The nature of internal conversion and intersystem crossing in exciplexes

Abstract

1 More and more data to suggest that many types of electron transfer processes, in particular photoinduced electron transfer between organic molecules in the kinetic region (at a free energy of excited-state electron transfer of ∆ > -0.4 eV), occur via the mechanism of intermediate formation of exciplexes [1][2][3][4][5][6], rather than the generally accepted mechanism of the preceding thermally activated solvent reorganization [7][8][9][10], are appearing in the literature. According to the mechanism of intermediate formation of exciplexes, the apparent rate constant of quenching of excited molecules is equal to k Q = 1/[1/ k 1 + exp ( ∆ / RT )] , where k 1 is the rate constant of exciplex formation, which is close to the diffusion rate constant; = 1/( + + + ) is the exciplex lifetime; ∆ is the Gibbs energy of exciplex formation; and , ,, and are the rate constants of exciplex emission, internal conversion and intersystem crossing of exciplexes, and exciplex dissociation into radical ions, respectively. Competition between different processes of exciplex decay determines not only the quantum yields of primary products of electron transfer but also the magnitude of the rate constants of quenching of excited molecules. Internal conversion and intersystem crossing, which involve partial back charge transfer and lead to either the initial singlet ground state or the triplet state of reactants, are the major processes of the exciplex decay. The back electron transfer in the radical ion pairs has been extensively investigated by many researchers [11][12][13][14][15][16][17][18][19][20][21], especially in view of the fact that it is for these exciplexes that a decrease in the rate of electron transfer with an increase in the reaction exoergicity (free energy of back electron transfer ∆ G BET = electron transfer) predicted by the Marcus theory (inverted region) [11][12][13][14][15][16][17][18][19][20][21] was observed.In order to reveal most important factors governing the rate of internal conversion and intersystem crossing in exciplexes, we will consider two alternative mechanisms, the thermally activated electron transfer and radiationless quantum transition. An analysis of experimental data shows that it is more sound to consider both internal conversion and intersystem crossing (back electron transfer) in exciplexes as radiationless electronic-vibrational diabatic quantum transitions rather than thermally activated electron transfer.In the previous works [22,23], we measured the quantum yields of fluorescence ( ) of exciplexes of 9-cyanophenanthrene (CP) with 1,2,3-and 1,3,5-trimethoxybenzenes (123TMB and 135TMB), the quantum yields of formation from these of triplet states ( ) and of radical ions ( ), and the rate constants of internal conversion and intersystem crossing of these exciplexes in solvents of different polarities. The experimental methods and the procedures of the determination of the quantum yields of formation of triplets, radical ions, and internal conversion were described elsewhere [22,23]. The d...

A small cationic donor–acceptor iridium complex with a long-lived charge-separated state

“…28 Given the relatively small reorganisation energy of triarylamines vs. the free energy of À2.23 eV for charge recombination from either 3 CS or 1 CS state to S 0 , both the spin-allowed 1 CS -S 0 and the spin-forbidden 3 CS -S 0 processes are likely to be in the Marcus inverted region. 29,30 This is in agreement with the observation that back electron transfer is much faster in the polar MeCN (t 1 = 7 ns (94%); t 2 = 218 ns (6%)) in which both the reorganisation energy is larger and the free energy of back electron transfer is smaller. Both terms should decrease the inverted region effect as observed.…”

Spin Chemistry Investigation of Peculiarities of Photoinduced Electron Transfer in Donor–Acceptor Linked System

“…Then it might be reasonable to consider the possibility of processes in the exciplex of the dyads to influence the rate of fluorescence quenching. The abovementioned internal singlettriplet conversion of exciplex has to be highly sensitive to both energetic and steric effects [42].…”

Recent progress on exciplex-emitting OLEDs

“…The transition rate constants in exciplex-forming systems are also influenced by the degree of CT of exciplexes, f CT [9,12,13,[23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. The fluorescence rate constants (k f ), nonradiative rate constants (k nr ), and intersystem crossing (ISC) rate constants (k isc ) of the singlet exciplex states between 2,6,9,10-tetracyanoanthracene (TCA) and DCA as electron acceptors and various alkylbenzenes as electron donors in various solvents are plotted as a function of the average emission energy of singlet exciplex states in Figure 1(d) [12].…”