Elucidation of TICT Behavior of Styryl Pyridinium Dyes via Supramolecular Confinement With Cucurbit[7]uril

Styryl pyridinium derivatives have been extensively explored as chromophoric dyes in the areas of biomolecular imaging and nonlinear optics. However, these chromophores are weakly emissive in aqueous medium and is generally assumed that it is due to the involvement of nonemissive TICT state formed through the torsinal motion of various moieties of these dyes in high polarity protic solvents. Here, two structurally correlated styryl pyridinium derivatives namely, DEASPI and JuSPI have been synthesized and the intrigiung photophysical aspects, especially, TICT behavior of these dyes were elucidated in free and confined medium. Complex formation between these dyes and cucurrbit[7]uril (CB7) is proposed from the absorption and emission studies and is further substantiated by ¹H-NMR, ITC and geometry optimization data. Time-resolved emission, in the absence and presence of CB7, revealed the effect of encapsulation on the excited state dynamics, thereby indicating the structural restrictions toward formation of nonemissive TICT state in solution. Formation of different stoichiometric complexes of styryl pyridinium dyes were effectively utilized to supress the features of intramolecular torsions, demonstrating the tunability of the emissive properties for further applications. Moreover, this manuscript clarifies the nature of the TICT state involved in the fluorescence enhancement of these dyes upon CB complexation.