Singlet vibronic coupling

Vibronic Coupling in Singlet States of Linear Molecules... 

Using these molecular states, the weak absorption observed between 490 and 640 nm for Cbo in solution (Fig. 6) [67] is assigned to transitions between the singlet ground state So and the lowest excited singlet state 5i (associated with the tiu orbital and activated by vibronic coupling). 

A much more interesting example of the intermediate case is encountered when the density of states is rather small but the vibronic coupling elements are large (due to favorable Franck-Condon vibrational overlap factors). The consequences of this type of intramolecular vibronic coupling are seen in the anomalously long radiative lifetimes of the first singlet states of N02, S02, and CS2 86-99 and in the many extra unexpected lines in the spectra of these molecules. 

The polarized phosphorescence spectra of 1,5-naphthyridine and its d6 isomer in durene and in durene-d14 mixed crystals have been obtained at 4°K. The lowest singlet state is at 27123 and 27200 cm-1, whereas the corresponding triplet state is at 23215 and 23288 cm 1 for the proto and deutero compounds, respectively. The phosphorescence lifetime in durene crystals is 0.23 sec.140 The polarized spectra (4°K) of 1,5-naphthyridine and its d6 isomer in naphthalene have also been examined. There is a difference between the spectra in naphthalene and in durene attributable to a decrease in the strong vibronic coupling that is considered to exist between the n- n state and the higher n -> n states.141 TheEPR spectrum of 1,6-naphthyridine in its lowest triplet state has been observed for solid solutions in single crystals of durene. The nuclear hyperfine structure allows an estimate of 0.14 to be made for the spin density on the nitrogen atom in the 1-position.142... 

Fig. 17. Vibronic coupling mechanisms (Herzberg-Teller couplings). The purely electronic transition between the excited state I (triplet substate) and the ground state 0 (Sq) is spin and symmetry forbidden, i.e. no intensity is found at the electronic origin I. Two mechanism are proposed. Coupling route (a) is probably more important for vibrations of metal-ligand character, while mechanism (b) preferentially induces satellite intensities by internal ligand vibrations. The electronic state S is a singlet, for which an electronic transition is dipole forbidden to the electronic ground state Sq. On the other hand, the state S , represents a singlet that carries sufficient transition probability. For detailed explanations see the text...

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