Final Report Summary - ENPHOC (Environmental & Dynamical Effects in Computational Photochemistry) The molecular and supra-molecular scale set a limit for the design and engineering of controllable devices, and the development of such molecular machines and the technology to build and manipulate them at the nanometer scale has emerged as a major pursuit of chemical research in the past decade. In this context, photochemical processes are particularly appealing, either because they represent a way of converting photon energy into work at a molecular scale, but also because they provide ways to control molecular processes using light sources. This control may be in the form of an active manipulation of chemical reactions through the fine tuning of the time and frequency of laser pulses, but also, and more relevantly for the purposes of this project, by using lasers to trigger a chemical process in a system designed to perform a given function by promoting it to a reactive excited electronic state. The design of such light-induced molecular devices requires a detailed understanding of the excited electronic state properties of chromophores but also how they evolve in time and how their intrinsic photochemical properties change and are determined by the molecular environment. This project aims to go beyond the individual molecule static picture, and advance our knowledge of the dynamics of photochemical systems, but also to consolidate methodology and procedures that allow predictability and transferability of simulation results.While the ever increasing available computational power has made possible the simulation of complex photochemical systems, involving potential energy surface calculation of several electronic states, often including an environment, as well as its time evolution to some degree of approximation, such increased availability of results has also created new challenges: how to compare, assess, and sometimes even interpret, simulation results that currently often reach sizes of many gigabytes of data? The current EnPhoC project aims to increase the knowledge and understanding of photochemical reactivity by working in two fronts: first is to devise simplified theoretical models and concepts which are transferable between systems but still capture the essence of the underlying dynamical phenomena in photochemistry; and second, to integrate different state of the art electronic structure and dynamics methods, balancing in a systematic way computational cost and accuracy, in order to standardize procedures and correctly assess the reliability of the different approximations involved photochemical simulations. Such tools and concepts are used to address currently unexplained experimental results.The project is set out to address a number of specific scientific goals:Analytic description of non-decay decay at a conical intersectionConical intersections play a crucial role in the relaxation of electronically excited molecular species. This effort aims at providing a closed form analytical description for the probability of decay in the vicinity of a conical intersection. The result obtained is able to rationalize, and to some extent contradict current knowledge, of which conical intersection properties influence the efficiency of electronic de-excitation.Mechanism of protonated Schiff basis (PSB) photoisomerizationcis-trans photoisomerization is an important photochemical reaction, and it has been a common molecular component in the design ofmolecular machinery, both artificial or biological. The chromophores of some of the most efficient biological molecular devices are PSBs, and the mechanism of this important photochemical process is still not completely understood. Via electronic structure studies, both in isolation and coupled to dynamics methods, the project aims to bring a better understanding about the mechanism of the reaction. So far, our studies have identified the likelihood of different possible mechanisms for isolated molecules. Dynamical calculations will follow to confirm these assessments.Improved polyene electronic structure methodology for photochemistryPolyenes have a rich photochemistry and the constitute a common benchmark for such studies. PSBs are also characterized by a polyene chain. Standard methodologies present limitations in the description of the challenging electronic structure of short polyene's excited states. Improvement of the excited electronic structure of these systems often requires the use of computationally costly methodologies that are unsuited for the study of their dynamics and photochemistry. One aim of the project is to develop a consistent, accurate and cost-effective procedure to describe the excited state of such molecules. This was achieved comparing two short polyenes of open and closed structures, highlighting the effect of ring closure on the electronic structure and exposing in a simple way the necessary ingredients such calculation should include. These studies contribute to a deeper understanding of dynamics effects in photochemistry, which is one step before the successful engineering of excited state molecular behaviour, and unleashing the promises of nanotechnology with far reaching technological and economic consequences.João Pedro Malhadomalhado@imperial.ac.uk
