Dual-channel deep-NIR-emissive N-embedded PAHs with hybridized local and charge-transfer excited-state

Abstract

Polycyclic aromatic hydrocarbon (PAH) molecules have been extensively investigated, and they showcase excellent optoelectronic properties, which are promising for optical applications, including deep-penetration bioimaging and NIR lasers. However, constructing PAHs with deep-NIR (800–1700 nm) photoluminescence is a long-standing challenge, owing to the limitation of the energy gap law. Herein, three N-atom-doped PAHs APAH-a–c with electronic acceptor–donor–acceptor (A–D–A) configuration were produced via a facile sandwich-like peri-fusion pathway. On the basis of these three model molecules, their electronic characters and physicochemical properties were comprehensively studied by X-ray crystallographic analyses, various spectroscopic analyses and theoretical calculations. Our outcomes revealed that core-twisted APAH-b and APAH-c displayed unique dual-emission with reasonable photoluminescence quantum yield (PLQY) in low-polarity solvents, which were further red-shifted to ∼975 nm with increasing solvent-polarity due to their hybridized local and charge-transfer (HLCT) character. In sharp contrast, compound APAH-a with a planar π-skeleton did not show any charge-transfer (CT) character or fluorescent solvatochromism effect, indicating a synergistic effect of skeletal nonplanarity and electron-withdrawing N-doping for formation of an HLCT excited-state. Additionally, varying the π-configuration also gives rise to other differentials, such as aromaticity, band gap and open-shell characteristics. In conclusion, our findings offer a valid strategy for the development of deep-NIR-emissive PAH-derivatives via fine-tuning the geometrical structure of the π-framework.

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Introduction

Organic near-infrared (NIR)-emitting (700–2500 nm) materials have garnered significant attention due to their unique advantages, including low cost, mechanical flexibility, deep penetration, and biocompatibility, making them highly promising for broad applications, such as high-resolution bioimaging, NIR organic light-emitting diodes (OLEDs), and NIR organic lasers.^1–4 Among these materials, π-conjugated polycyclic aromatic hydrocarbons (PAHs) represent a class of particularly fascinating molecules due to their excellent photostability, high absorbance, tunable band gap, and emissive properties.^5–8 Conventional PAHs typically exhibit fluorescence via radiative transitions from a locally excited (LE) state to the ground state (S₀), characterized by a large transition moment with a significant orbital overlap, resulting in a high photoluminescence quantum yield (PLQY) but short emission wavelength within visible regions.⁹

To modulate the light-emitting properties of PAHs, such as the wavelength and PLQY, current approaches primarily involve the enhancement of π-conjugation or the introduction of a charge-transfer (CT) state.^10–12 For example, systematically extending the π-framework facilitates electron delocalization and π-conjugation, giving rise to a narrower band gap accompanied by red-shifted absorption and emission bands.^13–16 However, in view of the energy gap law, a small energy gap (S₀–S₁) could enhance internal conversion and thus block the radiative transition pathway.^17–19 Additionally, extending PAHs always involves tedious chemical syntheses. Conversely, fabricating PAHs with a charge-transfer (CT) character might enable greatly redshifted emission through electron-withdrawing heteroatom-doping like N or B atoms into the π-frameworks (Fig. 1b).^20–24 However, CT-type PAHs generally exhibit low-efficiency photoluminescence, especially in high-polarity environments, due to a decreased transition dipole moment and a slow radiative transition rate.^25–28 Consequently, preparing PAHs with deep-NIR (>800 nm) photoluminescence remains a challenge.

Fig. 1 (a) Illustration of relaxation pathways of a D–A-type π-conjugated system from the excited state (S₁) to the ground state (S₀). (b) Scheme of geometrical twisting-governed HLCT character of APAH-a–c.

