Heavy Atom Substituents as Molecular Probes for Solvent Effects on the Dynamics of Short‐lived Triplet Exciplexes

Abstract

The influence of heavy atom substituents (Br, I) in the electron donor aniline on the electron transfer reaction with thiopyronine triplet is investigated by flash spectroscopy in solvents of different viscosity and polarity. Triplet quenching constants and radical yields are determined. The results are analysed in terms of decay constants of an intermediate triplet exciplex where the heavy atom substituents significantly enhance the intersystem crossing process leading to singlet ground state formation and thus diminishing the radical yield due to exciplex dissociation. The sensitivity of the radical yield to solvent effects is strongly enhanced by heavy atom substitution though the solvent effects proper on the exciplex dynamics are substituent independent. The heavy atom effect is used to study the solvent cage effect on exciplex dissociation by means of viscosity variation. The exciplex dissociation lifetime is proportional to solvent viscosity, however, it contains a constant contribution which may be attributed to exciplex bonding. By means of the heavy atom probes it is found that increasing solvent polarity intersystem crossing in the exciplex is favored over exciplex dissociation into radicals. A tentative explanation is given in terms of solvent polarity dependence of the electronic energy gap between exciplex and corresponding Franck-Condon ground state. (1) and (2) it follows that the radical yield is determined by three factors, characterizing the behaviour of the triplet exciplex (Eq. (4» where 'Hal is the atomic spin-orbit coupling constant of the halogen and C 2 (POSHaJ is the Hiickel spin density transferred to the position of the halogen atom by the electron transfer. The latter factor readily explains the positional dependence of the substituent effect (para > ortho > meta).(1)The rate determining step in the quenching of the excited cationic dye triplet eA +) by the electron donor (D) is the formation of a triplet exciplex with radical-pair-like electronic structure e(A'D +». The triplet exciplex has two decay channels: dissociation to the free radicals (rate constant k fr ) and intersystem crossing with back transfer of the electron to the donor leading to the formation of the ground state of the reaction partners (rate constant k isc ) ' The radical yield observed is determined by the two decay constants of the exciplex:triplet exciplexes can be detected by their phosphorescence under such conditions. In a recent paper [22] we provided indirect evidence for the formation of triplet exciplexes as intermediates in electron transfer reactions. We studied the reaction of thionine triplet as an electron acceptor with aniline and its monohalogen derivatives as electron donors in methanol. In these reactions the halogen substituents have systematic influence on the radical yield (extrapolated to complete triplet quenching) which could be explained satisfactorily by the following mechanism, strongly supported by a recently determined magnetic field effect [23]:The analysis of the radical yield...

Magnetokinetic Investigations of Spin-Forbidden Electron Back Transfer in Exciplexes and Radical Pairs

“…In concluding, we point out that the determination of abso lute values of individual rate constants characterizing the de cay of the short lived triplet exciplexes, as has been demon strated in this paper, confirms Dur previous interpretation of the solvent viscosity and polarity dependence of ~fr [6]. The argument provided for an increase of kisc with solvent polarity was that increasing the solvent polarity should have a decre asing effect on the Franck-Condon energy gap between exciplex and ground state pair of reactants.…”

“…Nevertheless one might assume that k er mainly reflects solvent viscosity effects, whereas kisc charac terizing a process that involves electron transfer (BET) should also depend on solvent polarity. It was on the basis of these assumptions that a reasonable interpretation of the solvent de pendence of ~fr for the reaction of thiopyronine triplet with p-I-aniline and p-Br-aniline could be given previously [6]. As will be shown in the following, however, with the help of the magnetokinetic technique one can determine the absolute values of the rate parameters of the triplet exciplex and, in addition, assess the relative importance of BET within the ex ciplex and the correlated RP.…”

“…as an indicator of solvent effects on or kisc' Note, however, k er that relative changes of these rate constants are reflected as changes of ~fr only if kisc > For this reason the applica k er . tion of heavy atom substituents proves to be most useful [6]. The experimental results reported and analyzed in this paper have been obtained by investigating the electron transfer reac tion between methylene blue triplet and p-I-aniline, the iodine substituent providing for a strong SOC in the redox pair.…”

“…It is therefore difficul t to obtain direct information on subnanosecond dynamics of short-lived electron transfer intermediates in such cases. This is exactly the situation encountered when studying pho toelectron transfer reactions with typical sensitizer dyes as, for example, the phenothiazine dyes thionine or methylene blue in which we have been interested for some time [3][4][5][6]. These dyes are cations and acting as photoelectron acceptors they form neutral radicals so that no Coulombic inter action prevents the primary redox pair from dissociation.…”

Photo-induced inactivation of viruses: adsorption of methylene blue, thionine, and thiopyronine on Qbeta bacteriophage.

“…TP (TP+Cl-= TP) (Fluka) was twice recrystallized from ethanol. Hydrogen chloride associated with crystalline TP (TP+Cl-x 1/2 HCl) was removed by heating at 100°C in vacuo at 0.1 torr (1 torr = 133 Pa) (13).…”