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.     

Interlayer triplet-triplet energy transfer in nanosized molecular layers

The results of the study of interlayer triplet-triplet energy transfer from anthracene molecules to Nile Red molecules in Langmuir-Blodgett films are presented. The observed sensitized delayed fluorescence of the energy acceptor is shown to be due to annihilation of migrating triplet excitons. It has been found that the decay kinetics of delayed fluorescence of the donor and the acceptor has a complex form and is described by a combination of the power and exponential functions. The dependence of the energy transfer efficiency on the distance between the donor and acceptor layers was studied.     

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.     

Origin of simultaneous donor-acceptor emission in single molecules of peryleneimide-terrylenediimide labeled polyphenylene dendrimers

F?rster type resonance energy transfer (FRET) in donor-acceptor peryleneimide-terrylenediimide dendrimers has been examined at the single molecule level. Very efficient energy transfer between the donor and the acceptor prevent the detection of donor emission before photobleaching of the acceptor. Indeed, in solution, on exciting the donor, only acceptor emission is detected. However, at the single molecule level, an important fraction of the investigated individual molecules (about 10-15%) show simultaneous emission from both donor and acceptor chromophores. The effect becomes apparent mostly after photobleaching of the majority of donors. Single molecule photon flux correlation measurements in combination with computer simulations and a variety of excitation conditions were used to determine the contribution of an exciton blockade to this two-color emission. Two-color defocused wide-field imaging showed that the two-color emission goes hand in hand with an unfavorable orientation between one of the donors and the acceptor chromophore.     

Theory of excitation energy transfer regarded as nonadiabatic transition 2: Calculational evidence of nonadiabatic interaction causing intermolecular excitation energy transfer

To clarify whether the excitation energy transfer from a donor molecule or aggregate to a remote acceptor molecule or aggregate can be caused by nonadiabatic interaction as expected in our previous studies 4 ; 5 , we carried out ab initio calculations for three donor–acceptor systems. Even when the acceptor is separated from the donor by 15 Å, it was found that nonadiabatic coupling elements have moderately large values in the nuclear configuration region where the potential energy surfaces at two excited states for the donor–acceptor system are close to each other; otherwise, the conical intersection between the two excited‐state potential energy surfaces appears. In addition, it was found that the adiabatic approximation for the donor–acceptor system holds in the nuclear configuration region in which the initial and final wave packets in the process of the excitation energy transfer lie. These findings lead to the conclusion that the excitation energy transfer between two remote molecules or aggregates can be caused by the nonadiabatic interaction. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem 94: 36–43, 2003     

Quenching of singlets and triplets by reversible ionization followed by charge recombination

The reversible electron transfer from donor to excited molecule (acceptor of electron) is shown to be the irreversible energy quenching, if it is completed by subsequent irreversible recombination radical-ions which are produced. The Stern-Volmer constant of fluorescence as well as the Markovian rate constant of triplet quenching are calculated analytically, assuming the electron transfer is contact. The multiple Rehm-Weller effect is shown to be peculiar to both constants.     

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.     

Radiationless transitions in electron donor-acceptor complexes: selection rules for S1 → T intersystem crossing and efficiency of S1 → S0 internal conversion

Evidence is presented which indicates that singlet → triplet intersystem crossing from the charge-transfer singlet state of electron donor-acceptor complexes is efficient only when a locally excited triplet state of a component molecule (donor or acceptor) lies below the charge-transfer singlet state.     

Effect of substituents on the T1 energy of trans-stilbenes

Oxygen-induced singlet—triplet absorption and electrochemiluminescence quenching experiments with substituted stilbenes indicate a small influence of monosubstitution or donor—acceptor disubstitution on the triplet energy. This implies a decrease of the S₁T₁ energy difference particularly in the case of donor—acceptor substituted stilbenes.     

Triplet excitation transfer through the walls of hemicarcerands: dependence of the electronic coupling on the size of the molecular cage.

Triplet excitation transfer from biacetyl trapped inside three hemicarcerands of different size (1, 2, and 3) to acceptors in the surrounding medium was investigated. The largest hemiracerand 1 employs four butyl linkers and the intermediate hemicarcerand 2 four o-xylyl linkers. The smallest hemicarcerand 3 contains only three methylene linkers. Both neat liquid triplet acceptors and acceptors dispersed in solvents were used. The primary objective of this work was to determine the dependence of the energy transfer rate on the size and the electronic structure of the molecular cages. There is a pronounced, more than 10-fold, increase of triplet energy transfer rates with decreasing size of the cage. The corresponding electronic coupling, /V/, increases approximately by a factor of approximately 3.5 from the largest hemicarceplex 1 to the smallest hemicarceplex 3. This increase of the electronic interaction is similar to that observed in covalently bound systems when the distance between the triplet donor and the acceptor is reduced by one carbon-carbon sigma-bond. The electronic structure of the hemicarcerand appears to be of secondary importance, at least when T(1) states of the donor and the acceptor are far from a resonance with the T(1) state of the cage. A very good agreement between the results obtained in neat acceptors and in solution was found, indicating that the association between the acceptors and the molecular cages is negligible, if at all present. An unexpectedly large interaction between the guest and the polarizable walls of the hemicarcerands manifested by emission red-shifts was observed in all cases. This suggests that the entrapment within the molecular cage gives rise to an environment considerably different from that of a single molecule in the gas phase. An interesting correlation between the magnitude of the phosphorescence spectral shift, Deltanu(0-0), and the guest-to-external acceptor electronic coupling, /V/, was found.     