The emerging concept of a hybridized local and charge-transfer (HLCT) state might offer a promising solution. For a quasi-rigid donor–acceptor (D–A) π-conjugated system, an appropriate twisting angle can fine-tune the orbital mixing and the interconverted LE and CT configuration, thereby generating a unique state with admixed LE and CT characters, a so-called HLCT excited-state.^29–31 As a result, the HLCT characteristic guarantees compatibility of the long-wavelength photoluminescence and the large radiative decay rate. Despite the significant potential of HLCT behavior in achieving high-efficiency fluorescence, developing PAHs with intrinsic HLCT behavior remains extremely rare due to the unmanageable equilibrium of the LE and CT states. Until very recently, when Lu et al. reported a series of D–A-type heterocycloarenes with a curved π-skeleton, in which one species exhibited a typical HLCT character while the others displayed CT-dominant fluorescence.³² In this case, these PAHs obviously possess a non-planar π-framework, leading to an appropriate twisting angle between electron-rich and electron-deficient segments upon photoexcitation, which plausibly provides an extra opportunity for the formation of HLCT character. However, the sophisticated mechanism for precise excited-state manipulation of PAHs is as yet ambiguous.

Based on the abovementioned considerations, herein we designed and prepared three doubly N-embedded model molecules, APAH-a, APAH-b and APAH-c, featuring deep-NIR emission tuned by HLCT characters (Fig. 1b). Topologically, the sandwich-like peri-fusion of two electron-withdrawing quinoline units (weak acceptor) and one electron-donating tetracene/pyrene (weak donor) moiety might exhibit moderate electronic pull–push interaction and thus prevent an over-expressed CT effect. Conversely, from APAH-a to APAH-c, increasing cove-regions of the π-skeleton give rise to facile variation in molecular geometry upon photoexcitation, which facilitates the disruption of excited-state symmetry and polarized charge separation.^33,34 In this way, the HLCT character could be well controlled by the synergistic effect of electron-withdrawing N-doping and a topologically edged structure. As expected, optical measurements of APAH-b and APAH-c demonstrated unique polarity-tuned dual emission at the deep-NIR region, attributed to their HLCT characteristic, which was absent for planar APAH-a. Additionally, experimental studies accompanied by theoretical calculations unveiled significant differences in their electronic structures and physicochemical properties, such as band gaps and aromaticity. More intriguingly, APAH-c showcased a thermally-switched diradicaloid character. Overall, our investigation opens new avenues for exploring excited-state dynamics of aza-PAHs with HLCT characteristics and provides a feasible protocol for developing PAH-based deep-NIR emitters.

Results and discussion

Synthesis and structural characterization

The sophisticated integration of cove-edged topologies can impart twisted geometries, increased solubility, and unique π-electronic structures to PAHs.^35–38 However, constructing small-sized PAHs with multiple cove-edges, such as the dibenzoterrylene (DBT) molecule, is still a tough challenge because of the robust structural strains.^39–42 Fortunately, the incorporation of a heteroatom into the π-framework might offer an approach for tailoring novel PAHs with greatly twisted π-backbones.^43,44 Herein, via a universal two-step synthetic method, three aza-PAHs with either planar (APAH-a) or varied degrees of backbone distortion (APAH-b and APAH-c) were prepared (Scheme 1). Briefly, starting from bis((trimethylsilyl)ethynyl-substituted pyrene/tetracene derivatives that were pre-equipped with solubilizing groups, such as n-butyl phenyl or an n-butoxy chain, key precursors 1–2 and 2–2 were first prepared via a one-pot cascade reaction involving a tandem desiliconization, Povarov [4 + 2] heterocyclization, and oxydehydrogenation with overall yields of 78%/63%.

Scheme 1 Synthesis of APAH-a–c (TBAF: tetrabutylammonium fluoride; TBAB: tetrabutylammonium bromide; ⁱPrCOOH: pivalic acid).

