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Unformatted text preview: Insights for Light-Driven Molecular Devices from Ab Initio Multiple Spawning Excited-State Dynamics of Organic and Biological Chromophores TODD J. MARTNEZ* Department of Chemistry, Beckman Institute, and The Frederick Seitz Materials Research Laboratory, University of Illinois, 600 S. Mathews Avenue, Urbana, Illinois 61801 Received May 2, 2005 ABSTRACT We discuss the basic process of photoinduced isomerization as a building block for the design of complex, multifunctional molecular devices. The excited-state dynamics associated with isomerization is detailed through application of the ab initio multiple spawning (AIMS) method, which solves the electronic and nuclear Schro d- inger equations simultaneously . This first-principles molecular dynamics approach avoids the uncertainties and extraordinary effort associated with fitting of potential energy surfaces and allows for bond rearrangement processes with no special input. Further- more, the AIMS method allows for the breakdown of the Born- Oppenheimer approximation and thus can correctly model chem- istry occurring on multiple electronic states. We show that charge- transfer states play an important role in photoinduced isomerization and argue that this provides an essential design rule for multi- functional devices based on isomerizing chromophores. Introduction The design of functional molecular architectures has emerged as one of the major goals of chemistry in the past decade, as evidenced by three recent special issues of this journal. 1- 3 With the advent of modern lasers, it is now possible to simultaneously control spatial and tem- poral characteristics of light at a level that approaches natural molecular length (nanometer) and time (femto- second) scales. Carefully designed optically responsive molecules can be envisioned as a means to transfer this detailed control to molecular transport and function. We are not talking primarily about active control schemes in this context, where the detailed time-frequency profile of a laser pulse is optimized to drive a particular chemical reaction. 4,5 Rather, we are thinking of molecules that behave as devices, containing one or more optically addressable components surrounded by a scaffolding. The scaffolding must protect the chromophores, but it also should direct their photodynamics and amplify the effects of any photochemical transformation. The simplest examples would be optically responsive polymers that might transform optical signals into me- chanical work. Such examples have been reported re- cently, building on the photoinduced cis- trans isomer- ization of azobenzene. Incorporating azobenzene into polymers, Hugel et al. were able to demonstrate the use of light to power mechanical work in the form of motion of an atomic force microscope (AFM) tip....
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