Matrix-isolation infrared spectrum of acenaphthene in the lowest electronically excited triplet (T1) state

Since acenaphthene isolated in an Ar matrix at 15 K emits green-yellow phosphorescence for ∼10 s after stopping UV-light irradiation, its transient IR spectrum is expected to be measured with an FTIR spectrometer during UV irradiation. The transient spectrum observed is found to be consistent with the simulated spectrum of acenaphthene in the lowest electronically excited triplet (T1) state calculated at the B3LYP/6-31++G(d,p) level. The spectral pattern obtained using the B3LYP functional, which is the most popular DFT hybrid functional, reproduces the observed spectrum more satisfactorily than that obtained using the recently proposed M06-2x functional, although the optimized geometrical parameters in the both calculations are not so different from each other. The IR-band assignments of the T1 state and the structural changes from the S0 state to the T1 state are discussed based on the results of B3LYP calculation.