The molecular exciton model has received its most extensive development and application in the field of molecular crystals1'2. More recently, numerous applications to non-crystalline molecular composite systems have been made, including van der Waals and hydrogen-bonded dimers, trimers, and higher order aggregates. Another type of composite system has also been investigated, namely the composite molecule consisting of covalently bonded molecular units, with intrinsic individual unsaturated electronic systems so isolated by single bonds that but little or insignificant electronic overlap between units may occur. It is now well established that in molecular aggregates and in composite molecules, exciton effects may be observed if sufficiently strong electronic transitions exist in the component sub-units. The result of exciton splitting of excited states in the composite molecule may be the appearance of strong spectral shifts or splittings (which may be of the order of 2000 cm—1) of the absorption bands for the component molecules. At the same time, as a consequence of the exciton splitting of the excited state manifold, an enhancement of triplet state excitation may result. The purpose of this paper is to present a summary of the various type cases for molecular dimers, trimers and double and triple molecules in the description of the molecular exciton strong-coupling model. Then it will be shown by new experimental examples that, even in those cases where no significant exciton effect is observable in the singlet—singlet absorption spectrum for the composite molecule (intermediate and weak coupling cases), the enhancement of lowest triplet state excitation may still be conspicuous and significant. The ideas which are summarized in this paper have a curious history. Long ago, Kautsky and Merkel3 demonstrated experimentally that aggregation of dyes facilitated their action as photophysical sensitizers in photochemical reactions, at the same time diminishing their fluorescence efficiency. Kautsky attributed these easily demonstrated effects to enhancement of metastable state excitation in the aggregate dye. There is no doubt today that the metastable state he described is the lowest triplet state of the molecules studied. However, he did not distinguish between intrinsic and enhanced metastable (triplet) state excitation, so his interpretations were largely overlooked. Forster in l946 used the quasi-classical vector model to

/pdf/the-exciton-model-in-molecular-spectroscopy-2vwkowivjh.pdf

The exciton model in molecular spectroscopy

Quartz crystal microbalance: a useful tool for studying thin polymer films and complex biomolecular systems at the solution-surface interface.

Multimode Molecular Dynamics Beyond the Born‐Oppenheimer Approximation

Knowing the underlying photophysics of thermally activated delayed fluorescence (TADF) allows proper design of high efficiency organic light-emitting diodes. We have proposed a model to describe reverse intersystem crossing (rISC) in donor–acceptor charge transfer molecules, where spin–orbit coupling between singlet and triplet states is mediated by one of the local triplet states of the donor (or acceptor). This second order, vibronically coupled mechanism describes the basic photophysics of TADF. Through a series of measurements, whereby the energy ordering of the charge transfer (CT) excited states and the local triplet are tuned in and out of resonance, we show that TADF reaches a maximum at the resonance point, substantiating our model of rISC. Moreover, using photoinduced absorption, we show how the populations of both singlet and triplet CT states and the local triplet state change in and out of resonance. Our vibronic coupling rISC model is used to predict this behaviour and describes how rISC and TADF are affected by external perturbation.

/pdf/revealing-the-spin-vibronic-coupling-mechanism-of-thermally-20s1drezx3.pdf

Revealing the spin-vibronic coupling mechanism of thermally activated delayed fluorescence.

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 spin-allowed 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 independe...

Spin-Vibronic Mechanism for Intersystem Crossing

Photophysical properties of biologically important molecules related to proximity effects: psoralens

Fluorescence lifetimes of naphthalene, β‐naphthol, and β‐naphthylamine have been measured under collision‐free conditions in the vapor. The lifetimes were then combined with the measured quantum yield (for naphthalene), or with the estimated radiative decay rate (for β‐naphthylamine), to deduce the nonradiative decay rate of the molecule as a function of excitation energy. The results indicate that (1) the nonradiative decay probability increases, first slowly then very rapidly, as the excess vibrational energy in the lowest excited singlet state (S1) is varied over a range of nearly 20 000 cm−1, and (2) a plot of nonradiative decay rate versus excitation energy exhibits a break at photon energy corresponding to the second excited singlet state of a molecule. Observation (1) is interpreted in terms of two different pathways of radiationless transition that compete in the singlet deactivation of the molecules. It is argued that, in the region of low excess energies, the intersystem crossing to the triplet ...

Radiationless singlet deactivation in isolated large molecules. I Naphthalene, naphthol, and naphthylamine

Radiationless transitions in electron donor-acceptor complexes: selection rules for S1 → T intersystem crossing and efficiency of S1 → S0 internal conversion

The S2→S1 fluorescence of azulene in a naphthalene mixed crystal has been measured from 740 to 1000 nm at 4.2 K. Of the 25 in‐plane fundamentals (exclusive of C–H stretches) of the S1 state, 20 are readily identifiable in emission. A number of other bands are present in the spectrum and these are tentatively identified as S1 out‐of‐plane vibrations induced by vibronic coupling to σπ* and/or πσ* excited electronic states. An emission previously assigned as T1→S0 phosphorescence is shown to be a combination band of S2→S1 fluorescence. The branching ratio of S2 →S0 fluorescence to S2→S1 fluorescence is approximately 7500 for xenon arc lamp excitation; this is an order of magnitude larger than the value reported for short pulse laser excitation. A summary of photophysical parameters for the low‐lying electronic states of azulene, based on the most recent and reliable experimental data, is also presented.

Vibrational analysis of the S2→S1 fluorescence of azulene in a naphthalene mixed crystal at 4.2 K

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Papers

Photophysical properties of biologically important molecules related to proximity effects: psoralens

Radiationless singlet deactivation in isolated large molecules. I Naphthalene, naphthol, and naphthylamine

Radiationless transitions in electron donor-acceptor complexes: selection rules for S1 → T intersystem crossing and efficiency of S1 → S0 internal conversion

Vibrational analysis of the S2→S1 fluorescence of azulene in a naphthalene mixed crystal at 4.2 K

Photophysical properties of biologically important molecules related to proximity effects: psoralens