Activation Parameters for the Formation and Decay of Partial-Charge-Transfer Exciplexes of 9-Cyanoanthracene, 1,12-Benzperylene, and Pyrene

Temperature effect on the competition between the processes of 9-cyanophenanthrene exciplex decay

“…In the case of exciplex decay via internal conversion or intersystem crossing, the activation enthalpy is close to zero, and the formal activation entropy defined as R( ln(k B T/h)) is -50 to -120 J mol -1 K -1 [13,14]. However, this quantity is not a thermodynamic characteristic of the transition state but actually characterizes the probability of the transition and corresponds to the value of the Franck-Condon factor of 10 -3 -10 -7 .…”

“…It has been found that the formation rate for exciplexes with partial charge transfer is determined by the diffusion rate and orientation of the reactants, and the free energy of exciplex formation is determined by their electronic structure and solvent polarity. The mean values of the enthalpies and the entropies of the exciplex decay were also calculated [13,14]. These parameters are usually calculated by two methods.…”

“…The reason behind this difference is the stabilization of exciplexes with partial charge transfer as a result of a high value of the matrix element of electronic coupling between locally excited and chargetransfer states (0.2-0.4 eV) [8,9,13]. The values of and found in [13,14] demonstrate that the strongly negative activation entropy of the exciplex decay (about -(50-150) J mol -1 K -1 ) is the major factor determining the activation barrier in the exciplex decay, while is close to zero.…”

“…Determining the thermodynamic and activation parameters for exciplex formation and decay, we assumed [13][14][15] that the exciplex decay could be characterized by the same mean activation entropy and enthalpy over the entire temperature range examined, although their contribution could vary with temperature because of competition between several processes. The aim of this work was to find and to analyze the activation parameters that determine the rates of internal conversion and intersystem crossing in exciplexes and the rate of exciplex dissociation into radical ions.…”

Transient Exciplex Formation Electron Transfer Mechanism

“…Another approach [43] considered some average values of ΔG Ex * and τ 0 along the reaction coordinate, that implied very shallow potential minimum for an exciplex and also provided erroneous dependence of ΔG Ex * on ΔG ET * . Experimental width of exciplex emission spectra [44,45], close to that for ordinary aromatic compounds, and wellknown exponential kinetics of their decay indicate that exciplexes represent ordinary chemical species with distinct energy minimum, and kinetics of their formation and decay can be treated in terms of ordinary chemical kinetics and requires no complications related to the averaging which could be necessary for very flat potential surface. Model plots of ΔG along the reaction coordinate are shown in Figure 2.…”