Singlet Energy Migration Research Articles

Donor-Acceptor-Collector Ternary Crystalline Films for Efficient Solid-State Photon Upconversion.

It is pivotal to achieve efficient triplet-triplet annihilation based photon upconversion (TTA-UC) in the solid-state for enhancing potentials of renewable energy production devices. However, the UC efficiency of solid materials is largely limited by low fluorescence quantum yields that originate from the aggregation of TTA-UC chromophores and also by severe back energy transfer from the acceptor singlet state to the singlet state of the triplet donor in the condensed state. In this work, to overcome these issues, we introduce a highly fluorescent singlet energy collector as the third component of donor-doped acceptor crystalline films, in which dual energy migration, i.e., triplet energy migration for TTA-UC and succeeding singlet energy migration for transferring energy to a collector, takes place. To demonstrate this scheme, a highly fluorescent singlet energy collector was added as the third component of donor-doped acceptor crystalline films. An anthracene-based acceptor containing alkyl chains and a carboxylic moiety is mixed with the triplet donor Pt(II) octaethylporphyrin (PtOEP) and the energy collector 2,5,8,11-tetra- tert-butylperylene (TTBP) in solution, and simple spin-coating of the mixed solution gives acceptor films of nanofibrous crystals homogeneously doped with PtOEP and TTBP. Interestingly, delocalized singlet excitons in acceptor crystals are found to diffuse effectively over the distance of ∼37 nm. Thanks to this high diffusivity, only 0.5 mol % of doped TTBP can harvest most of the singlet excitons, which successfully doubles the solid-state fluorescent quantum yield of acceptor/TTBP blend films to 76%. Furthermore, since the donor PtOEP and the collector TTBP are separately isolated in the nanofibrous acceptor crystals, the singlet back energy transfer from the collector to the donor is effectively avoided. Such efficient singlet energy collection and inhibited back energy transfer processes result in a large increase of UC efficiency up to 9.0%, offering rational design principles toward ultimately efficient solid-state upconverters.

Comments on the through-space singlet energy transfers and energy migration (exciton) in the light harvesting systems

Recent findings on the photophysical investigations of several cofacial bisporphyrin dyads for through space singlet and triplet energy transfers raised several serious questions about the mechanism of the energy transfers and energy migration in the light harvesting devices, notably LH II, in the heavily studied purple photosynthetic bacteria. The key issue is that for simple cofacial or slipped dyads with controlled geometry using rigid spacers or spacers with limited flexibilities, the fastest possible rates for singlet energy transfer for three examples are in the 10 × 10 9 s −1 (i.e. just in the 100 ps time scale) for donor–acceptor distances approaching 3.5–3.6 Å. The reported time scale for energy transfers between different bacteriochlorophylls, notably B800 ∗ → B850, is in the picosecond time scale despite the long Mg⋯Mg separation of ∼18 Å. Such a short rate drastically contrasts with the well accepted Förster theory. This article reviews the modern knowledge of the structure, bacteriochlorophyll a transition moments, and photophysical processes and dynamics in LH II, and compares these parameters with the recently investigated model bisporphyrin dyads build upon octa- etio-porphyrin chromophores and rigid and semi-rigid spacers. The recently discovered role of the rhodopin glucoside residue called carotenoid will be commented as the possible relay for energy transfer, including the possibility of uphill processes at room temperature. In this context, the concept of energy migration, called exciton, may also be affected by relays and uphill processes. Also, it is becoming more and more apparent that the presence of an irreversible electron transfer reaction at the reaction center, i.e. electron transfer from the special pair to the phyophytin macrocycle and so on, renders the rates for energy transfer and migration more rapid precluding all possibility of back transfers.

S 2- and S 1-states properties of zinc porphyrin polypeptides

S 2- and S 1-states properties of zinc porphyrin polypeptides were investigated using various fast spectroscopic techniques. The B band of zinc porphyrin polypeptides showed broadening and shift to the longer wavelength side with the polymerization degree, indicating the interaction between the porphyrin units. The lifetime of S 2-fluorescence became shorter with increasing the polymerization degree. This phenomenon can be explained by applying the energy gap law to the S 2–S 1 internal conversion. From the picosecond transient absorption measurements, singlet–singlet annihilation was confirmed, indicating the intrachain singlet energy migration in the polypeptides. The annihilation rate became faster with the polymerization degree because of the shorter distance between the zinc porphyrin units of longer polypeptides.

