Energy and Charge Transfer in Organic Materials and Its Spectroscopic Signature: An Ab Initio Approach

[摘要] Energy and charge transfer processes in organic materials have received a tremendous amount of attention in recent years, due to their impact on functionality within a wide range of applications. One prominent example is the field of organic photovoltaics (OPVs), where significant improvements in power conversion efficiency and durability have been achieved over the last decade. Another example is organic scintillators, which have seen a renewed interest due to the constrained supply of helium–3 gas, as well as their ability to discriminate between types of ionizing radiation. Advancement in the design of organic photovoltaic and luminescent materials can be facilitated by molecular level insights into the processes of energy transfer, gained through both experimental observations and theoretical and computational modeling. Thus, this thesis utilizes computational techniques to investigate excited states, and their spectroscopic signatures, in molecular systems that are experimentally relevant for OPVs and organic scintillators.In Chapter II of this thesis, a computational protocol based on density functional theory (DFT) is presented for calculating the dependence of the vibrational frequency of a carbonyl reporter mode on the electronic state of the molecular system, in the context of charge transfer (CT) in organic molecules. This protocol was utilized to study a system consisting of a phenyl–C61–butyric acid methyl ester electron acceptor with a N,N–dimethylaniline donor, in which small frequency shifts of less than 4 cm−1 were observed between the ground state and the CT excited state. A Stark tuning rate of 0.768 cm−1/(MV/cm) was calculated between the vibrational frequency and the electric field.In Chapter III of this thesis, the CT process in a carotenoid–porphyrin–C60 molecular triad was investigated in its two primary conformations (bent/linear) with an explicit tetrahydrofuran solvent via molecular dynamics. Vibrational frequency distributions were calculated for the amide I mode and found to be sensitive to the three electronic states relevant to CT: the Pi–Pi* excited state, the porphyrin-to-C60 CT state, and the carotenoid-to-C60 charge-separated state, with shifts as large as 40–60 cm−1 observed between the CT1 and CT2 states. Rate constants between these states were calculated with a hierarchy of approximations based on the linearized semiclassical method. The CT process was determined to occur via a two-step mechanism, Pi–Pi* -> CT1 -> CT2, where the second step is mediated by the bent-to-linear conformation change.In Chapter IV of this thesis, the role of intersystem crossing (ISC) from S1 to Tn in the pulse-shape discrimination (PSD) ability of single-crystal trans–stilbene was investigated. Time-dependent DFT was used with the newly developed OT– SRSH–PCM method to calculate the excited states, and an equilibrium Fermi’s golden rule approach was employed to calculate transition rate constants. The ISC rates were found to be too slow to compete with prompt fluorescence, and thus do not significantly impact the PSD ability. Deuteration of trans–stilbene was found to have a retarding effect on the ISC rates, with rate constants reduced by as much as 30%.Finally, in Chapter V of this thesis, a novel compute-to-learn pedagogy is presented, in which students design and develop interactive demonstrations of physical chemistry concepts in a peer-led studio environment. The rationale behind the pedagogy and improvements made over the course of three iterations are discussed, as well as an initial assessment of the pedagogy conducted via end-of-semester interviews.

[发布机构] University of Michigan

[效力级别] Organic photovoltaics

[关键词] Charge transfer;Organic photovoltaics;Organic scintillators;Pulse shape discrimination;Chemistry education;Chemistry;Science;Chemistry