Experimental observation of preferential solvation on a radical ion pair using MARY spectroscopy

Abstract

The effect of preferential solvation on the exciplex luminescence detected magnetic field effect has been studied using magnetic-field-effect-on-reaction-yield (MARY) spectroscopy. By designing solvent mixtures which can provide a micro-environment around the magneto-sensitive radical ion pair (RIP) from highly heterogeneous to quasi-homogenous, the effect of the polarity scan on an absolute magnetic field effect (χ(E)) and B(1/2) (the field value marking half saturation) has been studied on the system 9,10-dimethylanthracene (fluorophore)/N,N'-dimethylaniline (quencher). While the trend in χ(E) (although with subtle differences) follows the usual norm of passing through maxima with increasing polarity, the B(1/2) values show either a large monotonic decrease (for heterogeneous solvents) or remain constant (for quasi-homogenous systems) with increasing polarity. The observations have been interpreted invoking the concept of amplification of the "cage-effect" as a result of preferential solvation in binary solvents and its influence on the decaying exciplex. The use of ternary solvents further confirms the proposed mechanism. Additionally electron hopping from the radical ion pair to the surrounding neutral donor molecules could also possibly contribute to the observed trend.

Magnetic Field-Sensitive Radical Pair Dynamics in Polymethylene Ether-Bridged Donor–Acceptor Systems

“…We observe that for n = 8 and 10, the B 1/2 -values are markedly larger than the corresponding value for the free fluorophore–quencher-pair DMAnt/DMA in the limit of low donor concentration ( B 1/2 = 5.3 mT). 5,6 This can be attributed to a reduced lifetime of the radical pair and/or singlet–triplet dephasing. In fact, the lifetime effect has been suggested to lead to a broadening of the MARY lines in a study of the micro-heterogeneous solvation of the radical pair of DMAnt/DMA in DMSO/toluene mixtures, which likewise promote large MFEs by increasing the reencounter probability.…”

“…By systematically studying the MFE of the exciplex emission, details of the dynamics of the spin-correlated RIP can be revealed indirectly. 5,6,9−11 The MFEs are theoretically accounted for in the framework of the so-called radical pair mechanism . 4,12,13,17−20 It relies on the quantum-coherent mixing, by the hyperfine interaction (HFI) and its interplay with the Zeeman interaction, of the singlet (S) and three triplet sublevels (T ± , T 0 ) describing the spin configurations of the two uncompensated electron spins on the radical ions (hyperfine mechanism).…”

Impact of chirality on the photoinduced charge transfer in linked systems containing naproxen enantiomers

“…34,[37][38][39][40] This impact of polarity on CIDNP efficiency has been shown earlier for the photoinduced ET in the short dyad NPX -Pyr 4(a,b). 22,24 Dyads NPX-AA-Pyr 2(a,b) and NPX-CyAA-Pyr 3(a,b) also demonstrate the dependence of 1 H CIDNP effects at N-methylpyrrolidine fragments, obtained with the help of a pseudo-steady state pulses sequence, 41 on the solvent permittivity (Fig.…”

Spectral and thermodynamic properties for the exciplexes of N-alkyl carbazoles with dicyanobenzenes in THF

“…The exciplex, on the other hand, is formed as a result of bimolecular interactions between an excited fluorophore (acting as acceptor (A)) and a ground state quencher acting as electron donor (D) and vice versa [1]. Formation of exciplex is accompanied by the change in the electronic structure of excited fluorophore and quencher along with the change in medium reorganization [2][3][4].…”