Singlet Energy Migration along an Alternating Block Copolymer of Oligothiophene and Oligosilylene in Solution

The singlet excited-state properties of the block copolymers of oligothiophene and oligosilylene in solution were investigated with several fast spectroscopic methods. Time-resolved fluorescence measurements at room temperature and in a glassy matrix revealed that the singlet excited states of the block copolymers are deactivated accompanying structural changes of the polymer. It became clear from the transient absorption spectroscopy that the absorption peak of the singlet excited state shifted to the longer wavelength side compared to that of the corresponding oligothiophenes because of the sigma-pi conjugation of the oligothiophene and oligosilylene. The intersystem crossing process generating the triplet excited state was also revealed by the transient absorption spectroscopy. Energy migration along the polymer chain was revealed by the fluorescence anisotropy measurements. The time constant for the energy migration became faster as the size of the oligothiophene in the polymer repeating unit became shorter. From comparison with the Förster theory, the energy migration process was attributed to an incoherent hopping mechanism.

Self-Assembled Fibrillar Networks through Highly Oriented Aggregates of Porphyrin and Pyrene Substituted by Dialkyl l-Glutamine in Organic Media

Microfibrous self-aggregation of chromophoric groups of porphyrin and/or pyrene substituted by didodecyl l-glutamic acid in organic media is confirmed by transmission electron microscopic (TEM) observation. Chromophoric probes of porphyrin and pyrene moieties enable evaluation of their assembling behavior photophysically through UV−vis, circular dichroism (CD), and fluorescence spectroscopic characterization. This spectroscopic characterization was able to compensate the lack of TEM observation for the aggregation even at a low concentration below the critical gel concentration. The temperature affects the salient features of the photophysics of porphyrin or pyrene in the microfibrous assemblies. Highly oriented network structures were formed at low temperature since the CD intensities of the porphyrin and pyrene systems increased with lowering the temperature. Fluorescence spectroscopic characterization confirmed the monomer excitation of porphyrin itself, and efficient excimer formation for the pyrene−pyrene charge transfer was detected at low temperature. In particular, we also obtained the preliminary results of fluorescence spectroscopic measurement on singlet−singlet energy migration from pyrene to porphyrin in the mixed assemblies for mimicry of the efficient energy transfer process of the photosynthetic antenna complex.

Intramolecular Excimer Formation of Poly(γ-(1-naphthylmethyl)-L-glutamate)

Poly(γ(1-naphthylmethyl)-L-glutamate) (PNLG) was synthesized and fractionated into samples with narrow molecular weight distributions. Absolute molecular weights of all the PNLGs used in this study were determined by time-of-flight matrix-assisted laser desorption ionization mass spectrometry. The circular dichroism measurements of PNLG showed that all the PNLGs with a degree of polymerization larger than 7 had a righ-handed helical structure in tetrahydrofuran (THF) solution. The effects of molecular weight on the photostationary and transient fluorescence of PNLG were examined in order to obtain information on excimer formation and singlet energy migration. Because the structure of a PNLG is rigid and regular, a one-dimensional random walk model can be used to describe singlet energy migration among side-chain naphthyl groups of PNLG. Consequently, the singlet energy migration coefficient of PNLG in THF was found to be higher than 1.6 x 10 - 5 cm 2 /s.

Vinyl monomers bearing chromophore moieties and their polymers. XI. Synthesis and photochemical behavior of carbazole-containing methacrylic monomers and their polymers