A new algorithm for predicting triplet-triplet energy-transfer activated complex coordinate in terms of accurate potential-energy surfaces

The new algorithm presented here allows, for the first time, the determination of the optimal geometrical distortions that an acceptor molecule in the triplet-triplet energy-transfer process undergoes, as well as the dependence of the activation energy of the process on the triplet energy difference of donor and acceptor molecules. This algorithm makes use of the complete potential-energy surfaces (singlet and triplet states), and contrasts with the first-order approximation already published [L. M. Frutos, O. Castano, J. L. Andres, M. Merchan, and A. U. Acuna, J. Chem. Phys. 120, 1208 (2004)] in which an expansion of the potential-energy surfaces was used. This algorithm is gradient based and finds the best trajectory for the acceptor molecule, starting from S(0) ground-state equilibrium geometry, to achieve the maximum variation of the singlet-triplet energy gap with the minimum energy of activation on S(0). Therefore, the algorithm allows the determination of a "reaction path" for the triplet-triplet energy-transfer processes. Also, the algorithm could also serve eventually to find minimum-energy crossing (singlet-triplet) points on the potential-energy surface, which can play an important role in the intersystem crossing process for the acceptor molecules to recover their initial capacity as acceptors. Also addressed is the misleading use of minimum-energy paths in T(1) to describe the energy-transfer process by comparing these results with those obtained using the new algorithm. The implementation of the algorithm is illustrated with different potential-energy surface models and it is discussed in the frame of nonvertical behavior.     

Electrogenerated Chemiluminescence of Donor–Acceptor Molecules with Thermally Activated Delayed Fluorescence

The electrochemistry and electrogenerated chemiluminescence (ECL) of four kinds of electron donor–acceptor molecules exhibiting thermally activated delayed fluorescence (TADF) is presented. TADF molecules can harvest light energy from the lowest triplet state by spin up‐conversion to the lowest singlet state because of small energy gap between these states. Intense green to red ECL is emitted from the TADF molecules by applying a square‐wave voltage. Remarkably, it is shown that the efficiency of ECL from one of the TADF molecule could reach about 50 %, which is comparable to its photoluminescence quantum yield.     

Intramolecular electronic excitation energy transfer in donor/acceptor dyads studied by time and frequency resolved single molecule spectroscopy

Electronic excitation energy transfer has been studied by single molecule spectroscopy in donor/acceptor dyads composed of a perylenediimide donor and a terrylenediimide acceptor linked by oligo(phenylene) bridges of two different lengths. For the shorter bridge (three phenylene units) energy is transferred almost quantitatively from the donor to the acceptor, while for the longer bridge (seven phenylene units) energy transfer is less efficient as indicated by the occurrence of donor and acceptor emission. To determine energy transfer rates and efficiencies at the single molecule level, several methods have been employed. These comprise time-correlated single photon counting techniques at room temperature and optical linewidth measurements at low temperature (1.4 K). For both types of measurement we obtain broad distributions of the rate constants of energy transfer. These distributions are simulated in the framework of Forster theory by properly taking into account static disorder and the flexibility of the dyads, as both effects can substantially contribute to the distributions of energy transfer times. The rate constants of energy transfer obtained from the calculated distributions are smaller on average than those extracted from the experimental distributions, whereby the discrepancy is larger for the shorter bridge. Furthermore, by plotting the experimentally determined transfer rates against the individual spectral overlaps, approximately linear dependencies are found being indicative of a Forster-type contribution to the energy transfer. For a given single molecule such a linear dependence could be followed by spectral diffusion induced fluctuations of the spectral overlap. The discrepancies between measured energy transfer rates and rates calculated by Forster theory are briefly discussed in light of recent results of quantum chemical calculations, which indicate that a bridge-mediated contribution is mainly responsible for the deviations from Forster theory. The availability of the inhomogeneous distributions of donor and acceptor electronic transition frequencies allows for comparing the energy transfer process at liquid helium and room temperature for the same set of molecules via simple simulations. It is found that on average the energy transfer is by a factor of approximately 3 faster at room temperature, which is due to an increase of spectral overlap.     

