Ha signal. The NMR data further indicate that the benzo unit becomes more electron-deficient upon protonation, thereby enhancing its donor–acceptor interaction with the DTF group. This interpretation is consistent with the redshift observed in the UV-Vis titration experiment.

Fig. 8 ¹H NMR (300 MHz, CDCl₃) spectra monitoring the titration of 4c (3.18 × 10⁻³ M) with TFA at room temperature. The cyan curves highlight the shifts of Hb and Hc during titration.

Electrochemical properties of DTF-BTTD chromophores

To characterize the redox properties of the DTF-BTTD framework, cyclic voltammetry (CV) was first performed on compound 4a. The phenyl appendage in 4a exerts minimal electronic influence on the core DTF-BTTD π-system; therefore, its CV response primarily reflects the intrinsic redox activity of the DTF-BTTD unit.

The CV profiles of 4a in the positive and negative potential windows are shown in Fig. 9A and B, respectively. In the positive window, the first anodic scan exhibits a well-defined oxidation peak at +1.01 V, which is reasonably assigned to the one-electron oxidation of the DTF moiety to form a radical cation. The subsequent cathodic scan shows a significant reduction peak at +0.97 V. This reduction peak is attributed to a species generated by a follow-up chemical reaction of the initially formed radical cation. This interpretation is supported by the evolution of the voltammogram in subsequent cycles: the original oxidation peak at +1.01 V disappears, and a new anodic peak emerges at +1.44 V in the second and third scans.

In contrast to 4a, the voltammograms of 5a do not exhibit the characteristic features of an EC (electrochemical-chemical) process. This difference in reactivity is likely due to the structural modification in 5a, where the site β to the DTF group is incorporated into a dithiole ring. As shown in Fig. 9C, the first anodic scan displays two oxidation peaks at +1.14 V and +1.62 V, assigned to the sequential one-electron oxidations of the DTF and dithiole groups, respectively. In subsequent cycles, the first peak (DTF oxidation) disappears, while the second peak (dithiole oxidation) persists. No significant reduction peaks are observed in the corresponding cathodic scans. This irreversible electrochemical behavior suggests that facile chemical reactions follow the initial oxidation of the DTF moiety. A detailed mechanistic understanding of these redox pathways will require further investigation.

Fig. 9 Cyclic voltammograms of compound 4a scanned in (A) the positive and (B) negative potential windows, respectively. Cyclic voltammograms of compound 5a scanned in (C) the positive and (D) negative potential windows, respectively. CV experimental conditions: Bu₄NPF₆ (0.1 M) as the electrolyte, glassy carbon as the working electrode, Ag/AgCl as the reference electrode, Pt wire as the counter electrode, CH₂Cl₂ as the solvent, and scan rate = 0.20 V s⁻¹. Arrows indicate the directions of initial scans.

In the negative potential window, the cyclic voltammograms of 4a exhibit three irreversible reduction peaks at −0.86 V, −1.15 V, and −1.50 V (Fig. 9B). In contrast to the behavior in the positive window, these voltammograms remain unchanged over multiple scan cycles. The three peaks are assigned to sequential one-electron reductions of the BTTD unit. Similar reduction patterns are observed for 5a, although the peak potentials are slightly shifted relative to those of 4a.

To investigate the substituent effects of the appended phenyl group on the redox activity of DTF-BTTD derivatives, we selected compounds 4b and 4d for CV analysis. These compounds feature a strong electron-donating group (OMe) and an electron-withdrawing group (CF₃), respectively, on the phenyl ring. The cyclic voltammogram of 4b in the positive potential window (Fig. 10A) exhibits significant EC features similar to those of 4a. The first anodic scan shows a distinct oxidation peak at +0.95 V. The corresponding reverse scan reveals two reduction peaks at +1.45 V and +1.08 V. By the third cycle, the initial oxidation peak at +0.95 V has nearly vanished, and a new oxidation peak is clearly present at +1.56 V. In contrast to 4a, the cathodic peaks in the reverse scans of 4b also diminish progressively. This behavior suggests that the oxidized product(s) of 4b undergo further chemical reactions, likely involving the activated anisole group, leading to irreversible electrochemical features.

Fig. 10 Cyclic voltammograms of compound 4b scanned in (A) the positive and (B) negative potential windows, respectively. Cyclic voltammograms of compound 4c scanned in (C) the positive and (D) negative potential windows, respectively. CV experimental conditions: Bu₄NPF₆ (0.1 M) as the electrolyte, glassy carbon as the working electrode, Ag/AgCl as the reference electrode, Pt wire as the counter electrode, CH₂Cl₂ as the solvent, and scan rate = 0.20 V s⁻¹. Arrows indicate the directions of initial scans.

The CF₃-substituted derivative 4d exhibits markedly different voltammetric behavior in the positive potential window (Fig. 10C). The first cycle shows two oxidation peaks at +1.01 V and +1.38 V, with corresponding reduction peaks at +0.57 V and +1.03 V. In subsequent cycles, the first oxidation peak (+1.01 V) disappears, while the other peaks persist. Although a detailed electrochemical mechanism remains unclear, these results clearly demonstrate that the substituent on the phenyl ring of the DTF-BTTD framework can significantly modulate its redox activity in the positive potential window.

