Radiationless deactivation transition

The results obtained from thermal spin equilibria indicate that AS = 1 transitions are adiabatic. The rates, therefore, depend on the coordination sphere reorganization energy, or the Franck-Condon factors. Radiationless deactivation processes are exothermic. Consequently, they can proceed more rapidly than thermally activated spin-equilibria reactions, that is, in less than nanoseconds in solution at room temperature. Evidence for this includes the observation that few transition metal complexes luminesce under these conditions. Other evidence is the very success of the photoperturbation method for studying thermal spin equilibria intersystem crossing to the ground state of the other spin isomer must be more rapid than the spin equilibrium relaxation in order for the spin equilibrium to be perturbed. 

See emission, energy tranfer, internal conversion, radiationless deactivation and transition, radiative transition. 

See radiationless deactivation, radiationless transition, radiative transition. 

As shown in Fig. 20, a vibrational fine structure of the phosphorescence due to the V=0 double bond of the vanadyl group is clearly observed at 77 K. However, the fine structure is not observed at 298 K because of the significant contribution of an efficient radiationless deactivation arising from various types of vibrational interaction on the surfaces. From an analysis of the vibrational fine structure, the energy gap between the (0 -> 0) and (0 -> 1) vibrational transitions is determined to be about 1035 cm in good agreement with the vibrational energy of the surface V=0 bond obtained by IR and Raman measurements (727, 722). 

The participation of higher excited singlet states (Sn, n > 1) of molecules in photophysical (Sn SQ fluorescence (FL)) or photochemical (photoinduced electron transfer (PET), isomerization, etc.) processes, which compete with radiationless deactivation, manifests itself in the dependence of the quantum yield (q>) and FL spectra on the wavelength of the exciting light (the violation of the Vavilov law). Such processes were first shown for the FL of azulene solutions due to the transition from the second excited level to the ground state S2 -> S0. ... 

Fig. 1.1. Electronic and vibrational energy levels (schematic, rotational are omitted for simplicity), So singlet ground state, Sj excited singlet state (higher electronically excited states are omitted in this figure), T triplet electronically excited energy state. Straight lines symbolise radiative processes (absorption (Ab) as well as emission F, fluorescence P, phosphorescence). Wavy lines give the radiationless transitions, ic, internal conversion isc, intersystem crossing sd, radiationless deactivation te, thermal equilibration.

Besides the excited singlet states some excited tripled states Ti, T2, etc. exist, which cannot be reached directly by absorption of radiation starting from the ground state Sq. This direct transition is forbidden [3]. In the case of radiationless transition this limitation is not valid. For this reason the molecule A can radiationlessly deactivate to the first triplet state A" (Tj) (see Jablonski s diagram in Fig. 1.1)... 

Some reports on fluorescence occurring in, for instance, porous materials such as Nafion or aluminophosphates, " do not refer to azobenzene but to protonated azobenzene, which is classified as a pseudostilbene see Section 1.5). Emission from nonprotonated, isolated azobenzene-type molecules is still very rare. Aggregated systems, however, seem more prone to sho%v fluorescence emission. Shinomura and Kunitake have detected fluorescence bands with a maximum of near 600 nm in bilayer systems built from the monomers of 15. They have shown that the ability to emit is tied to the type of aggregation Head-to-tail aggregates emit relatively strongly, with quantum yields of up to < ) = 10" and lifetimes below 2 ns. Their prototype of card-packed dimers does not emit at all. This is expected because of the low transition probability at the lower band edge, which favors radiationless deactivation, probably via the Si state (see Figure 1.7). 

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