Anti-Kasha Conformational Photoisomerization of a Heteroleptic Dithiolene Metal Complex Revealed by Ultrafast Spectroscopy.

We investigated the anti-Kasha photochemistry and anti-Kasha emission of d8-metal donor-acceptor dithiolene with femtosecond UV-vis transient absorption spectroscopy and molecular modeling. Experimentally, we found a lifetime of 1.4 ps for higher excited states, which is exceptionally long when compared to typical values for internal conversion (IC) (10 s of fs or less). Consequently, a substantial emission originates from the second excited state. Molecular modeling suggests this to be a consequence of the spatially separated molecular orbitals of the first and second excited states, which gives a charge transfer character to the IC. More surprisingly, we found that the inherent flexibility of the molecule allows the metal complex to access different configurations depending on the photoexcited state. We believe that this unique manifestation of anti-Kasha photoinduced conformational isomerization is facilitated by the exceptionally long lifetime of the second excited state.