In the negative potential window, both 4b and 4d exhibit similar irreversible reduction profiles, with two cathodic peaks at −0.73 V and −1.08 V for 4b and −0.68 V and −1.00 V for 4d. The potentials for the electron-donating derivative 4b are slightly shifted cathodically relative to those for the electron-withdrawing derivative 4d, indicating that the appended phenyl group exerts only a weak influence on the reduction potentials. In summary, the electrochemical redox properties of DTF-BTTD derivatives are highly sensitive to repeated scanning in the positive potential window. In contrast, the reduction features in the negative window show minimal variation. Detailed CV data for the remaining DTF-BTTD derivatives are provided in the SI (Fig. S31–S35).

The cyclic voltammograms measured in the positive and negative potential windows were used to determine the electrochemical band gaps (E_g) of these compounds. The E_g was calculated as the difference between the onset potential of the first oxidation (E¹_ox) and the onset potential of the first reduction (E¹_red) from the first CV cycle. As listed in Table 2, the DTF-BTTD compounds 4a–h exhibit band gaps ranging from 1.315 eV to 1.416 eV. This narrow range indicates that attaching various substituents to the phenyl group only slightly adjusts the band gaps, which is consistent with the relatively weak substitution effects observed in the UV-Vis analysis. This behavior reflects a limited ability to tune the electronic band gaps of the DTF-BTTD chromophores by modifying the phenyl appendage. Conversely, it suggests that the DTF-BTTD chromophores are tolerant to structural modifications such as the attachment of anchoring groups for surface functionalization or the addition of synthetic handles for structural extension without significantly disrupting their core electronic properties.

Table 2 Summary of the first oxidation and reduction potentials (in V) and electrochemical band gaps (E_g, in eV) for 4a–h

Entry	E^1ox	E^1red	Eg
4a	+0.773	−0.619	1.392
4b	+0.771	−0.606	1.377
4c	+0.756	−0.597	1.353
4d	+0.779	−0.556	1.335
4e	+0.740	−0.575	1.315
4f	+0.728	−0.688	1.416
4g	+0.780	−0.594	1.374
4h	+0.781	−0.588	1.369

---

Covalent functionalization of TiO₂ nanoparticles with 4c

Having elucidated the fundamental properties of the DTF-BTTD compounds, we next explored their applicability in materials science. Organic dyes with strong visible to near-infrared (Vis-NIR) absorption are extensively used in dye-sensitized solar cells and photocatalytic systems.^8,28,65–67 A particularly common strategy for generating new functional composites involves combining such dyes with inorganic semiconductor nanoparticles, such as titanium dioxide (TiO₂).⁶⁸

Effective immobilization of an organic dye on a metal oxide surface typically requires an anchoring group, such as a carboxylic acid or cyanoacrylic acid group. The DTF-BTTD molecular structure contains multiple heteroatoms that can potentially act as ligand sites for binding to transition metals. We therefore hypothesized that DTF-BTTD compounds might exhibit sufficient affinity for metal oxide surfaces to form dye-functionalized hybrid materials. To test this, we performed IR analysis on mixtures of anatase TiO₂ nanoparticles with compound 4c. In our experiments, TiO₂ nanoparticles were first dispersed in ethanol to form a suspension, to which different amounts of 4c were added. The resulting mixtures were dried and analyzed by IR spectroscopy. As shown in Fig. 11, pristine TiO₂ nanoparticles exhibit a broad band at 1638 cm⁻¹, assigned to the O–H bending mode of adsorbed surface water.^69–71 When mixed with a small amount of 4c, this water O–H bending peak disappears. The IR spectrum of this mixture is drastically different from the spectra of pristine TiO₂ and 4c, indicating covalent functionalization of the TiO₂ nanoparticle surface by 4c. In contrast, when TiO₂ nanoparticles were mixed with an excess of 4c, the observed IR modes resemble those of pure 4c, suggesting the 4c/TiO₂ hybrids were fully encapsulated by 4c molecules via non-covalent interactions.

Fig. 11 FTIR spectra of TiO₂ nanoparticles, compound 4c, and their mixtures.

To better understand the covalent interactions between 4c and TiO₂ nanoparticles, we computationally modeled a simplified system consisting of a 4c molecule and a (TiO₂)₁₅ cluster. We first searched for stable structures of this assembly using the CREST (conformer-rotamer ensemble sampling tool) program.^72–74 The most stable conformer identified by CREST was then subjected to geometry optimization using density functional theory (DFT) calculations with the Gaussian 16 software package.⁷⁵ Fig. 12A shows the optimized structure of the 4c–(TiO₂)₁₅ complex. One of the triazolyl nitrogen atoms forms a Ti–N bond with a length of 2.06 Å. This metallation site is consistent with the preferred protonation site observed in X-ray analysis, indicating that this specific triazole nitrogen is the most active ligand for interacting with Ti. The formation of the Ti–N bond causes significant elongation of a nearby Ti–O bond to 3.35 Å (see the dashed bond in Fig. 12A). The weakening of this Ti–O bond allows the oxygen atom to form a Ti=O double bond (1.63 Å), which is much shorter than the other Ti–O bonds in the cluster. In addition to this covalent Ti–N bond, the DTF-BTTD segment of 4c is notably bent, enabling the edge of 4c to form S⋯O and H⋯O interactions with the TiO₂ cluster.

Fig. 12 (A) Optimized geometry of the complex of 4c and a (TiO₂)₁₅ cluster. Selected interatomic distances are highlighted in Å. (B) DFT-simulated IR spectra of 4c (red trace) and the complex of 4c and (TiO₂)₁₅ (blue trace). Calculations were done at the B3LYP-D3/6-31G(d)/LanL2DZ level of theory with a scaling factor of 0.97.

