Exciplexes versus Loose Ion Pairs: How Does the Driving Force Impact the Initial Product Ratio of Photoinduced Charge Separation Reactions?

Hao Minh Hoang,Günter Grampp,Daniel R. Kattnig,Thi Bich Van Pham

doi:10.1021/jz501575r

Hao Minh Hoang, Günter Grampp + Show 2 more

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https://doi.org/10.1021/jz501575r

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Abstract

Many donor–acceptor systems can undergo a photoinduced charge separation reaction, yielding loose ion pairs (LIPs). LIPs can be formed either directly via (distant) electron transfer (ET) or indirectly via the dissociation of an initially formed exciplex or tight ion pair. Establishing the prevalence of one of the reaction pathways is challenging because differentiating initially formed exciplexes from LIPs is difficult due to similar spectroscopic footprints. Hence, no comprehensive reaction model has been established for moderately polar solvents. Here, we employ an approach based on the time-resolved magnetic field effect (MFE) of the delayed exciplex luminescence to distinguish the two reaction channels. We focus on the effects of the driving force of ET and the solvent permittivity. We show that, surprisingly, the exciplex channel is significant even for an exergonic ET system with a free energy of ET of −0.58 eV and for the most polar solutions studied (butyronitrile). Our findings demonstrate that exciplexes play a crucial role even in polar solvents and at moderate driving forces, contrary to what is usually assumed.

Highlights

Photoinduced electron transfer (ET) reactions have been extensively studied for many years
While the loose ion pairs (LIPs) is formed at distances longer than or equal to the contact distance of A* and D, the exciplex formation typically involves tight stacking and a well-defined relative orientation,[5−9] which can be inferred from the correlated motion of the donor and the acceptor in the complex.[10]
In nonpolar solvents, exciplex fluorescence is often observed, suggesting the contribution of the exciplex formation in the ET deactivation.[11−18] Only a few exceptions to this empirical rule are known in the literature