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The overall aim of our ACS-PRF proposal, and the work of our group in general, is the elucidation of fundamental aspects of exciton generation and charge transfer in organic molecular systems. Our studies are first and foremost a basic research effort, yet the implications could lead to the discovery of transformational improvements in organic molecular photovoltaics. The development of organic-based semiconductors that are inexpensive, processable, and stable replacements for silicon-based semiconductors would significantly advance photovoltaics and their use in energy production.

Our group reached two milestones with the discovery of charge separation in one self-assembled organic system, and the discovery of >100% quantum yield for triplet excited state generation in another. Our findings in 2008-2009 are the topics of two manuscripts that are now in preparation for ACS journals, and will also be disseminated in two posters and three talks that my students and I will present in the Spring 2010 ACS and Materials Research Society (MRS) meetings.

The terahertz (THz) research which was highlighted in the original proposal is in our sights, however for pragmatic and scientific reasons, other time-resolved optical pump-probe techniques were the focus of our initial work. Vibrational-electronic spectroscopy was necessary to distinguish various excited state species which could never have been identified by THz spectroscopy at the outset. Therefore, we built a picosecond pump-probe resonance Raman spectrometer immediately following the completion of the lab renovation. We also significantly improved an existing nanosecond transient absorption spectrometer for our studies. Preliminary femtosecond transient absorption studies have been performed in our lab and off-campus, and we anticipate that these kinds of experiments will be done routinely in our lab in the near future.

We also found it advantageous in the first year to focus our work on organic molecular fibers and aggregates in solution prior to work on organic molecular films. The fibers and aggregates are intimately connected to molecular films because intermolecular interactions dominate the photophysics in all of these systems. One can form different kinds of aggregates in solution as described below, analogous to control of a thin film morphology. However the presence of a solvent environment provides some practical advantages. First, a liquid solution allows one to probe the effect of environment (e.g. solvent polarity) on photophysics in a ways that are more difficult or impossible with thin films. Second, it is possible to explore size effects with aggregates. Third, all of our measurements are “non-contact” and a liquid offers advantages in pump-probe spectroscopy including the ability to flow fresh sample to the laser focus.

The following briefly describes specific achievements for each of the two systems mentioned above:

1) Charge separation and delocalization within an insulated organic nanowire comprised of perylene diimide subunits (Students with partial support from ACS-PRF: Brendan Connors & Maria Angelella)

Chromophores that can form extended structures are of central interest to our proposal from the standpoint of basic photophysics and charge-transfer behavior. Recently, an unusual amphiphilic derivative of perylene diimide (PDI) with polyethylene glycol substituents was synthesized and shown to self-assemble into robust micron-length fibers in a binary water/THF solution (Baram et al. J. Am. Chem. Soc. 2008, 130, 14966–14967). We have investigated the photophysics of the aggregates by femtosecond-to-millisecond transient absorption spectroscopy. We discovered that the aggregates undergo prompt charge separation upon excitation. The charge separation lasts into the microsecond time regime and is surprisingly insensitive to the addition of external quenchers including oxygen. Furthermore, the EPR spectra of chemically reduced aggregates reveal charge-sharing over multiple PDI units on a timescale that exceeds 10 MHz. The insights from both optical and magnetic resonance spectroscopic probes reveal that the self-assembled PDI nanofibers act as an organic molecular wire, with a core that facilitates charge transport and an insulating exterior shield which protects the charges from the environment. The time-scale and yield for the formation of separate charges, and the nature of the intermediate excited states in the process are currently under investigation.

2) Femtosecond-to-microsecond excited state dynamics of carotenoid aggregates in solution (students with partial support from the ACS-PRF: Chen Wang, Maria Angelella)

The array of photophysical and electronic properties that derive from the close coupling of nearby chromophores could lead to transformational improvements in inexpensive organic solar cells. Zeaxanthin aggregates with differing sizes, couplings, and optical properties can be readily formed in binary aqueous/organic solutions. At the end of the first year of ACS support, we found a remarkable result with picosecond resonance Raman spectroscopy that triplet excited states of zeaxanthin in one kind of aggregate can be formed in quantum yields that exceed 100%. In other words, most of the absorbed photons form a singlet excited state which then creates two triplet excited states on neighboring molecules (singlet fission) within 10 psec. These experiments reveal that the excited state dynamics of the aggregates differ dramatically from monomeric zeaxanthin which has a triplet yield of only 0.2 percent. Our results are the first observation of singlet fission in an organic molecular aggregate.

For the second half of the ACS support, we will focus on the mechanistic aspects of both the charge separation in the first project, and the triplet excited state dynamics in the second. Although the aim of the ACS-PRF work is specifically fundamental in nature, we emphasize that a “wire” of organic chromophores could open the way for new designs of organic molecular solar cells. Furthermore, the generation of multiple excitons (including multiple triplets) is well known to enhance solar cell efficiency, thus a better understanding of singlet fission would also be expected to benefit organic photovoltaics and energy production in the future.