Fig. 12B compares the calculated IR spectra of pristine 4c and the 4c–(TiO₂)₁₅ complex. In the 1100–1700 cm⁻¹ region, 4c is predicted to exhibit three significant vibrational bands at 1641, 1476, and 1370 cm⁻¹. The band at 1641 cm⁻¹ is assigned to C=C stretching in the benzo unit, while the bands at 1476 and 1370 cm⁻¹ correspond to C=N stretching modes in the triazole and dithiazine units, respectively. The experimental spectrum of 4c is more complex, likely due to the presence of multiple conformers and aggregates in the solid sample. For the 4c–(TiO₂)₁₅ complex, five significant vibrational bands are predicted in this region. These show excellent agreement with the experimental IR spectrum of the 4c/TiO₂ nanoparticle mixture (highlighted bands in Fig. 11), validating our computational model for simulating the surface interactions. The DFT calculations indicate that upon covalent bonding to the TiO₂ cluster, the benzo C=C stretching vibration shifts to a lower frequency (1603 cm⁻¹), consistent with the experimental trend. Three peaks predicted at 1416, 1389, and 1351 cm⁻¹ are assigned to C=N vibrations of 4c. In the experimental spectrum, corresponding peaks are clearly observed at 1447, 1416, and 1359 cm⁻¹, showing very good agreement. Notably, the unique Ti=O bond in the complex is predicted to vibrate at 1197 cm⁻¹. A characteristic band appears in the experimental spectrum at 1151 cm⁻¹, providing strong evidence for covalent interaction between 4c and TiO₂. This assignment is supported by a previous literature report on Ti=O vibrational properties.⁷⁶

Following the IR analysis, we examined the UV-Vis absorption properties of 4c-functionalized TiO₂ nanoparticles via a titration experiment. As shown in Fig. 13, incremental additions of TiO₂ nanoparticles were made to a solution of 4c in ethanol. During the titration, the broad visible absorption band of 4c (ca. 450–680 nm) showed only slight attenuation. In contrast, the absorbance in the UV region (300–450 nm) increased significantly due to the intrinsic absorption of TiO₂. Furthermore, three relatively sharp absorption bands emerged at 335, 319, and 272 nm, providing additional evidence for covalent interactions between 4c and the TiO₂ nanoparticles. Overall, the UV-Vis study indicates that surface functionalization of TiO₂ nanoparticles with 4c yields a hybrid material capable of absorbing a broad spectrum of light. This enhanced absorption profile is beneficial for applications in photovoltaics and visible-light-promoted photocatalysis.

Fig. 13 UV-Vis spectral analysis monitoring the titration of 4c (1.05 × 10⁻⁵ M in EtOH) with TiO₂ nanoparticles from 0.00 to 4.22 mole equiv. Arrows indicate the trend of spectral changes with the progress of the titration. The trace depicted by the dashed line is the UV-Vis absorption spectrum of TiO₂ nanoparticles suspended in EtOH.

Finally, we examined the effect of surface functionalization with 4c on the size of TiO₂ nanoparticles. Various amounts of 4c were added to a suspension of TiO₂ nanoparticles in ethanol, and the particle size distributions of the resulting mixtures were measured by dynamic light scattering (DLS). As shown in Fig. 14, the pristine TiO₂ nanoparticles have diameters ranging from ca. 50 to 500 nm. The addition of 4c caused only minor changes in the size distribution. While a small fraction of particles increased in size to 500–600 nm, the majority remained within the original 50–500 nm range. These DLS results suggest that 4c forms only a monolayer or a few layers on the TiO₂ surface and that the functionalization does not induce significant interparticle aggregation.

Fig. 14 DLS analysis showing the particle size distributions for anatase TiO₂ nanoparticles (7.50 mg L⁻¹) mixed with various amounts of 4c.

In summary, our investigation into the interactions between 4c and TiO₂ nanoparticles demonstrates their significant potential for use in optoelectronic devices, particularly dye-sensitized solar cells (DSSCs). The DTF-BTTD chromophores exhibit several advantageous properties: (1) remarkably high molar absorptivity (exceeding 10⁵ M⁻¹ cm⁻¹; see Table S1, SI) in the visible region, enabling strong light harvesting; (2) relatively small band gaps (Table 2), which facilitate broader spectral coverage; (3) a triazolyl unit that acts as an effective anchoring group, promoting direct interaction between the dye's π-conjugated framework and the TiO₂ surface, likely enhancing electron injection efficiency; and (4) minimal aggregation on the TiO₂ surface, as shown by DLS, a factor known to improve DSSC performance by reducing intermolecular energy transfer and charge recombination. This combination of properties positions these dyes as promising candidates for achieving enhanced power conversion efficiencies in DSSCs, warranting further experimental exploration.

Tests of antibacterial activities

The global spread of bacterial drug resistance necessitates the discovery of new and potent antimicrobial agents.^77–79 Heterocyclic compounds containing a triazole ring are of particular interest due to their broad bioactivities, including antibacterial and antifungal properties.^80–83 Our synthesized DTF-BTTD chromophores incorporate a 1,2,4-triazole ring fused to a benzothiadiazine framework. As 1,2,4-triazole is a well-known pharmacophore with many derivatives exhibiting antibacterial activity,^80,82 we evaluated the antibacterial properties of 4c against representative Gram-negative (Escherichia coli) and Gram-positive (Staphylococcus aureus) bacteria. While we did not design these chromophores de novo as antibacterial agents following certain established molecular guidelines, the well-known pharmacophoric nature of the 1,2,4-triazole motif prompted us to explore whether this structural feature might impart additional functionality. Fortuitously, we discovered that compound 4c exhibited notable antibacterial activity against these two representative bacteria. We therefore present this antibacterial property as an unexpected, yet valuable, multifunctional attribute of our chromophores.

