Lifetimes of partial charge transfer exciplexes of 9-cyanophenanthrene and 9-cyanoanthracene

Abstract

The fluorescence decays of several exciplexes with partial charge transfer have been investigated in solvents of various polarity. The measured lifetimes are found to be in reasonable agreement with the activation enthalpy and entropy of exciplex decay obtained earlier from the temperature dependence of the exciplex emission quantum yields. For exciplexes with 9-cyanophenanthrene substantial contribution of the higher local excited state into the exciplex electronic structure is found and borrowed intensity effect enhances the exciplex emission rate constants.

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

“…The values of range from 23 to 33 kJ mol -1 and exceed the values for the free energy of activation of exciplex formation approximately by 10-15 kJ mol -1 [24]. Hence, the conclusion that the rate-limiting step of photoinduced electron transfer is the exciplex decay step is confirmed.…”

“…The values of the activation enthalpies and entropies for the exciplex decay processes and the rate constants for each of these processes at ambient temperature are listed in Table 2. The use of experimental ratios determined in [22,24] allowed us to define more accurately the values of for the CP-135TMB and CP-123TMB exciplexes. As follows from the data given in Table 2, the activation entropies for all the exciplex decay processes are also close to .…”

Transient Exciplex Formation Electron Transfer Mechanism

“…Besides, exciplex emission spectra can overlap significantly with parent reactant fluorescence, especially when ΔG ET * > 0, and special ingenuity is often necessary to reveal them. Radiative transitions in exciplexes are forbidden by symmetry and k F is usually proportional to the contribution of the locally excited state (1−z) [66,72]. Exciplex lifetimes can be very short because of efficient radiationless decay (internal conversion into the ground state and intersystem crossing into the triplet state) [28,45] (see, Section 3).…”

“…(2) Nonexponential fluorescence decay of parent M * is frequently observed because of reversible exciplex formation (M * + Q Ex). In such a case the decay of M * is biexponential [19,66]…”

“…It should be mentioned that frequently only one of these exponents can be observed experimentally because of strong difference in their amplitudes or decay times. For instance, quenching of anthracene fluorescence by N,N,dimethylaniline [66] demonstrates substantially greater slope of the plot τ 0 /τ 1 versus [Q] relative to ϕ 0 /ϕ versus [Q] and almost constant τ 0 /τ 2 τ 0 /τ 1 . But in the case of pyrene [66] τ 1 is several hundred times smaller than τ 2 and can be missed in actual measurements in spite of its greater amplitude.…”