The resulting brominated quinoline moieties in 1–2/2–2 allow a further intramolecular cyclodehydrohalogenation process via a twofold Heck coupling reaction assisted by a palladium catalyst and a base.⁴⁵ Specifically, fully planar APAH-a was easily obtained with a satisfying yield of 73% under mild Heck reaction conditions (Pd(dba)₂/ⁱPrCOOH/Cs₂CO₃/90 °C). However, likely due to the increased steric hindrance upon the simultaneous formation of multiple spatially crowded cove-regions, such conditions failed to afford APAH-b and APAH-c. Therefore, a systematic screening of reaction conditions (Table S1) was conducted, ultimately revealing that harsher conditions (Pd(^tBu₃P)₂/Cs₂CO₃/130 °C) were required to afford the two cove-region-containing aza-PAH molecules as a mixture, which was inseparable by conventional purification techniques, such as column chromatography or recrystallization. Considering the possible production of stereoisomers with differential geometries, we then chose gel permeation chromatography (GPC) to separate the mixtures to successfully obtain the two stereoisomers with a total yield of 70% (Fig. S1). It is also worth noting that APAH-b and APAH-c were obtained in an approximate molar ratio of 1 : 2, likely indicating that the C atoms at the inner ring of tetracene have higher reactivity toward the intramolecular cyclodehydrohalogenation process. This separation process indicated that GPC is an effective tool for the isolation of structural isomers, which has rarely been reported in the past.

The resulting aza-PAHs showed moderate solubility in common organic solvents (such as toluene and dichloromethane) and high stability both in the solid state and solution state while stored under ambient conditions. These compounds were also characterized by ¹H nuclear magnetic resonance (NMR) spectroscopy and high-resolution mass spectrometry (HRMS), and the results agreed with their corresponding chemical structures. As the ¹³C NMR spectra of APAH-b and APAH-c were indistinguishable, 2D ¹H–¹H NOESY (nuclear Overhauser effect spectroscopy) NMR was further conducted to corroborate their chemical structure (Fig. S2 and S3).

X-ray crystallography

Single crystals suitable for X-ray crystallographic analysis of APAH-a and APHA-b were successfully obtained by either slow evaporation solution growth or vapor diffusion techniques. However, attempts to grow single crystals of APAH-c were unsuccessful. Fortunately, our long-term attempts have led to the serendipitous finding of its dealkylated product (Fig. S4), thereby also providing evidence for the structural accuracy of the APAH-c compound. As expected, the resolved single-crystal X-ray structure uncovered an almost planar π-conjugated backbone of APAH-a with negligible twisting at its bay-regions (Fig. 2a). Despite its bulky benzoic substituents, it was found that two adjacent APAH-a molecules are organized into dimeric assemblies with multiple short π–π contacts (3.330–3.354 Å, Fig. S5a), which were further stacked in a tight brickwork packing pattern (Fig. S5b). In sharp contrast, APAH-b exhibited a prominently distorted backbone skeleton featuring three cove regions and one bay area with large dihedral angles of 19.0°–40.2° (Fig. 2b), induced by the substantial steric repulsion between the adjacent hydrogen atoms. As a result, in the crystal lattice, an identical number of stereoisomeric enantiomers were found for APAH-b without any close π–π contacts (Fig. S6). Moreover, the DFT-optimized structure of APAH-c showed an extremely twisted π-backbone with large dihedral angles of 36.8°–38.7° (Fig. S7). Bond-length analyses revealed that the lengths of linking C–C bonds between two quinoline and the central pyrene units of APAH-a were almost equal (1.466–1.467 Å), while those of APAH-b displayed very different lengths (e.g., 1.462 vs. 1.403 Å), indicating intense strains for the bridged six-membered rings.

Fig. 2 X-ray crystallographic structures of APAH-a (a) and APAH-b (b) with highlighted bond-lengths (Å) (black) of the linkage bonds and twisted angles (green) in bay and cove regions; hydrogen atoms are omitted for clarity.

