Singlet Fission - Department of Chemistry

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$X-ray powder diffraction characterization of the elusive ...$

AL Chemical Reviews, XXXX, Vol. xxx, No. xx Smith and Michl 60 fs, 1.45 ps, and 5.3 ps, respectively, with an additional long-lived component that was assigned to the triplet of 15. The fast formation (within 5.3 ps) of the triplet state suggested singlet fission as the source, and a yield of 35% at room temperature was estimated. Triplet formation has also been observed in Chromatium Vinosum 221 and attributed to the carotenoids, 15 and rhodopin (19, 11 conjugated double bonds), that it contains. When studied by transient absorption and ps time-resolved resonance Raman spectroscopy with 8 ps pulses, the initially excited carotenoid 1 1 Bu state was found to decay into the 2 1 Ag state in
Singlet Fission Chemical Reviews, XXXX, Vol. xxx, No. xx AM polymers, discussed in the present section, are the only known cases of this situation. Electronic states of polyenes are usually classified more formally according to the C2h symmetry group, although only the all-anti (“all-s-trans”) conformers of the all-trans polyenes actually possess this symmetry. In this notation, the S1 state of the shorter polyenes is 2Ag and their S2 state is 1Bu. Further classification is possible using an approximate pairing symmetry operation that exists within the framework of the semiempirical Pariser-Parr-Pople (PPP) model. 44,45 This allows an assignment of electronic states into “plus” and “minus” categories 47 and provides a selection rule according to which only transitions between states of opposite character are one-photon allowed. A closed-shell ground state is of “minus” symmetry. Although the pairing symmetry is not truly exact, it holds well enough that in practice only transitions from the ground state to states of the “plus” type can have substantial intensity in an ordinary absorption spectrum of an alternant hydrocarbon such as a polyene. In common notation, the ground state is 1 1 Ag - and the S1 state, which can be viewed as singlet-coupled combination of two locally excited triplet states, 9-11,223 is 2 1 Ag - . The first optically allowed state is 1 1 Bu + and in the shorter polyenes it is S2, but in the longer ones forbidden transitions to other states intervene between transitions to the 2 1 Ag - and 1 1 Bu + states. For a sufficiently long polyene, it only takes a lattice distortion for the optically forbidden 2 1 Ag state (S1, TT) to dissociate to form two localized triplets. 223 5.1. Polydiacetylenes Polydiacetylenes can be viewed as polyacetylenes dehydrogenated by removal of both hydrogen atoms on every other single bond. Thus, newly introduced additional π bonds are formed by orbitals that are perpendicular to the conjugated π system of the polyacetylene and do not interact with its conjugated π system, but they make every other single bond of the parent polyacetylene shorter and thus modify the usual bond length alternation. In parent 22, the remaining substituents are hydrogen atoms, but in actually studied systems they are alkyls or substituted alkyls, as shown in formulas 22a-e. These polymers are available in a sol form in solution (presumably as coiled chains) and a gel form formed by adding a poor solvent to the solution (presumably aggregated straight chains with few or no interchain interactions). The polymers 22c, d, and e occur in the “red form” and the “blue form”. The blue form has fewer defects and a greater extent of conjugation than the red form. 224 The structure is determined by the method of polymerization. The red form is formed in solution and can be cast into films, whereas the blue form is produced by polymerization in KBr pellets or using a mechanically aligned monomer. 113 Isolated linear polymer chains can be generated at low concentrations (∼0.01% by weight) by polymerization within crystals of the diacetylene monomer. 111 They are isolated, linearly aligned, and have lengths of ∼2.6 µm, making them good approximations to one-dimensional conjugated systems. 111 In polydiacetylenes, triplets are believed to be formed by fission of the S1 state. 225 Singlet fission triplet yields for 22 are typically low: for 22b, on the order of 0.1%, 22 and for 22c, 0.4%. 125 Variations in singlet and triplet energies occur with conformational defects introduced by twists in the polymer backbone. A rise in the quantum efficiency of photogenerated triplet excitons in 22c is observed over several tenths of an eV increase in excitation energy. 112 This has been modeled in terms of inhomogenous broadening due to varying conjugation lengths within the polymer and the release of vibrational energy in the formation of triplet excitons. From the model, E(T1) was calculated to be 1.1 eV and E(S1) was calculated to be 1.9 eV. The intensity dependence of triplet formation has been used to estimate the threshold energy for singlet fission. In energy ranges where the intensity dependence is quadratic, triplet formation by fission has been proposed to occur by an initial two-photon absorption process, 22,111 which allows the energy requirements to be met even when the photon energy is below 2 E(T1). Triplet formation in isolated chains of 22a and 22b exhibits this type of intensity dependence. 111 In 22b triplet formation has a quadratic dependence on intensity at 1.82 eV and the exponent steadily decreases to a value of 1.5 at energies greater than ∼2 eV. The corresponding triplet yield increases by 2 orders of magnitude between 1.82 and 2 eV. The variation of the power law exponent with excitation energy is due to simultaneous quadratic and linear processes with similar yields. A fit for 22b indicated that at 2 eV ∼50% of singlet fission is due to a one-photon process. In 22a, there is a similar trend in the intensity dependence and an order of magnitude increase in triplet yield between 1.82 and 1.9 eV. On the basis of the threshold for a linear intensity dependence, E(T1) is between 0.9 and 0.95 eV in 22b and 0.95 to 1.0 eV in 22a. In both of these cases, the activation energy for singlet fission is
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