EPR of photo-excited triplet states: A personal account

Abstract

A sketch is presented of the path that has led from Zavoisky's pioneering experiments to modern investigations by electron paramagnetic resonance (EPR) of the phosphorescent (S = 1) triplet state of polyatomic molecules or ions. The group-theoretical method first introduced by Wigner in his analysis of the multiplets of atomic spectroscopy, likewise provides a key for understanding the zero-field splitting and selection rules for radiative decay of the phosphorescent triplet state. Examples to illustrate the progress made through EPR experiments are selected from three fields. (i) Conformational instability on excitation. Both the zero-field splitting and the electron spin density distribution provide unique fingerprints of a triplet state's geometry -structural information of a kind that is nonexistent for singlet states! Illustrations are provided by benzene C 6 H6 and fullerene C60 . (ii) The optical pumping cycle. The spin selectivity of singlet-to-triplet intersystem crossing and radiative decay of the individual spin components of the triplet state is discussed. In practice this selectivity is put to advantage by performing EPR on triplet states in zero-field by means of optical detection. In turn, such experiments have led to a detailed insight into the spin-orbit coupling mechanisms responsible for the spin selectivity of the above processes. The high sensitivity attainable with optical detection has recently culminated in EPR experiments on single molecules. (iii) Quantum interference. In a triplet state of low symmetry two of the spin sublevels may decay to the ground state by the emission of photons of a common polarization (i.e., out of plane for an aromatic hydrocarbon). In such a situation quantum interference between the two decay channels can be induced by an appropriate preparation of the excited state. An example is shown where flash-excitation in the singlet manifold followed by rapid intersystem crossing causes the S = 1 spin angular momentum to be created in a spin state which is not an eigenstate of the zero-field splitting tensor. This nonstationary character of the initial triplet state, which reflects the spin-orbit coupling pathway, is observed through the detection of a spontaneous microwave signal following the 25 ps laser flash.

Photoexcited Triplet State Kinetics Studied by Electron Paramagnetic Resonance Spectroscopy

“…Many time‐resolved triplet studies have been reported in the field of photosynthesis . In his review of 2001, a personal account given on the occasion of his Zavoisky award lecture, Van der Waals highlights the historical perspectives of the EPR of excited triplet states . Another technique for the study of excited triplets is optically detected magnetic resonance (ODMR), which, as an optical method, is extremely sensitive, such that triplet state kinetics can be studied even for single molecules .…”

“…About 10 years later, van der Waals and co‐workers detected the phenomenon of optical spin polarization (OSP) in such systems, that is, the fact that the triplet spin sublevels were not created with initial Boltzmann distribution. This discovery marked the beginning of an era of intense studies on excited triplet states, EPR spectroscopy figuring most prominent among the methods to understand and characterize such states …”

“…This discoverym arked the beginning of an era of intense studies on excited triplet states, EPR spectroscopy figuring most prominenta mong the methods to understand and characterize such states. [3] Photoexcitedt riplet states have found practical applications in several areas, of which photosensitization [4] may be the most important. Other significant fields are photovoltaics, [5] organic electroluminescent devices [6] and photon upconversion at low light intensities via triplet-triplet annihilation.…”

Spin-chemistry concepts for spintronics scientists

“…In extended molecules, owing to zero-field splitting (ZFS), the three triplet states are not degenerate. Under these conditions, as proven by Wolf et al [ 142 ] as well as Schmidt and van der Waals [ 145 – 146 ] in the late 1960s, the pathways of spin–orbit coupling which enable ISC are strongly spin selective. As a result, often only that triplet state gets populated, matching the symmetry of the preceding excited singlet state.…”

Coherent Storage of Photoexcited Triplet States UsingSi29Nuclear Spins in Silicon

“…The use of an optically driven mediator spin has been suggested as a way to control coupling between donor electron spins in silicon: the donor spins exhibit weak direct coupling, but mutually couple through the optically excited state of the mediator [8]. Such ideas could similarly be applied to couple nuclear spins, and, if the mediator spin is a photo-excited triplet with a spinzero single ground state, it would have the added advantage that it avoids long-term impact on the nuclear spin coherence [9][10][11].Photoexcited triplets are optically-generated electron spins (S = 1) which often exhibit large (positive or negative) spin polarization, thanks to preferential population of each of the triplet sub-levels following intersystem crossing and/or the differing decay rates of these sublevels to the ground singlet state [12,13]. Nuclear spins, in contrast, have weak thermal spin polarization at experimentally accessible conditions, due to its small magnetic moment.…”