Theoretical calculations of aromaticity

Based on their crystallographic structures, the nucleus-independent chemical shift (NICS) values and anisotropy of the induced current–density (ACID) plots of APAH-a and APAH-b were calculated to probe ring aromaticity upon backbone twisting (Fig. S7 and S8).^46,47 It was revealed that, while the other benzene rings are aromatic, the bridged hexagons of APAH-a showed an evident antiaromatic character with positive NICS(1)_ZZ values of 12.4 ppm and 12.5 ppm, respectively. For APAH-b, however, the NICS(1)_ZZ values of the corresponding hexagons were 6.7 ppm and 2.5 ppm, respectively. Such a trend from antiaromatic to nonaromatic character of the two bridged hexagons is in line with the increased ring strains within APAH-b. The other hexagons of APAH-a and APAH-b maintained largely negative values (−11.0 to −23.7 ppm) except for the central ring on APAH-b. As depicted in Fig. S7, the ring displayed weak aromaticity (−7.1 ppm) resulting from the bending geometry of the tetracene unit. For APAH-c, its DFT-optimized structure was adopted for comparison, which showed that the central rings of its tetracene moiety exhibited weak aromaticity, with NICS(1)_ZZ values of −8.8 and −9.4 ppm. The calculated ACID plots further revealed that the planar APAH-a exhibited global π-conjugation, as evidenced by a clockwise (diatropic) ring current delocalized along the periphery of the entire π-backbone. In contrast, APAH-b showed separately delocalized ring currents over the periphery of both the quinoline and tetracene subunits, likely due to the interference from its backbone twisting. Similarly, APAH-c also displayed evident fragmentally delocalized aromaticity for the quinoline and tetracene moieties, as suggested by the ACID plots, agreeing well with its calculated NICD results. Clearly, these results suggest the substantial impact of geometry twisting on the π-electron delocalization pathway of these aza-PAHs.

Optical and electrochemical properties

Fig. 3a depicts the UV-vis-NIR absorption spectra of the three PAHs in their dilute toluene solutions. Compound APAH-a displayed a well-resolved absorption band with a maximum (λ_abs) at 592 nm (ε = ∼108 000 M⁻¹ cm⁻¹, ε: molar extinction coefficient), which can be assigned to the HOMO to LUMO transition on the basis of TD-DFT calculation (Fig. S9 and Table S2). Going on to the two nonplanar aza-PAHs, similar absorption patterns were observed; however, their major absorption bands were significantly red-shifted into the near-infrared regions, with maxima at 772 (ε = ∼98 000 M⁻¹ cm⁻¹) and 814 nm (ε = ∼97 000 M⁻¹ cm⁻¹) for APAH-b and APAH-c, respectively, also arising from their HOMO to LUMO transitions (Fig. S10 and S11, Tables S3 and S4). Notably, despite its more twisted molecular geometry, APAH-c displayed a 42 nm redshift compared to its regioisomer APAH-b, resulting in a narrowed optical energy band-gap (1.25 eV for APAH-cversus 1.45 eV for APAH-b), as estimated from the onset of the lowest energy absorptions. DFT calculations of the frontier molecular orbitals (FMOs) further revealed that the LUMO energy levels of APAH-a–c were almost equivalent, whereas the HOMO levels greatly increased from −4.86 eV for APAH-a to −4.35 eV for APAH-b and −4.29 eV for APAH-c (Fig. S12), giving gradually narrowed band gaps and correlating well with the variations in the values obtained from their optical spectra.

Fig. 3 UV-vis-NIR absorption spectra (a) and normalized photoluminescence spectra (b) of APAH-a–c in toluene (c = 10⁻⁵ M). (c) Cyclic voltammograms of APAH-a–c in DCM/PhCl₂ with 0.1 M ⁿBu₄N⁺PF₆⁻ as the supporting electrolyte; the electrode potential was externally referenced to the ferrocene/ferrocenium redox couple.

The emission behaviors of PAHs were then examined by recording both the steady-state and time-resolved photoluminescence (PL) spectra of APAH-a–c in toluene (Table 1). APAH-a exhibited intense red fluorescence with emission maximum (λem) at 612 nm (Fig. 3b). Due to its rigid and planar π-framework, the emission profiles of APAH-a also resembled its absorption pattern in a mirror-symmetric fashion with a relatively small Stokes shift of 552 cm⁻¹. Compared to APHA-a, a broad emission band, covering a broad range of 750–1400 nm, with λ_em at 914 nm and a distinct shoulder at ∼845 nm was observed upon excitation at 700 nm for APAH-b. Similar to APAH-b, the fluorescence spectrum of APAH-c displayed a λ_em at 915 nm and an obscure shoulder at ∼870 nm upon excitation at 720 nm. The excitation spectra of APAH-a–c were measured (Fig. S13), which are coincident with their absorption spectra, proving their intrinsically stimulated radiation behavior. To the best of our knowledge, the fluorescence of APAH-b and APAH-c accesses the longest wavelength among the previously available N-embedded PAHs, including those with greatly extended π-skeletons (Fig. S16). It is noteworthy that, in comparison with the pristine DBT molecule without N-doping, the absorption and emission bands of APAH-c exhibit bathochromic-shifts of ∼74 nm and ∼133 nm, respectively, resulting from the enhanced charge-transfer upon electron-withdrawing N-doping.⁴⁸

