Rationalizing the Fluorescence Behavior of Core-Substituted Naphthalene Diimides.

We study the internal conversion (IC) and intersystem crossing (ISC) pathways of low-lying excited electronic states of three core-substituted naphthalene diimides (bNDI, yNDI, and gNDI) using wavepacket simulations within the linear vibronic coupling method. Our wavepacket simulations reproduce the experimental electronic absorption spectra very well. All molecules decay rapidly to S 2 upon populating a higher dipole-allowed singlet excited-state. The S 2 → S 1 IC dynamics and singlet-triplet energy gap, spin-orbit coupling strength trends suggest a favorable S 2 → T 4 ISC in gNDI. The efficient ultrafast T 4 formation and its decay to lower triplet states make gNDI nonfluorescent. Such triplet formation pathways are not operative in both bNDI and yNDI; hence, these molecules emit fluorescence from S 1 after a slower S 2 → S 1 IC.