Time-resolved fluorescence spectroscopy of solvatochromic 2,3-diarylindenes

The fluorescence decay kinetics of the Stokes-shifted emission bands in two strongly solvatochromic diarylindenes, 2-(3-nitrophenyl)-3-(4-methoxyphenyl)-6-methoxyindene (1) and 2-(4-nitrophenyl)-3-(4-methoxyphenyl)-6-methoxyindene (2) have been investigated through time-correlated photon counting in several solvents with varying polarity. Neither of the indenes exhibits single-exponential decay. The 1/e emission lifetimes of indene 2 (which exhibits smaller fluorescence Stokes shifts than indene 1) are typically ∼3 ns and vary little with solvent polarity. The corresponding lifetimes for indene 1 (which displays exceptionally large emission Stokes shifts of 11 000-16 000 cm^-1) are strongly solvent-dependent, varying from several nanoseconds in nonpolar solvents to several hundred picoseconds in polar solvents. The lifetime trend in indene 1 is consistent with S₁-S₀ nonradiative decay in a twisted intramolecular charge-transfer (TICT) state across an energy gap whose width is controlled by the solvent polarity.