The Formation and Dissociation of the Solvated Ion-pair in the Excited Pyrene–Dicyanobenzene System

Abstract

Nanosecond laser photolysis has been applied to investigate the photochemical primary processes of the pyrene-p-dicyanobenzene system in some solvents. The solvated ion-pair is formed by the excitation and its yield increases as the dielectric constant of the solvent does; this was explained with the empirical equation proposed previously. The dissociation of the solvated ion-pair to free ions was directly observed in moderately polar solvents and the rate constant of the dissociation was determined, which is in agreement with the values calculated by Eigen’s equation. The role of the ion-pair formation in the fluorescence quenching was discussed on the basis of the present results, and it was concluded that some unknown quenching processes other than the triplet formation compete with the solvation to ion-pairs.

Radical ions in photochemistry. 15. The photosubstitution reaction between dicyanobenzenes and alkyl olefins

“…The radical cations of both pyrene (22) and 11 (70) have been detected by means of laser flash photolysis in acetonitrile in the presence of 1. Furthermore, Pac and co-workers (46) have calculated the theoretical free energy change for the transfer process, using the Weller equation, and came to the conclusion that transfer from the excited singlet state of several aromatic hydrocarbons to 1 should be a highly favoured process.…”

Investigation of geminate recombination of radical ion pairs generated by dissociation of exciplexes in moderately polar solvents using the photoconductivity technique

“…Recently, it has been shown [24] that measurements of transient photocurrents enable one to find the recombination rates of solvent-separated radical ion pairs in the case that the ions arise from dissociation of exciplexes, a primary product of the photoinduced ET reactions. It has been known that for many exciplex systems the exciplex fluorescence decay observed after short light pulses is accompanied by a build-up of the d.c. photocurrent attributed to dissociation of exciplexes into ions [9,[13][14][15][20][21][22]. The decay curves of the exciplex fluorescence and the photocurrent rise curves following a short photoexcitation pulse at time t = 0 were reproduced approximately by simple exponential functions, I fl (t) / exp(Àt/t fluor ) and J(t) / 1Àexp(Àt/t rise ), respectively.…”

“…The decay curves of the exciplex fluorescence and the photocurrent rise curves following a short photoexcitation pulse at time t = 0 were reproduced approximately by simple exponential functions, I fl (t) / exp(Àt/t fluor ) and J(t) / 1Àexp(Àt/t rise ), respectively. However, the photocurrent rise time (t rise ) was found to be, as a rule, less than the decay time (t fluor ) of the exciplex fluorescence [9,[13][14][15][20][21][22]. As shown [24], the explanation of the distinction between t rise and t fluor is that the life time of the exciplexes, precursors of ions, is comparable to the diffusion separation time of the geminate ion pair A -+ D + generated by dissociation of exciplexes.…”

“…The photoconductivity technique is of frequent use for determination of quantum yields of free ion formation in the photoinduced electron transfer (ET) reactions [9,[13][14][15][16][17][18][19][20][21][22] and for measurements of excited-state dipole moments and exciplex dipole moments [23]. Recently, it has been shown [24] that measurements of transient photocurrents enable one to find the recombination rates of solvent-separated radical ion pairs in the case that the ions arise from dissociation of exciplexes, a primary product of the photoinduced ET reactions.…”

“…The present paper is devoted to analysis of the recombination kinetics of radical ion pairs formed in the solutions of pyrene and p-dicyanobenzene (DCNB) in weakly polar solvents. The pyrene/ DCNB system is of interest for determination of the recombination rate of the ions because both the photocurrent rise curves and the fluorescence decay curves were studied in the same experimental conditions by Mataga and co-workers [13][14][15] for this donoracceptor system. It has been shown that the photoconductivity rise time can be either rather close to the t fluor time or distinctly less than that depending on solvent [15].…”