The relationship between microwave spectroscopy,coherent triplet excitons,and density of states functions in molecular crystals

A theory of microwave band-to-band transitions in triplet Frenkel excitons in the coherent migration limit for a one-dimensional exciton associated with translationally equivalent molecules is developed. Because of selective spin-orbit coupling to the magnetic sublevels, an anisotropy in the zero field splitting across the band (k = 0 to k = ±π/a) occurs which, on a reduced energy scale, “mirrors” the energy dispersion of triplet exciton states. As a result, if the coherence time of a k state of the triplet band is longer than the intrinsic time associated with a microwave field connecting the magnetic sublevels of the band, one can measure both the bandwidth and density of states function with a zero field electron spin resonance experiment.     

Supramolecular polymorphism of the 1:1 molecular salt (adamantane-1-carboxylate-3,5,7-tricarboxylic acid)·(hexamethylenetetraminium). A "failed" crystal engineering attempt

The tetrahedral arrangement of hydrogen bonding donor and acceptor groups is used to rationalise the design of a diamondoid network; however, a single proton transfer renders the four sites inequivalent, and results in two polymorphs of the title molecular salt utilizing similar intermolecular synthons.     

ESR and ENDOR of triplet states in the charge-transfer crystal naphthalene-tetracyanobenzene

The ENDOR spectrum of localized triplet states (X-traps) in napthalene-tetracyanobenzene crystals at 4.2 J has been analyzed. From the symmetry of the spin density distribution on the donor and acceptor, it is concluded that the chargetransfer state is distributed over one donor and two acceptors. Between 130 and 300 K, the ESR spectrum or mobile triplet excitons is measured.     

Delayed-fluorescence ODMR and EPR studies of triplet excitons in charge-transfer molecular crystals

Triplet excitons in electron donor—acceptor charge-transfer (CT) molecular crystals are generated through the intersystem crossing process by excitation in the CT visible band and give rise to delayed fluorescence. Delayed-fluorescence optically detected magnetic resonance (DF ODMR) in magnetic field is analyzed in terms of microwave-induced transitions between energy levels of either the isolated triplet excitons or the annihilating triplet exciton pair. The spin polarization of the triplet excitons plays an important role in the described phenomena. A comparison between DF ODMR and EPR spectra of the anthracene—tetracyanobenzene and biphenyl—tetracyanobenzene systems is presented. In the former case the microwave transitions occurring between free exciton sublevels are predominantly responsible of the DF ODMR signal, whereas the transitions between energy levels of the exciton pair are the most important for biphenyl—TCNB.     

Long-range electron and excitation energy transfer in donor–bridge–acceptor systems

Donor–bridge–acceptor (D-B-A) systems, either as supermolecules or on surfaces, have been extensively studied with respect to long-range electron (ET) and excitation energy (EET) transfer. In more recent years, the main research objective has been to develop knowledge on how to construct molecular-based devices, with predetermined electron transfer properties, intended for application in electronics and photovoltaics. At present, such construction is in general hampered for several reasons. Most importantly, the property of a D-B-A system is not a simple linear combination of properties of the individual components, but depends on the specific building blocks and how they are assembled. An important example is the ability of the bridge to support the intended transfer process. The mediation of the transfer is characterized by an attenuation factor, β, often viewed as a bridge specific constant but which also depends on the donor and the acceptor, i.e. the same bridge can either be poorly or strongly conducting depending on the donor and acceptor. This review gives an account of the experimental exploration of the attenuation factor β in a series of bis(porphyrin) systems covalently linked by bridges of the oligo(phenyleneethynylene) (OPE) type. Attenuation factors for ET as well as for both singlet and triplet EET are discussed. A report is also given on the dependence of the transfer efficiency on the energy-gap between the donor and bridge states relevant for the specific transfer process. The experimental variation of β with varying donor and acceptor components is shown for a range of conjugated bridges by representative examples from the literature. The theoretical rationalization for the observed variation is briefly discussed. Based on the Gamow tunneling model, the observed variations in β-values with varying donors and acceptors for the same bridges is simulated successfully simultaneously as the observed energy-gap dependence is modelled.     

