Horizontal radiationless transitions

In Chapters 2 and 4, the Franck-Condon factor was used to account for the efficiency of electronic transitions resulting in absorption and radiative transitions. The efficiency of the transitions was envisaged as being related to the extent of overlap between the squares of the vibrational wave functions, /2, of the initial and final states. In a horizontal radiationless transition, the extent of overlap of the /2 functions of the initial and final states is the primary factor controlling the rate of internal conversion and intersystem crossing. 

In 1969, Suppan reviewed experimental data about dipole moment changes in excited states of substituted aromatic molecules and suggested a theoretical approach according to which charge transfer occurs if the lowest vacant orbitals are very close in energy.41 In 1978, Birks introduced the term horizontal radiationless transition, which was applied to intramolecular rotation in stilbene and polyene derivatives.37 In this... 

In the last two cases, we would still expect a behavior of the short-wavelength FB band similar to that of DMABN There, the kBA channel toward the TICT state leads to efficient fluorescence quenching of FB. This can be seen directly in the low-temperature range, for example, in Fig. 2.1 (Section II.A), below 180 K, where the quantum yield of FB fluorescence increases upon cooling because the horizontal radiationless transition kBA is slowed down. The simplified Kinetic Scheme II, also valid for any other monomolecular quenching process, describes this... 

The theory of radiationless transition considers the transition to occur in two steps (i) horizontal transition from one energy state to the other at the isocnergetic point, for the two combining states and (ii) vibrational relaxation of the lower energy state. The step (i) is the rate determining step. The rate constant is given by the theory of Robinson and Frosch as,... 

Figure 7. Barrier widths (horizontal lines with double arrows) for radiationless transition for cases in which the vibronic coupling is weak (left), strong (center), and very strong (right). (From ref. [32] with permission.) The dashed curves represent the potential energy curves in the absence of vibronic coupling.

Radiative and radiationless (nonradiative) transitions may be pictured as competing vertical and horizontal crossings, respectively, between the... 

Fig. 3 Schematic excited state scheme for a Cr(lll) complex. Q and D denote quartet and doublet state, respectively light horizontal lines indicate vibrational levels. Superscripts zero denote thermally equilibrated states subscripts zero and one denote ground state and excited state, and PC denotes Franck-Condon state. Optical transitions are shown as vertical arrows, and radiationless processes, as wavy arrows.

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