Engineering Sensitized Photon Upconversion Efficiency via Nanocrystal Wavefunction and Molecular Geometry

Triplet energy transfer from inorganic nanocrystals to molecular acceptors has attracted strong attention for high‐efficiency photon upconversion. Here we study this problem using CsPbBr₃ and CdSe nanocrystals as triplet donors and carboxylated anthracene isomers as acceptors. We find that the position of the carboxyl anchoring group on the molecule dictates the donor‐acceptor coupling to be either through‐bond or through‐space, while the relative strength of the two coupling pathways is controlled by the wavefunction leakage of nanocrystals that can be quantitatively tuned by nanocrystal sizes or shell thicknesses. By simultaneously engineering molecular geometry and nanocrystal wavefunction, energy transfer and photon upconversion efficiencies of a nanocrystal/molecule system can be improved by orders of magnitude.     

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.     

Excitation Energy Transfer in Dendritic Host–Guest Donor–Acceptor Systems

We report on a study of a physically formed host–guest system, which was designed to be investigated by fluorescence energy transfer. All donor and acceptor molecules used were cyanine dyes. Investigation was performed at the ensemble level as well as at the single‐molecule level. The ensemble measurements revealed a distribution of binding sites as well for the donor as for the acceptor. Accordingly, we found a distribution of the energy transfer efficiency. At the single‐molecule level, these distributions are still present. We could discriminate entities that show very efficient energy transfer, some that do not show any energy transfer and systems whose energy transfer efficiency is only about 50 %. The latter allowed the time‐resolved detection of energy transfer of single entities through the acceptor decay. Finally, we discuss the observation that the energy transfer efficiency fluctuates as a function of time.     

Spectral identification of specific photophysics of cy5 by means of ensemble and single molecule measurements

The triplet-state characteristics of the Cy5 molecule related to trans-cis isomerization are investigated by means of ensemble and single molecule measurements. Cy5 has been used frequently in the past 10 years in single molecule spectroscopic applications, e.g., as a probe or fluorescence resonance energy transfer acceptor in large biomolecules. However, the unknown spectral properties of the triplet state and the lack of knowledge on the photoisomerization do not allow us to interpret precisely the unexpected single molecule behaviors. This limits the application of Cy5. The laser photolysis experiments demonstrate that the trans triplet state of Cy5 absorbs about 625 nm, the cis ground state absorbs about 690 nm, and the cis triplet state also absorbs about 690 nm. In other words, the T1-Tn absorptions largely overlap the ground-state absorptions for both trans and cis isomers, respectively. Furthermore, the observation of the cis triplet state indicates an important isomerization pathway from the trans-S1 state to the cis-T1 state upon excitation. The detailed spectra presented in this article let us clearly interpret the exact mechanisms responsible for several important and unexpected photophysical behaviors of single Cy5 molecules such as reverse intersystem crossing (RISC), the observation of dim states with a lower emission intensity and slightly red-shifted fluorescence, and unusual energy transfer from donor molecules to dark Cy5 molecules acting as acceptors in single molecule fluorescence resonance energy transfer (FRET) measurements. Spectral results show that the dim state in the single molecule fluorescence intensity time traces originated from cis-Cy5 because of a lower excitation rate, resulting from the red-shifted ground-state absorption of cis-Cy5 compared to that of the trans-Cy5.     

Accelerated energy transfer between molecules near a solid particle

The Forster-Dexter theory of energy transfer between molecules is generalized to include the effects of a nearby solid state particle. It is found that the energy transfer rate between a donor and acceptor molecule may be enhanced by many orders of magnitude when the molecular transition frequencies lie in the vicinity of the resonance frequency of the particle and when the particle possesses sharp features. Due to increased damping near the particle, however, this may or may not lead to increased acceptor molecule radiation.     

Density functional theory study of 1,2‐dioxetanone decomposition in condensed phase

The decomposition of 1,2‐dioxetanone into a CO₂ molecule and into an excited state formaldehyde molecule was studied in condensed phase, using a density functional theory approach. Singlet and triplet ground and excited states were all included in the calculations. The calculations revealed a novel mechanism for the chemiluminescence of this compound. The triplet excitation can be explained by two intersystem crossings (ISCs) with the ground state, while the singlet excitation can be accounted by an ISC with the triplet state. The experimentally verified small excitation yield can then be explained by the presence of an energy barrier present in the potential energy surface of the triplet excited state, which will govern both triplet and singlet excitation. It was also found that the triplet ground state interacts with both the triplet excited and singlet ground states. A MPWB1K/mPWKCIS approach provided results in agreement with the existent literature. © 2012 Wiley Periodicals, Inc.