Deciphering the donor–acceptor order: impact of acceptor positioning on the optical properties of anthraquinone-carbazole-cyanostilbenes

Abstract

We report a systematic investigation of anthraquinone–carbazole–cyanostilbene conjugates arranged in two distinct connectivity patterns: AQCZCS and CZCSAQ. Despite having identical donor and acceptor units, subtle differences in donor–acceptor ordering profoundly impact their optical and excited-state properties. Both isomers exhibit broad, structureless emission characteristic of charge-transfer (CT) states, with maxima at 635 nm (AQCZCS) and 700 nm (CZCSAQ), corresponding to a pronounced bathochromic shift (Δλ ∼65 nm). Solution-phase emission reflects connectivity-dependent shifts, confirming strong donor–acceptor communication. In the solid state, AQCZCS emits vivid orangish red (610 nm) that shifts to deep red (645 nm) upon mechanical stimulation, while CZCSAQ shows red emission that further bathochromically responds under force, highlighting the tunable mechanochromic behavior. Quantum yields of 2.7% (AQCZCS) and 1.8% (CZCSAQ) demonstrate the effect of topology on emissive efficiency. These findings establish robust structure–property correlations and provide design principles for multifunctional solid-state emitters with highly tunable CT-driven emission across the visible spectrum.

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Introduction

Organic π-conjugated luminophores are central to modern optoelectronics, finding widespread use in organic light-emitting diodes (OLEDs),^1–6 sensing platforms,^7–10 and bioimaging.^11–15 The continuous drive toward next-generation materials have inspired strategies to overcome classical limitations such as aggregation-caused quenching (ACQ), while also enabling advanced photophysical phenomena like aggregation-induced emission (AIE),^16,17 solvatochromism,^18–20 mechanochromism,^21–25 and thermally activated delayed fluorescence (TADF).^26–30 Central to these efforts is the donor–acceptor (D–A) design principle, where the careful interplay between electron-rich and electron-deficient fragments allows tuning of emission color, charge-transfer character, and excited-state dynamics.^31–35 Among the wide variety of building blocks, carbazole has emerged as an important electron donor due to its strong donating ability, rigid framework, and good photostability.^36–40 Cyanostilbene is a versatile π-acceptor/spacer, well known for its twisted structure that suppresses ACQ and promotes AIE.^41–44 On the other hand, anthraquinone (AQ) is a classic diketone acceptor with strong electron-withdrawing carbonyl groups and a rigid fused backbone that efficiently lowers the LUMO and enhances spin–orbit coupling.^45–50 The combination of these units provides a robust molecular platform to engineer solid-state emitters with finely tunable optical properties.

Over the years, we have systematically explored carbazole-based scaffolds to understand how structure governs solid-state emission. Early studies on carbazole–anthracene conjugates showed that subtle changes in acceptor unit substitutions can affect emission, quantum yield, and aggregation.⁵¹ Extending this to carbazole–cyanostilbene systems with varied π-spacers revealed that conjugation length effectively tunes charge transfer, AIE and MFC behavior.^52,53 Recently, we have focused on how positional changes of the benzothiazole acceptor influence emission color, molecular packing, and mechanochromic behavior.⁵⁴ While donor–acceptor chemistry has been widely explored, the role of positional isomerism i.e., the order in which donor and acceptor units are connected within the conjugated backbone remains comparatively underexplored. Even when the same fragments are present, their sequence can dictate the extent of orbital overlap, conjugation, and intermolecular packing, leading to pronounced differences in photophysical behavior. Positional variations can modulate emission wavelengths, solvatochromic shifts, AIE activity, and even the singlet-triplet energy gap (ΔE_ST), offering a subtle yet powerful lever for materials design. In this work, we focus on anthraquinone–carbazole–cyanostilbene conjugates, where the same donor (carbazole), cyanostilbene, and acceptor (anthraquinone) units are arranged in two distinct connectivity patterns (AQCZCS and CZCSAQ) (Chart 1). By systematically comparing these positional isomers, we uncover how subtle differences in donor–acceptor ordering governs their optical and excited-state properties. Through experimental photophysics, AIE and solvatochromism studies, and theoretical calculations, we establish clear structure–property correlations that highlight the critical role of connectivity in dictating emission behavior. These findings not only deepen the understanding of positional effects in organic luminophores but also provide guiding principles for the rational design of multifunctional solid-state emitters.

Chart 1 Molecular structures of the isomers.

