60th Annual Report on Research 2015

The research supported by this grant examines excited-state dynamics of aryl-substituted thiophenes and other conjugated organic molecular building blocks following optical excitation in solution. Optical excitation induces dramatic structural changes of conjugated molecules due to the shifting of π-bonding patterns in the excited state. Although the details of the structural response of a conjugated system are inherently sensitive to specific environmental conditions, optical excitation of small model compounds in solution provides an insightful model for examining in exquisite detail the structural dynamics of conjugated molecules in response to external electronic stimuli. Ultimately, the evolution of the atomic structure influences electronic properties of the molecule, including charge conduction properties that are relevant for applications in organic electronics and molecular wires.

Conjugated molecules are ubiquitous in organic electronics applications that require light-weight, flexible, and efficient substrates for charge generation and transport, and also hold great promise for novel molecular electronics applications involving single molecular wires. Delocalized π-conjugation along the backbone of the molecule facilitates charge transport over relatively long distances, but the efficiency of charge transport (or of charge separation in an optically active material) sensitively depends on the structure of the underlying molecular framework. Subtle deformations in response to the electronic perturbation of a passing charge carrier or exciton may inhibit the transport properties of the material through localization and trapping. To better understand these effects, the work supported by this grant uses transient electronic and vibrational spectrosopy to examine the ultrafast structural dynamics of small conjugated molecules. Mapping the response of well-defined model systems provides a convenient framework for understanding the underlying structural evolution of much larger systems that are subject to external electronic perturbations.

The experimental approach is two-fold, using transient absorption spectroscopy to monitor the excited-state dynamics of conjugated molecular building blocks based on changes of the electronic structure, as well as transient stimulated Raman measurements that more directly probe the evolving structure of the molecule in the excited electronic state via the vibrational frequencies and intensities. Conjugated molecules typically have strong excited-state absorption bands that evolve along with the structural and vibrational relaxation, and here are also used for resonance enhancement of the transient stimulated Raman measurement. Notably, the aryl-substituted thiophenes exhibit a clear separation of timescales between the vibrational relaxation dynamics of high-frequency modes and the slower, large-amplitude structural response of the molecule to the perturbation of the π-bonding framework. Collectively, these measurements provide a sensitive window on the excited state potential energy surfaces that govern the vibrational dynamics of model conjugated systems in response to electronic perturbations. The transient absorption experiments directly monitor low frequency motions along the relaxation coordinate, whereas transient stimulated Raman measurements sample the higher frequency vibrations as a probe of the transient structure of the molecule.

A key feature of these solution phase studies is the role of conformational disorder. More specifically, the ensemble dynamics of conjugated systems following optical excitation depends on the mapping of ground-state conformational populations onto the excited-state potential energy surface upon excitation. For example, a central focus of the work during year two of this grant has been the excited-state strucutral dynamics of diphenylthiophene, a model compound that typifies many of the fundamental features of conjugated systems. The ground-state potential energy surface of this molecule is very shallow along the inter-ring torsional coordinates, leading to a very broad distribution of conformational states with inter-ring dihedral angles spanning more than 60 degrees at room temperature. However, increased π-bonding across the inter-ring C–C bond upon optical excitation leads to a substantial stabilization of the planar geometry in the excited state, and a steep potential along that coordinate. Impulsive excitation of molecules from non-planar geometries therefore leads to significant torsional motion along the steep excited-state potential, resulting in a strong quantum beating signal in the transient absorption spectrum. The frequency of the quantum beat is surprisingly insensitive to the excitation wavelength, and the coherent motion is dominated by a single ~110 cm^–1 quantum beat that represents the torsional motion of the molecule around the intra-ring bond. This motion plays a central role in tuning the conductivity of a molecule in the optically excited state, because the planarity of the molecule directly influences the degree of conjugation along the backbone of the molecule. More fundamentally, the dynamics measurements provide new insight on the competition between steric effects that inhibit planar geometries, and conjugation effects that favor a planar structure, and how they control the restructuring of the molecular backbone in both the ground and excited electronic states upon perturbation of a molecule.

Complementing the transient absorption measurements, transient stimulated Raman spectroscopy offers a sensitive probe of the structural dynamics in the excited-state. The vibrational spectra reveal structural details of both the singlet and triplet excited states, which provides new benchmarks for electronic structure calculations. However, the primary goal is using high-frequency vibrations as a structural probe of the excited state dynamics. Not only are the frequencies of the vibrations valuable in determining bonding patterns and structure, but also the Raman intensities are sensitive to delocalization and conjugation effects, and therefore offer additional information about the excited-state dynamics of conjugated molecules. Although Raman spectroscopy presents significant challenges, this grant has made possible a combined experimental-theoretical approach by facilitating a new collaboration with an electronic structure group at KU, including support for a co-advised graduate student. This fruitful collaboration is providing key insight on the experimental results through calculations of the excited-state potential energy surfaces, as well as calculations of the excited-state Raman spectra. Probing the structural and electronic dynamics of small conjugated molecules provides a bottom-up approach to understanding, and ultimately predicting, the behavior of advanced materials in response to electronic perturbations.

This grant has had a substantial impact on the development of the overall research program in many ways. Student support provided by the grant has made possible the implementation of new experimental methodologies, as well as the development of a robust new collaboration. Specifically, two graduate students were supported during the second year of this grant.