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Current Highlight 

When a crystal of perylene molecules is illuminated by light, the molecule’s electrons can be lifted to a higher energy level. In this excited state the molecules in the crystal lattice suddenly experience a pair-wise attraction. They form short-lived (~100 nanoseconds) excited-state pairs, called excimers, which has profound effects on the properties of the crystal.

Researchers from CMM set out to capture this process in a “Molecular Movie” in an effort to better understand the fundamental structural dynamics of organic semiconductor materials under operating conditions (e.g. excitation by light). This may help to efficiently employ such light-sensitive organic molecules for thin, cheap and bendable solar cells.

While the five-ringed structure of perylene and its crystalline phases were determined close to 50 years ago, the transient structures formed after light excitation remained elusive due to their short-lived nature for many years. Early theoretical results (Cohen 1978), however, indicated a substantial shortening of the distance between the two molecules forming an excimer and thus the prospects of actually capturing the contraction process “in action” with time-resolved synchrotron radiation (link) appeared to be good. The CMM research team with PhD student Henrik Lemke as primary investigator performed the first time-resolved X-ray measurements on these thin organic films in July 2006 at the ID09 beamline at the European Synchrotron Radiation Facility and again at the Japanese Photon Factory in February 2007. After these measurements, spirits were high as strong, time-dependent signals were detected on the sub-nanosecond time scale but surprisingly, these results could not be explained within the old theory from the seventies.

These discrepancies inspired a group of theoreticians, headed by Post. Doc Gustavo Velardez, within CMM’s, to launch an investigation of the excimer structure of perylene using some of the most powerful methods of modern quantum chemistry. It was discovered that the earlier theory significantly overestimated the structural changes associated with the excimer formation and new predictions were obtained (Velardez, 2008). Based on these results it became clear that the experimental accuracy would have to be significantly increased to observe the transient structure of perylene excimers.

Meanwhile, the early experimental results obtained at ESRF and KEK were reproduced with other thin-film perylene samples with different geometries, and through detailed analysis of the results Henrik Lemke and colleagues determined that the nanosecond dynamics observed had an altogether different origin than expected: Rather than being the signal from excimer formation, the dynamics observed in the time-resolved experiments were coherent lattice vibrations, phonons, an effect which had never previously been detected with X-rays in thin organic films (Lemke 2008, in preparation).

So while the elusive structure of the perylene excimer has yet to be determined, the efforts of the CMM researchers have yielded new insights, both theoretical and experimental, into the detailed dynamics of photo-excited organic semiconductors. The interplay between the Centre´s cross-disciplinary capabilities within sample processing and characterization, advanced X-ray techniques and theoretical modelling has proven crucial in developing this understanding of the perylene system and the continued efforts are expected to further extend the knowledge of fundamental structural dynamics in organic semiconductor materials.