Photon upconversion based on sensitized triplet–triplet annihilation

Photon upconversion, the process wherein light of long wavelength is frequency converted to photons of higher energy, is readily achieved at low incident power through sensitized triplet–triplet annihilation (TTA) in various chromophore combinations spanning the UV to the near-IR. This emerging wavelength-shifting technology truly represents a viable route towards converting low energy terrestrial solar photons into light adequate to drive electron transfer in operational photovoltaics. Generalized molecular design constraints, all operational examples reported to date, and measurement techniques applied to these low power nonlinear processes are reviewed in this contribution. In many instances, direct visualization of this phenomenon is presented in solution and within various polymeric host materials.     

Photodissociation of N2O: triplet states and triplet channel

The role of triplet states in the UV photodissociation of N(2)O is investigated by means of quantum mechanical wave packet calculations. Global potential energy surfaces are calculated for the lowest two (3)A' and the lowest two (3)A' states at the multi-reference configuration interaction level of electronic structure theory using the augmented valence quadruple zeta atomic basis set. Because of extremely small transition dipole moments with the ground electronic state, excitation of the triplet states has only a marginal effect on the far red tail of the absorption cross section. The calculations do not show any hint of an increased absorption around 280 nm as claimed by early experimental studies. The peak observed in several electron energy loss spectra at 5.4 eV is unambiguously attributed to the lowest triplet state 1(3)A'. Excitation of the 2(1)A' state and subsequent transition to the repulsive branch of the 2(3)A' state at intermediate NN-O separations, promoted by spin-orbit coupling, is identified as the main pathway to the N(2)((1)Σ(g)(+))+O((3)P) triplet channel. The yield, determined in two-state wave packet calculations employing calculated spin-orbit matrix elements, is 0.002 as compared to 0.005 ± 0.002 measured by Nishida et al. [J. Phys. Chem. A 108, 2451 (2004)].     

Lifetimes of triplet formaldehyde

By direct optical excitation into the ³A₂ state of H₂CO (D₂CO) with a tunable laser, phosphorescence is observed and lifetimes are measured for this state. Very short lifetimes, apparently limited by intersystem crossing, are observed.     

Homogeneous and heterogeneous triplet — triplet annihilation in anthracene-doped phenanthrene crystals

Delayed and prompt fluorescence spectra and delayed fluorescence decay data for 10^?5?10^?6 M/M crystals of anthracene in phenanthrene are reported. It is shown that these data indicate that a heterogeneous (host-dopant) annihilation process takes place which produces the excited singlet state of the host molecule.     

Luminescence and triplet—triplet absorption spectra of rhodium (III) porphyrins

Photophysical studies of four rhodium(III) porphyrins [RhTPPS (H₂O)₃^3?, RhTPP (Cl) (L), RhOEP (Cl) (L) and RhMesoPMEI(CI) (L)] show that these porphyrins are characterised by a moderate phosphorescence (φ ? 10^?2) and a very weak fluorescence (φ ? 5 × 10^?4) in solution at room temperature. TPP derivatives also have moderately intense triplet—triplet absorption extending to 900 nm.     

Magnetic field effects on heterogeneous triplet—triplet annihilation in anthracene-doped phenanthrene

The orientation of an 8 kG magnetic field with respect to the crystallographic axes is shown to affect the intensity of anthracene delayed fluorescence in an anthracene-doped phenanthrene crystal. These results are consistent with the calculated locations of level-crossing resonances for anthracene trapped-phenanthrene triplet exciton heterofusion. The line shapes of these level crossing resonances appear to be rather different from those reported for other cases of heterofusion or homofusion magnetic field resonances.     

Influence of triplet—triplet excitation transfer on the decay function of donor luminescence

The triplet—triplet electronic excitation transfer from benzophenone to naphthalene in rigid solutions has been investigated by measuring the non-exponential decay curve of benzophenone photphorescence using a laser pulse for excitation. The results of measurements were satisfactorily reproduced by Inokuti—Hirayama's formula based on Dexter's theory of exchange interaction.     

Carotenoid triplet state lifetimes

Carotene and xanthophyll triplet lifetimes are found to depend on the concentration of the parent molecule. These results account for some of the variations in carotenoid triplet lifetimes reported previously. The rate constants obtained for ground state quenching correlate with the number of conjugated double bonds, the longer chain systems having higher quenching rate constants.     

Intersystem crossing of triplet formaldehyde

Triplet benzene (³B_1U sensitized decomposition of H₂CO in the gas phase gives an appreciable yield of “molecules” H₂ and CO. This evidence supports that T₁?S₀ intersystem crossing in formaldehyde is an efficient and important process. The implication of this is that S₁?T₁ intersystem crossing could be important, more so than recognized previously, in the photochemistry of (n,π^*) state formaldehyde.     