Conformer-dependent electronic coupling for long-range triplet energy transfer in donor-bridge-acceptor porphyrin dimers

The electronic coupling for triplet energy transfer is calculated by time-dependent density functional theory (TD-DFT) for a set of tri-chromophoric systems based on a zinc(II) porphyrin donor and the corresponding free base acceptor covalently connected by different hydrocarbon bridging chromophores. The calculated electronic coupling, for systems with identical donor acceptor distances, is sensitive to the bridge electronic structure and shows a significant dependence for the bridge and donor-bridge conformations. The computational results compare quantitatively to measurements of triplet energy transfer rates in the corresponding donor-bridge-acceptor systems.     

Spin coherence studies of a quasi-one-dimensional crystal: Trap-to-trap energy migration and stimulated echo decay

Optically detected spin coherence experiments including spin locking, two-pulse Hahn echoes and three-pulse stimulated echoes were performed on h₂ traps in isotopically mixed 1,2,4,5-tetrachlorobenzene. Values for the rate constants for trap-to-trap triplet energy migration are obtained for crystals of several different trap concentrations, and the relative importance of various energy transfer pathways is estimated. The results suggest direct superexchange as an important mechanism for triplet energy migration in these crystals. We also find that some out-of-chain energy transfer must occur to account for our results.     

Pseudo cage effect in double energy transfer

In some systems, the donor of a triplet—triplet energy transfer can be sensitized in its singlet state through a singlet—singlet energy transfer (Dexter mechanism), where the donor is the acceptor of the triplet transfer itself. As a consequence an extra acceptor molecule in the triplet energy transfer is present in the vicinity of the donor, thus enhancing the efficiency of the transfer process. Experiments show clearly this effect and a diffusional model gives semi-quantitative agreement with the experimental data.     

An investigation of the triplet state dynamics of zinc chlorophyll b by microwave-induced changes in the intensity of fluorescence and singlet-singlet absorption

Optical detection of magnetic resonance experiments on the triplet state of zinc-substitution chlorophyll b has provided the zero-field splitting and depopulation rate constants for the individual triplet spin sublevels. The zero field triplet state EPR transitions could be observed at 890 MHz and 1085 MHz as either microwave-induced changes in the fluorescence intensity or in the intensity of S₀ → S_n absorption. The dynamics experiments show that intersystem crossing from the Zn chlorophyll b triplet state into the ground state occurs primarily through the out-of-plane (lowest energy) spin sublevel.     

Ab initio MO study for electron transfer matrix element in hydrogen-bonded system

The electronic coupling matrix element of electron transfer between donor and acceptor connected with hydrogen bonds has been studied in a model system. The calculated matrix element depends largely on the relative rotational conformation of the electron-donor and electron-acceptor sites and a simple orbital analysis has been presented. Along the approximate proton transfer coordinate, the energy potential is a double well and the matrix element has a single maximum at the center of the double well.     

Anisotropic spin—lattice relaxation in the triplet state of naphtralene-h8 in n-pentane

The temperature dependence of the individual spin—lattice relaxation rate constants (W_ij) between the lowest triplet sublevels of naphthalene-h₈ in a polycrystalline Shpol'skii matrix of n-pentane have been measured between 1.2 and 2.4 K in zero field. The total sublevel decay constants are evaluated and found to be independent of temperature. The W_ij are found to be highly anisotropic, but the anisotropy pattern differs from that observed previously in a durene matrix.     

