The life and times of excited states of organometallic and coordination compounds

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Abstract

The photochemistry of transition metal organometallic and coordination compounds is discussed from the perspective of time domains, in which excited-state reactions occur. Ultrafast photochemical processes, which are competitive with nuclear motion, are distinguished from reactions of long-lived, thermally equilibrated, excited states. Many special photochemical features of transition metal complexes and organometallic compounds stem from the simultaneous presence of excited states of different localisations and orbital origins in the same chromophoric molecule, together with high state densities. A brief survey of recently studied systems and reactions shows the present level of understanding and highlights the scientific challenges and possible applications of photoprocesses occurring on different time scales. Factors that influence excited-state lifetimes are discussed and differences in the chemical properties of electronically excited and ground states demonstrated using the radical-like behavior of [PtII2(P2O5H2)4]4− and electron-transfer reactivity of [Ru(bpy)3]2+ as typical examples. It is shown that reactions of long-lived excited states can become ultrafast when a chromophore is inserted into a redox-active supramolecular assembly or attached to an electrode surface. This has important implications for light-energy conversion and manipulation of information at a molecular level. Optically prepared Franck–Condon excited states of many transition metal complexes have an ultrafast chemistry of their own. This includes relaxation to lower excited states, electron transfer, energy transfer or bond splitting. Typical examples are discussed with an emphasis on phenomena occurring only on very short time scales. A final outlook points to the most challenging questions in mechanistic inorganic photochemistry and to possible applications.

Section snippets

Introduction: the two time scales

The best known natural photochemical processes, photosynthesis and vision, are nature's ways to convert light energy into chemical energy and to capture, store and process optical information. Recent vigorous research in photochemistry and photophysics has essentially the same ultimate goals. On top of that, the pure understanding of chemistry and dynamics of electronic excited states is of a great fundamental importance, not only because of its intellectual appeal but also as an enabling force

What is special about photochemistry of transition metal coordination and organometallic compounds?

The issue of time scales of excited-state processes is relevant to all photochemistry, regardless of the detailed nature of the photoactive species. However, the presence of a transition metal in a molecule introduces new types of excited states and reactivity patterns which give rise to unique photochemistry and photophysics: (i) Photochemistry of transition metal complexes can often be triggered by irradiation with low-energy visible light since excitation energies are generally lower than in

Photochemistry involving long-lived, thermally equilibrated excited states

A long lived excited state, which achieves a thermal equilibrium with respect to internal degrees of freedom and to the medium, differs from the respective ground state in its energy and electron distribution. The higher energy content allows for reactions which are thermodynamically impossible in the ground state. A different electron distribution is responsible for the very different nature, kinetics, and selectivity of excited-state reactions as compared with their ground state counterparts.

Ultrafast photochemistry

Optical excitation, which takes place in about 1 fs, prepares a Franck–Condon excited state. This is an electronically excited state which, because of the ‘vertical character’ of electronic excitation, retains the geometry of the ground state. Hence, the Franck–Condon states are, in most cases, excited both electronically and vibrationally and molecular surroundings are in a highly nonequilibrated configuration. Energy dissipation inevitably follows optical excitation. It can occur as a

Ultrafast, fast, or slow photoprocesses: challenges and perspectives

Based on their dynamics, two broad types of photochemical processes emerge: (i) ultrafast, which occur on a time scale comparable with that of vibrational or even electronic motion in the active chromophore and its surroundings, and (ii) relatively slow reactions of relaxed, thermally equilibrated excited states. Processes occurring on different photochemical time scales are closely interrelated, since every reaction of a relaxed excited state is preceded by ultrafast relaxation steps. This can

Acknowledgements

This work is part of a European COST D14 program. Support from the Ministry of Education of the Czech Republic (grant OC.D14.20) is much appreciated.

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