Table 1 Photophysical properties of APAH-a–c

Compounds	λ Abs (nm)	λ em (nm)	ϕ F (%)	τ (ns)
APAH-a	592	612	74.3	2.86
APAH-b	772	914	5.11	0.48, 16.20
APAH-c	814	915	5.19	0.52, 16.36

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The absolute photoluminescence quantum yields (PLQYs) of the three aza-PAHs were then determined based on an integrating sphere technique. It was found that APAH-a has a PLQY of 74.3% with an excited-state fluorescence lifetime (τ) of 2.86 ns in toluene (Fig. S14), whereas those of APAH-b and APAH-c in toluene were estimated to be 5.11% and 5.19%, respectively. The depressed PLQYs of APAH-b/c compared to APAH-a might stem from their combination of flexible π-frameworks and narrowed band-gaps, which cooperatively enhance nonradiative decay channels through vibronic coupling and intramolecular relaxation processes. Intriguingly, the excited-state fluorescence lifetime (τ) of APAH-b (τ₁/τ₂ = 0.48/16.20 ns) and APAH-c (τ₁/τ₂ = 0.52/16.36 ns) in toluene displayed typically biexponential decay (Fig. S14), implying dual-channel radiative transitions upon photoexcitation, correlating well with their emission profiles, as depicted in Fig. 3b.⁴⁹ Such short lifetimes accompanied demonstrations that no triplet excited-state species were formed for either molecule. Moreover, the calculated singlet and triplet energy levels of APAH-b and APAH-c revealed that both had large S₁–T₁ and S₁–T₂ energy gaps (ΔE_S1–T1 and ΔE_S1–T2) of 0.39–1.16 eV (Fig. S15), which resulted in ineffective intersystem cross (ISC) pathways. Tentatively, dual radiative transition pathways are plausibly attributed to the hybridization of a local and charge-transfer (HLCT) excited-state (vide infra).

The electrochemical characteristics of APAH-a–c were investigated by cyclic voltammetry (CV) and differential pulse voltammetry measurements (DPV), showing evident ambipolar redox profiles for the three aza-PAHs (Fig. 3b and S17). Each of them exhibited two reversible reductive curves and two reversible oxidative profiles. More specifically, APAH-a/b/c showed first reduction and oxidation potentials of −1.41/−1.24/−1.24 V and 0.33/0.16/0.04 V (vs. Fc/Fc⁺), respectively. Based on the onset potentials of the first redox transitions, the estimated HOMO energy levels were upshifted from −5.13 eV for APAH-a, −4.86 eV for APAH-b, to −4.71 eV for APAH-c, whereas an identical LUMO energy level of −3.67 eV was found for APAH-b and APHA-c, both of which were downshifted compared to that of APHA-a (−3.49 eV). Accordingly, while an electrochemical band gap of 1.64 eV was calculated for APAH-a, the two aza-PAHs, APAH-b and APAH-c, exhibited profoundly smaller band gaps of 1.19 eV and 1.04 eV, respectively (Table S5). Such narrowed band gaps might give rise to the open-shell electronic characteristics in the PAH-frameworks due to the intensified coulombic repulsions between their valence electrons (vide infra).^50,51

Open-shell characteristics

To verify the abovementioned assumption, spin-unrestricted DFT calculations were performed at (UCAM-B3LYP/6-31G(d,p)) level. These calculations revealed that APAH-b and APAH-c displayed weak diradical character (y₀ = 6.9% and 9.1%) with large energy gaps between the singlet and triplet states (ΔE_S–T = −10.89 and −8.95 kcal mol⁻¹, respectively), thus suggesting a minor contribution from the open-shell resonance structures for those two twisted molecules at room temperature. The diradical character y₀—a dimensionless parameter quantifying the contribution of open-shell resonance configurations to molecular electronic structure (0–100%)—is calculated from the occupation number of the lowest unoccupied natural orbital (LUNO) in spin-projected wavefunction analyses.⁵² To validate their open-shell characteristics, variable-temperature ¹H NMR and variable-temperature electron spin resonance (ESR) measurements were then carried out. It was found that heating APAH-b solution to 373 K led to negligible NMR broadening due to the large ΔE_S–T (Fig. S18). Meanwhile, APAH-b powder also showed an ESR-silent signal with increasing temperature, confirming that APAH-b is more likely to be a fully closed-shell molecule.