Intrinsic photogeneration of long-lived charges in a donor-orthogonal acceptor conjugated polymer

Efficient charge photogeneration in conjugated polymers typically requires the presence of a second component to act as electron acceptor. Here, we report a novel low band-gap conjugated polymer with a donor/orthogonal acceptor motif: poly-2,6-(4,4-dihexadecyl-4H-cyclopenta [2,1-b:3,4-b′]dithiophene)-alt-2,6-spiro [cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene]-2′,7′-dicarbonitrile, referred to as PCPDT-sFCN. The role of the orthogonal acceptor is to spatially isolate the LUMO from the HOMO, allowing for negligible exchange energy between electrons in these orbitals and minimising the energy gap between singlet and triplet charge transfer states. We employ ultrafast and microsecond transient absorption spectroscopy to demonstrate that, even in the absence of a separate electron acceptor, PCPDT-sFCN shows efficient charge photogeneration in both pristine solution and film. This efficient charge generation is a result of an isoenergetic singlet/triplet charge transfer state equilibrium acting as a reservoir for charge carrier formation. Furthermore, clear evidence of enhanced triplet populations, which form in less than 1 ps, is observed. Using group theory, we show that this ultrafast triplet formation is due to highly efficient, quantum mechanically allowed intersystem crossing between the bright, initially photoexcited local singlet state and the triplet charge transfer state. Remarkably, the free charges that form via the charge transfer state are extraordinarily long-lived with millisecond lifetimes, possibly due to the stabilisation imparted by the spatial separation of PCPDT-sFCN''s donor and orthogonal acceptor motifs. The efficient generation of long-lived charge carriers in a pristine polymer paves the way for single-material applications such as organic photovoltaics and photodetectors.

The spatial separation of PCPDT-sFCN''s donor and orthogonal acceptor motifs allows efficient photogeneration of extraordinarily long-lived charge carriers in the pristine polymer, providing an important step towards single-material optoelectronics.     

On the singlet-triplet splitting of geminate electron-hole pairs in organic semiconductors

Because of their unique photophysical properties, organic semiconductors have shown great promise in both light-emitting devices (LEDs) and photovoltaic systems. In particular, the question of spin statistics looms large in these applications: the relative energetics and rates of formation for singlet versus triplet excited states can have a significant impact on device efficiency. In this Article, we study the singlet and triplet charge-transfer (CT) configurations that can be thought of as the immediate precursors to the luminescent states in organic LEDs. In particular, we find that the CT singlet-triplet energy gap (deltaE(ST)) of organic dyes and oligomers depends sensitively on both the material and the relative orientation of the donor/acceptor pair. Furthermore, in contrast with the commonly held view, we find that the singlet CT states nearly always lie energetically below the triplet CT states (deltaE(ST) < 0). This trend is attributed to two physical sources. First, the relatively close contact between the donor and acceptor leads to a strong kinetic exchange component that favors the singlet. Second, Coulombic attraction between the separated charges favors inner-sphere reorganization that brings the donor and acceptor closer together, further enhancing the kinetic exchange effect. We discuss the implications of these results on the design of organic LEDs.     

Controlling Coherence Transfer of Dimer Interacting with Independent Environments

We study the coherence transfer between two pigments (donor and acceptor) of a dimer interacting with two independent environments. By means of a prior weak measurement on the donor and a post measurement, which is either a reversal measurement or a weak measurement, on the acceptor, we present a scheme to optimally control the transfer of the donor's coherence to the acceptor. We construct explicit relationships for the two measure-ment strengths and the evolution time, by which the coherence degree of the acceptor can approach the maximum value 0.5 at the cost of a decreased probability.     

Asymptotic quantum yield of triplet sensitized decomposition

The asymptotic quantum yield of triplet energy transfer is found by calculating the fraction of acceptor molecules with energy above the minimum energy for decomposition. This is done by allowing for a statistical energy distribution among the internal modes in the collision complex. It is found that for a monatomic triplet donor most of the triplet energy is transferred to the acceptor molecule, while for a polyatomic donor molecule only a fraction of it is available for future decomposition of the acceptor.     

