Vibronic lifetimes

At sufficiently low temperatures, it is possible to imagine an electronic transition between a ground state 0g and an excited state where the ground state has no thermally occupied phonon levels. The lowest possible frequency of absorption involves an electronic transition from (n = 0, y = 0, / = 0) to ( = 1,7 = 0,/ = 0). (Note that the usual selection rules that describe atomic and molecular transitions in vacuum are violated in condensed phases.) Within the absorption band corresponding to this electronic transition, excitations of vibrons will lead to a number of sharp lines since (at least for the lowest values forj) the vibrational levels consist of a series of relatively sharp lines of approximately 1 cm line width, whose width is the convolution of the excited state vibron lifetime and the electronic state lifetime. 

Figure C3.5.10. Frequency-dependent vibronic relaxation data for pentacene (PTC) in naphthalene (N) crystals at 1.5 K. (a) Vibrational echoes are used to measure VER lifetimes (from [99]). The lifetimes are shorter in regime I, longer in regime II, and become shorter again in regime III. (b) Two-colour pump-probe experiments are used to measure vibrational cooling (return to the ground state) from [1021.

A different example of non-adiabatic effects is found in the absorption spectrum of pyrazine [171,172]. In this spectrum, the, Si state is a weak structured band, whereas the S2 state is an intense broad, fairly featureless band. Importantly, the fluorescence lifetime is seen fo dramatically decrease in fhe energy region of the 82 band. There is thus an efficient nonradiative relaxation path from this state, which results in the broad spectrum. Again, this is due to vibronic coupling between the two states [109,173,174]. 

The line widfh Av of a rofafional fransifion accompanying an elecfronic or vibronic fransifion is related to fhe lifetime t of fhe excited sfafe and fhe firsf-order rate consfanf k for decay by... 

Pelletier and Reber315 present new luminescence and low-energy excitation spectra of Pd(SCN)42 in three different crystalline environments, K2Pd(SCN)4, [K(18-crown-6)]2Pd(SCN)4, and (2-diethylammonium A -(2,6-dimethylphcnyl)acetamide)2Pd(SCN)4, and analyze the vibronic structure of the luminescence spectra, their intensities, and lifetimes as a function of temperature. The spectroscopic results are compared to the HOMO and LUMO orbitals obtained from density functional calculations to qualitatively illustrate the importance of the bending modes in the vibronic structure of the luminescence spectra. 

Exciplexes are complexes of the excited fluorophore molecule (which can be electron donor or acceptor) with the solvent molecule. Like many bimolecular processes, the formation of excimers and exciplexes are diffusion controlled processes. The fluorescence of these complexes is detected at relatively high concentrations of excited species, so a sufficient number of contacts should occur during the excited state lifetime and, hence, the characteristics of the dual emission depend strongly on the temperature and viscosity of solvents. A well-known example of exciplex is an excited state complex of anthracene and /V,/V-diethylaniline resulting from the transfer of an electron from an amine molecule to an excited anthracene. Molecules of anthracene in toluene fluoresce at 400 nm with contour having vibronic structure. An addition to the same solution of diethylaniline reveals quenching of anthracene accompanied by appearance of a broad, structureless fluorescence band of the exciplex near 500 nm (Fig. 2 )... 

However, often the minimum in Si or Ti which is reached at first is shallow and thermal energy will allow escape into other areas on the Si or Ti surface before return to So occurs (Fig. 3, path e). This is particularly true in the Ti state which has longer lifetimes due to the spin-forbidden nature of both its radiative and non-radiative modes of return to So-The rate of the escape should depend on temperature and is determined in the simplest case by the height and shape of the wall around the minimum, similarly as in ground state reactions (concepts such as activation energy and entropy should be applicable). In cases of intermediate complexity, non-unity transmission coefficients may become important, as discussed above. Finally, in unfavorable cases, vibronic coupling between two or more states has to be considered at all times and simple concepts familiar from ground-state chemistry are not applicable. Pres-... 

