Crystallization solvent-induced fluorescence tuning by subtle conformational change in a conformationally flexible fluorophore

The structural flexibility of fluorophores and their ability to display different conformations provides an opportunity to develop fluorescent polymorphs that exhibit tunable solid-state fluorescence. In this study, a conformationally flexible organic fluorophore (4-(diphenylamino)benzaldehyde (TPA-CHO)) was crystallized from polar to non-polar solvents, and the solid-state fluorescence and structural assembly were explored. TPA-CHO, a typical donor-acceptor fluorophore, showed solvent polarity-dependent locally excited (LE) state blue fluorescence in toluene and charge transfer (CT) state greenish-yellow fluorescence in polar DMF. In contrast, crystals obtained from polar DMF exhibited strong blue fluorescence (LE state, lambda max = 453 nm, phi f = 2.54%), whereas crystals grown from hexane displayed greenish-yellow fluorescence (CT state) at 532 nm (phi f = 7.68%) along with a weak emission at shorter wavelengths. Depending upon the solvent polarity, the crystals displayed dual fluorescence with varied LE and CT state fluorescence intensity between greenish-yellow (longer) and blue (shorter) wavelength emission. CIE chromaticity displayed fluorescence tuning from blue-greenish-yellow. Single-crystal structural analysis did not show any drastic change in the conformation or packing (polymorphism), and the same molecular packing was exhibited. However, a closer examination of the structure revealed subtle conformational differences dependent upon the solvent polarity. Although highest occupied molecular orbital (HOMO)-lowest unoccupied molecular orbital (LUMO) calculations showed a similar intramolecular charge transfer (ICT) from the diphenylamine donor to the benzaldehyde acceptor in all crystals, the optical band gap comparison revealed a clear difference between the crystals and supported subtle conformation-controlled tunable fluorescence. Thus, the present work provides insight into the solvent-solute interactions of conformationally flexible molecules for designing tunable fluorescent functional materials.