Review Cite This: Chem. Rev. XXXX, XXX, XXX−XXX
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Spin-Vibronic Mechanism for Intersystem Crossing Thomas J. Penfold,*,† Etienne Gindensperger,‡ Chantal Daniel,‡ and Christel M. Marian§ †
Chemistry - School of Natural and Environmental Sciences, Newcastle University, Newcastle Upon-Tyne NE1 7RU, United Kingdom ‡ Laboratoire de Chimie Quantique, Institut de Chimie UMR-7177, CNRS - Université de Strasbourg, 1 Rue Blaise Pascal 67008 Strasbourg, France § Institut für Theoretische Chemie, Heinrich-Heine-Universität Düsseldorf, Universitätsstrasse 1, 40225 Düsseldorf, Germany ABSTRACT: Intersystem crossing (ISC), formally forbidden within nonrelativistic quantum theory, is the mechanism by which a molecule changes its spin state. It plays an important role in the excited state decay dynamics of many molecular systems and not just those containing heavy elements. In the simplest case, ISC is driven by direct spin− orbit coupling between two states of different multiplicities. This coupling is usually assumed to remain unchanged by vibrational motion. It is also often presumed that spinallowed radiationless transitions, i.e. internal conversion, and the nonadiabatic coupling that drives them, can be considered separately from ISC and spin−orbit coupling owing to the vastly different time scales upon which these processes are assumed to occur. However, these assumptions are too restrictive. Indeed, the strong mixing brought about by the simultaneous presence of nonadiabatic and spin−orbit coupling means that often the spin, electronic, and vibrational dynamics cannot be described independently. Instead of considering a simple ladder of states, as depicted in a Jablonski diagram, one must consider the more complicated spin-vibronic levels. Despite the basic ideas being outlined in the 1960s, it is only with the advent of high-level theory and femtosecond spectroscopy that the importance of the spin-vibronic mechanism for ISC in both fundamental as well as applied research fields has been revealed with significant impact across chemistry, physics, and biology. In this review article, we present the theory and fundamental principles of the spinvibronic mechanism for ISC. This is followed by empirical rules to estimate the rate of ISC within this regime. The most recent developments in experimental techniques, theoretical methods, and models for the spin-vibronic mechanism are discussed. These concepts are subsequently illustrated with examples, including the ISC mechanisms in transition metal complexes, small organic molecules, and organic chromophores. 3.3. On-the-f ly Dynamics Methods 3.3.1. Trajectory Surface Hopping 3.3.2. Methods Based upon Gaussian Wavepacket 3.4. Excited State Dynamics of Sulfur Dioxide: A Case Study 4. Experimental Observation of Spin-Vibronic Dynamics 4.1. Transient Optical Absorption and TimeResolved Emission Spectroscopy 4.2. Time-Resolved Vibrational Spectroscopy 4.3. Time-Resolved Photoelectron Spectroscopy 4.4. Multidimensional Electronic Spectroscopy 4.5. Time-Resolved X-ray Spectroscopy 5. Spin-Vibronic Intersystem Crossing in Transition Metal Complexes 5.1. Jahn−Teller and Spin−Orbit Coupling Effects in Transition-Metal Trifluorides 5.2. Fe(II) Complexes
CONTENTS 1. Introduction 2. Background and Theory 2.1. The Coupling Elements 2.1.1. Spin−Orbit Coupling 2.1.2. Nonadiabatic Coupling 2.1.3. Spin-Vibronic Coupling 2.2. Qualitative Rules for Intersystem Crossing 2.2.1. El-Sayed Rules 2.2.2. The Energy Gap Law 2.2.3. Qualitative Estimation of Spin-Vibronic Coupling 3. Theoretical Methods 3.1. Perturbation Theory Approaches 3.1.1. Evaluation of ISC Rates in the Energy Domain 3.1.2. Time-Dependent Methods 3.1.3. Including Temperature 3.2. Quantum Dynamics Methods 3.2.1. Multi-Configurational Time-Dependent Hartree Approach 3.2.2. Including Temperature 3.2.3. Spin-Vibronic Model Hamiltonian
© XXXX American Chemical Society
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Special Issue: Theoretical Modeling of Excited State Processes Received: October 16, 2017
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DOI: 10.1021/acs.chemrev.7b00617 Chem. Rev. XXXX, XXX, XXX−XXX
Chemical Reviews 5.3. Cu(I) Complexes 5.4. Re(I) α-Diimine Complexes 6. Spin-Vibronic ISC in Small Molecules and Organic Chromophores 6.1. Small Aromatic Molecules 6.2. Deactivation of DNA Bases by ISC 6.3. Heteroaromatic Photosensitizers for Photodynamic Therapy 6.4. Thermally Activated Delayed Fluorescence (TADF) 7. Environment Effect on Spin-Vibronic Dynamics 7.1. Heteroaromatic Compounds 7.2. Transition Metal Systems 7.3. Environment Effects on Metal-Free TADF Emitters 8. Summary and Outlook Author Information Corresponding Author ORCID Notes Biographies Acknowledgments Glossary References
Review
photoexcited molecules and to decipher complicated excited state dynamics. In this context, one of the most striking examples is the ultrafast ISC kinetics in transition metal complexes, which are often characterized by a high number of various electronic excited states of different multiplicities in a limited domain of energy.25 This is exemplified by the ultrafast light-induced spincrossover dynamics of [Fe(bpy)3]2+.26−29 Photoexcitation of the singlet metal-to-ligand charge transfer (1MLCT) band of [Fe(bpy)3]2+ leads to population of a nonemissive quintet metal-centered (MC, 5T2) state within 100 fs.30−32 Such dynamics involve two spin transitions and therefore are remarkable given that the time scales involved are significantly faster than the time scale usually expected for a single ISC event. This is accompanied in other molecules by intriguing effects such as ISC rates varying with the vibrational period of important normal modes rather than heavy atom effects33 or which are strongly solvent dependent.34 For a long time it was believed that spin-forbidden nonradiative transitions in purely organic compounds were slow in comparison to their spin-allowed counterparts and often their presence in the interpretation of excited state dynamics was neglected. However, there is increasing evidence that even in compounds composed of only light elements, ISC can occur on the subpicosecond time scale and is thus competitive with IC. This is illustrated by aromatic carbonyl compounds of the xanthone type and nitro aromatic compounds. The unusual photophysical and photochemical properties of nitroarenes stimulated a series of femtosecond-resolved experiments.35−46 Their excited state decay kinetics are characterized by a biphasic decay of the lowest excited singlet state with time constants of ≤100 fs and