Results and discussions

Synthesis and characterisation

The synthetic route toward the anthraquinone–carbazole–cyanostilbene conjugates (AQCZCS and CZCSAQ) is drawn in Scheme 1. Both positional isomers were constructed via a stepwise strategy including Suzuki–Miyaura cross-coupling and Knoevenagel condensation. For AQCZCS, 2-bromoanthracene-9,10-dione was first coupled with 9-hexyl-6-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-9H-carbazole-3-carbaldehyde using Suzuki–Miyaura reaction conditions to afford the corresponding aldehyde intermediate. Subsequent Knoevenagel condensation with benzylacetonitrile furnished the desired conjugate in moderate yield. In contrast, the CZCSAQ isomer was obtained by first synthesizing the carbazole-cyanostilbene intermediate. This was achieved by condensing 9-hexyl-9H-carbazole-3-carbaldehyde with 4-bromobenzylacetonitrile to yield a carbazole-cyanostilbene derivative, which was then subjected to Suzuki–Miyaura borylation. In this pathway, the key boronic ester (Z)-3-(9-hexyl-9H-carbazol-3-yl)-2-(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)acrylonitrile, undergoes Suzuki cross coupling reaction with 2-bromoanthracene-9,10-dione to yield CZCSAQ. The chemical structures of AQCZCS and CZCSAQ were confirmed by ¹H and ¹³C NMR spectroscopy together with high-resolution mass spectrometry (HRMS). The ¹H NMR spectra of both isomers displayed sharp, well-resolved resonances corresponding to the product. The integration values were fully consistent with the proposed structures, while the characteristic downfield-shifted protons of the anthraquinone moiety and the vinylic proton signals of the cyanostilbene unit provided clear analytic evidence of successful conjugation. The ¹³C NMR spectra likewise exhibited well-defined signals for all aromatic and aliphatic carbons, including the carbonyl carbons of the anthraquinone group and the cyano carbon of the cyanostilbene. Importantly, no solubility issues were encountered, allowing the complete assignment of all major carbon resonances. Further support came from ESI-HRMS analysis, which revealed molecular ion peaks ([M]⁺ or [M + H]⁺) in agreement with the calculated exact masses, thereby confirming the molecular formulations of both positional isomers. Taken together, the combined NMR and HRMS data firmly establish the successful synthesis of AQCZCS and CZCSAQ.

Scheme 1 Synthetic scheme for the target isomers.

Photophysical properties in solution

The absorption and emission spectra of AQCZCS and CZCSAQ were first recorded in THF (Con. = 1.0 × 10⁻⁵ M) (Fig. 1). Both compounds display intense absorptions in the 240–300 nm region arising predominantly from localized π–π* transitions of the aromatic units, while the additional bands extending into the 300–350 nm region can be attributed n–π* transitions commonly observed in anthraquinone-based chromophores.⁵⁵ A weaker, broadened band extending into the visible region (370–430 nm) is evident, which can be assigned to charge-transfer (CT) interactions involving the anthraquinone acceptor. Interestingly, CZCSAQ exhibits a more red-shifted and broader absorption profile compared to AQCZCS, reflecting stronger donor–acceptor coupling when the anthraquinone unit is positioned at the terminal end of the π-conjugated pathway. Upon excitation at the ICT band, both AQCZCS and CZCSAQ exhibited broad, structureless emission profiles characteristic of charge-transfer (CT) states, with maxima at 635 nm and 700 nm, respectively. The positional variation of the anthraquinone fragment induced a pronounced bathochromic shift of Δλ ∼65 nm, underscoring the sensitivity of excited-state energies to donor–acceptor topology. Notably, the emission profile of CZCSAQ is broader and extends further into the near-infrared region, indicative of enhanced charge delocalization and stronger electronic communication across the conjugated backbone. These findings establish that although both isomers are constructed from identical donor–acceptor components, the specific placement of anthraquinone critically governs their electronic structures, resulting in distinct optical bandgaps and emission colors. Absolute quantum yields (ϕ_f) in THF were relatively low, ϕ = 7% (AQCZCS) and 1.8% (CZCSAQ), consistent with the strong electron-accepting nature of anthraquinone and the prevalence of nonradiative deactivation pathways.

Fig. 1 (A) Normalized absorption and (B) normalized emission spectra of isomers AQCZCS and CZCSAQ in THF (Con. 10 µM, λ_ex = 380 nm for AQCZCS and λ_ex = 400 nm for CZCSAQ).