In sharp contrast to APAH-b, the ¹H NMR spectra of APAH-c showed a gradual broadening of aromatic resonance signals were observed with the increase in temperature from 273 K to 353 K, likely arising from its thermally excited paramagnetic character (Fig. 4a).^53,54 The resonances of H_c and H_e were found to have almost disappeared at 353 K, which should be attributed to the larger spin populations on their neighboring carbon atoms than the other aromatic protons (Fig. 4c, S19 and S20). For its solid ESR spectra, it was also found that APAH-c displayed a negligible ESR signal at room temperature, whereas the intensity increased continuously as the temperature increased from 293 K to 423 K (Fig. 4b), which is a typical characteristic for singlet diradicaloids.^55,56 The g_e factor of APAH-c powder was estimated as 2.0028, suggesting that the unpaired spins are ascribed to carbon-based radicals.

Fig. 4 (a) Variable-temperature ¹H NMR spectra (aromatic region) of APAH-c recorded in C₂D₂Cl₄ (the chemical shift of the solvent molecule is at 6.00 ppm). (b) Variable-temperature ESR spectra of APAH-c in solid state. (c) The Mulliken spin density distribution of APAH-c (isovalue = 0.008).

To further confirm the thermal-activated open-shell characteristic of APAH-c, its variable-temperature absorption spectra were measured in a strictly deaerated tetrachloroethane solution (Fig. S21). As the temperature increased from 293 K to 393 K, the main absorption centered at 815 nm was slightly attenuated, accompanied by an emerging broad band tailing up to 1050 nm. This weak absorbance at the lowest-energy edge can be ascribed to the characteristic forbidden double-excitation state for open-shell diradicaloids.⁵⁷ More importantly, such spectral band evolution was found to be totally reestablished with high reversibility after cooling to room temperature. Overall, these results confirm the thermally accessible diradicaloid character of APAH-c, which can be as attributed to its greater number of cove-regions, leading to robust steric repulsion and intensified coulombic repulsions between their valence electrons compared to the APAH-b molecule (which could be relieved by adopting the open-shell form).^51,58

Photoexcitation dynamics

To shine light onto the possible CT state of these aza-PAHs, their absorption and emission spectra in different solvents with varied polarities were carefully measured. The shapes and positions of the absorption and emission profiles for APAH-a were almost maintained in all solvents despite their distinct polarities (Fig. S22 and Table S6). For APAH-b and APAH-c, however, while their absorption spectra also showed negligible variations as the polarity increased (Fig. S23), obvious variations in emission bands were clearly observed for both molecules (Fig. 5). For instance, in lowest-polar hexane, APAH-b exhibited a broad emission band with a maximum at 810 nm and a shoulder at ∼898 nm (Fig. 5a). Moving on to tetrachloromethane (CCl₄) with higher polarity, a well-resolved dual-emission band with two peaks at 820 nm and 910 nm was clearly observed. However, with a further increase in solvent polarity, the dual-emission character gradually disappeared, accompanied by a redshift of the emission band with a λ_em from 910 nm in CCl₄ to 935 nm in DMF. It was also found that the increase in solvent polarity also caused a gradual decrease in PLQYs from 9.63% in hexane to 0.31% in DMF, as well as significantly enlarged Stokes shifts (from 777 cm⁻¹ in hexane to 2342 cm⁻¹ in DMF) (Table S7). Such evolutions in emission, including the positive solvatochromism phenomenon (from 845 nm in hexane to 975 nm in DMF), decreased PLYQ (from 14.32% in hexane to 0.23% in DMF), and boosted Stokes shifts (from 496 cm⁻¹ in hexane to 2028 cm⁻¹ in DMF), were also found for APAH-c, revealing the similar impact of backbone twisting on their photophysical properties (Table S8). Moreover, double-exponential analysis of the excited-state decay kinetics for APAH-b and APAH-c revealed that the fluorescence lifetime components τ₁ and τ₂ exhibit pronounced and contrasting trends with increasing solvent polarity (Fig. S24, Tables S9 and S10). Specifically, the fraction of the shorter-lived component (τ₁) decreases significantly, while that of the longer-lived component (τ₂) increases concomitantly. This evolution in decay kinetics aligns closely with the observed changes in their steady-state emission spectra, providing further compelling evidence for the polarity-dependent dual excited-state characteristics of APAH-b and APAH-c.

