Determination of the Energy Levels of Radical Pair States in Photosynthetic Models Oriented in Liquid Crystals with Time-Resolved Electron Paramagnetic Resonance

Abstract

We report the results of time-resolved electron paramagnetic resonance (TREPR) studies of photoinduced charge separation in a series of biomimetic supramolecular compounds dissolved in oriented liquid crystal solvents. The molecules contain a chlorophyll-like (zinc 9-desoxomethylpyropheophorbide a) electron donor, D (ZC), and two electron acceptors with different reduction potentials, i.e., pyromellitimide, A1 (PI), and 1,8:4,5-naphthalenediimide, A2 (NI). The compounds investigated are ZCPI, ZCNI, and ZCPINI, and they have small but well-defined differences of their ion-pair energies. Temperature-dependent TREPR studies on this series of compounds permit the determination of the radical pair energy levels as the solvent reorganization energy increases from the low-temperature crystalline phase, through the soft glass phase, to the nematic phase of the liquid crystal. As the temperature is increased, the radical pair with the lowest energy is the first to exhibit triplet-initiated charge separation as the solvent reorganization energy increases in the liquid crystal. The energy levels of the radical pairs and the solvent reorganization energy are determined by using the known singlet and triplet excited state energy levels of ZC, the electrochemically determined relative energies between the radical ion pairs in polar isotropic solvents, and the TREPR data. All these yield information about the ordering of the radical ion pair energy levels relative to the excited-state energy levels of ZC.

Triplet porphyrins as donors in intramolecular electron transfer and their intermolecular interaction with free radicals

“…The latter mechanism is observed in D-s-A systems with center-to-center distance r b 12 Å . This conclusion is supported by comparing the spectra of 3 For the pairs with short center-to-center distance a spectral width of $1.6 mT corresponds, by using the point dipole approximation method of TRPs [24,25], to a chargeseparated distance of $11.5 Å , which agrees well with the molecule dimension of 11 Å . In contrast to this, by assuming a TRP spectrum and with a spectral width of 0.65 mT for ZC Á PINI ÁÀ , a value of 15 Å is calculated for the pair separation.…”

“…In contrast to this, by assuming a TRP spectrum and with a spectral width of 0.65 mT for ZC Á PINI ÁÀ , a value of 15 Å is calculated for the pair separation. This value does not agree with the molecule dimension, which supports the conclusion that a CRP is generated in this case [24,25].…”

“…Thus it is important to develop experimental methods to explicitly determine the absolute values of the radical ion pair energy levels. This goal can be achieved by blending together molecular architecture, solvent properties (LCs) and fast TREPR detection of paramagnetic transients [24,25]. This powerful approach has permitted the elucidation of photochemical mechanisms that otherwise could not be explored.…”

“…In general terms, for a center-to-center distance r`12 Å the TRP dominates the spectrum (the electron exchange interaction J is larger than the dipolar interaction d), while for r b 12 Å the dipolar interaction is dominant and leads to the correlated radical pair mechanism (CRPM) [24,25]. The width of the RP spectrum can provide us with valuable information regarding the type of RP, i.e.…”

Singlet and Triplet Excited States and Intersystem Crossing in Free‐Base Porphyrin: TDDFT and DFT/MRCI Study

“…Considerable efforts have recently focused on the development of metalloporphyrin assemblies and porphyrin-containing mimics of the photosynthetic reaction centers for light-harvesting purposes, as well as on the creation of molecular candidates for duplicating the photoinduced processes involved in solar-energy conversion. [4][5][6] In addition, the triplet excited states of porphyrins are often used in cancer treatment and phototherapy for oxidation of a tumor. [7,8] Finally, the strong triplet excited-state absorption of porphyrins has generated considerable interest in their use for nonlinear absorption and optical-storage applications.…”