Role of viscosity in the magnetic field effect on pyrene–DMA exciplex emission at different permittivities Amit Kumar Jana, Partha Roy, Deb Narayan Nath ⇑ Department of Physical Chemistry, Indian Association for the Cultivation of Science, 2A and 2B Raja S.C. Mullick Road, Jadavpur, Kolkata 700032, India article info Article history: Received 30 September 2013 In final form 3 January 2014 Available online 10 January 2014 abstract Effect of viscosity variation on the magnetic field effect in pyrene-N,N-dimethylaniline exciplex lumines- cence has been studied at different permittivity values. The data is compatible to the model of Krissinel et al. (1999) [10] reported earlier to explain the effect probing the escape yield of radical pairs. It is shown that the data can also be explained on the basis of a simple model. It is interesting to note that the present letter also demonstrates the positive slope of MFE with diffusivity at extremely high viscous condition as predicted by Krissinel et al. (1999) [10] which has not been observed in earlier experiments. Ó 2014 Elsevier B.V. All rights reserved. 1. Introduction Studies on the role of viscosity in the magnetic field effect (MFE) in radical pair (RP) systems are not many in literature. To the best of our knowledge the pioneer work has been made by Steiner et al. [1] in the electron-transfer quenching of methylene blue triplet by p-iodoaniline in methanol–ethyleneglycol mixtures of various vis- cosities by monitoring free radical yield (/ ce ) using laser flash spec- troscopy and a photo-stationary flow technique. In this letter the MFE (relative change in the yield of free radical in presence of mag- netic field) shows an increase with decrease in diffusivity (D). It should be noted that the inter-system-crossing (ISC) mechanism involved in RP in this case is of Dg type and it required high mag- netic field on the order of a few Tesla. Similar studies has been made on the escape yield (survival probability) / ce created in the electron transfer reaction between photo-excited [Ru(bpy) 3 ] 2+ and methyl viologen (MV 2+ ) observed for solvent mixtures (H 2- O:ACN, 1:1, with ethyleneglycol) of different viscosities [2]. Here also the ISC mechanism is of Dg type but MFE on / ce exhibits char- acteristic minima in the MFE versus viscosity curves. Recent stud- ies on the MFE and its viscosity dependence on the hydrogen abstraction process in benzophenon, Dg mechanism, [3,4] also shows non-monotonous signature. It is to be noted that the viscosity effect on the MFE in singlet recombination yield, like exciplex luminescence, in the RP has not been studied yet. The case of hyperfine interaction (HFI) induces ISC which operates at low field is also untouched and systematic study of the viscosity effect at different permittivity (e) values also has not been made. In this letter, we have studied the viscosity effect on MFE by monitoring the singlet exciplex luminescence of unlinked pyrene-N,N-dimethylaniline (Py-DMA) system at different e values. The Noyes approach of classic Smoluchowksi equation of sto- chastic motion states that as the diffusivity increases the recombi- nation probability of geminate RP should decrease [5]. We know that to have MFE the RIP must diffuse out to the extent of S-T degeneracy where HFI induced ISC can be operative and at the same time to observe the effect on singlet exciplex luminescence there should be also appreciable recombination probability. The nature of electron transfer (ET) in the basic photo-induced process is also very important. Earliest concept is contact transfer between excited acceptor and donor. Later in early 50’s Marcus introduced the concept of solvent mediated ET (super-exchange); i.e. in addi- tion to sandwich configuration of acceptor and donor (overlapping of MO’s), ET can also takes place through solvent molecule by tun- neling (steric orientation of MO of solvent may be also important). The concept of Marcus is the backbone of exponential model where the ET efficiency decreases exponentially with donor–acceptor dis- tance monotonously. The electron transfer rate thus is given by k et ðrÞ¼ w 0 exp½ðr  aÞ=L ð1Þ where w 0 is the rate constant at contact distance a, and L is a char- acteristic length parameter depending on the strength of super-ex- change. Burshtein et al. [6–9] has considered the spatial dispersion of free energy (DG) and reorganization energy (E r ) and the simpli- fied concept of exponentially decreasing ET probability, according to Marcus, changes to bell shaped curve (non-monotonous, the probability is maximum not at contact but at a certain distance) for both ionization and recombination. According to Burshtein mod- el; at very high diffusivity when the system is kinetically controlled the transfer rate is approximated to be contact. Whereas, when ion- ization is controlled by diffusion (under viscous condition) the spherical reaction layer, where ions are mainly born (r 0 ), is shown 0009-2614/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2014.01.003 ⇑ Corresponding author. Fax: +91 33 2473 2805. E-mail address: pcdnn@iacs.res.in (D.N. Nath). Chemical Physics Letters 593 (2014) 145–149 Contents lists available at ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett