Triplet Energy Transport Research Articles

Sweet Spot of Intermolecular Coupling in Crystalline Rubrene: Intermolecular Separation to Minimize Singlet Fission and Retain Triplet–Triplet Annihilation

Singlet fission is detrimental to NIR-to-vis photon upconversion in the solid rubrene (Rub) films, as it diminishes photoluminescence efficiency. Previous studies have shown that thermally activated triplet energy transport drives singlet fission with nearly 100% efficiency in closely packed Rub crystals. Here, we examine triplet separation and recombination as a function of intermolecular distance in the crystalline films of Rub and the t-butyl substituted rubrene (tBRub) derivative. The increased intermolecular distance and altered molecular packing in tBRub films cause suppressed singlet dissociation into free triplets due to slower triplet energy transport. It was found that the formation of correlated triplet pairs 1(TT) and partial triplet separation 1(T···T) occurs in both Rub and tBRub films despite differences in intermolecular coupling. Under weak intermolecular coupling as in tBRub, geminate triplet annihilation of 1(T···T) outcompetes dissociation into free triplets, resulting in emission from the 1(TT) state. Essentially, increasing intermolecular distance up to a certain point (a sweet spot) is a good strategy for suppressing singlet fission and retaining triplet–triplet annihilation properties.

Triplet Energy Transport in Platinum-Acetylide Light Harvesting Arrays.

Light harvesting and triplet energy transport is investigated in chromophore-functionalized polystyrene polymers featuring light harvesting and energy acceptor chromophores (traps) at varying loading. The series of precision polymers was constructed via reversible addition-fragmentation transfer polymerization and functionalized with platinum acetylide triplet chromophores by using an azide-alkyne "click" reaction. The polymers have narrow polydispersity and degree of polymerization ∼60. The chromophores have the general structure, trans-[-R-C6H4-C≡C-Pt(PBu3)2-C≡C-Ar], where R is the attachment point to the polystyrene backbone and Ar is either -C6H4-C≡C-Ph or -pyrenyl (PE2-Pt and Py-Pt, respectively, with triplet energies of 2.35 and 1.88 eV). The polychromophores contain mainly the high-energy PE2-Pt units (light absorber and energy donor), with randomly distributed Py-Pt units (3-20% loading, energy acceptor). Photophysical methods are used to study the dynamics and efficiency of energy transport from the PE2-Pt to Py-Pt units in the polychromophores. The energy transfer efficiency is >90% for copolymers that contain 5% of the Py-Pt acceptor units. Time-resolved phosphorescence measurements combined with Monte Carlo exciton dynamics simulations suggest that the mechanism of exciton transport is exchange energy transfer hopping between PE2-Pt units.

Benzophenone Photosensitized DNA Damage

Although the carcinogenic potential of ultraviolet radiation is well-known, UV light may interact with DNA by direct absorption or through photosensitization by endogenous or exogenous chromophores. These chromophores can extend the "active" fraction of the solar spectrum to the UVA region and beyond, which means that photosensitizers increase the probability of developing skin cancer upon exposure to sunlight. Therefore researchers would like to understand the mechanisms involved in photosensitized DNA damage both to anticipate possible photobiological risks and to design tailor-made photoprotection strategies. In this context, photosensitized DNA damage can occur through a variety of processes including electron transfer, hydrogen abstraction, triplet-triplet energy transfer, or generation of reactive oxygen species. In this Account, we have chosen benzophenone (BP) as a classical and paradigmatic chromophore to illustrate the different lesions that photosensitization may prompt in nucleosides, in oligonucleotides, or in DNA. Thus, we discuss in detail the accumulated mechanistic evidence of the BP-photosensitized reactions of DNA or its building blocks obtained by our group and others. We also include ketoprofen (KP), a BP-derivative that possesses a chiral center, to highlight the stereodifferentiation in the key photochemical events, revealed through the dynamics of the reactive triplet excited state ((3)KP*). Our results show that irradiation of the BP chromophore in the presence of DNA or its components leads to nucleobase oxidations, cyclobutane pyrimidine dimer formation, single strand breaks, DNA-protein cross-links, or abasic sites. We attribute the manifold photoreactivity of BP to its well established photophysical properties: (i) it absorbs UV light, up to 360 nm; (ii) its intersystem crossing quantum yield (ϕ(ISC)) is almost 1; (iii) the energy of its nπ* lowest triplet excited state (E(T)) is ca. 290 kJ mol(-1); (iv) it produces singlet oxygen ((1)O(2)) with a quantum yield (ϕ(Δ)) of ca. 0.3. For electron transfer and singlet oxygen reactions, we focused on guanine, the nucleobase with the lowest oxidation potential. Among the possible oxidative processes, electron transfer predominates. Conversely, triplet-triplet energy transfer occurs mainly from (3)BP* to thymine, the base with the lowest lying triplet state in DNA. This process results in the formation of cyclobutane pyrimidine dimers, but it also competes with the Paternò-Büchi reaction in nucleobases or nucleosides, giving rise to oxetanes as a result of crossed cycloadditions. Interestingly, we have found significant stereodifferentiation in the quenching of the KP triplet excited state by both 2'-deoxyguanosine and thymidine. Based on these results, this chromophore shows potential as a (chiral) probe for the investigation of electron and triplet energy transport in DNA.

