When "Donor-Acceptor" Dyes Delocalize: A Spectroscopic and Computational Study of D-A Dyes Using "Michler's Base"

In this study, we show that the "Michler's base" motif can be combined in a donor-acceptor arrangement with a range of acceptor units (indandione, indandione with cyano substituents, barbituric acid, or rhodanine) to give photophysical properties that are dominated by delocalized excited states. By changing the acceptor unit and by altering the planarity of this system, it is possible to tune the low-energy absorption feature in terms of intensity from 23 000 to 67 000 M^-1 cm^-1 and energy between 500 and 700 nm. Resonance Raman spectroscopy and time-dependent density functional theory indicate that this absorption feature has two underlying transitions: a weaker charge-transfer transition around 500 nm and a strong mixed or delocalized transition between 550 and 700 nm. Generally, these compounds are not strongly emissive; however, dual emission is observed, and the relative intensity of the two states can be modulated by solvent polarity. The energy of these emissive states does not correlate with the Lippert-Mataga analysis in which the Stokes shift is related to the solvent polarity (Δf).