Molecularly Dispersed Donors in Acceptor Molecular Crystals for Photon Upconversion under Low Excitation Intensity

For real‐world applications of photon upconversion based on the triplet–triplet annihilation (TTA‐UC), it is imperative to develop solid‐state TTA‐UC systems that work effectively under low excitation power comparable to solar irradiance. As an approach in this direction, aromatic crystals showing high triplet diffusivity are expected to serve as a useful platform. However, donor molecules inevitably tend to segregate from the host acceptor crystals, and this inhomogeneity results in the disappointing performance of crystalline state TTA‐UC. In this work, a series of cast‐film‐forming acceptors was developed, which provide both regular acceptor alignment and soft domains of alkyl chains that accommodate donor molecules without segregation. A typical triplet sensitizer, Pt^II octaethylporphyrin (PtOEP), was dispersed in these acceptor crystals without aggregation. As a result, efficient triplet energy transfer from the donor to the acceptor and diffusion of triplet excitons among regularly aligned anthracene chromophores occurred. It resulted in TTA‐UC emission at low excitation intensities, comparable to solar irradiance.     

Triplet-triplet energy transfer in Langmuir-Blodgett films

The temperature effect on the efficiency of the triplet energy transfer between different molecules included in molecular layers by the Langmuir-Blodgett (LB) procedure was studied. The efficiency of the triplet energy transfer from the LB film of the donor to the LB film of the acceptor is determined by the homogeneous broadening of the energy donor levels.     

Chiral self-assembly regulated photon upconversion based on triplet-triplet annihilation

Photon upconversion (UC) based on triplet-triplet annihilation (TTA) in quasi-solid or solid state has been attracting much research interest due to its great potential applications. To get effective UC, precisely controlled donor-acceptor interaction is vitally important. Chiral self-assembly provides a powerful approach for sophisticated regulation of molecular interaction. Here we report a chiral self-assembly controlled TTA-UC system composed of chiral acceptor and achiral donor. It is found that racemic mixture of acceptors could form straight fibrous nanostructures, which show strong UC emission, while chiral assemblies for homochiral acceptors emit weak upconverted light. The racemic assemblies allow efficient triplet-triplet energy transfer (TTET) and further realize efficient UC emission, while the homochiral assemblies from chiral acceptor produce twisted nanostructures, suppressing efficient triplet energy transfer and annihilation. The establishment of such chiral self-assembly controlled UC system highlights the potential applications of triplet fusion in optoelectronic materials and provides a new perspective for designing highly effective UC systems.     

Energy transfer in polymers—IV. Singlet and triplet energy transfer in polymethylvinylketone films

The efficiency of transfer from the triplet of polymethylvinylketone (PMVK) to naphthalene has been measured at 77°K. The critical distance of transfer according to Hirayama is 11 · 3 Å. This is the usual value of R_o for exchange interaction without migration. Singlet energy transfer from the polymer to benzophenone occurs when donor and acceptor are very close together, at room temperature. In the system PMVK-anthracene, the emission spectrum shows that microcrystals of the additive are formed in the polymer even at low concentration.     

Mono‐ and Bis(tetrathiafulvalene)‐1,3,5‐Triazines as Covalently Linked Donor–Acceptor Systems: Structural,Spectroscopic, and Theoretical Investigations

Reaction of 2,4,6‐trichloro‐1,3,5‐triazine with lithiated tetrathiafulvalene (TTF) in stoichiometric conditions, followed by treatment with sodium methanolate, provides mono‐ and bis(TTF)–triazines as new covalently linked (multi)donor–acceptor systems. Single‐crystal X‐ray analyses reveal planar structures for both compounds, with formation of peculiar segregated donor and acceptor stacks for the mono(TTF)–triazine compound, while mixed TTF–triazine stacks establish in the case of the bis(TTF) derivative. Cyclic voltammetry measurements show reversible oxidation of the TTF units, at rather low potential, with no splitting of the oxidation waves in the case of the dimeric TTF, whereas irreversible reduction of the triazine core is observed. Intramolecular charge transfer is experimentally evidenced through solution electronic absorption spectroscopy. Time‐dependent DFT calculations allow the assignment of the charge transfer band to singlet transitions from the HOMO of the donor(s) to the LUMO of the acceptor. Solution EPR measurements correlated with theoretical calculations were performed in order to characterize the oxidized species. In both cases the spectra show very stable radical species and contain a triplet of doublet pattern, in agreement with the coupling of the unpaired electron with the three TTF protons. The dication of the bis(TTF)–triazine is paramagnetic, but no spin–spin exchange interaction could be detected.