Three carbazole-containing methacrylic monomers, 2-(N-carbazolyl)ethyl methacrylate(CzEMA), 6-(N-carbazolyl)hexyl methacrylate(CzHMA), and 11-(N-carbazolyl)undecyl methacrylate (CzUMA), and their saturated model compounds, 2-(N-carbazolyl)ethyl isobutyrate, 6-(N-carbazolyl)hexyl isobutyrate, and 11-(N-carbazolyl)undecyl isobutyrate, were synthesized and polymerized. UV absorption spectra showed that there was either negligible or no interaction between the carbon–carbon double bond of the methacrylic group and the carbazolyl chromophore moiety in the ground state for these monomers. Fluorescence spectra of the monomers, their model compounds, and the polymers were recorded in the solvents with different polarities. CzEMA exhibited the fluorescence structural self-quenching effect (SSQE), but CzHMA and CzUMA did not. In addition, the SSQE of CzEMA depended strongly on the polarity of the solvents. That is, the stronger the polarity of a solvent was, the more obvious the SSQE was. Therefore, the SSQE of CzEMA mainly was caused by the intramolecular charge-transfer interaction between the excited electron-donating carbazolyl chromophore moiety and the electron-accepting carbon–carbon double bond of the methacrylic group. This was confirmed by the fluorescence-decay curves and the fluorescence lifetimes of the monomers, their model compounds, and the polymers. The monomers, their model compounds, and the polymers initiated the photopolymerization of methyl methacrylate (MMA) upon UV irradiation. CzEMA showed greater initiation ability than the other two monomers and their model compounds; this was ascribed to the photoinduced intramolecular charge-transfer interaction. The higher initiation efficiency of the homopolymers compared to that of the copolymers with MMA was interpreted as the result of singlet energy migration of the excited carbazolyl chromophores along the polymer chains. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 679–688, 2000

Vinyl monomers bearing chromophore moieties and their polymers. XI. Synthesis and photochemical behavior of carbazole‐containing methacrylic monomers and their polymers

Three carbazole-containing methacrylic monomers, 2-(N-carbazolyl)ethyl methacrylate(CzEMA), 6-(N-carbazolyl)hexyl methacrylate(CzHMA), and 11-(N-carbazolyl)undecyl methacrylate (CzUMA), and their saturated model compounds, 2-(N-carbazolyl)ethyl isobutyrate, 6-(N-carbazolyl)hexyl isobutyrate, and 11-(N-carbazolyl)undecyl isobutyrate, were synthesized and polymerized. UV absorption spectra showed that there was either negligible or no interaction between the carbon–carbon double bond of the methacrylic group and the carbazolyl chromophore moiety in the ground state for these monomers. Fluorescence spectra of the monomers, their model compounds, and the polymers were recorded in the solvents with different polarities. CzEMA exhibited the fluorescence structural self-quenching effect (SSQE), but CzHMA and CzUMA did not. In addition, the SSQE of CzEMA depended strongly on the polarity of the solvents. That is, the stronger the polarity of a solvent was, the more obvious the SSQE was. Therefore, the SSQE of CzEMA mainly was caused by the intramolecular charge-transfer interaction between the excited electron-donating carbazolyl chromophore moiety and the electron-accepting carbon–carbon double bond of the methacrylic group. This was confirmed by the fluorescence-decay curves and the fluorescence lifetimes of the monomers, their model compounds, and the polymers. The monomers, their model compounds, and the polymers initiated the photopolymerization of methyl methacrylate (MMA) upon UV irradiation. CzEMA showed greater initiation ability than the other two monomers and their model compounds; this was ascribed to the photoinduced intramolecular charge-transfer interaction. The higher initiation efficiency of the homopolymers compared to that of the copolymers with MMA was interpreted as the result of singlet energy migration of the excited carbazolyl chromophores along the polymer chains. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 679–688, 2000

Spatial Imaging of Singlet Energy Migration in Perylene Bis(phenethylimide) Thin Films

The migration length of singlet electronic energy excitations (excitons) in thin films of perylene bis(phenethylimide) (PPEI) has been measured by an imaging technique that incorporates near-field optical excitation and far-field fluorescence imaging. The observed energy migration lengths fall in the range of ≤50 to ∼500 nm depending on sample morphologies. Sample morphology was controlled by “solvent annealing” and characterized by fluorescence NSOM and scanning force microscopy. The potential role of sample morphology in singlet energy migration is discussed. Additionally, the results reported herein are compared with a previous determination of singlet energy migration in PPEI.

Singlet and Triplet Energy Transfer in Tetracyanobenzene-Biphenyl Crystals

Singlet and triplet excitonic energy migration in mixed-stack charge-transfer crystals of tetracyanobenzene-biphenyl (TCNB-B) doped with anthracene (10−4-10−2 M/M) have been studied within temperature range 5-300 K by using optical, spectroscopic and kinetic techniques. It has been found, that the motion of excitons is thermally activated with the activation energies of 450 ± 60 and 550 ± 60 cm−1 for the singlet and triplet excitons, respectively. The later value differs from the activation energy in undoped TCNB-B crystals (where it is 360 ± 60 cm−1) and suggests that doping with anthracene perturbs the host crystal lattice.

