Singlet Energy Transfer Research Articles

Supramolecular-Surface Photochemistry: Supramolecular Assembly Organized on a Clay Surface Facilitates Energy Transfer between an Encapsulated Donor and a Free Acceptor

We report the occurrence of efficient energy transfer reaction in a novel host–guest assembly composed of an anionic clay nanosheet, cationic porphyrin, and neutral aromatic molecule encapsulated within a cationic organic cavitand. The supramolecular assembly was prepared by the coadsorption of tetracationic Zn–porphyrin (acceptor) and 2-acetylanthracene (donor) enclosed within cationic organic cavitand (octaamine in its protonated form) on anionic clay nanosheets. In this arrangement under the interguest distance of 2.4 nm, almost 100% efficiency of singlet–singlet energy transfer was achieved. Detailed time-resolved fluorescence measurements revealed that the energy transfer rate constant could be attributed to a single component (1.9 × 109 s–1). This strongly suggests that the adsorption distribution of porphyrin and cavitand is rather uniform, not segregated. This is a progress from our previous study that involves energy transfer between two encapsulated neutral molecules. The use of Zn–porphyrin as an energy acceptor in this study enables to connect this energy transfer system to charge separation processes in the same manner as natural photosynthetic systems do; moreover, the efficiency of energy transfer reaction improved to almost 100% from 85% in the previous system between two cavitands.

Slow and Fast Singlet Energy Transfers in BODIPY-gallium(III)corrole Dyads Linked by Flexible Chains

Red (no styryl), green (monostyryl), and blue (distyryl) BODIPY-gallium(III) (BODIPY = boron-dipyrromethene) corrole dyads have been prepared in high yields using click chemistry, and their photophysical properties are reported. An original and efficient control of the direction of the singlet energy transfers is reported, going either from BODIPY to the gallium-corrole units or from gallium-corroles to BODIPY, depending upon the nature of the substitution on BODIPY. In one case (green), both directions are possible. The mechanism for the energy transfers is interpreted by means of through-space Förster resonance energy transfer (FRET).

Triplet harvesting in poly(9‐vinylcarbazole) and poly(9‐(2,3‐epoxypropyl)carbazole) doped with CdSe/ZnS quantum dots encapsulated with 16‐(N‐carbazolyl) hexadecanoic acid ligands

ABSTRACTSemiconductor quantum dots (QDs) can be used as alternative for transition metal complexes to harvest the nonemissive triplet excitons in organic light‐emitting diodes (OLEDs). In search for a QD‐based OLED material generating blue emission, poly(9‐vinylcarbazole) (PVK) and poly(9‐(2,3‐epoxypropyl) carbazole) (PEPK) are chosen as host for blue‐emitting CdSe/ZnS core/shell QDs. The QDs are encapsulated with 16‐(N‐carbazolyl) hexadecanoic acid (C16), a ligand terminated by a carbazole moiety. As alternative for PVK, PEPK, where the lower molecular weight and less extensive excimer formation could promise a better film formation and more extensive exciton hopping, is explored. The efficiencies of singlet ( ) and triplet ( ) energy transfer to the C16 capped QDs are estimated by combining stationary photoluminescence spectra and fluorescence decays of pristine polymer films with those of polymer films doped with the QDs. At a loading of 30 wt % of the QDs, increases from 12 ± 1% in PVK to 41 ± 2% in PEPK while increases from 37 ± 22% in PVK to 72 ± 48% in PEPK. The investigation of the film morphology by atomic force microscopy confirms that the main factor limiting the triplet transfer efficiency in the PVK matrix is the clustering of the C16 capped QDs. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014, 52, 539–551

Origin of the temperature dependence of the rate of singlet energy transfer in a three-component truxene-bridged dyads

We report a truxene-based dyad built upon one donor (tri-meso-phenylzinc(II)porphyrin) and two acceptors (octa-β-alkylporphyrin free base) in which the donor exhibits free rotation around a Ctruxene-Cmesosingle bond at 298 K in fluid solution but not at 77 K in a glass matrix, whereas the acceptors have very limited motion as they are blocked by β-methyl groups. This case is interesting because all the structural and spectroscopic parameters affecting the rate for singlet energy transfer according to a Förster Resonance Energy Transfer are only weakly temperature dependent, leaving only the Dexter mechanism explaining the larger variation in rate of energy transfers with the temperature hence providing a circumstantial evidence for a dual mechanism (Föster and Dexter) in truxene-based dyads (or polyads) in the S1excited states.

