Study of photochemical reaction of 2,4,2',4'-tetranitrodiphenylmethane by time-resolved resonance raman spectroscopy

A transient species of 2,4,2',4'-tetranitrodiphenylmethane produced by irradiation with ultraviolet light in solution has been detected by time-resolved resonance Raman spectroscopy. From the time dependence of the spectra, the decay time of the transient species is estimated to be 1–2 ms in acetone solution. The comparison of the spectrum of the transient species with that of the anion of this compound indicates that the transient species is identical with the anion.     

A theoretical study on the low-lying excited states of 2,2':5',2'-terthiophene and 2,2':5',2':5',2'-quaterthiophene.

The nature and properties of the low-lying singlet and triplet valence excited states of 2,2':5',2'-terthiophene (terthiophene) and 2,2':5',2':5',2'-quaterthiophene (tetrathiophene) are discussed on the basis of high-level ab initio computations. The spectroscopic features determined experimentally for short alpha-oligothiophenes are rationalised on theoretical grounds. Special attention is devoted to the nonradiative decay process through intersystem crossing (ISC) from the singlet to the triplet manifold, which is known to be relatively less efficient in tetrathiophene. Along the geometry relaxation of the S1 state of terthiophene, the S1 and T2 states become degenerate, which leads to a favourable situation for the occurrence of ISC. The parallel process is expected to be less favoured in tetrathiophene because of the less efficient spin-orbit coupling and the increase of the S1-T2 energy gap.