Several groups have studied naphthalene substituted anthracene derivatives as hosts or emitter materials in blue OLEDs (121, 202-205) (Scheme 3.63). The Kodak group used ADN as a host and TBP as a dopant in ITO/CuPc/NPD/ADN TBP/Alq3/Mg Ag [241]. They achieved a narrow vibronic emission centered at 465 nm with CIE (0.154, 0.232) and a luminescent efficiency as high as 3.5 cd/A. In comparison, the undoped device shows a broad and featureless bluish-green emission centered at 460 nm with CIE (0.197, 0.257) and an EL efficiency below 2.0 cd/A. The operational lifetimes of the doped device and the undoped device were 4000 and 2000 h at an initial luminance of 636 cd/m2 and 384 cd/m2, respectively. 

One way to conciliate all these findings is to assume that the A adiabatic state is the only one responsible of the EP process, acting as the doorway state. However, because there are important S — If vibronic couplings among the A,B and B states, the rovibrational states are mixed, thus sharing the absorption intensity and also the EP lifetimes. 

The vibronic decay pathway to the D oscillator is much less efficient. Thus, by comparison of the luminescence lifetimes in H2O and D2O an approximation for the number of coordinated solvent water molecules, q, in aqueous solution can be calculated by Eq. (1). 

A possible explanation for this increase in lifetime is a reduction of the nonradiative processes. As pointed out by Robinson (95), these radiationless rates must depend upon the magnitudes of the product of the vibrational overlap integrals between the initial and final states. The substitution of deuterium for hydrogen results in lower vibronic amplitudes, yielding a smaller overlap product. 

In the nanosecond biphotonic photolysis the vibronically excited level reached by absorption of the first photon relaxes by IC to the first excited state (Figure 3), which is stronger acid than the ground state by up to six orders in magnitude [11]. Quantum chemical calculations showed that the O - H bond becomes a bit longer and the C - OH bond becomes shorter and more rigid. The lifetimes of the first excited singlet state of the sterically hindered phenols... 

In conclusion, a typical time of 300 fs has been found for the excited-state intramolecular double proton transfer in TAB and DAC. The proton transfer dynamics is not influenced by aggregation. In addition, a vibronic cooling time of 20 ps has been measured for the probe molecules in the molecular and stacked configurations. Finally, aggregation is found to almost completely hamper the rotational diffusion motions of the molecules during the fluorescence state lifetime of 4 ns. 

To discover smaller specific effects on the intramolecular dynamics after attachment of an Ar atom to the benzene molecule, we performed lifetime measurements of single rovibronic states in the 6q band of the benzene-Ar and the benzene-84 Kr complex. No dependence of the lifetime on the J K> quantum number within one vibronic band was found [38]. This is in line with the results in the bare molecule and points to a nonradiative process in the statistical limit produced by a coupling to a quasi-continuum, for example, the triplet manifold. 

A much more interesting example of the intermediate case is encountered when the density of states is rather small but the vibronic coupling elements are large (due to favorable Franck-Condon vibrational overlap factors). The consequences of this type of intramolecular vibronic coupling are seen in the anomalously long radiative lifetimes of the first singlet states of N02, S02, and CS2 86-99 and in the many extra unexpected lines in the spectra of these molecules. 

Now, in aromatic hydrocarbons intramolecular skeletal vibrations, rather than C—H vibrations, dominate the vibronic coupling contribution to the term J m = — . Furthermore, intermolecular vibrations will have negligible effect on the coupling of the electronic states of interest. Thus, in the case of internal conversion, where the (relatively large) matrix elements are solely determined by intramolecular vibronic coupling, no appreciable medium effect on the nonradiative lifetime is to be expected. On the other hand, intersystem crossing processes are enhanced by the external heavy atom effect, which leads to a contribution to the electronic coupling term. 

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