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Reports: DNI653045-DNI6: Ultrafast Dynamics of Organic and Molecular Electronics Components

Christopher G. Elles, PhD, University of Kansas

The research supported by this DNI grant from the ACS PRF examines the ultrafast dynamics of conjugated organic molecules that are basic building blocks for materials in molecular and organic electronics. Probing the structural and electronic dynamics of these small-molecule systems provides a bottom-up approach to understanding, and ultimately predicting, the behavior of advanced materials in response to electronic perturbations. Whether a compound is acting as a conductor, such as a molecular wire, or acting as an optically-active material, the properties and efficiency depend sensitively on the structural response of the underlying molecular framework. To this end, we have probed the excited-state dynamics of a number of model systems during the first year of the grant, including several two- and three-ring aryl-substituted thiophene compounds and even some preliminary measurements of a unique tail-to-tail biazulene compound. Additional work includes an examination of the quantum yields for destructive rearrangement reactions of arylthiophenes, as well as the addition of new experimental and computational capabilities for probing the excited state dynamics in even greater detail.

Of particular interest are the initial structural dynamics of conjugated compounds following optical excitation, and the role that those initial motions play in determining the intersystem crossing rates in these molecules, which typically form triplet states within tens to a few hundred picoseconds. Non-planar excited-state geometries favor intersystem crossing, therefore increasing conjugation length decreases the intersystem crossing rate by stabilizing the planar structure. On the other hand, and the addition of substituents can sterically block the planarization, and therefore increase intersystem crossing. We have used broadband transient absorption measurements to probe the ultrafast structural relaxation dynamics and intersystem crossing rates for a series of aryl-substituted thiophene compounds, as illustrated for diphenylthiophene in Figure 1. The ultrafast relaxation dynamics following the initial excitation of a molecule represents the structural reorganization in response to changing the electronic configuration upon excitation. Each of the compounds we have studied relax with characteristic timescales for vibrational relaxation and cooling in the excited state, and several of the compounds also exhibit coherent vibrational motions.

Figure 1. Transient absorption spectroscopy of diphenylthiophene following optical excitation. Spectral evolution within the first few picoseconds reveals structural relaxation and vibrational cooling, followed by the picosecond-scale intersystem crossing that is responsible for the spectral evolution in the figure.

Impulsive vibrational motions in the excited state provide a direct probe of the excited-state potential energy surface of the molecule, including especially the effects of changing the bond order along the conjugated backbone of the molecule. Increased pi-bonding across the inter-ring C–C bond leads to substantial structural rearrangement in the excited state. The coherent motion is typically dominated by a single ~100 cm–1 mode that represents the torsional motion of the molecule around the intra-ring bond, as observed in Figure 2. 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.

Figure 2. Coherent oscillations in the transient absorption spectrum reveal vibrational frequencies in the excited electronic state. The probe wavelength dependence of the oscillations reveals underlying structure in the excited state absorption spectrum.

A key factor that determines the ensemble dynamics of the conjugated systems we study is the mapping of ground-state conformational populations onto the excited-state potential energy surface upon excitation. The inter-ring torsional motions in both the ground and excited states are sensitive to steric effects that inhibit planar geometries, as well as conjugation effects that favor the planar structure. We are currently exploring the role of conformational disorder in the ground state, and how that affects impulsive vibrational motions in the excited states. This work has spawned a new collaboration that is providing opportunities for students to receive training in advanced computational techniques.

In addition to the transient absorption measurements, transient stimulated Raman spectroscopy offers a sensitive probe of the structural dynamics in the excited-state, and we have observed both the ultrafast structural relaxation and the intersystem crossing behavior from the complementary perspective of excited-state vibrational spectroscopy. The vibrational spectra reveal structural details of both the singlet and triplet excited states, which provides new benchmarks for electronic structure calculations, and might also be able to reveal the specific modes responsible for vibronic coupling among the singlet and triplet states.

The durability of the aryl-thiophenes is another important consideration for applications in organic and molecular electronics, therefore we have explored the role of low-yield decomposition reactions from the perspective of the excited-state dynamics. For example, rearrangement of the attachment position for aryl-substituted thiophenes only occurs in one direction (position 2 to 3 on the thiophene), and has profound consequences for the conductivity of the molecule. These rearrangement reactions occur in very low yield, making them challenging to study. However, we have measured rearrangement yields for a series of compounds. Surprisingly, the quantum yields for rearrangement seem to be anti-correlated with the intersystem crossing rate, despite the fact that rearrangement has been shown to occur only in the singlet states. The mechanism of the rearrangement reaction has been debated for several decades, but our measurements offer a more quantitative reference point for comparison with theory.

Funds from this grant have been used to provide salary and research support for two graduate students, as well as supporting the research efforts of an additional graduate student and an undergraduate summer research student. The impact on the overall research program has been substantial, making possible the addition of new experimental capabilities and enabling a wide range of ultrafast measurements for probing the fundamental dynamics of conjugated organic molecules. This work has also spurred a new collaboration with a theory group at KU that will provide new computational training opportunities for students whose main focus is experimental.