The disk diffusion method (DDM)⁸⁴ was used to assess the antibacterial performance of 4c at various concentrations (Table 3). To probe the influence of the DTF functional group, we also tested the reference compound 2i, a stable, nearly planar aryl-substituted BTTD (Fig. 15). A comparison of their activities reveals whether the DTF group enhances or attenuates bioactivity. As shown in Table 3, both 4c and 2i exhibited inhibitory activity against both bacteria at low concentrations (0.1–2.5 mM), with a stronger effect observed against E. coli than S. aureus. Notably, 4c produced larger inhibition zones than 2i under identical conditions, confirming that DTF functionalization enhances antibacterial performance.

Table 3 Results of inhibition zones of various test solutions against two bacterial strains measured by the filter paper disc diffusion method

Entry	Concentration (mM)	Zone of Inhibition (mm)
		E. coli	S. aureus
4c	2.5	17.8 ± 0.4	15.7 ± 0.3
	0.5	14.0 ± 0.4	13.3 ± 0.5
	0.1	12.1 ± 0.5	12.2 ± 0.4
2i	2.5	14.3 ± 0.4	12.2 ± 0.5
	0.5	13.7 ± 0.6	11.3 ± 0.4
	0.1	10.0 ± 0.3	9.9 ± 0.4

---

Fig. 15 Chemical structure (left) and ORTEP drawing (at 50% ellipsoid probability) of compound 2i.

Following the antibacterial assays, we explored the application of 4c as a functional dye to impart self-cleaning properties to glass surfaces. Fluorine-doped tin oxide (FTO) glass was selected as the substrate due to its widespread use in optoelectronic devices (e.g., solar cells, touchscreens, and smart windows). For such applications, maintaining surface transparency is critical, and bacterial adhesion is a common cause of surface fouling. We examined the attachment of two model bacteria, E. coli and B. subtilis, to four types of FTO substrates: (I) neat FTO, (II) FTO coated with 4c, (III) FTO coated with TiO₂ particles, and (IV) FTO coated with both 4c and TiO₂. Fig. 16 summarizes the results of these attachment tests after the substrates were immersed in bacterial cultures for 12 h under dark or ambient light conditions, while detailed experimental conditions and procedures are provided in the SI (see Fig. S37–S52).

Fig. 16 Bacterial attachment assays on FTO glass substrates with (A) E. coli and (B) B. subtilis. Substrate conditions: I, neat; II, coated with 4c; III, coated with TiO₂; IV, coated with 4c-TiO₂. N_cell represents the number of bacteria counted within a defined square area on the surface.

As shown in Fig. 16A, the 4c-coated FTO substrate exhibited the lowest E. coli attachment under dark conditions, consistent with the enhanced anti-E. coli activity of 4c observed in prior antibacterial assays. Under ambient light, substrates coated with 4c and TiO₂ showed significantly reduced E. coli attachment, indicating light-induced self-cleaning behavior. The best bacterial resistance was observed for the surface coated with both 4c and TiO₂, which retained the fewest E. coli cells after 12 h of immersion in the bacterial culture. This performance can be attributed to a synergistic effect between 4c and TiO₂. Specifically, 4c can be photoexcited by visible light, given its strong absorptivity in the visible spectral range, and subsequently transfer electrons to the TiO₂ surface, promoting the generation of bactericidal reactive oxygen species (ROS).⁸⁵

In the B. subtilis tests (Fig. 16B), the four substrates exhibited similar levels of bacterial attachment under dark conditions, with the TiO₂-coated glass showing the highest bacterial density. This indicates that neither 4c nor TiO₂ provides significant antibacterial activity against B. subtilis in the dark. Under ambient light, however, all coated substrates showed reduced bacterial attachment compared to the neat glass. Among these, the substrate coated with both 4c and TiO₂ demonstrated the most pronounced resistance, highlighting a strong photo-induced synergistic effect. Collectively, these bacterial attachment results demonstrate that hybrids of the DTF-BTTD chromophore with TiO₂ can serve as antibacterial coatings that effectively protect optically transparent substrates from biofouling.

Conclusions

In summary, we have developed a new family of DTF-BTTD chromophores that exhibit strong visible-light absorption and versatile redox activities, driven by intramolecular push–pull effects across their extended π-conjugated frameworks. By modifying the aryl group attached to the triazolyl moiety, we can tune the molecular conformation, crystal packing, and electronic band gaps, establishing the DTF-BTTD system as a flexible molecular platform for advanced optoelectronic applications. The fused 1,2,4-triazole ring provides an active site for acid interaction, resulting in pronounced acidochromic effects. Protonation enhances and redshifts the visible-region absorption, making DTF-BTTD compounds robust organic dyes that retain strong visible-light absorptivity under acidic conditions. Electrochemical studies show that while these compounds exhibit consistent reduction behavior (indicating reliable acceptor performance), their oxidation properties vary significantly. For some derivatives, oxidation follows complex EC mechanisms, likely due to the reactivity of the generated DTF radical cation–a subject that merits further investigation. The triazole unit also functions as an effective ligand, binding to TiO₂ nanoparticle surfaces via covalent Ti–N linkages. This suggests promising applications in surface-functionalized metal oxides for devices such as dye-sensitized solar cells and photocatalytic systems. Furthermore, DTF-BTTD structures display enhanced antibacterial activity compared to their BTTD precursors, and hybrids of DTF-BTTD with TiO₂ demonstrate superior inhibition of bacterial attachment to glass surfaces under ambient light. These properties offer a novel light-powered self-cleaning mechanism for designing and fabricating multifunctional, regenerative, optically transparent surfaces.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: detailed synthetic procedures, spectroscopic characterizations of new compounds, single-crystal X-ray crystallographic data, UV-Vis absorption, cyclic voltammetric data, and results of DFT calculations as well as antibacterial activity measurements. See DOI: https://doi.org/10.1039/d6qm00098c.

