Efficient singlet fission has been reported in polycrystalline thin films of 1,3-diphenylisobenzofuran. Two conformational polymorphs have been characterized by various spectroscopic techniques. Thin films exhibit 200% triplet quantum yield at 77 K, as determined by ultrafast transient absorption measurements. Rapid excimer formation presents a significant energetic barrier to triplet formation.
Sensitizer dyes capable of producing two triplet excited states from a singlet excited state produced by the absorption of
a single photon would allow an increase of the efficiency of photovoltaic cells by up to a factor of 1.5, provided that
each triplet injects an electron into a semiconductor such as TiO2. Although singlet fission in certain crystals and
polymers was reported long ago, little is known about its efficiency in dyes suitable for use as sensitizers of
photoinduced charge separation on semiconductors surfaces. Biradicaloids and large alternant hydrocarbons are
desirable parent structures likely to meet the requirement E(T2), E(S1) > 2E(T1) for the excitation energies of the lowest
excited singlet (S1) and the two triplet (T1, T2) states. We report results for 1,3-diphenylisobenzofuran, a model
compound of the biradicaloid type. Its energy levels satisfy the desired relation, and in solution it shows no triplet
formation by intersystem crossing. In the neat solid state, it forms triplets efficiently, and indirect evidence suggests that
this is due to singlet fission. This appears to be the first compound displaying SF by design. When two such
chromophores were combined into dimers, triplet formation yields of up to 9% were observed in polar solvents, possibly
due to singlet fission, but possibly due to intersystem crossing. The triplet formation occurs in two steps, via an
intermediate assigned as an intramolecular charge-transfer state and responsible for most of the observed excitation loss.
The spectra of porphyrin isolated in low temperature noble gas and nitrogen matrices reveal a multiplet structure consisting of pairs of sites (A and B) which may be interconverted as a result of photo-induced double proton transfer. Single-site spectra are obtained for IR, electronic absorption, fluorescence and phosphorescence. Analysis of different magnitudes and signs of site-splittings in the IR spectra may be helpful for vibrational assignments.
In designing chromophores for second harmonic generation applications one would like to have a large hyperpolarizability (beta) and an absorption well away from the second harmonic frequency. Unfortunately, there appears to be a connection between the experimentally observed absorption maximum energy and the observed zero-frequency hyperpolarizability (beta) 0, when comparing different molecules within a class, such as donor/acceptor substituted benezenes and stilbenes, that makes this difficult to achieve. In this paper, the origins of this relationship are investigated by comparing results from semi-empirical calculations with experimental values. A significant contribution to the observed relationship between absorption maximum and (beta) 0 is seen to arise from frequency dependences of the ground and excited state dipole moments. Since the two-level model for (beta) is used both as an extrapolation technique to obtain (beta) 0 and as a tool for analyzing the data, a discussion of the validity of this model is also presented.
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