Multiple Intermolecular Exciplexes in Highly Polar Solvents

Exciplexes of 2,6,9,10-tetracyanoanthracene (TCA) with alkylbenzenes were investigated in solvents ranging from cyclohexane to acetonitrile. Plots of the reduced emission maxima or the average emission frequency (hv(av)) versus redox potential differences (E-redox) were linear with a slope of in all solvents, which is consistent with the highly ionic character of the exciplexes. The exciplex spectra were analyzed in terms of the energy gap between the exciplex minimum and the AD pair (Delta G), the energy difference between Delta G and E-redox (delta(Ex) )and the total reorganization energy (Sigma lambda). A plot of (E-redox - hv(av)) equivalent to (Sigma lambda - delta(Ex)), versus a solvent polarity function showed a linear dependency for the low-to-moderate polarity solvents, whereas highly polar solvents deviated significantly. delta(Ex) showed a smooth linear dependency for all solvents. Thus, the deviation of the polar solvents is due to a larger-than-expected Sigma lambda. Additionally, the full width at half-maximum (fwhm) of the emission spectra in polar solvents deviates significantly from the extrapolated trend in less-polar solvents. The deviations of Sigma lambda and fwhm in highly polar solvents can plausibly be explained by composite emissions from two exciplex structures, with the donor overlapping with the inner or outer ring of TCA.