Polyene spectroscopy: Vibronic evidence for a forbidden transition in DECA-2,4,6,8-tetraene
✍ Scribed by John R. Andrews; Bruce S. Hudson
- Publisher
- Elsevier Science
- Year
- 1978
- Tongue
- English
- Weight
- 479 KB
- Volume
- 57
- Category
- Article
- ISSN
- 0009-2614
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✦ Synopsis
The dimethylpolyene deu-2.4,6,8-tetraene was studied by absorption, fluorescence excitation and fluorescence spectroscopy in gkzsses at 77 K and in n-alkane crystals at 4.2 K. A strong transition to a lB, exci?ed state is observed with an origin at 32400 cm-' in isopentane at 77 K and at 31280 cm-l in n-undecane at 4.2 K. A weak transition to a r Ag excited state is observed with an origin at 28738 cm-r ixz the n-undecane matrix. The radiative fluorescence lifetime zs 500 ns. In undecane the transition from the groucd state to the 'Ag excited state exhibits a chssic Herzberg-Teller viirotic pattern indicating a symmetry forbidden transition_ Linear conjugated polyenes have a characteristic strongly allowed transition in the near W or visx%le region [ 1,2] _ According to any independent particle model this is the lowest energy singlet-singlet transition for these species. The observation of structured weak absorption at an energy lower than that of this strong absorption [3,4] has led to a re-evaluation of inde_oendent particle models [S] and a new interpietation of previous experimental data on polyenes [ 1, 3,4] _ The large Stokes shift 123, anomalously long radiative lifetimes f l&,7] and differentiaI solvent shift of absorption relative to emission [ 1,3,4] provide strong evidence that this low-lying weak absorption is due to the presence of a distinct electronic state lying Iower in energy than the 'B, state_ In the (2% point group the ir-eIectronic states of linear polyenes are of + and B, symmetry_ The totally symmetric Ag ground state (designated 'Ai) has one-photon allowed transitions only to the B, states, those to 4 excited states being electric dipole for-