Environment-sensitive fluorescent probes are a specific category of dyes that are designed to track changes in the environment by detecting alterations in fluorescence intensity or color [1-3]. A variety of fluorophores have been developed for monitoring pH [4-7], reactive oxygen species (ROS) [8,9], viscosity [10], ions [6,11,12], bioconjugation reaction [13,14], cellular organelles [15,16], polarity [17] etc. These fluorophores operate through mechanisms such as charge transfer, twisted intramolecular charge transfer [18,19], intramolecular proton transfer [20], conformational changes [21,22], and changes in aggregation states [23]. AIEgen, for example, which has a rotatable molecular structure, serves as an efficient probe for tracking microenvironmental viscosity and aggregation states [24-26]. Moreover, fluorophores with dynamic molecular structures can measure changes in free volume in polymer matrices. There are also other fluorophores available for monitoring extremely harsh environments, such as high pressure or high temperature [27,28]. Solvatochromic dyes, which fall under this category, are capable of changing their color in response to the polarity of the solvent [29,30].

Classical solvatochromic dyes that consist of both donor and acceptor moieties, known as push-pull fluorophores [31]. This push-pull structure facilitates the transfer of charge from the donor moiety to the acceptor upon light absorption, leading to the formation of a highly dipolar excited state. The excited state, which is highly dipolar, interacts with the dipole of the solvent and relaxes, resulting in a redshift in wavelength in solvents with high polarity. Various push-pull dyes, such as Prodan [32,33], Dansyl [34], Anthradan [35], FR0 [36], PA [37,38], 4DMP [39], NBD [40], Coumarine 153, Dapoxyl [41], Nile Red, DCDHF-6 [42] and 3MC-2 (Fig. 1a) [43], have been extensively studied in the literature. However, the selection of the most suitable fluorescent tool requires careful consideration of several factors, including quantum yield, absorption and emission wavelengths, photostability, chemical stability, and Stokes shift. To address these requirements, scientists have designed a variety of dyes with distinct structural features. Each dye presents its own set of advantages and disadvantages, which must be carefully evaluated for specific applications. Consequently, the continuous development of new small molecule probes is crucial to encompass all necessary elements while minimizing compromises [44].

The development of new solvatochromic dyes currently relies on modifying existing fluorophores. In a recent study, we reported a series of bisbenzo[f]isoindolylidenes compounds from dipropargyl benzenesulfonamides (Fig. 1b) [45]. Through the modification of dipropargyl benzenesulfonamides, herein we have successfully synthesized a series of novel biisoindolylidene compounds (Fig. 1c). In comparison to previous work, the new probes also offer the advantage of tunable emission and superior photostability. Furthermore, these new probes exhibit large stokes shift and more sensitive environmental responses. By introducing an electron-donating group at the 3,3′ position, it becomes feasible to transition the excited state of these compounds from a locally excited state to a charge-transfer excited state, resulting in remarkable solvatochromic properties.

With 1s as the model substrate, we optimized the conditions for the synthesis of (E)-3,3′-diphenyl-1,1′-biisoindolylidene (1). Various catalysts were screened using K₂CO₃ as a base and ⁿBuOH as a solvent at 80 ℃ in the presence of air (Table 1, entries 1–4). The dimeric product 1 could be obtained in 33% yield without any catalyst (Table 1, entry 5). None of the other bases and solvents resulted in an improvement in the reaction yield (Table 1, entries 6–13). Interestingly, the use of NaClO as a co-oxidant increased the yield of the target product 1 to 41% (Table 1, entry 14). Furthermore, altering the temperature did not have any significant impact on the yield of product 1 (Table 1, entries 15–17).

The structures of compound 1 was confirmed using X-ray crystallography. The crystal structure shown in Fig. 2, compound 1 is observed to be a rigid and coplanar compound with C_2h symmetry in its solid state. The dihedral angle between the phenyl group and the biisoindolylidene skeleton is measured to be 37.2°. The packing model reveals that both molecules are closely packed together in a J-type aggregation, with a distance of approximately 3.37 Å between neighboring molecules, indicating π-π surface overlap (Fig. 2c). The phenyl group at the 3,3′-position effectively extends the π-conjugation of the biisoindolylidene skeleton, providing electronic diversity. Consequently, modifying substituent groups, specifically the aryl substituent groups at the 3,3′-position, allows for the synthesis of various intriguing biisoindolylidene derivatives with distinct photophysical properties [46].

Based on the results of condition screening and crystal analysis, we synthesized a series of biisoindolylidene derivatives with various substituents in moderate yields (Fig. 3). Subsequently, we investigated their UV–vis absorption spectra, fluorescence spectra, and photoluminescence quantum yield in DCM. As shown in Fig. 3 and Table 2, the photophysical properties of the compounds 1–5 remained largely unchanged when substituent groups R¹ and R² were altered. The compounds exhibited a maximum absorption wavelength around 450 nm, while the maximum emission wavelengths ranged from 545 nm to 584 nm. However, the photoluminescence quantum yield was found to be very low. The introduction of bromine atoms (6) on the benzene ring of R³ did not significantly affect the photophysical properties. In contrast, the introduction of an electron-donating group on R³ resulted in noticeable changes in photophysical properties (7–10). The emission wavelength of the compounds gradually red-shifted, the photoluminescence quantum (PLQY) efficiency was significantly improved, and have a large Stokes shift (reached a maximum of 138 nm). Furthermore, the introduction of an electron-rich thiophene group in R³ also led to significant changes in photophysical properties (11 and 12). These findings suggest that the substitution at 3,3′-position does have a significant effect on the photophysical properties of biisoindolylidene derivatives. Additionally, the structure of compounds 2, 4, 5, 7, 8, and 9 were also confirmed by X-ray single crystal diffraction.