Fluorescence and Singlet Energy Migration in Conformationally Restrained Acrylate Polymers Bearing Pendant Chromophores

Fluorescence and singlet energy migration can be studied in chromophore-appended polyacrylates in which conformationally restricted ring structures in the polymer backbone prevent excimer formation. Specifically, two substituted α-methylene-γ-butyrolactones (1a bearing a naphthyl group and 1b bearing a phenanthryl group) and a naphthyl-substituted diethyl 2,6-dimethyleneheptanedioate (2) yield linear polymers with five-membered rings fixed perpendicular to the main chain (poly(1)) or cyclopolymers with six-membered rings in the main chain (poly(2)) upon radical or group transfer polymerization. 4-Methyl-4-(2-naphthyl)butyrolactone (3a), 4-methyl-4-(3-phenanthryl)butyrolactone (3b), and 1-(1-naphthylmethoxycarbonyl)-1-(ethoxycarbonyl)cyclohexane (4) were prepared as spectroscopic models for these polymers. No evidence for excimer emission could be seen in poly(1a) or poly(1b), but poly(2) exhibited weak excimer emission. Defects in the polymer structure of poly(2) correlate with observed sites for excimer formation. Steady-state fluorescence quenching experiments by CCl 4 demonstrate that singlet energy migration occurs intramolecularly along these polymer chains.

Migration of electronic excitation energy in the charge-transfer crystal of pyromellitic dianhydride-phenanthrene

Singlet and triplet exciton energy migration in the charge-transfer crystal of pyromellitic dianhydride-phenanthrene, undoped and doped with anthracene (10 −4−10 −2M/M), have been studied in the 5–300 K temperature range. At low temperatures singlet and triplet excitons are self-trapped. They become mobile with increasing temperature with activation energies of 380 ± 70 and 400 ± 80 cm −1 for singlet and triplet excitons, respectively. The motion of triplet excitons in the temperature range 50–100 K can be described by one-dimensional hopping within cages, limited by structural defects of the host crystal. Mobile singlet excitons are observed for temperatures above 100 K and their motion is well described by a time-independent excitation transfer rate constant.

Energy transfer in fluid solutions of poly(styrene), both labelled and unlabelled, and poly(2-vinylnaphthalene)

The question of singlet energy migration in excimer-forming synthetic polymers is readdressed in the light of new data, with improved temporal resolution, on poly(styrene) in methylene chloride and cyclopentane solutions, and poly(2-vinylnaphthalene) in benzene. In these selected samples, simple kinetics for the population of excimers were observed, and thus in these cases, it was not necessary to invoke facile singlet energy migration to explain the results. The concept of rapid rotational sampling could be utilized to explain the data.

Fluorescence and energy migration in the random copolymers of (1-naphthylmethyl)methacrylamide and sodium 2-(acrylamido)-2-methylpropanesulfonate

Fluorescence spectra of the copolymers in organic solution showed predominant monomer fluorescence of the naphtyl (Np) residues even when the Np content in the copolymers is as high as 58 mol%. In aqueous solutions the copolymers showed a strong preference for excimer emission. The excimer intensity increased with increasing Np content. The results imply that little or no self-quenching site exists and rapid singlet energy migration occurs solely to preformed excimer sites in the copolymers with a higher Np content in aqueous solutions.

Monte Carlo simulation of singlet energy migration and trapping in nonrandom chromophore distributions generated by photoreaction in glassy polymer matrixes

Les fonctions de transfert d'energie simulees sont utilisees pour construire des courbes de declin de fluorescence qui sont comparees avec les courbes experimentales obtenues precedemment avec un poly(methacrylate de methyle) vitreux contenant de l'acetate de 2-naphtyle

One-dimensional singlet energy migration in the columnar liquid crystal of a triphenylene derivative

The fluorescence properties of 2,3,6,7,10,11-hexa-n-hexyloxytriphenylene have been studied in solution and in its crystalline and columnar liquid-crystalline phases by steady-state and time-resolved spectroscopy. When an energy acceptor, perylene, is added to the organized phases, photosensitized fluorescence is observed. The triphenylene fluorescence decay curves obtained with the columnar phase are well described by a random walk of the excitation on a linear chain containing traps. The kinetic analysis permits the determination of the exciton hopping time (32 fs), the exciton diffusion coefficient (2.0 × 10–2 cm2 s–1) and the intrinsic trap molar fraction (10–3).

