Abstract
1,3,5,7-tetramethylcyclo-octatetraene dianion (TMCOT−2) is employed as a metalloporphyrin model. Its UV-VIS absorption spectrum shows two electronic bands, an intense band (Soret or B) at ∼215 nm and a weaker state (Q band) at 382 nm, while cyclo-octatetraene dianion (COT−2) only has a B band at 236 nm. An exact Pariser–Parr–Pople (PPP) calculation of COT−2 predicts a strong Soret band at 231 nm and a vanishing Q band intensity at 366 nm. Resonance Raman spectrum with excitation near the Q and Soret bands of TMCOT−2 shows enhancement patterns remarkably similar to those of metalloporphyrins. Nontotally symmetric modes are enhanced in resonance with the Q band, while the totally symmetric modes are enhanced in resonance with the B state. The cyclic polyene TMCOT−2 spectrum thus provides direct evidence for Herzberg–Teller coupling of the Q band and B state.
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