Solvatochromism

The UV-vis absorption spectra of both AQCZCS and CZCSAQ remained invariant across solvents of varying polarity, with the lowest-energy absorption band (ICT) consistently centered at the same wavelength (Fig. S15). This insensitivity of the ground-state absorption to solvent environment suggests that the lowest-energy transitions possess predominantly locally excited (LE) character, with minimal redistribution of charge in the ground state. In contrast, the emission spectra revealed strong positive solvatochromism, characteristic of excited states with pronounced charge-transfer (CT) character. For AQCZCS, the emission maximum shifted from 550 nm in toluene to 635 nm in THF, while CZCSAQ showed an even more dramatic shift from 530 nm to >700 nm across the same solvent range. The greater solvatochromic response of CZCSAQ indicates that positioning the anthraquinone at the opposite end of the π-conjugated framework enhances donor–acceptor electronic communication, leading to stronger stabilization of the CT excited state in polar environments. The photoluminescence quantum yields (PLQYs) of both compounds were measured in various solvents to correlate emission efficiency with solvent polarity (Table S1). AQCZCS exhibited a high PLQY of 56.1% in toluene, which decreased significantly to 7% in THF, while CZCSAQ showed comparatively lower efficiencies of 5.0% and 1.8%, respectively. The pronounced drop in PLQY with increasing polarity further confirms the solvent-dependent charge-transfer nature of the emissive states. These findings reinforce the notion that while the ground states of the two isomers are relatively unaffected by solvent polarity, their excited states are highly sensitive to both molecular topology and solvent polarity effects.

Aggregation-induced emission (AIE) studies

The aggregation-induced emission (AIE) properties of AQCZCS and CZCSAQ were examined in DMSO/H₂O mixtures with varying water fractions (f_w), where the addition of water gradually reduces solubility and induces molecular aggregation (Fig. 2). For AQCZCS, the emission intensity showed no change at low water fractions but began to rise noticeably at f_w = 30%, reaching a maximum at f_w = 40%, after which a steady decrease was observed at higher water fractions. In contrast, CZCSAQ exhibited an aggregation-induced emission response, with the luminescence peaking at f_w = 60% and again showing a decline at higher water fractions. Notably, at f_w = 90%, CZCSAQ displayed a secondary enhancement in emission intensity with a red-shifted emission maximum, suggesting the formation of more compact and emissive aggregates at very high-water content. Importantly, the maximum emission intensity of AQCZCS was nearly fifteen times higher than that of CZCSAQ, underscoring the much greater efficiency of aggregate-induced restriction of intramolecular motions in the former. The different aggregation behaviors of the two isomers highlight the role of anthraquinone positioning in dictating molecular packing preferences and aggregation pathways. The emission maxima closely correlated with morphological observations from scanning electron microscopy (SEM) and particle size distributions obtained by dynamic light scattering (DLS) (Fig. S17 and S18). In both cases, the strongest emission coincided with the largest aggregate sizes, indicating that the restriction of intramolecular motions (RIM) in densely packed aggregates is the primary contributor to the observed AIE activity.⁵⁶ These results underscore that while both positional isomers display aggregation-enhanced emission, the distinct water-fraction dependence and large disparity in emission efficiencies reflect different aggregation modes governed by donor–acceptor topology.

Fig. 2 Fluorescence spectra of (A) AQCZCS and (B) CZCSAQ in DMSO-water mixtures with different water fractions. Digital photos taken for the various water fractions under UV 365 nm lamp are displayed at the top of the spectra. I/I₀ plots of (C) AQCZCS and (D) CZCSAQ with different fractions of water (Con. 10 µM, λ_ex = 380 nm for AQCZCS and λ_ex = 400 nm for CZCSAQ). (Instrumental parameters: excitation slit width and emission slit width = 20 nm).

Solid state emission and mechanochromism

In the solid state, both isomers displayed distinct photophysical characteristics influenced by the position of the anthraquinone unit. AQCZCS exhibited vivid-orangish red emission centered at 610 nm with a quantum yield of 2.7%, while CZCSAQ showed a further red-shifted emission at 620 nm with a quantum yield of 1.8% (Fig. 3). The red-shifted profile of CZCSAQ pushes its emission closer to the deep-red region, underscoring how anthraquinone positioning governs the extension of conjugation and stabilization of the excited state. Upon mechanical grinding, both isomers reveal mechanochromic fluorescence responses. AQCZCS undergoes a remarkable red-shift from 610 to 645 nm, whereas CZCSAQ shifts from 620 to 635 nm. PXRD analysis confirms that these optical transitions originate from a crystalline-to-amorphous phase transformation, where disrupted molecular packing enhances intermolecular interactions (Fig. S19).⁵⁷ Notably, the mechanochromic response is irreversible in both systems. Taken together, these results show that the donor–acceptor sequence not only dictates emission color: from orange to near deep-red, but also modulates mechanochromic sensitivity. The larger shift of AQCZCS contrasts with the intrinsically redder emission of CZCSAQ, underscoring a trade-off between mechanochromic amplitude and spectral positioning. Such insights provide valuable guidelines for designing responsive solid-state luminophores.

Fig. 3 (A) Solid-state emission spectra of isomers, MFC responses of (B) AQCZCS and (C) CZCSAQ.

