Some areas of active interest are discussed below. Topics include: adaptive femtosecond pulse shaping, synthetic control of electron transfer photochemistry, statistical analysis methods developed for control results, 2D electronic spectroscopy and other phase-sensitve tools using pulse shaping.
Our coherent control research centers around manipulation of photochemistry using adaptive femtosecond pulse shaping. This methodology implements a 'learning loop' as suggested by Rabitz in 1992 as an experimental analogy to optimal control theory. The technique has begun to blossom in the last decade with numerous experimental demonstrations in the chemistry and physics communities.
We became interested because the technique is amenable to electronically, structurally, and reactively complex systems: it approaches photochemical control in much the same way that many complex biological problems are solved; i.e., through adaptation within massive parameter spaces. We seek control of interesting photochemical and photophysical control subjects often centering around manipulation of electron or charge transfer phenomena. We focus on molecules and materials of specific interest in solar energy conversion efforts. Our synthetic program plays an integral role in these efforts.
At the same time we are developing experimental and analysis tools to understand the physiochemical implications of control results. As we discover controllability we are asking and studying why certain dynamics take place, what kinds are controllable, what it means for understanding excited states, how discovered intermediates react, and what we can do to redesign molecules so that they do interesting and potentially important photochemistry with incoherent radiation.
Statistical Analyses: In our initial published work (JPCA2006, 110, 6391) we developed a new general tool for analysis of adaptive pulse shaping results that extracts the essential experimental variables that are being manipulated when control is achieved. For example we found only sevenknobs are important out of an original 208 when controlling the excited state population of an emissive Ru(II) coordination complex (the control mechanism is explored/understood in the context of JPCA 2007, 111, 1426). This dimension reduction heralds significant opportunities to understand the physical implications of a control mechanism without experimentally biasing for a particular control outcome by limiting the search space of the algorithm.
We next realized this general dimension reduction methodology suggests a procedure for extracting a singlecontrol ‘knob’ capable of manipulating the quality of a laser pulse at achieving a particular control outcome. When we applied this idea to our emission control result, we found we could linearly manipulate the quality (fitness) of laser pulses and that their quality is directly linked to the mechanism we had previously uncovered (JPCA 2007, 111, 5126). This was an exciting result suggesting that unbiased adaptive control also reveals the necessary information for testing hypotheses about control mechanisms.
In recent work using simulated control experiments (manuscript in preparation) we have identified patterns in the phase functions of optimal shaped laser fields that explain the mechanisms of control and why reduced dimensions are sufficient for robust manipulation of fitness.
Phase Sensitive Spectroscopies: Our research has also lead us to develop pulse-shaper based phase-sensitive spectroscopic tools to interrogate complex systems and to identify opportunities for control. For example we recently collaborated with the Zanni group at Wisconsin to develop methodologies for facile collection of 2D electronic (2DE) spectra (Opt. Express 2007, 15, 16681: We showed 2DE for the gaseous Rb system).
We are now exploiting this to deconstruct broad electronic spectra of transition metal complexes in solution and would like to identifying excitation bandwidths where multiple electronic states are interrogated. We hope to use this in efforts to control charge transfer direction.
We have also recently developed (manuscript in preparation) a general methodology to test when laser pulse phase manipulations are able to alter the outcome of a light-matter interaction. This will be a front-line screening tool to identify opportunities for coherent control. This has now been used to show that multiple exciton generation (MEG) in quantum dots is dependent on the phase conditions of the excitation field.
In our synthetic/photophysical control research we manipulate the structure and resultant dynamics of chromophoric coordination complexes. We focus on systems that rearrange internally upon light absorption with the intention of exploiting molecular motions to enhance the rate of ET across an internal bridge while hindering energy-wasting charge recombination processes.
In systems described in our first extensive manuscript on these efforts (IC 2008, 47, 4060), dynamical ligand structures can alter excited-state properties by changing the ‘range’ over which electrons delocalize and the extent of nuclear rearrangement. This then impacts the speed at which electrons transfer over long distances which is monitored using fast transient absorption spectroscopy in our lab (an example of a kinetics trace we collected is shown in the scheme above). These studies concerned the molecules shown below which we designed and synthesized.
Upon pushing these design motifs in new directions we have recently discovered cases where modification of structure intended to slow ET using steric hinderance at the bridge between donor and acceptor actually enhances its rate. These observations defy our understanding based on applications of Marcus theory and hint at underutilized principles for efficiently separating charge with minimal reliance on driving force. Fundamental explorations are forthcoming and we anticipate important implications in the context of heterojunction solar cells. These systems are also important subject matter in our adaptive femtosecond pulse shaping explorations aiming to control photoinduced electron transfer rates.
We have also recently synthesized several new donor-bridge-acceptor systems for exploration of structure/ photochemical function relationships and for quantum control purposes. One is related to the compounds shown above where we remove the saturated spacer. Under comparable excitation conditions we observe forward electron transfer rates more than an order of magnitude larger than in (3) or (4). Another set of systems juxtaposes an acceptor next to a metal-aquo complex. These systems are designed to understand electron transfer and control of electron transfer in competition with significant nonradiative pathways.