Fig. 5 Photoluminescence profiles of (a) APAH-b and (b) APAH-c in solvents (1 × 10⁻⁵ M) with varying polarities.

Given their intriguing variations in emission profiles in different solvents, the excited-state evolutions of APAH-b and APAH-c were then revealed by classical Lippert–Mataga solvatochromic models.²⁶ For this purpose, the absorption and emission bands of APAH-b and APAH-c were measured in various solvents with different orientation polarizability (f), and the corresponding Stokes shifts were estimated (Fig. S25–S27; Tables S10 and S11). The Lippert profiles of APAH-b and APAH-c displayed a typical two-section linear relation, which is characteristic of the HLCT excited-state.³⁰ Upon transitioning from low-polarity solvents to medium-to-high-polarity media, both compounds exhibited a marked discontinuity in the slope of their Stokes shift–polarity correlation plots. The non-monotonic Stokes shift dependence on solvent polarity, coupled with their pronounced fluorescent solvatochromism, suggests the emergence of polarity-dependent excited-state characteristics—demonstrating progressive evolution from an LE-dominated HLCT state to a CT-dominated configuration as solvent polarity increases.

Moreover, natural transition orbital (NTO) analyses revealed that the hole and electron species of the S₁ state for APAH-a were distributed over the whole molecule, while the hole and electron distributions of APAH-b and APAH-c exhibited partial spatial separation, especially on the tetracene subunit, revealing LE-dominant HLCT characteristics, as mentioned above (Fig. S28).^59–61 Notably, APAH-b and APAH-c, featuring a multi-cove architecture and conformational flexibility in their π-conjugated backbones, undergo substantial dihedral angle reorganizations between the ground state (S₀) and excited state (S₁) (Fig. S29). This structural feature correlates with their unique excited-state dynamics, similar to DBT, underscoring the critical role of molecular topology in modulating photophysical pathways.⁴⁸

To further confirm the existence of HLCT states of APAH-b and APAH-c, the excited-state relaxation dynamics were investigated using femtosecond transient absorption (TA) spectroscopy. Following a 400 nm laser pulse with 50 fs duration, the time-resolved TA spectra of APAH-b and APAH-c in CCl₄ and DCM, respectively, were recorded, spanning a wavelength range from 750 nm to 1500 nm (Fig. 6a and b; S30). As shown in Fig. 6a, the spectra in the low-polarity solvent CCl₄ display two broad photo-induced absorption (PIA) features centered at 1152 nm and 1340 nm, which are attributed to the charge-transfer (CT) state and the locally excited (LE) state, respectively. The strong and long-lived ground-state photo-bleaching (PB) peak at 830 nm correlates with the intense photoluminescence (PL) and high PLQY observed for APAH-c in CCl₄. Interestingly, as shown in Fig. 6c, the two PIAs and the PB share the same decay dynamics over a timescale of up to 5000 ps. This behavior reflects the slow vibrational relaxation between the LE and CT states, which facilitates the equilibrium between the two states due to an energy barrier separating them. These findings highlight the role of the solvent in stabilizing the LE and CT states, enabling radiative transitions and extending the excited-state lifetimes.

Fig. 6 Pseudo-color representation of fs-TA spectra of APAH-c recorded in CCl₄ (a) and DCM (b) following photoexcitation at 400 nm. TA decay dynamics of ground-state photo-bleaching (PB) at 830 nm and photo-induced absorptions (PIAs) for APAH-c in CCl₄ (c) and DCM (d). Schematic relaxation pathways in low-polarity (e) and high-polarity (f) solvents after excitation.