Theoretical studies

Quantum-chemical calculations were undertaken to provide a molecular-level understanding of the structure–property relationships in AQCZCS and CZCSAQ. Ground-state geometries were optimized using DFT (B3LYP/6-31G(d,p)), and the absence of imaginary frequencies confirmed that both molecules are true minima.⁵⁸ Frontier molecular orbital (FMO) analysis revealed a pronounced donor–acceptor separation. The HOMO density is largely distributed over the carbazole and cyanostilbene segments, reflecting their strong electron-donating and conjugated nature, whereas the LUMO is entirely localized on the anthraquinone core. This orbital distribution highlights the charge-transfer (CT) character of the lowest-energy transitions. Interestingly, the HOMO–LUMO energy gap was calculated to be 2.63 eV for AQCZCS and slightly narrower at 2.54 eV for CZCSAQ, in line with the experimentally observed red-shifted emission of the latter. Excited-state properties were further examined by TD-DFT, using both B3LYP and M06 functionals in the presence of solvent (PCM, THF). While both functionals reproduced the main CT absorption band, M06 delivered excitation energies and oscillator strengths in closest agreement with experiment. Natural transition orbital (NTO) analysis further confirmed the intramolecular charge transfer nature of the S₁ state, with both isomers displaying clear spatial separation between the hole and particle densities localized on the donor and acceptor segments, respectively (Fig. 4).

Fig. 4 FMO analysis and calculated energy gaps of isomers (A) AQCZCS and (B) CZCSAQ. Natural transition orbitals (NTOs) calculated for S₁ of (C) AQCZCS and (D) CZCSAQ using M06 functional and 6-31G(d, p) basis set.

Thermal properties

The thermal stability of AQCZCS and CZCSAQ was evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). As shown in Fig. S22(A), both compounds exhibited good stability with decomposition temperatures (T_d, defined at 5% weight loss) well above 380 °C. Specifically, AQCZCS retained its structural integrity up to ∼410 °C, while CZCSAQ showed an onset of decomposition at ∼380 °C. Such high decomposition temperatures underscore the determined nature of the carbazole–cyanostilbene–anthraquinone framework, which is advantageous for practical applications in optoelectronic devices that require long-term thermal endurance. The DSC thermograms (Fig. S22(B)) further revealed distinct thermal transitions. AQCZCS displayed a relatively broad endothermic feature around 150 °C, likely corresponding to a phase transition. In contrast, CZCSAQ showed a sharp and well-defined endothermic peak at ∼210 °C, corresponding to its melting point, followed by a secondary thermal event near ∼230 °C that is attributed to the onset of thermal degradation.⁵⁹ During the cooling cycle, CZCSAQ also displayed prominent exothermic crystallization peaks, indicating strong recrystallization tendencies. The presence of multiple endothermic and exothermic features for CZCSAQ highlights its greater propensity for phase reorganization compared to AQCZCS. Inclusive, the thermal analyses confirm that both isomers are thermally robust, with decomposition temperatures well beyond those typically required for device fabrication.

Conclusions

This study demonstrates that the positional arrangement of anthraquinone within carbazole–cyanostilbene conjugates exerts a decisive influence on their photophysical behavior. AQCZCS and CZCSAQ, despite having identical donor, and acceptor units, display different emission profiles: in the solid state, AQCZCS shifts from vivid orangish red to deep red upon mechanical stimulation, whereas CZCSAQ exhibits red emission with slight bathochromic shift. These shifts highlight the tunability of solid-state emission through subtle positional connectivity of the acceptor unit. Solution-phase emission mirrors these connectivity effects, confirming strong donor–acceptor electronic communication. The pronounced bathochromic shifts, broad CT-character emission, and topology-dependent quantum yields underscore the critical role of donor–acceptor ordering in dictating excited-state properties. Altogether, these findings provide a clear structure–property blueprint for designing multifunctional solid-state luminophores with tunable orange-to-deep-red emission and responsive mechanochromic behavior.

Conflicts of interest

The authors declare no competing financial interest.

Data availability

The data underlying this study are available in the published article and its supporting information (SI). Supplementary information: ¹H NMR, ¹³C NMR, HRMS, and characterization data for the isomers, photophysical data, PXRD, SEM, DLS, DFT and TD-DFT results (PDF). See DOI: https://doi.org/10.1039/d5ra07931d.

Acknowledgements

CAS P thanks, SERB/EEQ/2021/000180 for funding and support. A. A thanks the National Institute of Technology, Calicut, for GATE-SRF fellowship. The authors acknowledge the Centre for Materials Characterisation NIT Calicut for NMR, PXRD facilities, HRMS (DST-FIST) and Centre for Computational Modelling and Simulations NIT Calicut. We would like to thank the SEM Centre, the Department of Materials Science and Engineering, National Institute of Technology Calicut for the SEM facility.

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