Prototypical triplet alkyl phosphonatocarbenes

The current case study focuses on the generation, identification, and characterization of two representative mono- and disubstituted alkyl phosphonatocarbenes by means of matrix isolation techniques in conjunction with density functional theory [B3LYP/6-311++G(d,p)] and coupled cluster [CCSD(T)/cc-pVXZ, X = D, T] computations. The EPR measurements identify both carbenes as triplet ground-state species with D values of 0.660 and 0.623 cm(-1), respectively, exhibiting persistency toward intramolecular reactions (the EPR signal observable in perfluoromethylcyclohexane up to around 70 K for the disubstituted molecule). While the reaction of the carbene center of the conformationally rich tetramethyl bisphosphonatocarbene with the CH bonds of the methyl groups leads to phosphaoxetane at room temperature, its fragmentation via a Wittig-type reaction during high vacuum flash pyrolysis (HVFP) results in dimethyl vinylphosphonate and methyl metaphosphate. The latter has been observed for the first time as an isolated entity.     

Molecular weight dependence of triplet—triplet processes in poly(2-vinylnaphthalene)

Delayed fluorescence and phosphorescence spectra and decay curve measurements at 77 K are reported for samples of poly(2-vinylnaphthalene) with molecular weights determined by viscometry to be in the range 15 600 to 505 000. The intensity of delayed fluorescence relative to phosphorrescence was found to increase with molecular weight up to the highest molecular weight sample of solution polymerized material (MW ≈ 100 000) with a leveling off for the higher molecular weight bulk polymers. It is argued that these results imply a triplet exciton migration distance of up to 700 chromophore units. Decay curves imply that most mobile triplet excitons are trapped within approximately 0.3 s. The excitation dependence of the delayed emissions imply that unimolecular processes dominate the kinetics of triplet state chromophores.     

Evaluation of triplet repulsion energies

The equation is suggested for predicting triplet repulsion energies. X=R–R^o, while R^o and¹V^o are the equilibrium bond length and the spectroscopic dissociation energy, respectively. Parameters and may be estimated from known bond properties.

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( X=R–R^o, a R^{o 1}V^o , . , .

     

Stochastic model for triplet yields

A stochastic model of triplet yields is considered where the singlet S₁ is initially excited and subsequently feeds the triplet T₁. Both S₁ and T₁ have Montroll—Shuler step ladder vibrational relaxation mechanisms and radiative and non-radiative decay rates that vary linearly with increasing vibrational energy. Assuming the S₁ → T₁ rates also have this linear variation, the kinetic model is exactly solved in terms of integrals of simple functions of hyperbolic functions. The predictions of the model are illustrated by application to naphthalene. The model parameters are chosen; wherever possible, from experimental data. The predictions are in gross qualitative agreement with available experiments on triplet yields, and they indicate more detailed future experiments to separate the S₁ → T₁ and S₁ → S₀ (ground singlet) decays (and their energy dependence) in aromatic hydrocarbons.     

A stable triplet diradical emitter

Molecules with luminescence have been extensively investigated, but the luminescence of a stable molecule with a triplet ground state has not been observed. Synthesis of boron-containing radicals has attracted lots of interest because of their unique electronic structures and potential applications in organic semiconductors. Though some boron-based diradicals have been reported, neutral boron-containing diradicals with triplet ground states are rare. Herein two borocyclic diradicals with different substituents (3 and 4) have been isolated. Their electronic structures were investigated by EPR and UV spectroscopy, and SQUID magnetometry, in conjunction with DFT calculations. Both experiment and calculation suggest that 3 is an open shell singlet diradical while 4 is a triplet ground state diradical with a large singlet–triplet gap (0.25 kcal mol⁻¹). Both diradicals show multi fluorescence peaks (3: 414, 431, and 470 nm; 4: 420, 433, and 495 nm). 3 displays multiple redox steps and is a potential material towards the design of high-density memory devices. 4 represents the first example of a neutral triplet boron-containing diradical with a strong ferromagnetic interaction, and also is the first stable triplet diradical emitter.

Stable borocyclic diradical emitters with a tunable ground state.     

The triplet state of 4-nitronaphthylamine

Nanosecond spectroscopic and kinetic studies of 4-nitronaphthylamine (4-NO₂NA) in aerated and deaerated nonpolar solvents at room temperature show a transient species with absorption maxima at 470 and 665 nm. The rate constant for the decay of this species in deaerated benzene is 6.7 × 10⁵ sec^?1, while in aerated benzene solutions the species is quenched by oxygen with arate constant k = 2.0 × 10⁹M^?1·sec^?1. The transient absorption at 470and 665 nm is assigned to the lowest triplet excited state of 4-NO₂NA. In polar solvents, however, electronic excitation of 4-NO₂NA does not lead to any detectable transient absorption between 400 and 800 nm for the temperature range of 25 to ?150°C. This is attributed to lack of intersystem crossing of 4-NO₂NA in polar solvents.