Ultrafast Intersystem-Crossing Dynamics and Breakdown of the Kasha-Vavilov's Rule of Naphthalenediimides

The fluorescence quantum yield of a red naphthalenediimide dye (rNDI) with amino and Br core substituents has been found to decrease by a factor of almost 2 by going from S-1 <- S-0 to S-2 <- S-0 ( So excitation. Time-resolved spectroscopic measurements reveal that this deviation from the Kasha-Vavilov's rule is due to an ultrafast, < 200 fs, intersystem-crossing (ISC) from the S-2 state to the triplet manifold, due to the pi pi*-> n pi* character of the transition and to the presence of the heavy Br atom. In non-core substituted naphthalenediimide (pNDI), ISC is slower, similar to 2 ps, and was found to be reversible on a time scale shorter than that of vibrational cooling. The fluorescence and triplet quantum yields of rNDI, thus, can be substantially changed by a simple variation of the excitation wavelength.