Role of Bridge Energetics on the Preference for Hole or Electron Transfer Leading to Charge Recombination in Donor-Bridge-Acceptor Molecules

The impact of donor-acceptor electronic coupling and bridge energetics on the preference for hole or electron transfer leading to charge recombination in a series of donor-bridge-acceptor (D-B-A) molecules was examined. In these systems, the donor is 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) and acceptor is naphthalene-1,8:4,5-bis(dicarboximide) (NI), while the bridges are either oligo(p-phenyleneethynylene) (PE(n)P, where n = 1-3) 1-3 or oligo(2,7-fluorenone) (FN(n), where n = 1-3) 4-6. Photoexcitation of 1-3 and 4-6 produces DMJ(+•)-An-PE(n)P-NI(-•) and DMJ(+•)-An-FN(n)-NI(-•), respectively, which undergo radical pair intersystem crossing followed by charge recombination to yield both (3*)An and (3*)NI, which are observed by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. (3*)NI is produced by hole transfer from DMJ(+•) to NI(-•), while (3*)An is produced by electron transfer from NI(-•) to DMJ(+•), using the agency of the bridge HOMOs and LUMOs, respectively. By monitoring the initial population of (3*)NI and (3*)An in 1-6, the data show that charge recombination occurs preferentially by selective hole transfer when the bridge is PE(n)P, while it occurs by preferential electron transfer when the bridge is FN(n). Over time, the initial population of (3*)NI decreases, while that of (3*)An increases, indicating that triplet-triplet energy transfer (TEnT) occurs. The observed distance dependence of TEnT from (3*)NI to An is weakly exponential with a decay parameter β = 0.08 Å(-1) for the PE(n)P series and β = 0.03 Å(-1) for the FN(n) series. In the PE(n)P series, this weak distance dependence is attributed to a transition from the superexchange regime to hopping transport as the energy gap for triplet energy injection onto the bridge becomes significantly smaller as n increases, while in the FN(n) series the corresponding energy gap is small for all n resulting in triplet energy transport by the hopping mechanism.

The effect of radiation-induced traps on triplet energy transport in polyguanylic acid

Transport of triplet excitations on polyguanylic acid (Poly(G)) at 77 K has been studied by observing the efficiency of sensitization of Tb 3+ ions, using both steady-state and time-dependent luminescence. Past work has revealed that energetic disorder within the polymer must be taken into account by using Monte Carlo (MC) simulations for energy transport. A self-consistent fit of the experimental data can be achieved to yield the energy hopping rate, mean number of intrinsic traps per polymer and the maximum number of Tb 3+ ions in contact with Poly(G). We have applied this same experimental approach to the study of Poly(G) that has been exposed to γ-irradiation. Irradiation diminishes the efficiency of Tb 3+ sensitization and MC simulations to fit the data suggest that the density of Poly(G) triplet states is modified and that the number of deep traps on Poly(G) is increased. This latter quantity correlates with the change of the molecular weight of the polymer, as determined by viscosity.

Low temperature tunneling of triplet excitation energy

Triplet energy transport (ET) in p-dibromobenzene (DBB) crystals doped with p-dichlorobenzene (DCB) at concentrations above 1% results in energy trapping exhibiting dispersive transport kinetics for some of the triplet excitations. This is monitored by observing the phosphorescence decay. Coherent ODMR techniques, especially stimulated echo experiments, reveal that some excitation energy is also migrating among DCB monomers without being affected by trapping processes. This happens via the superexchange (tunneling) mechanism, with a typical monomer to monomer jump rate of 3 × 10 8 s −1. The combination of these two experimental approaches reveals strong disorder in the energy transport kinetics of the chemically mixed crystal DCB/DBB.

Characterization of energy transport in molecular crystals by ultrametric spaces

Dispersive triplet energy transport in the chemically mixed p-dichlorobenzene/p-dibromobenzene singe crystal is dependent on doping concentration c and on temperature above 4.2 K. It can be described by a Kohlrausch type decay law Iẽxp(- t β). Below about 6 K the dispersivity parameter β is independent of c, and constant with temperature. The concept of ultrametric spaces applied to energy transport with hierarchies of energies or distances predicts β = 0 for T ≃ = 0 or c ≃ 0, which is experimentally not observed. Probably this is due to a cross-over between different transfer mechanisms.

Optical excitation of a three-dimensional triplet energy funnel

Triplet excitation spectroscopy at 4.2 K of X-traps induced in para-dibromobenzene (DBB) by para-xylene guest molecules reveals that several DBB host states split off from the triplet exciton band. These states from an energy funnel with a depth of 122 cm -1 for DBB triplet energy transport. The relative trap energies within the funnel suggest a three-dimensional structure.