CCDC 2132937–2132944 contain the supplementary crystallographic data for this paper.^86a–h

Acknowledgements

We acknowledge the funding support from the Natural Sciences and Engineering Research Council of Canada (NSERC) and the Canada Foundation for Innovation (CFI). FS and HF acknowledge the School of Graduate Studies (SGS), Memorial University of Newfoundland for providing a graduate scholarship. KN thanks the Zhejiang Provincial Natural Science Foundation of China for funding support (Grant no. LTGS24B070001). Dr. Jian-bin Lin at C-CART, Memorial University of Newfoundland is acknowledged for collecting the single-crystal crystallographic data and solving the X-ray structures reported in this work. The Digital Research Alliance of Canada (the Alliance) and ACENET are acknowledged for providing computational resources.

References

1. H. Zollinger, Color chemistry: syntheses, properties, and applications of organic dyes and pigments, John Wiley & Sons, 2003.
2. W. Herbst and K. Hunger, Industrial organic pigments: production, properties, applications, John Wiley & Sons, 2006.
3. M. Gsänger, D. Bialas, L. Huang, M. Stolte and F. Würthner, Organic semiconductors based on dyes and color pigments, Adv. Mater., 2016, 28, 3615–3645.
4. W. Cheng, H. Chen, C. Liu, C. Ji, G. Ma and M. Yin, Functional organic dyes for health-related applications, View, 2020, 1, 20200055.
5. M. Saleem, M. Rafiq, M. Hanif, M. A. Shaheen and S.-Y. Seo, A brief review on fluorescent copper sensor based on conjugated organic dyes, J. Fluoresc., 2018, 28, 97–165.
6. E. Oliveira, E. Bértolo, C. Núñez, V. Pilla, H. M. Santos, J. Fernández-Lodeiro, A. Fernández-Lodeiro, J. Djafari, J. L. Capelo and C. Lodeiro, Green and red fluorescent dyes for translational applications in imaging and sensing analytes: A dual-color flag, ChemistryOpen, 2018, 7, 9–52.
7. Z. Jin, W. Yim, M. Retout, E. Housel, W. Zhong, J. Zhou, M. S. Strano and J. V. Jokerst, Colorimetric sensing for translational applications: from colorants to mechanisms, Chem. Soc. Rev., 2024, 53, 7681–7741.
8. Y. Ooyama and Y. Harima, Molecular designs and syntheses of organic dyes for dye-sensitized solar cells, Eur. J. Org. Chem., 2009, 2903–2934.
9. M. Liang and J. Chen, Arylamine organic dyes for dye-sensitized solar cells, Chem. Soc. Rev., 2013, 42, 3453–3488.
10. J. Wang, K. Liu, L. Ma and X. Zhan, Triarylamine: Versatile platform for organic, dye-sensitized, and perovskite solar cells, Chem. Rev., 2016, 116, 14675–14725.
11. H. Mustroph, M. Stollenwerk and V. Bressau, Current developments in optical data storage with organic dyes, Angew. Chem., Int. Ed., 2006, 45, 2016–2035.
12. S. Fukuzumi and K. Ohkubo, Organic synthetic transformations using organic dyes as photoredox catalysts, Org. Biomol. Chem., 2014, 12, 6059–6071.
13. S. G. Amos, M. Garreau, L. Buzzetti and J. Waser, Photocatalysis with organic dyes: Facile access to reactive intermediates for synthesis, Beil. J. Org. Chem., 2020, 16, 1163–1187.
14. M. Forchetta, F. Valentini, V. Conte, P. Galloni and F. Sabuzi, Photocatalyzed oxygenation reactions with organic dyes: State of the art and future perspectives, Catalysts, 2023, 13, 220.
15. P.-T. Chou and Y. Chi, Phosphorescent dyes for organic light-emitting diodes, Chem. – Eur. J., 2007, 13, 380–395.
16. K.-H. Kim and J.-J. Kim, Origin and control of orientation of phosphorescent and TADF dyes for high-efficiency OLEDs, Adv. Mater., 2018, 30, 1705600.
17. J. Tagare and S. Vaidyanathan, Recent development of phenanthroimidazole-based fluorophores for blue organic light-emitting diodes (OLEDs): An overview, J. Mater. Chem. C, 2018, 6, 10138–10173.
18. L. Torsi, M. Magliulo, K. Manoli and G. Palazzo, Organic field-effect transistor sensors: A tutorial review, Chem. Soc. Rev., 2013, 42, 8612–8628.
19. S. Yuvaraja, A. Nawaz, Q. Liu, D. Dubal, S. G. Surya, K. N. Salama and P. Sonar, Organic field-effect transistor-based flexible sensors, Chem. Soc. Rev., 2020, 49, 3423–3460.
20. Y. Cai, W. Si, W. Huang, P. Chen, J. Shao and X. Dong, Organic dye based nanoparticles for cancer phototheranostics, Small, 2018, 14, 1704247.
21. B. Zhou, Y. Li, G. Niu, M. Lan, Q. Jia and Q. Liang, Near-infrared organic dye-based nanoagent for the photothermal therapy of cancer, ACS Appl. Mater. Interface, 2016, 8, 29899–29905.
