Intramolecular radiationless decay

Rh(bpyL3+ is an example of a complex that exhibits an almost pure n-n phosphorescence and demonstrates one of the limitations of nearly pure ligand localized emissions. At 77K, the complex is highly emissive with a beautifully structured blue ligand phosphorescence (Amax = 446 nmfor the first peak) having at in the tens of msec,(17) but it has no detectable room temperature emission. It is this very long radiative lifetime that causes the absence of room temperature emission. The radiative decay is so slow that it cannot compete effectively against inter- and intramolecular radiationless decay at room temperature. 

We consider three decay channels for D in addition to injection Fluorescence (rate constant k ), intramolecular radiationless decay (rate constant k ), and energy transfer quenching within the adsorbed layer (rate constant kg) ... 

In the case of 7-diethylamino-4-(trifluoromethyl)coumarin ( coumarin-35 ), which has an amino group that is free to rotate, another competitive solvent-dependent decay path has been proposed rotation of the amino group of the planar ICT excited-state molecule can lead to a twisted intramolecular charge-iransfer (TICT) excited-state molecule, from which a radiationless decay to the ground-state molecule occurs [341], Solvent-dependent rate constants for both the radiative and nonradiative decay of excited-state coumarin dyes have been determined [341]. For critical discussions concerning the electronic structure of the excited states of 7-(dialkylamino)coumarins and 7-aminocoumarin ( coumarin-151 ), see references [341d, 341e]. 

The benzene triplet has not been observed to phosphoresce in either the gas or liquid phase, as noted previously but in view of the lifetime measured In glassy media at low temperature this is not surprising, since very small amounts of quenching impurities would reduce emission to undetectable levels. Some detailed investigations have shown that there is an intramolecular process which Induces radiationless decay of the benzene triplet state, other than impurity quenching. 

Let the basis set still be the BO states starting points. Sim we wish to focus upon all the diverse molecular phenomena which are classify as involving radiationless processes, it is necessary to center attention upon th molecule. This focus is best obtained by considering the effective Hamiltoniar Hett, for the molecule which accounts for all relaxation mechanisms other tha the intramolecular nonradiative decay. (The use of effective Hamiltonians is popular in considering the relaxation processes associated with studies of magnetic resonance 37L) For the present case, the effective Hamiltonian is 16>17)... 

The radiationless decay has been investigated by ultrafast polarization spectros-copy [44] and time-resolved fluorescence [45,46]. The results confirm that the radiationless decay occurs by an ultrafast internal conversion, due to intramolecular motion about the bridging bond of the chromophore in the excited state, that the isomerization is nearly barrierless, and that there is only a very weak dependence on medium viscosity, thereby implying that the isomerization occurs by a volume-conserving motion such as a hula twist [47]. 

The language of bright state and dark state is central to population quantum beats and also to the related polyatomic molecule radiationless decay processes (Bixon and Jortner, 1968 Rhodes, 1983), Intramolecular Vibrational Redistribution (IVR) (Parmenter, 1983 Nesbitt and Field, 1996 Wong and Gruebele, 1999 Keske and Pate, 2000), Inter-System Crossing (ISC), and Internal Conversion (IC), discussed in Section 9.4.15. 

One reason that intersystem crossing from an excited singlet state ( Pi) to an excited triplet state C P ) usually occurs more rapidly than phosphorescence or radiationless decay of to the ground state is that the energy gap between the zero-point vibrational levels of f l and usually is much smaller than the gap between and the ground state. Within a related series of molecules, the rates of radiationless intramolecular transitions decrease approximately exponentially with the 0-0 energy difference between the initial and final states (AEoo)-... 

Laser induced optoacoustic spectroscopy, LIOAS, is a very suitable method to study the radiationless decay processes of excited states. We have applied this method of LIOAS to access the total balance of energy in the intramolecular exciplex systems.[32]... 

Lefebvre, R. (1971) Preparation and decay of initial states in intramolecular radiationless processes. Chem. Phys. Lett., 8, 306. 

A previous review provides a description of the theory of electronic relaxation in polyatomic molecules with particular emphasis on the vibronic state dependence of radiationless transition rates. A sequal review considers the general question of collisional effects on electronic relaxation, while the present one covers only the special phenomenon of collision-induced intersystem crossing. It departs from the other collisional effects review in presenting only a qualitative description of the theory the full theoretical details can be obtained from the previous review and the original papers.As a review of the basic concepts of radiationless transitions theory is necessary as a prelude to a discussion of collision-induced intersystem crossing, considerable overlap exists between this section and Section II of the previous collision effects review. However, since many concepts from radiationless transition theory, such as the nature and criteria for irreversible decay, the role of the preparation of the initial state, the occurrence of intramolecular vibrational relaxation, etc. pervade the other papers on laser chemistry in these volumes, it is useful to recall the primary results of the theory of electronic relaxation in isolated molecules and its relevance to the material in the present volume as well as to this review. 

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