Selectively Altering the Reactivity of Transient Organic Radical Ions via Their Solvation Environment

Abstract

Photoexcitation of the charge transfer band of electron donor−acceptor complexes composed of toluene and 1,2,4,5tetracyanobenzene yields organic radical ion pairs whose ultrafast reactive dynamics are determined by equilibrium solvent properties. A comparative study of ultrafast reaction rates in a series of alkane alcohols identified their dependence on the local polarizability and hydrogen bond donating/accepting character of the solvent. Because of the rapid and efficient equilibration of these radical ion pairs into solvent-separated species, simple modifications to bulk conditions can be used as a means to selectively alter their decay rates. Selectively altering distinct stages in this photochemical cycle via cosolutes or additives is a valuable step toward understanding and controlling the reactivity of organic radical ions in complex environments.

Quantum Coherent Phenomena in Energy Harvesting and Storage

“…A key recent result in this area by the Scholes group proposes that dephasing times can be used to differentiate functional coherences from spectator coherences . Other work in this VSI examining environmental impacts include simulation of bath effects on dynamic Stokes shifts, calculations on effects of interchain contacts in charge transfer polymers, impacts of confinement on aggregate dynamics, solvent effects on charge transfer in contact ion pairs, temperature-dependent singlet fission in hexacene, and impacts of coherent and incoherent energy transfer …”

“…In the development of our understanding of the possible uses of quantum coherences to increase light harvesting and energy conversion, experimental results aimed at probing the functional role of coherences have been crucial. These include 2-dimensional electronic spectroscopy (2DES), ,,, which is the leading experimental approach in this field, as well as ultrafast transient absorption spectroscopy ,,,, and femtosecond stimulated Raman spectroscopy. , This VSI includes a thoughtful review on 2DES, as well as several papers highlighting the ability to simulate 2DES measurements. ,, The role of careful experimental measurements such as temperature-dependent and magnetic field-dependent absorption and emission spectroscopy are critical in adding to our fundamental understanding. Recent breakthroughs have also come from advances in theoretical and computational methods, such as the ability to accurately simulate complex spectra from complex and heterogeneous chemical systems, as well as to model interactions between chromophores, polymers, and donor–acceptor complexes.…”

“…A key recent result in this area by the Scholes group proposes that dephasing times can be used to differentiate functional coherences from spectator coherences. 10 shifts, 11 calculations on effects of interchain contacts in charge transfer polymers, 12 impacts of confinement on aggregate dynamics, 13 solvent effects on charge transfer in contact ion pairs, 14 temperature-dependent singlet fission in hexacene, 15 and impacts of coherent and incoherent energy transfer. 16 Newer directions in quantum coherent light harvesting processes include the creation of polaritonic states, which are formed as a result of strong coupling between electromagnetic fields and molecular orbitals, along with the use of entangled light to generate and probe coherences.…”

“…20 In the development of our understanding of the possible uses of quantum coherences to increase light harvesting and energy conversion, experimental results aimed at probing the functional role of coherences have been crucial. These include 2-dimensional electronic spectroscopy (2DES), 6,16,21,22 which is the leading experimental approach in this field, as well as ultrafast transient absorption spectroscopy 1,14,15,20,7 and femtosecond stimulated Raman spectroscopy. 8,9 This VSI includes a thoughtful review on 2DES, as well as several papers highlighting the ability to simulate 2DES measurements.…”

Solvent Control of Chemical Identity Can Change Photodissociation into Photoisomerization

“…In addition to the special cases where reactions cannot occur without solvent mediation, the presence of solvent molecules can also strongly alter our gas-phase-like picture of solution-phase chemistry: solvent molecules can do much more than act as a reactive medium. For example, first-shell solvent molecules can “cage” the products of photodissociation reactions, inhibiting separation of the photofragments and promoting recombination. − Additionally, photoreaction pathways and photofragment relaxation time scales can differ depending on solvent polarity − or viscosity . Moreover, solvent interactions can alter the potential energy surface on which reactions take place, changing them significantly from what they were for an isolated gas-phase solute. − Previously, we have shown that Pauli repulsion interactions from surrounding solvent molecules can compress a solute’s bonding electrons, raising a solute’s bond vibrational frequency .…”