Nonradiative decay, of excited states

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 excited-state dynamics of the 2-hydroxypheylbenzotriazole (HPB) photostabilizer copolymerized with polystyrene (51) are reported in Ref. 195. The HPB fluorescence from these copolymer films is observed at 630 nm, characteristics of the proton-transferred excited state of HPB, and it has a rise time of < 10 psec and a decay time of 28 4 psec at room temperature. Measurement of the relative fluorescence quantum yield as a function of temperature gives the activation energy for nonradiative decay of this state as 259 25 cm-1. 

Fig. 5.6 A schematic representation of alternative pathways for formation and decay of excited states. Singlet states are labeled S and triplet states T superscripts 0, 1, 2,. .n,. .. denote the ground state and excited states of increasing energy. Radiative processes (absorption, fluorescence, phosphorescence) are indicated with solid arrows nonradiative processes (intersystem crossing, internal conversion, etc.), with wavy arrows. Internal conversion and intersystem crossing usually proceed via excited vibrational levels of the product state. Diagrams of this type were introduced by A. Jablonski in 1935 in a paper on the mechanism of phosphorescence [295]. The horizontal axis has no physical significance...

Sobolewski AL, Domcke W (2002) On the mechanism of nonradiative decay of DNA bases ab initio and TDDFT results for the excited states of 9H-adenine. Eur Phys J D 20 369... 

When the symmetry is reduced by replacement of a bpy ligand by another bidentate, the symmetry restrictions are relaxed and the initially formed excited state is expected to be localized (600, 601). The rates of nonradiative decay of a large range of Os(II)-polypyridine have been calculated successfully in terms of a modified energy gap law in which low-frequency modes are explicitly considered (148). 

The vibrational overlap is both important and difficult to guess. In polyatomic molecules the large number of excited states near resonance will tend to increase the rates of all nonradiative processes, including decay to both A and B. However, intuition tells us that in a very large molecule not all vibrational levels will be important. For example, fluorescence lifetimes of alkyl derivatives of aromatic hydrocarbons are essentially independent of the length of the attached alkyl chain.26... 

The imbedded nature of the potential curves in Figure 6 for electron transfer in the inverted region is a feature shared with the nonradiative decay of molecular excited states. In fact, in the inverted region another channel for the transition between states is by emission, D,A -> D+,A + hv, which can be observed, for example, from organic exciplexes,74 chemiluminescent reactions,75 or from intramolecular charge transfer excited states, e.g. (bipy)2Rum(bipyT)2+ - (bipy)2Run(bipy)2+ + hv. 

The energy released as heat in the course of the nonradiative decay of P to the ground state and detected as a pressure wave by laser-induced optoacoustic spectroscopy (LIOAS) exhibits positive deviations (i.e., a> 1 cf. Eq. (1)) from the values which were calculated on the basis of the absorption spectrum of Pr alone (Figure 15) [90,115]. This indicates that already within the 15-ns duration of the excitation flash, one or several intermediates must have been formed. These in turn, within the same interval, may again absorb light from an intense laser flash and (at least in part) dissipate heat upon their return to the ground state of the same species (internal conversion) and/or to Pr (photochemical back reaction). The formation of primary photoproducts within the nanosecond flash duration was of course to be expected in view of the much shorter lifetimes of the photochromic fluorescence decay compo-... 

Interrelationships of Excited-State Decay Routes. The iLV curves conveniently display the competitive nature of photocurrent and luminescence intensity as excited-state deactivation pathways. Our analysis is limited in the sense that we have obtained absolute numbers for X but have had to content ourselves with relative < > r measurements. We lack measures of nonradiative recombination efficiency (4>nr) although they now appear to be... 

From a theoretical point of view, the ultra-short excited state lifetimes observed for canonical structures have been attributed to the existence of easily accessible conical intersections between the excited state and the ground state efficiently promoting nonradiative decay [10,25, 30, 36,44,45, 53, 58, 60, 74,75, 85, 87, 102], Accurate prediction of excited state properties still presents a major challenge to... 

---