Hot Injection Processes in Optically Excited States: Molecular Design for Optimized Photocapture Gil Katz,* ,† Mark A. Ratner,* ,‡ and Ronnie Kosloff* ,§ † Afikim Science, Afikim Israel, Fritz Haber Research Center for Molecular Dynamics, Hebrew University of Jerusalem, Jerusalem 91904, Israel ‡ Department of Chemistry, Northwestern University, Evanston, Illinois 60208-3113, United States § Institute of Chemistry and the Fritz Haber Research Center for Molecular Dynamics, Hebrew University of Jerusalem, Jerusalem 91904, Israel * S Supporting Information ABSTRACT: Design principles for efficient solar photocapture using a single molecule are presented. The proposed molecular model is composed of ground and excited bright and dark electronic states. Once photoexcited to the bright states, vibrational relaxation and dissipation to the ground vibrational level of the bright state commonly occur. This degrades a substantial amount of the incoming photon energy into heat, further reducing the efficiency of molecular photocells. One way to circumvent this energy flow from electronic excitation into heat is the hot injection process, by which the original excited bright state undergoes a rapid crossing to an acceptor dark state, with a higher potential energy minimum, and is trapped in the region of that minimum. By choosing an appropriate pair of vibrational modes, the overall energy gain can be increased substantially and the constraints on the bath behavior substantially simplified. We present calculations in a two-dimensional vibrational space, along with energy relaxation and transfer to the bath (using a Stochastic Surrogate Hamiltonian model). We find that the second degree of vibrational freedom, if carefully chosen, strongly increases the efficiency and the possibility of successful hot injection. In addition, the same molecular model can be designed to utilize the red part of the solar spectrum. Excited state absorption can recycle the wasted bright state population thus increasing the efficiency of solar capture. ■ INTRODUCTION Solar energy harvesting is challenged by the broad energy spectrum of solar radiation spanning approximately ∼2.5 eV. Existing devices that convert the energy into charge carriers lose the low frequency end of the spectrum. This part has either photons that are lower in energy than the bright state or insufficient energy for generating charge upon photoexcitation. The high frequency end of the solar spectrum actually dis- sipates heat, which has to be removed. An engineering solution is to construct a multilayer device with each layer possessing a different band gap. 1,2 Another suggestion is to find nano- materials where the high frequency end of the spectrum generates multiple charge carriers. 3 Here we propose a scheme where a single dye molecule can be designed to serve as a multilevel absorber, capturing the energy of a significant part of the solar spectrum energy. Dynamics of photoexcited molecules, following Franck- Condon vertical excitation to a bright state, occur on multiple time scales; these include vibrational relaxation to the lowest vibrational level in the hot electronic state, which was believed to be a dominant dynamical process, which could occur before intersystem crossings, fluorescence, or nonradiative decay. This generalization (that subsequent dynamics occurs from the vibrational minimum of the bright state) is known as Vavilov’s Law 4-7 and was an accepted assumption until the 1960s, when deeper understandings were gained about the actual time scales for dynamics in photoexcited manifolds. The Jablonski diagram of Figure 1 shows that the amount of energy lost from the vertical excitation to the vibrational minimum of the bright state, termed ΔE B , can be quite large. This suggests that if it were possible to undergo an essentially irreversible electronic transition to a dark state before vibrational relaxation to the bright state minimum occurs the energy ΔE B in the Vavilov’s Law regime is simply dissipated into the environment and can be Received: May 24, 2014 Revised: August 26, 2014 Published: August 29, 2014 Figure 1. Three diabatic potential energy surfaces: the ground V ̂ g , bright V ̂ b , and dark V ̂ d potentials. On the right, a schematic one- dimensional cut in the potentials. Article pubs.acs.org/JPCC © 2014 American Chemical Society 21798 dx.doi.org/10.1021/jp5051172 | J. Phys. Chem. C 2014, 118, 21798-21805