Characterization of aggregates and their importance for energy transport in chemically crystals

Chemically mixed crystals of dichlorobenzene (DCB) in dibromobenzene have been investigated over the concentration range from 0.01 to 10%. DCB guest molecules induce energy funnels for triplet energy transport. Phospherescence excitation spectroscopy reveals that guest aggregates formed at higher concentrations induce the same kind of energy funnels as monomers. Optical shifts of DCB aggregates relative to monomers are mainly determined by crystal field effects whereas changes in ODMR frequencies are due to exchange interactions.

Discrimination between thermally activated and direct energy transport mediated by induced energy funnels in disordered single crystals

A chemically mixed molecular crystal of p-dichlorobenzene (DCB) in p-dibromobenzene (DBB) at concentrations up to 0.15 mol/mol shows triplet energy transport phenomena, which can be described by (1) thermally activated hopping of localized DCB molecules in an induced energy funnel, and (2) dispersive energy transport among DCB aggregates at temperatures below 4.2 K and diffusive transport above 15 K.

Concentration dependent nondiffusive triplet energy transport on a disordered lattice in the chemically mixed crystal dichlorobenzene–dibromobenzene

Nondiffusive triplet energy transport is observed in a p-dibromobenzene single crystal doped with p-dichlorobenzene (DCB) at variable concentrations. Phosphorescence kinetics of DCB monomers and aggregates are strongly nonexponential and can be described assuming a stretched exponential exp[−(t/τ)β], where β varies from 1 at low concentrations to 0.28±0.05 at concentrations above 0.05 mol/mol.

Triplet energy transport and microwave-controlled triplet excitation density in doped dibenzofuran single crystals

Triplet energy transport and microwave-controlled triplet excitation density in doped dibenzofuran single crystals

Formation of energy funnels for triplet energy transport in a chemically mixed molecular crystal

Dichlorobenzene guest molecules form energy traps and additionally induce next-neighbour X-traps in a dibromobenzene host crystal. This shows up as a funnel for triplet energy transport. The width of the funnel is less than four lattice constants. Energy transport within and out of this funnel can be described by a thermally activated process.

Fractal-like energy transport and fusion in a naphthalene-doped polymeric glass

Singlet energy transport in naphthalene-doped poly (methylmethacrylate) (PMMA) samples has been found, by doping with anthracene, to follow Stern-Volmer kinetics with a transfer rate constant ca. 2x10 9. Delayed fluorescence is easily observed; triplet energy transport seems to have fractal-like properties.

Triplet exciton transport in isotopic mixed naphthalene crystals. II. Master equation analysis

The experimental data on triplet exciton transport in isotopically mixed crystals of naphthalene/perdeuteronaphthalene (paper I) are contrasted with singlet exciton transport in the same samples (20%–100%) and analyzed in terms of incoherent hopping models. The master equation approach is emphasized and extended. We modify the conventional continuum master equations via a physically plausible cutoff of the high-frequency transfer rates. This results in an experimentally acceptable functional form (transport linear with high power of concentration) and nearest-neighbor transfer time (100 ps). We also developed a lattice master equation (numerically soluble), using an experimentally tested exciton superexchange formula. The somewhat surprising result is that the lattice master equations do not fit the experimental functional form. The success of the continuum models and the failure of the lattice models are attributed to the latter’s neglect of the spread in transfer rates for a given intersite distance. We claim that clusterization as well as diagonal homogeneous and/or inhomogeneous disorder cause the above spread. On the other hand, these energy mismatches are small with respect to the thermal energy, in contrast to the singlet exciton transport case, where, due to larger energy mismatches, a percolation-like critical concentration is observed. Thus for the given concentration and temperature regimes, the triplet exciton transport is diffusive while the singlet exciton transport is percolative. Lower temperatures and/or concentrations are required for percolative triplet energy transport in these systems.

Quasiresonant excitation transfer in molecular aggregates

Excitation transfer in finite molecular aggregates is analyzed in the context of the Haken–Strobl model. Explicit solutions are presented for a trimer and a rectangular tetramer. Special emphasis is placed upon population transfer among subunits (monomers, dimers) and its relationship to energy transfer, and upon the problems associated with coherence of this transfer process. These aggregates serve as models for the problem of excitation transfer in disordered media, where partial coherence resulting from short-range interactions has been largely ignored. Our most intriguing result is the greatly diminished effectiveness of the longer-ranged transfer in the presence of short-range clusters. Under some conditions the ensuing energetic mismatches may well dominate the overall energy transport and render invalid the usual description in terms of hopping among individual sites. An application to triplet energy transport in isotopic mixed naphthalene crystals is given; it is seen that the reduced efficiency of non-nearest-neighbor transfer processes reinforces the two-dimensional characteristics of the energy transport.