In contrast, APAH-c in DCM, a high-polarity solvent, exhibits a significantly narrowed absorption feature centered at 1340 nm with much faster decay (Fig. 6b and d). The stronger solvent–solute polarization effect in DCM stabilizes the CT state, lowering its energy level relative to the LE state and favoring its population. The dominance of the CT state results in faster relaxation dynamics, as evidenced by the rapid decay of the photo-induced absorption (PIA) at 1340 nm, with lifetimes of approximately 10 ps. Furthermore, the weaker and faster-decaying PB signal at 810 nm in DCM contrasts with the stronger PB signal in CCl₄, supporting the conclusion that non-radiative decay pathways dominate in high-polarity environments. The narrower energy gap between the CT and ground states in DCM facilitates non-radiative deactivation processes, such as internal conversion (IC), explaining the significantly lower PLQY of APAH-c in DCM. Similar measurements were conducted for APAH-b (Fig. S30), and the results were consistent with those observed for APAH-c.

Based on systematic fs-TA measurements, the excited-state relaxation dynamics of APAH-b or APAH-c in solvents with different polarities are schematically described in Fig. 6e and f. In a low-polarity solvent, vibrational relaxation from the S₁ (LE) state to the S₁ (CT) state is relatively slow due to the energy barrier, and an equilibrium is established between them. Subsequently, the excited species return to the ground state via two radiative transition channels (PL₁ and PL₂) from the lowest, resulting in characteristic dual emission. Meanwhile, a stronger solvent–solute polarization effect in a polar environment lowers the energy of the CT state, and therefore, the excited molecules exhibited a greater population of the CT state rather than the LE state.

Conclusions

In summary, we demonstrated an efficient synthetic pathway to the peri-fusion of two electron-withdrawing quinoline segments with one electron-donating tetracene (or pyrene) unit, and consequently obtained a new class of aza-PAHs APAH-a–c. The electronic properties of these aza-PAHs were governed effectively by the increasingly twisted π-backbones, which were reflected by their band gaps, optical spectra, and redox potentials. Specifically, APAH-b and APAH-c with narrow band gaps exhibited deep-NIR photoluminescence with a maximum approximating 1000 nm, a feature very scarce for PAH molecules. Meanwhile, the unique HLCT characters of APAH-b and APAH-c were unveiled by their fluorescence solvatochromism effect and excited-state dynamics, as well as theoretical calculations, which were not observed for APAH-a due to its geometrically planar configuration. Our results also suggested that the thermally accessible open-shell characteristics are relevant to the unique topological twisting. Overall, this study has provided guidance for the future structural engineering of deep-NIR-emissive PAHs with polarity-dependent HLCT excited-states.

Author contributions

Z. Z., D. W. and K. Y. conceived the project. Z. L. conducted the synthesis and carried out the physical characterizations and the major of property studies. Y. Z. and Z. Z. performed the femtosecond transient absorption studies. K. Y. and Z. L. participated in writing the manuscript. Z. Z., K. Y., D. W. and Y. Y. discussed the results and commented on the manuscript. All authors have given approval to the final version of the manuscript.

Conflicts of interest

The authors declare that they have no conflict of interest.

Data availability

The data supporting this article have been included in the SI.

All synthetic protocols, spectroscopic data, SI figures and tables, and detailed crystallographic information can be found in the SI. In detail, these include: synthesis and characterization, NMR and HRMS spectra, data of X-ray crystallographic studies, quantum chemical calculations. See DOI: https://doi.org/10.1039/d5sc01897h.

Acknowledgements

The authors thank the National Natural Science Foundation of China (52525306 and 22375059), Special Funds for the Construction of the Science and Technology Project of Hunan Province (2024RC1027), Natural Science Foundation of Hunan Province (2024JJ4013), Guangdong Basic and Applied Basic Research Foundation (2025A1515010687), the National Natural Science Foundation of China (52273174), the National Natural Science Foundation of China (52403158) for financial support.

Notes and references

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