To investigate the impact of substituents on orbital energies, we conducted DFT theoretical calculations using the B3LYP/6–31G(d,p) [47] level for compounds 1, 9, and 10 (Fig. 4). The HOMO energy levels of compounds 9 and 10, which had electron-donating groups at the terminal aromatics, increased by 0.31 eV and 0.83 eV, respectively, compared to compound 1. Consequently, the energy gap of compounds 9 and 10 were reduced to 2.51 eV and 2.31 eV, respectively. These findings suggest that the HOMO of this π-conjugated skeleton is significantly influenced by the terminal aryl groups. Compound 1 exhibits local excitation (LE) character as both its HOMO and LUMO are completely delocalized over the conjugation plane. On the other hand, the LUMOs of compounds 9 and 10 are distributed on the biisoindolylidene backbone, indicating its high electron-accepting properties. In contrast, the HOMO is mainly distributed on the electron-donor substituted aryl group at the 3,3′-position and the conjugated structure. The observed charge separation from the distribution of HOMO and LUMO suggests that the HOMO/LUMO transitions of compounds 9 and 10 exhibit charge transfer (CT) character.

The solvatochromic properties of compounds 1, 9, and 10 were investigated in different polar solvents, based on their calculated LE or CT character (Fig. 5 and Table 3) [48]. Compound 1 exhibited no significant change in emission wavelength (Figs. 5d and g), whereas compounds 9 and 10 displayed a red-shift in their maximum emission wavelengths as the polarity of the solvent increased (Figs. 5e, f, h and i). The PLQY of compounds 1, 9, and 10 decreased as the solvent polarity increased. Compound 1 did not show a significant change in the Stokes shift with different polarities. However, compounds 9 and 10 displayed an increase in the Stokes shift as the solvent polarity increased. Notably, compound 10 had a maximum Stokes shift of 197 nm (Table 3). These large Stokes shifts and red-shifted fluorescence emissions hold great significance for biological applications [49]. This solvatochromic response can be attributed to the relaxes through interaction with the dipoles of solvent of the polar charge transfer excited states of compounds 9 and 10 in more polar solvents, resulting in larger Stokes shifts and redshifts. The new fluorophoreʼs photostability is of great importance. It has been tested against fluorescein using compounds 1, 9, and 10, and the results, as shown in Fig. S26 (Supporting information), indicate that it exhibits superior photostability (Figs. S42–S46 in Supporting information).

Trifluoroacetic acid (TFA) was chosen as the analyte to investigate the acid response behaviour of compounds 1, 9, and 10. In toluene, the fluorescence intensity of compounds 1, 9, and 10 gradually decreased as the TFA concentration increased (Figs. 6a–c). Compound 1, which appears as a yellow solution under natural light with a maximum absorption wavelength of 447 nm in toluene, exhibited a red-shifted absorption wavelength to 498 nm and changed from yellow to red with increasing TFA concentration (Fig. 6d and Fig. S47 in Supporting information). Similarly, when different concentrations of TFA were added, compounds 9 and 10 also showed a red-shifted maximum absorption wavelength of 712 nm and 835 nm, respectively (Figs. 6e and f, Figs. S49 and S51 in Supporting information). This change in absorption wavelength can be attributed to the nitrogen atom on the core biisoindolylidene and the alterations in H⁺ binding, which affect the electron acceptance of the receptor. Consequently, a redshift in the maximum absorption wavelength occurs [50,51]. Interestingly, the luminescent behaviour of the compounds and the colour of the solution were restored when triethylamine was used as a base to neutralize TFA (Fig. 6g). This reversible colour change behaviour can be valuable for the detection of volatile acids (Fig. 6h). A test paper was prepared by immersing filter paper in a DCM solution containing compound 1 and then drying it. When exposed to TFA vapor, the colour of the paper changed significantly to red within 10 s. After allowing the TFA on the test paper to evaporate naturally, the paper returned to its original colour and could be reused.

In conclusion, a new class of fluorophores with a biisoindolylidene skeleton was successfully synthesized. The crystal structure analysis and DFT calculations revealed that these fluorophores have a rigid, conjugated, and coplanar structure and are excellent electron acceptors. To gain a deeper understanding of the photophysical properties of these fluorophores, a series of compounds with the biisoindolylidene backbone were synthesized, incorporating various electron-absorbing and electron-donating groups. This systematic approach allowed us to create a library of fluorescent probes with rational design. Furthermore, we investigated the solvatochromic effect and acid response of these compounds, and explored their potential application in detecting volatile acids. The development of these novel fluorophores holds promise for future biological research.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

CRediT authorship contribution statement

Xiaoyu Chen: Methodology, Investigation, Data curation. Jiahao Hu: Validation, Methodology. Jingyi Lin: Methodology. Haiyang Huang: Validation. Changqing Ye: Writing – review & editing, Writing – original draft, Visualization, Funding acquisition, Formal analysis, Data curation, Conceptualization. Hongli Bao: Writing – review & editing, Writing – original draft, Project administration, Funding acquisition, Data curation, Conceptualization.

Acknowledgments

This paper is dedicated to the memory of Dr G. W. A. Milne. This work was supported by the National Natural Science Foundation of China (Nos. 22225107, 22301302).

Supplementary materials

Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.109923.