22. C. Yin, X. Li, Y. Wang, Y. Liang, S. Zhou, P. Zhao, C.-S. Lee, Q. Fan and W. Huang, Organic semiconducting macromolecular dyes for NIR-II photoacoustic imaging and photothermal therapy, Adv. Funct. Mater., 2021, 31, 2104650.
23. S. Luo, E. Zhang, Y. Su, T. Cheng and C. Shi, A review of NIR dyes in cancer targeting and imaging, Biomaterial, 2011, 32, 7127–7138.
24. J. Wu, Z. Shi, L. Zhu, J. Li, X. Han, M. Xu, S. Hao, Y. Fan, T. Shao, H. Bai, B. Peng, W. Hu, X. Liu, C. Yao, L. Li and W. Huang, The design and bioimaging applications of NIR fluorescent organic dyes with high brightness, Adv. Opt. Mater., 2022, 10, 2102514.
25. H. Meier, Conjugated oligomers with terminal donor–acceptor substitution, Angew. Chem., Int. Ed., 2005, 44, 2482–2506.
26. M. Kivala and F. Diederich, Acetylene-derived strong organic acceptors for planar and nonplanar push–pull chromophores, Acc. Chem. Res., 2009, 42, 235–248.
27. F. Bureš, Fundamental aspects of property tuning in push–pull molecules, RSC Adv., 2014, 4, 58826–58851.
28. C. Pigot, G. Noirbent, D. Brunel and F. Dumur, Recent advances on push–pull organic dyes as visible light photoinitiators of polymerization, Eur. Polym. J., 2020, 133, 109797.
29. A. Abbotto, L. Beverina, R. Bozio, S. Bradamante, C. Ferrante, G. A. Pagani and R. Signorini, Push–pull organic chromophores for frequency-upconverted lasing, Adv. Mater., 2000, 12, 1963–1967.
30. H.-T. Feng, J. Zeng, P.-A. Yin, X.-D. Wang, Q. Peng, Z. Zhao, J. W. Lam and B. Z. Tang, Tuning molecular emission of organic emitters from fluorescence to phosphorescence through push–pull electronic effects, Nat. Commun., 2020, 11, 2617.
31. E. V. Verbitskiy, G. L. Rusinov, O. N. Chupakhin and V. N. Charushin, Design of fluorescent sensors based on azaheterocyclic push–pull systems towards nitroaromatic explosives and related compounds: A review, Dyes Pigm., 2020, 180, 108414.
32. P. Kaur and K. Singh, Second-order nonlinear polarizability of push–pull chromophores. A decade of progress in donor-π-acceptor materials, Chem. Rec., 2022, 22, e202200024.
33. Y. Patil, H. Butenschön and R. Misra, Tetracyanobutadiene bridged push–pull chromophores: Development of new generation optoelectronic materials, Chem. Rec., 2023, 23, e202200208.
34. M. F. Abdollahi and Y. Zhao, Recent advances in dithiafulvenyl-functionalized organic conjugated materials, New J. Chem., 2020, 44, 4681–4693.
35. A. Yoshimura, H. Kimura, K. Kagawa, M. Yoshioka, T. Itou, D. Vasu, T. Shirahata, H. Yorimitsu and Y. Misaki, Synthesis and properties of tetrathiafulvalenes bearing 6-aryl-1,4-dithiafulvenes, Beil. J. Org. Chem., 2020, 16, 974–981.
36. M. Dekhtiarenko, S. Krykun, V. Carré, F. Aubriet, D. Canevet, M. Allain, Z. Voitenko, M. Sallé and S. Goeb, Tuning the structure and the properties of dithiafulvene metalla-assembled tweezers, Org. Chem. Front., 2020, 7, 2040–2046.
37. V. Bliksted Roug Pedersen, J. Granhøj, A. Erbs Hillers-Bendtsen, A. Kadziola, K. V. Mikkelsen and M. Brøndsted Nielsen, Fulvalene-based polycyclic aromatic hydrocarbon ladder-type structures: Synthesis and properties, Chem. – Eur. J., 2021, 27, 8315–8324.
38. R. Andreu, M. J. Blesa, L. Carrasquer, J. Garn, J. Orduna, R. Alcala and B. Villacampa, 1,3-Dithiole based quinoid systems: multiply proaromatic NLO-phores, Phosphorus, Sulfur Silicon Relat. Elem., 2005, 180, 1473–1474.
39. R. Andreu, M. J. Blesa, L. Carrasquer, J. Garn, J. Orduna, B. Villacampa, R. Alcalá, J. Casado, M. C. Ruiz Delgado and J. T. López Navarrete, et al., Tuning first molecular hyperpolarizabilities through the use of proaromatic spacers, J. Am. Chem. Soc., 2005, 127, 8835–8845.
40. S. Amriou, C. Wang, A. S. Batsanov, M. R. Bryce, D. F. Perepichka, E. Ort, R. Viruela, J. Vidal-Gancedo and C. Rovira, The interplay of inverted redox potentials and aromaticity in the oxidized states of new π-electron donors: 9-(1,3-dithiol-2-ylidene)fluorene and 9-(1,3-dithiol-2-ylidene)thioxanthene derivatives, Chem. – Eur. J., 2006, 12, 3389–3400.
41. F. Shahrokhi, R. F. Estabragh and Y. Zhao, Synthesis and comparative studies of K-region functionalized pyrene derivatives, New J. Chem., 2020, 44, 16786–16794.
42. A. Pratt, The photochemistry of imines, Chem. Soc. Rev., 1977, 6, 63–81.
43. C. D. Meyer, C. S. Joiner and J. F. Stoddart, Template-directed synthesis employing reversible imine bond formation, Chem. Soc. Rev., 2007, 36, 1705–1723.
