Nonradiative excited state decay

J. V. Caspar and T. J. Meyer, Application ofthe energy gap law to nonradiative, excited state decay, /. Phys. Chem. 87, 952-957 (1983). 

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... 

In the absence of photochemical reactions, the lowest excited states of [Re(L) (CO)3(N,N)]m complexes decay to the ground state both radiatively and nonradiatively. The lifetimes in fluid solutions range from tens of nanoseconds to microseconds, depending on L, N,N and the medium. Complexes where N,N = 1,4-diazabutadiene are mostly nonemissive in fluid solutions, having excited-state lifetimes of hundreds of picoseconds. Excited-state decay of Re complexes is a much studied phenomenon. It has been dealt with in several review articles [1, 91] and Chap. 2 of this book. Herein, we will only stress some crucial aspects ... 

Irrespective of the exact nature of the biexponential fluorescence decay of PdG (emission from two different conformers or bifurcation of the initial irir -state population to two nonradiative decay channels), it is important to note that the subpicosecond excited-state decay, characteristic of guanine or guanosine, is clearly absent in PdG. Thus, the presence of the exocyclic ring, which hinders the out-of-plane deformation of the six-membered ring (C2 in particular), leads to a dramatically reduced internal conversion rate. 

The excited-state decay kinetics of Ru(bpy)2(dcb) + -Ti02 immersed in neat acetonitrile, probed by transient absorption spectroscopy, exhibited nonexponential kinetics. By minimizing the excitation irradiance, near exponential kinetics were observed for excited-state decay. However, at high excitation irradiance, second-order kinetics were found to fit the experimental data well. These observations are consistent with competitive first- and second-order processes attributed to radiative and nonradiative excited-state deactivation, Eq. 21, proceeding in parallel with excited-state annihilation, Eq. 22 ... 

The vibronic spectra of Do — Di — D2 electronic states recoded by da Silva Filho et al. [45] revealed resolved vibrational structures of the Do and D2 electronic states and a broad and structureless band for the Di state. A slow ( 3-20 ps) and fast k, 200 fs) relaxation components are estimated for the Dq D2 transition in a (femto)picosecond transient grating spectroscopy measurements [16]. The fast component is attributed to the Do D2 transition and a nonradiative relaxation time of 212 fs is also estimated from the cavity ringdown (CRD) spectroscopy data [42]. Electronic structure results of Hall et al. [107] suggest that the nonradiative Do D2 relaxation occurs via two consecutive sloped type CIs [66,108]. We developed a global model PESs for the Do — Di— D2 electronic states and devised a vibronic coupling model to study the nuclear dynamics underlying the complex vibronic spectrum and ultrafast excited state decay of N +[20]. 

The rate of absorption (step i) can be expressed as the absorption intensity /abs (which depends on the concentration of the ground-state complex and the intensity of light used in excitation), while the rate of luminescence (step ii) and the rate of all the nonradiative decay paths (step iii) follow first-order rate laws that depend only on the concentration of the excited-state complex (nonradiative collisional quenching by solvent is not unimolecular, but follows first-order kinetics because of the large excess of solvent molecules). Dynamic quenching (step iv) results from an electron-transfer reaction during a collision between a quencher molecule and an excited-state complex. This bimolecular reaction has a... 

Figure 1. Cartoon outlining the steps involved in converting solar energy (hv) to stored chemical energy or electricity. Electron transfer reactions of the excited state (denoted by an asterisk) with donors (D) or acceptors (A) to yield charge-separated species (D or A-) take place in competition with nonproductive emission (hv ), nonradiative excited-state decay, and hack electron transfer, which...

Although this appears to be a simple experiment for systems in which the excited state decays via both fluorescence and dissociation, it is in fact not a simple matter to establish that k can be attributed exclusively to dissociation. The excited state can also decay nonradiatively by internal conversion to the ground electronic state, by intersystem crossing to a triplet state followed by collisional deactivation, etc. That is, dissociation is only one of several nonradiative paths that may be important in the decay of an excited state. Thus, considerable care must be exercised in the interpretation of such data. However, these data can always be used to place an upper limit on the dissociation rate constant. 

The recent theoretical approaches include a theory of barrierless electronic relaxation which draws on the model of nonradiative excited state decay, and a general treatment of the effect of solvent dielectric relaxation based on the theory of optical line shapes, as well as treatments based on classical and quantum rate theories. Equation(5) does not hold for all solvents and, more generally, may be frequency-dependent. Papers by Hynes, Rips and Jortner, Sumi and Marcus, and Warshel and Hwang " contain good overviews of the theoretical developments. 

The electronic relaxation of the excited singlet states of benzene vapor is rather typical of polyatomic molecules. Relaxation is predominantly nonradiative, and by paths that are sensitive to the degree of vibrational excitation in the decaying electronic state. Indeed, vibrational excitation appears as perhaps the most crucial parameter in setting the course of excited state decay. 

An attempt will be made to separate primary facts about relaxation parameters from the less secure (and sometimes transitory) inferences derived by mechanistic interpretation of the data. Accordingly, for each vibrational domain, attention will be directed first to the elementary separation of excited state decay into the channels of radiative and nonradiative relaxation without effort to further identify the nature of the nonradiative decay. This question will then be discussed separately, for it is an involved and incompletely resolved issue. 

Turning back to the processes shown in Figure 7.20, the excited state will return to the fundamental one in one of several ways. The most common is nonradiative thermal deactivation. In other cases, the initial excited state can nonradiatively decay to a somewhat lower one and from this to the fundamental state by photon emission. This emission, of a lower energy than the incident, is the fluorescence that occurs a short time after excitation. Another possible outcome is intersystem crossing, when the first excited state, which has the same multiplicity as the fundamental, usually a singlet state, evolves to a state of different multiplicity (usually a triplet). This state will eventually decay to the fundamental one, but this occurs at a much lower rate due to the different spin multiplicities this is the phosphorescence emission. Fluorescence can be applied for quantitative purposes, especially for organic compounds, for low concentrations. Phosphorescence, on the other hand, has low intensity and is generally not useful for such purposes. 

PiJip)) is called the Lorentzian lineshape function. Its fwhm is equal to y, and is inversely proportional to the lifetime t = Ijy. It approaches zero as o) + oo, and maximizes at co == coq (Fig. 8.1). Physically, y itself will have several components in any real absorption line, arising from spontaneous emission (fluorescence or phosphorescence), nonradiative excited-state decay (intersystem crossing, internal conversion, photochemistry), collisional deactivation, etc. ... 

Electronic relaxation in different excited vibronic levels corresponding to the same electronic configuration can be experimentally studied, provided that, as mentioned above, (1) single vibrational levels within the initial electronic state are populated and (2) the excited molecule decays nonradiatively on a timescale much shorter than the mean time between deactivating collisions or by other means such as infrared fluorescence [115]. For typical polyatomic molecules in the gas phase, a narrow-band optical excitation pulse (as small as 1 and shorter relative to the genuine decay times wiH result in the selection of a single vibronic state. U nder these conditions,... 

Energy Transfer. In addition to either emitting a photon or decaying nonradiatively to the ground state, an excited sensitizer ion may also transfer energy to another center either radiatively or nonradiatively, as illustrated in Figure 4. 

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