Excited state behavior of molecular and heterogeneous systems: electron transfer and vibrational cooling

This thesis consists of five chapters. Chapter 1 gives a brief introduction of the concepts and mechanisms regarding radiative and nonradiative decay processes of an excited state, followed by the discussion on how the overall electronic and optical properties of a system change upon alteration of these decay processes. Three examples in either molecular or heterogeneous materials that covered three projects were discussed in detail to enrich the context of this work, which can be found in Chapter 3 to Chapter 5. The experimental techniques used in this thesis were summarized in Chapter 2. In the first project (Chapter 3), excited state behavior of both single strand and bulk RR-P3HT in contact with an Au nanowire array was studied. By utilizing pump-probe spectroscopy and single molecule fluorescence spectroscopy, information about electron transfer of photoexcited P3HT both in a bulk layer (P3HT@AuNR) and at single molecular level (SMP3HT@AuNR) was obtained. The results showed multifaceted influence of AuNRs on energy and electron transfer processes in P3HT@AuNR. While single molecule measurements show strong enhancement of fluorescence of P3HT, the average lifetime of singlet excitons is dramatically reduced due to efficient quenching by AuNRs. The second project (Chapter 4) focused on the nonradiative, vibrational cooling in a series of oligovioloens consists of MV+• with different size and topology. Ultrafast transient absorption spectroscopy and classical molecular dynamics (MD) simulations were employed. The results show ballistic vibrational energy transfer at early times in all oligomers. Partially cooled molecules show more pronounced size dependence caused by diffusive vibrational energy transfer. Interference effects brought about by symmetric, equivalent pathways and slower cooling were observed in the meta linked “star” topology. Later we compared initial transient kinetics in singly reduced (MV+•-MV++) and doubly reduced (MV++-MV++) methyl viologen dimers to investigate the influence of electron transfer upon the rate of internal conversion of MV+• when coupled to MV++. The frequency mismatch between singly and doubly reduced methyl viologen moieties in MV+•-MV++ dimer was proved to cause a slower vibrational cooling rate. The final project looked into the excited state dynamics of an anthracenylboronic compound that contains sterically crowded anthracene and phenyl substituents. Transient absorption measurements and molecular orbital calculations as well as theoretical modeling by semi-classical Marcus-Jortner equation were employed to investigate solvent effect upon photoninduced intramolecular charge transfer and its emission. The results revealed the nature of the charge separated state in such compound: a slight electron density shift from anthracene groups to boron atom instead of a complete charge transfer. The corresponding fluorescence is quenched in more polar solvents, which is caused by the reverse of the dipole moment orientation in this compound upon the transition from S1 to S0. In addition, with the help of careful thermodynamic calculations and the comparison between transient decay kinetics in acetonitrile and in hexane, it was found that the transition between localized singlet excited state (SA) and partially delocalized state (S1) and intersystem crossing from S1 to T1 accelerate in more polar solvents.Ph.D.Includes bibliographical reference