Electronic Structure and Dynamics of the Excited State in CT Microcrystals As Revealed by Femtosecond Diffuse Reflectance Spectroscopy

Abstract

Transient absorption spectra of charge-transfer (CT) crystals between methyl- or methoxy-substituted benzene derivatives and pyromellitic dianhydride (PMDA) were measured by femtosecond diffuse reflectance spectroscopy, and the electronic structure and dynamics of the excited state depending on the nature of the electron donor (D) are discussed. For such weak CT complexes, it was confirmed that excitation energy is localized in one donor−acceptor pair and the mixing of CT and locally excited (LE) configurations is important in the excited CT singlet state. The CT degree of the excited state depends on the oxidation potential of D and on the mutual configuration of D and electron acceptor (A) molecules. The second-order decay constant of the excited state was observed under usual photolysis conditions, from which a motion-limited diffusion was considered. A first-order decay of the transient absorption was obtained, when the excitation intensity was weak, and ascribed to charge recombination to the ground state. The energy gap (−ΔG) dependence of the charge recombination rate constant (k CR) was confirmed to give a linear relationship between ln(k CR) and |−ΔG|.

Role of Polar Protic Solvents in the Dissociation and Reactivity of Photogenerated Radical Ion Pairs

“…The spectral relaxation of the TCNB •– ESA feature occurs on time scales commensurate with the amplitude growth described above (∼4–10 ps). TCNB •– ESA blueshifts as a result of the structural reorganization of geminate ion pairs and their solvation dynamics and its spectral bandwidth is related to the existence of mixed-state transitions at early times (whose wavefunctions include contributions from CT and locally excited states) , and to heterogeneity in the environment of the photoexcited CT states and resulting ions. While all samples equilibrate to the same energetic position for their absorption maxima (∼466 nm), TCNB •– in more-polar media experiences a larger amount of blueshift (Δ E MeOH = 29 meV, Δ E EtOH = 17 meV, and Δ E MeBz = 7 meV).…”

Charge-transfer biexciton annihilation in a donor–acceptor co-crystal yields high-energy long-lived charge carriers

“…[26][27][28][29] Several studies characterizing the CT ground state optical absorption as well as the steady-state and time-resolved photoluminescence of D-A co-crystals have been reported. 2,11,12,20,21,30,31 Also, studies using ultrafast transient diffuse reectance spectroscopy have been reported on a variety of co-crystal powders, including phenathrene 32 and methylated benzenes 33,34 co-crystallized with PMDA, and aromatic donors co-crystallized with tetracyanobenzene 35,36 and tetracyanoquinodimethane 37 acceptors. In addition, there have been several studies of exciton transport in polyacene single crystals of interest to singlet ssion.…”

Fs-dynamics of charge-transfer excited states in anthracene–PMDA single crystals

“…In our work on An-PMDA single crystals, we analyzed the CT-state formation in detail at fstime resolution in the temperature range between 295 and 10 K. Polarization-dependent transient absorption spectra measured on two crystal planes allow for the separation of differently polarized contributing signals, a measurement that is not possible for powder samples [9,10] or liquid solutions. We thus identified the expected three CT signal contributions and were able to separate an additional signal.…”