44. M. E. Belowich and J. F. Stoddart, Dynamic imine chemistry, Chem. Soc. Rev., 2012, 41, 2003–2024.
45. R. D. Patil and S. Adimurthy, Catalytic methods for imine synthesis, Asian J. Org. Chem., 2013, 2, 726–744.
46. K. Acharyya and P. S. Mukherjee, Organic imine cages: Molecular marriage and applications, Angew. Chem., Int. Ed., 2019, 58, 8640–8653.
47. C. Qian, L. Feng, W. L. Teo, J. Liu, W. Zhou, D. Wang and Y. Zhao, Imine and imine-derived linkages in two-dimensional covalent organic frameworks, Nat. Rev. Chem., 2022, 6, 881–898.
48. C. Prinzisky, I. Meyenburg, A. Jacob, B. Heidelmeier, F. Schröder, W. Heimbrodt and J. Sundermeyer, Optical and electrochemical properties of anthraquinone imine based dyes for dye-sensitized solar cells, Eur. J. Org. Chem., 2016, 756–767.
49. M.-D. Damaceanu, C.-P. Constantin and L. Marin, Insights into the effect of donor-acceptor strength modulation on physical properties of phenoxazine-based imine dyes, Dyes Pigm., 2016, 134, 382–396.
50. M. Tumer, F. Tumer, M. Kose, S. A. Gungor, S. Akar, I. Demirtas and G. Ceyhan, Structural characterizations, photophysical and biological properties of Disperse black 9 dye and π-extended imine derivatives, Dyes Pigm., 2018, 154, 62–74.
51. C. E. de Melo, C. R. Nicoleti, M. C. Rezende, A. J. Bortoluzzi, R. S. Heying, R. S. Oliboni, G. F. Caramori and V. G. Machado, Reverse solvatochromism of imine dyes comprised of 5-nitrofuran-2-yl or 5-nitrothiophen-2-yl as electron acceptor and phenolate as electron donor, Chem. – Eur. J., 2018, 24, 9364–9376.
52. J. Nakayama, N. Matsumaru and M. Hoshino, Preparation of stable quinone methide imines, J. Chem. Soc., Chem. Commun., 1981, 565–566.
53. B. R. Rani, M. Rahman and U. Bhalerao, A facile one pot synthesis of 3-substituted-1,2,4-triazolo[4,3-b][4,1,2]benzothiadiazine-8-ones, Org. Prep. Proced. Inter., 1991, 23, 157–162.
54. U. T. Bhalerao, C. Muralikrishna and B. R. Rani, Laccase enzyme catalysed efficient synthesis of 3-substituted-1,2,4-triazoto(4,3-b)(4,1,2)benzothiadiazine-8-ones, Tetrahedron, 1994, 50, 4019–4024.
55. B. A. Koscher, R. B. Canty, M. A. McDonald, K. P. Greenman, C. J. McGill, C. L. Bilodeau, W. Jin, H. Wu, F. H. Vermeire, B. Jin, T. Hart, T. Kulesza, S.-C. Li, T. S. Jaakkola, R. Barzilay, R. Gómez-Bombarelli, W. H. Green and K. F. Jensen, Autonomous, multiproperty-driven molecular discovery: From predictions to measurements and back, Science, 2023, 382, eadi1407.
56. C. A. Christensen, A. S. Batsanov and M. R. Bryce, Thiolated π-extended tetrathiafulvalenes: Versatile multifunctional π-systems, J. Org. Chem., 2007, 72, 1301–1308.
57. G. Steimecke, H.-J. Sieler, R. Kirmse and E. Hoyer, 1,3-Dithiole-2-thione-4,5-dithiolate from darbon disulfide and alkali metal, Phosphorus, Sulfur, Rel. Elem., 1979, 7, 49–55.
58. G. Chen, I. Mahmud, L. N. Dawe, L. M. Daniels and Y. Zhao, Synthesis and properties of conjugated oligoyne-centered π-extended tetrathiafulvalene analogues and related macromolecular systems, J. Org. Chem., 2011, 76, 2701–2715.
59. M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Muller, P. Liska, N. Vlachopoulos and M. Gratzel, Conversion of light to electricity by cis-X2bis(2,2′-bipyridyl-4,4′-dicarboxylate)ruthenium(II) charge-transfer sensitizers (X = Cl–, Br–, I –, CN–, and SCN–) on nanocrystalline TiO2 electrodes, J. Am. Chem. Soc., 1993, 115, 6382–6390.
60. A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo and H. Pettersson, Dye-sensitized solar cells, Chem. Rev., 2010, 110, 6595–6663.
61. A. Ajayaghosh, Chemistry of squaraine-derived materials: near-IR dyes, low band gap systems, and cation sensors, Acc. Chem. Res., 2005, 38, 449–459.
62. A. Mishra, R. K. Behera, P. K. Behera, B. K. Mishra and G. B. Behera, Cyanines during the 1990s: a review, Chem. Rev., 2000, 100, 1973–2012.
63. E. Buncel and S. Rajagopal, Solvatochromism and solvent polarity scales, Acc. Chem. Res., 1990, 23, 226–231.
64. C. Reichardt, Solvatochromic dyes as solvent polarity indicators, Chem. Rev., 1994, 94, 2319–2358.
65. C.-P. Lee, R. Y.-Y. Lin, L.-Y. Lin, C.-T. Li, T.-C. Chu, S.-S. Sun, J. T. Lin and K.-C. Ho, Recent progress in organic sensitizers for dye-sensitized solar cells, RSC Adv., 2015, 5, 23810–23825.
66. P. P. Kumavat, P. Sonar and D. S. Dalal, An overview on basics of organic and dye sensitized solar cells, their mechanism and recent improvements, Renew. Sustain. Energy Rev., 2017, 78, 1262–1287.
