- Yushchenko, O.; Licari, G.; Mosquera-Vazquez, S.; Sakai, N.; Matile, S.; Vauthey, E. “Ultrafast Intersystem-Crossing Dynamics and Breakdown of the Kasha-Vavilov’s Rule of Naphthalenediimides” J. Phys. Chem. Lett. 2015, 6, 2096-2100
The fluorescence quantum yield of a red naphthalenediimide dye (rNDI) with amino and Br core substituents (rNDI) has been found to decrease by a factor of almost 2 by going from S1←S0 to S2←S0 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 S2 state to the triplet manifold, due to the ππ*→nπ* character of the transition and to the presence of the heavy Br atom. In non-core substituted naphthalenediimide (pNDI), ISC is slower, ~2 ps, and was found to be reversible on a timescale shorter than that of vibrational cooling. The fluorescence and triplet quantum yields of rNDI can thus be substantially changed by a simple variation of the excitation wavelength.