Rate, internal conversion radiationless transitions

Once the excited molecule reaches the S state it can decay by emitting fluorescence or it can undergo a fiirtlier radiationless transition to a triplet state. A radiationless transition between states of different multiplicity is called intersystem crossing. This is a spin-forbidden process. It is not as fast as internal conversion and often has a rate comparable to the radiative rate, so some S molecules fluoresce and otliers produce triplet states. There may also be fiirther internal conversion from to the ground state, though it is not easy to detemiine the extent to which that occurs. Photochemical reactions or energy transfer may also occur from S. ... 

In Chapters 2 and 4, the Franck-Condon factor was used to account for the efficiency of electronic transitions resulting in absorption and radiative transitions. The efficiency of the transitions was envisaged as being related to the extent of overlap between the squares of the vibrational wave functions, /2, of the initial and final states. In a horizontal radiationless transition, the extent of overlap of the /2 functions of the initial and final states is the primary factor controlling the rate of internal conversion and intersystem crossing. 

Figure 5.3 The effect of energy gap in vibrational levels on Si VW> S0 internal conversion. Decreasing the vibrational energy gap leads to a radiationless transition in which the T overlap and Franck-Condon factor are reduced and the rate of internal conversion should be decreased...

The radiationless transitions are indicated by wavy lines with the corresponding rate constants or lifetimes. The rates of thermalization of a dye molecule after an absorptive or emissive transition, namely k in the excited singlet state Si and k" in the ground state So, axe so fast that they have not yet been reliably measured. From indirect evidence they are believed to be k k" rs 1012 — 1013 sec-1. The rate 21 °f radiationless internal conversion from S2 to Si has recently been measured for several dyes and was found to be in the range kzi = 1011 —1012 sec-1 5>6 While has not yet been measured directly, there is no reason to believe that it should be much different from A 21, and indirect evidence seems to... 

Various primary processes induced upon photon absorption by this molecule are also shown in Fig. 2-1. The photon absorption processes associated with the vibrational-electronic transitions from So to Si and S2 are represented by So ->Si Abs. and So —>82 Abs., respectively. By internal conversion (IC) we mean a radiationless process between two different electronic states of the same spin multiplicity. In Fig. 2-1, IC from S2 to S and IC from Si to So are shown. Usually, the rate constants of S2 Si IC and Si —>So IC are more than lO s and 10 -10 s , respectively. By intersystem crossing (ISC) we mean a radiationless process between two different electronic states of two different spin multiplicities. In Fig. 2-1, Si Ti ISC and Ti—>So ISC are shown. The rate constants of the... 

The rate of internal conversion (IC), a radiationless transition between isoenergetic levels of different states of the same multiplicity, may be of the same order of magnitude or even faster than vibrational relaxation. It depends, however, on the energy separation AEqo between the zero-vibrational levels of the electronic states involved (energy gap law, see Section 5.2.1). Similar relations hold for intersystem crossing transitions between states of different multiplicity, which are slower by 4-8 orders of magnitude. 

The questions to be addressed in this context have historically been characterized as photophysical and photochemical, categories whose boundaries are sometimes ill-defined. The photophysical steps include intrastate vibrational relaxation, photon emission (fluorescence and phosphorescence), and interstate radiationless transitions (internal conversion between states of the same multiplicity and intersystem crossing between states of different multiplicity). The major aim in this area is to determine the rates of the individual steps and the relationship between molecular structure and these rates. Other goals are to identify the photoactive state and to detail the reaction mechanism. 

In the gas phase or in non-interacting solvents and in the absence of other photophysical processes (cf. Fig. 6.23) the fluorescence intensity F detected over a certain emission wavelength range decays following a mono-exponential decay law with an average lifetime r. The rate constant of this fluorescence decay k = 1/t) represents the sum of the emissive rate of the fluorophore kg (= l/ o) the rate constants of the two radiationless processes, internal conversion and intersystem crossing, and kjsc, respectively. The radiative lifetime dg can be correlated with the transition dipole moment M by... 

Recent work by Lim and coworkers [11,12] on small thiones of C, and Cjv excited state symmetry has clearly demonstrated the symmetry requirements of vibronicaUy induced radiationless transitions. In the low pressure gas phase, Cjv molecules such as thioformaldehyde, HjCS, exhibit both strong Sj - Sq fluorescence and strong Tj - Sq phosphorescence. These molecules therefore exhibit rates of S, - Sq internal conversion and Tj - Sq intersystem crossing that are small in comparison with the excited state radiative decay rates, despite the fact that the density of states in Sq is sufficiently large that the molecule s radiationless transitions should fall into the statistical limit case. On the other hand similar thiones of C, excited state symmetry exhibit small quantum... 

The Sj radiationless transition rate is sensitive to both solvatochromic and substituent-induced shifts of the Sj - Sj energy gap. Figure 1.2 shows that an almost linear dependence of the logarithm of the rate constant of radiationless decay on the Sj - Sj energy gap is observed for a series of 1-and 1,3-fluorine-substituted azulenes in a range of solvents, as expected if the energy gap law of radiationless transition theory applies and internal conversion is the sole Sj decay process. 1,3-Difluoroazulene in ethanol... 

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