67. K. Sun, P. Xiao, F. Dumur and J. Lalevée, Organic dye-based photoinitiating systems for visible-light-induced photopolymerization, J. Polym. Sci., 2021, 59, 1338–1389.
68. Y. Ooyama and Y. Harima, Photophysical and electrochemical properties, and molecular structures of organic dyes for dye-sensitized solar cells, Chem. Phys. Chem., 2012, 13, 4032–4080.
69. D. Yates, Infrared studies of the surface hydroxyl groups on titanium dioxide, and of the chemisorption of carbon monoxide and carbon dioxide, J. Phys. Chem., 1961, 65, 746–753.
70. M. Primet, P. Pichat and M. V. Mathieu, Infrared study of the surface of titanium dioxides. I. Hydroxyl groups, J. Phys. Chem., 1971, 75, 1216–1220.
71. K. S. Finnie, D. J. Cassidy, J. R. Bartlett and J. L. Woolfrey, IR spectroscopy of surface water and hydroxyl species on nanocrystalline TiO₂ films, Langmuir, 2001, 17, 816–820.
72. C. Bannwarth, S. Ehlert and S. Grimme, GFN2-xTBAn accurate and broadly parametrized self-consistent tight-binding quantum chemical method with multipole electrostatics and density-dependent dispersion contributions, J. Chem. Theor. Comput., 2019, 15, 1652–1671.
73. S. Grimme, Exploration of chemical compound, conformer, and reaction space with meta-dynamics simulations based on tight-binding quantum chemical calculations, J. Chem. Theor. Comput., 2019, 15, 2847–2862.
74. P. Pracht, F. Bohle and S. Grimme, Automated exploration of the low-energy chemical space with fast quantum chemical methods, Phys. Chem. Chem. Phys., 2020, 22, 7169–7192.
75. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A. V. Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F. Ogliaro, M. J. Bearpark, J. J. Heyd, E. N. Brothers, K. N. Kudin, V. N. Staroverov, T. A. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. P. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C. Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas, J. B. Foresman and D. J. Fox, Gaussian 16 Revision C.01, 2016, Gaussian Inc., Wallingford CT.
76. C. Barraclough, J. Lewis and R. Nyholm, The stretching frequencies of metal–oxygen double bonds, J. Chem. Soc., 1959, 3552–3555.
77. M. N. Alekshun and S. B. Levy, Molecular mechanisms of antibacterial multidrug resistance, Cell, 2007, 128, 1037–1050.
78. C. Walsh, Molecular mechanisms that confer antibacterial drug resistance, Nature, 2000, 406, 775–781.
79. E. D. Brown and G. D. Wright, Antibacterial drug discovery in the resistance era, Nature, 2016, 529, 336–343.
80. F. Gao, T. Wang, J. Xiao and G. Huang, Antibacterial activity study of 1,2,4-triazole derivatives, Eur. J. Med. Chem., 2019, 173, 274–281.
81. B. Zhang, Comprehensive review on the anti-bacterial activity of 1,2,3-triazole hybrids, Eur. J. Med. Chem., 2019, 168, 357–372.
82. J. Li and J. Zhang, The antibacterial activity of 1,2,3-triazole-and 1,2,4-triazole-containing hybrids against Staphylococcus aureus: An updated review (2020-present), Curr. Top. Med. Chem., 2022, 22, 41–63.
83. P. Yang, J.-B. Luo, Z.-Z. Wang, L.-L. Zhang, J. Feng, X.-B. Xie, Q.-S. Shi and X.-G. Zhang, Synthesis, molecular docking, and evaluation of antibacterial activity of 1,2,4-triazole-norfloxacin hybrids, Bioorg. Chem., 2021, 115, 105270.
84. M. Balouiri, M. Sadiki and S. K. Ibnsouda, Methods for in vitro evaluating antimicrobial activity: A review, J. Pharm. Anal., 2016, 6, 71–79.
85. Z. Xu, J. Wu, B. Lovely, Y. Li, M. Ponder, K. Waterman, Y.-T. Kim, D. Shuai, Y. Yin and H. Huang, Visible light-activated dye-sensitized TiO₂ antibacterial film: A novel strategy for enhancing food safety and quality, J. Hazard. Mater., 2024, 480, 136296.
86. (a) CCDC 2132937: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc29lhfv; (b) CCDC 2132938: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc29lhgw; (c) CCDC 2132939: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc29lhhx; (d) CCDC 2132940: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc29lhjy; (e) CCDC 2132941: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc29lhkz; (f) CCDC 2132942: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc29lhl0; (g) CCDC 2132943: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc29lhm1; (h) CCDC 2132944: Experimental Crystal Structure Determination, 2026 DOI:10.5517/ccdc.csd.cc29lhn2.

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