Intramolecular decay time

In larger molecules the phase-coherence time of excited levels may be shorter than the population lifetime because of perturbations between closely spaced levels of different electronic states, which cause a dephasing of the excited-level wave functions. One example is the NO2 molecule, where the width of the Hanle signal turns out to be more than one order of magnitude larger than expected from independent measurements of population lifetime and Landd factors [851, 852]. This discrepancy is explained by a short intramolecular decay time (dephasing time), but a much larger radiative lifetime [853]. 

Figure 5.2. Grabowski s model of TICT formation in DMABN the locally excited (LE) state with near-planar conformation is a precursor for the TICT state with near perpendicular geometry. The reaction coordinate involves charge transfer from donor D to acceptor A. intramolecular twisting between these subunits, and solvent relaxation around the newly created strong dipole. Decay kinetics of LE and rise kinetics of the TICT state can be followed separately by observing the two bands of the dual fluorescence. For medium polar solvents, well-behaved first-order kinetics are observed, with the rise-time of the product equal to the decay time of the precursor, but for the more complex alcohol solvents, kinetics can strongly deviate from exponentiality, interpretable by time-dependent rate constants. 52 ...

There are many molecular interactions which influence the fluorescence decay times. The measured fluorescence lifetime r is usually shorter than the radiative lifetime tr because of presence of other decay rates which can be dependent on intramolecular processes and intermolecular interactions (Figure 10.3). The measured fluorescence lifetime (r) is given by the inverse of the total rate of dynamic processes that cause deactivation from the excited (mostly singlet Si) state... 

Even in a molecule the size of benzene the resolution achieved in this way is sufficient to investigate the dynamic behavior of individual rotational states. For this it is necessary to eliminate the Doppler broadening of the rovibronic transitions. Two methods have been applied (i) the elimination of Doppler broadening in a Doppler-free two-photon-transition and (ii) the reduction of Doppler broadening in a molecular beam. Measurements of the dynamic behavior have been performed in the frequency [3] and time domain [4]. We will briefly summarize the results from high-resolution measurements and discuss the conclusions on the intramolecular decay mechanism. Then it will be discussed how the intramolecular dynamics is influenced by the attachment of an Ar or Kr atom to the benzene molecule, leading to a weakly bound van der Waals complex. 

Although the general theory outlined provides a satisfying unified interpretation of the many relaxation processes mentioned, at present reliable numerical predictions are not possible. This must be considered the most serious technical limitation of the analysis we have reviewed. Because of the technical difficulties encountered in the a priori calculation of matrix elements, densities of states, etc., it is tempting to reverse the analysis to obtain information about the relevant intramolecular matrix elements, densities of states, etc., from line shape data and the several luminescence decay times. For example, it seems likely that the complex spectrum of a molecule such as NOa could be analyzed in this fashion, and thereby provide information not now available from any other source. 

Compounds 268a and 269a showed large Stokes shifts in polar solvents. Such large Stokes shifts have been observed in many TICT (twisted intramolecular charge transfer) molecules. Fluorescence decay time measurement indicated that there were two kinds of excited states, fast and slower decaying components, and the latter was the emission from the more polar state. 

The photophysics of aniline has been discussed in Section 4 of this chapter. The fluorescence decay time of /9-naphthylamine vapour has been shown to decrease from 14.0 to 0.50 ns for excitation wavelengths of from 347 to 215 nm.29 The shortening is ascribed to the onset of a new non-radiative decay mode (internal conversion to S0) in the upper levels of the molecule, and a discontinuity is observed at the onset of S2 excitation. Intramolecular vibrational redistribution is shown to be slow compared with electronic relaxation. The... 

In this scheme A is not a molar concentration term since the excimers are formed intramolecularly, but is related to the statistical conq>osltion of the polymer (47) (see below). Using an analysis based upon that given earlier, individual rate constants in the above scheme were obtained from quantum yields and decay times from the literature (46). Values are shown in Table 3. 

Figure 3.39 contains S(u), So v), 5j(w), and S = S — So — iS j plotted against u. The quantities Si u) are the components of the integral scattering intensity. For small u s, 5 So, and the time correlation function is defined by the translational diffusion coefficient only. When n > 3, the effect of time decay in the fluctuations is substantially dependent on the intramolecular relaxation time. 

A classic example of a three-state system is the intramolecular excimer formation with l,r-dipyrenylpropane [lPy(3)lPy], a dipyrenyl oligomer with three carbon atoms connecting the two pyrenes (see Fig. 15.14). Three species are observed one monomer and two excimers (sandwich-like and twisted conformations). It is worth noting that in the case of 2,2 -dipyrenylpropane [2Py(3)2Py] only one monomer and one excimer (less stable with a parallel sandwich-like geometry and decay time of 150 ns, [59]) are present, because the C2 symmetry of the pyrene-chain bond axis allows only one excimer conformation. Also, with the longer (ten carbon atoms chain) of l,r-dipyrenyldecane [lPy(10)lPy], see Rg. 15.13 above, only one monomer and one excimer are present, because the longer chain is sufficiently flexible to allow relaxation to the most stable conformation of the excimer (two-state system). 

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