Efficient Singlet Energy Migration in Amorphous Copolymer Film Having No Excimer Forming Site: Poly[2‐(9‐carbazolyl)ethyl methacrylate‐co‐methyl methacrylate

AbstractEnergy Transfer / Fluorescence / PolymersSinglet energy migration in the solid state for the homopolymer of 2‐(9‐carbazolyl)ethyl methacrylate and the copolymers with methyl methacrylate was investigated using perylene as an acceptor molecule. In this system, the carbazole (Cz) chromophore is dispersed uniformly even in the homopolymer and a deep trap is not formed. The fluorescence spectra and their decays were measured for these samples and the energy migration was analyzed on the basis of three models, i.e., random hopping model, Smoluchowski diffusion model, and Yokota‐Tanimoto (Y‐T) Padé model. Among these models, the Y‐T Padé model gave the most reliable diffusion coefficient. The diffusion coefficient was obtained in a wide range of carbazole concentrations from 0.14 to 4.4 mol/l and we obtained the following conclusions.1. The energy diffusion coefficient D is proportional to [Cz]4/3 as predicted from the Förster mechanism even for the samples of Cz concentration higher than 1 mol/l.2. At a low temperature, the energy migration rate becomes slow owing to shallow traps. The activation energy in a low temperature region is about 4 kJ/mol.3. Fast energy migration (D = 2.0·10−4 cm2/s) was found for the polymers whose concentration of Cz‐chromophore is high, but do not form excimer.

Photon harvesting vinylphenanthrene-methacrylic acid polymers: singlet-state migration and trapping

Random and alternating copolymers of 9-vinylphenanthrene and methacrylic acid have been prepared. These polymers have been doped with low mole fractions of anthryl energy traps. The photophysics of these polymers with and without these traps have been studied as a function of phenathryl content in tetrahydrofuran (THF) and water. The fluorescence properties of the alternating polymers indicate an absence of excimer-forming sites in THF but not in water. Significant singlet energy migration has been implicated by fluorescence quenching methods. The quantum efficiency of anthracene sensitization has been found to be ca. 0.4 for the alternating polymer in THF and water. There is a very strong dependence of the photophysical properties of the random copolymers on pH, which is reflected in the energy-transfer efficiency.

Intramolecular excimer formation of diastereoisomeric model compounds of polystyrene in fluid solution: Their local molecular motion and photophysical properties

The dynamic process of intramolecular eximer formation in diastereoisomeric oligamers [dimers: meso ( m)- and racemic ( r)-4,6-diphenylnonane (PS2), trimers: mm-, mr- and rr-4, 6, 8-triphenylundecane (PS3), and tetramers: mmm-, mmr-, mrm-, rrm-, and rrr-4, 6, 8, 10-tetraphenyltridecane (PS4)], model compounds of polystyrene, was investigated by using a picosecond pulse radiolysis technique. Monomer fluorescence of all-racemic isomers ( r PS2, rr PS3, rrr PS4) and rrm PS4 decays single-exponentially, while that of other isomers decays dual-exponentially. Multicomponent fluorescence decay curves are supposed to be mainly induced by conformational changes. The results suggest that the excimer in oligostyrenes (or polystyrene) is formed mainly in meso diad. It is definitely proved that there exists singlet energy migration in styrene trimer and tetramer systems. The conformational change in PS3 and PS4 is concluded to occur by way of cooperative motions in backbone chains bond such as a crankshaft transition, not by way of independent rotation around each carbon-carbon bond of the backbone chain.

Fluorescence quenching and singlet energy migration in polystyrene and its copolymers

A study of fluorescence quenching of polystyrene and statistic styrene copolymers with methyl methacrylate, ethyl acrylate and acrylonitrile by carbon tetrachloride in 1,2-dichloroethane has been carried out. The results of the study have been interpreted on the basis of the modified Stern-Volmer relation, which takes into account the fact that isolated styrenes do not participate in the process of energy migration. The coefficients of energy migration were determined for the systems studied. It was shown that the intramolecular excimers restrict the process of energy migration in polystyrene and its copolymers. The results obtained are discussed.