Unravelling the photophysics of triphenylamine and diphenylamine dyes: a comprehensive investigation with ortho-, meta- and para-amido substituted derivatives

The triphenylamine (TPA) and diphenylamine (DPA) derivatives constitute an important class of dyes that are suitable for many fluorescence-based applications. This study provides a thorough understanding about the photophysics of ortho-, meta- and para-amido substituted TPA (TPA-o, TPA-m, TPA-p) and DPA (DPA-o, DPA-m, DPA-p) dyes in several solvents and solvent mixtures over a wide polarity range. Correlation of the fluorescence emission maxima, quantum yields, and fluorescence lifetimes with the solvent polarity parameter, Delta f, reveal unusual solvent polarity-dependent photophysics of both series of dyes, that, to the best of our knowledge, have not been reported before. The present study, additionally, outlines the effect of functional group position (ortho-, meta- or para-) on the excited state properties of the derivatives, and also brings out the similarities and differences between the photophysics of the TPA and DPA series of dyes. For the TPA dyes, the observed results suggest that in the nonpolar solvent region (Delta f < 0.05), the emission arises from a nonpolar S-1 state, while in the moderate to higher polarity solvent region (Delta f similar to 0.05-0.3), the emission arises from an intramolecular charge transfer (ICT) state. For the DPA dyes, the observed photophysical parameters indicate that in addition to the participation of a nonpolar S-1 state (Delta f < 0.01) and ICT state (Delta f similar to 0.01-0.2), there is also contribution from another kind of charge transfer state at higher polarities (Delta f > 0.2) that may be likened to a twisted intramolecular charge transfer (TICT) state. Participation of the TICT state opens up an efficient nonradiative decay channel for the excited DPA dyes in highly polar solvents, and modifies their emission properties. These findings are significant for optimizing the performance of the TPA and DPA class of dyes for their various fluorescence functionalities.