Isomerization-Induced Excimer Formation of Pyrene-Based Acylhydrazone Controlled by Light- and Solvent-Sensing Aromatic Analytes.

Pyrene is a fluorescent polycyclic aromatic hydrocarbon, and it would be interesting to determine whether its C═N-based conjugate can be used for sensing of aromatic analytes at its supramolecular aggregated state. For this purpose, we have synthesized ()-3,4,5-tris(dodecyloxy)-'-(pyren-1-ylmethylene)benzohydrazide (Py@B) by alkylation, substitution, and the Schiff base reaction methodology. The -isomer of Py@B (-Py@B) exhibits a bright fluorescence due to excimer formation in nonaromatic solvents. Upon photoirradiation with λ = 254 nm, it exhibits isomerization across the C═N bond at a low concentration (10 M), resulting in a quenched fluorescence intensity, and interestingly, upon photoirradiation with λ = 365 nm, the -isomer of Py@B returns to the -isomer again, indicating that isomerization of Py@B is reversible in nature. The thick supramolecular aggregated morphology of Py@B changes to a flowery needlelike morphology after photoirradiation with λ = 254 nm. The UV-vis absorption band at 370 nm for 10 M Py@B in methyl cyclohexane (MCH) is due to excimer formation for closer proximity of pyrene moieties present in -Py@B and changes to the absorption peak at 344 nm for its -isomer formation. The fluorescence spectroscopy results also support the fact that the optimum concentration of the -isomer of Py@B is 2 × 10 M in MCH for excimer formation. From spectral results, it may be concluded that nonaromatic solvents assist in constructing the excimer, but aromatic solvents resist forming an excimer complex of Py@B. The fluorescent emission of -Py@B in MCH is quickly quenched on addition of different aromatic analytes through both static and dynamic pathways. In the solid state, -Py@B also senses aromatic vapors efficiently via fluorescence quenching. Absorbance spectra of a model molecule obtained using time-dependent density functional theory (TDDFT) calculations on a DFT-optimized structure indicate complex adduct formation between -Py@B and aromatic analytes from the well-matched theoretical and experimental UV-vis spectra on addition of different analytes with -Py@B.