Light-Harvesting Polymers: Ultrafast Energy Transfer in Polystyrene-Based Arrays of π-Conjugated Chromophores

Energy transfer along a nonconjugated polymer chain is studied with a polystyrene-based copolymer of oligo(phenylene-ethynylene) (OPE) donor and thiophene-benzothiadiazole (TBT) acceptor pendants. The graft copolymers are prepared from reversible addition-fragmentation transfer polymerization (RAFT) and copper(I)-catalyzed azide-alkyne "click" reaction. The singlet energy transfer from donor to accept is studied via fluorescence emission and ultrafast transient absorption spectroscopy. Near unity quenching of the OPE excited state by the TBT moiety occurs on multiple time scales (2-50 ps) dependent on where the initial exciton is formed on the polymer.

Thiaporphyrins: from building blocks to multiporphyrin arrays

21-Thiaporphyrins and 21,23-dithiaporphyrins resulting from the replacement of one and two pyrrole rings respectively, of tetrapyrrolic porphyrins possess singlet state energy levels which are lower than porphyrins and metalloporphyrins. This specific feature of thiaporphyrins is advantageous when thiaporphyrins are connected either covalently or non-covalently to porphyrins and metalloporphyrins for unidirectional flow of energy transfer/electron transfer. In recent times, the synthetic routes were developed for the synthesis of functionalized thiaporphyrin building blocks which were not accessible earlier. The functionalized thiaporphyrins have been used to synthesize several unsymmetrical thiaporphyrin-based arrays containing two or more different types of porphyrin subunits whose singlet state energy levels are arranged in a favourable way for the energy transfer/electron transfer. In this review, we describe different synthetic approaches being employed for the synthesis of functionalized thiaporphyrin building blocks and their use in the construction of several covalently and non-covalently linked thiaporphyrin-based multiporphyrin arrays ranging from dyads to octads. The photophysical studies are also described to show the possibility of singlet–singlet energy transfer from one porphyrin unit to another in some selected thiaporphyrin-based multiporphyrin arrays.

A Selective Metal‐Mediated Approach for the Efficient Self‐Assembling of Multi‐Component Photoactive Systems

AbstractWe report on the efficient use of different coordinative affinities to quantitatively construct a three‐component multichromophore system containing one free‐base porphyrin, one aluminium(III) monopyridylporphyrin, and one ruthenium(II) tetraphenylporphyrin in a single step. The AlIII monopyridylporphyrin plays the pivotal role in directing the correct self‐assembly process, and it behaves as the antenna unit for the photoinduced processes of interest. A two‐component system, containing the same free‐base porphyrin and one aluminium(III) tetraphenylporphyrin, was prepared for comparative studies. Both systems are remarkably stable in chlorinated solutions. Their photophysical behaviour was studied in detail by stationary and time‐resolved emission techniques. In the two‐component system, fast and efficient singlet energy transfer from the Al–porphyrin to the free‐base porphyrin is observed; in the three‐component system, upon excitation of the Al–porphyrin, a fast (<250 ps) electron transfer process from the Ru–porphyrin to the aluminium–porphyrin is observed.

Highly Efficient Blue Organic Light-Emitting Diodes Based on Intermolecular Triplet–Singlet Energy Transfer

Diphenilamino-substituted carbazoles were used as guest compounds for the preparation of highly efficient blue organic light-emitting diodes based on the phenomenon of delayed fluorescence. It was shown that the spectra of the delayed fluorescence of host–guest systems are identical to those of the prompt fluorescence and in general coincide with the photoluminescence spectra of the guest films. The congruence of the prompt and delayed fluorescence spectra is explained by the effective intermolecular triplet–singlet (T → S) energy transfer from the excited T states of the host to the S states of the guest molecules. High external electroluminescence efficiency of the fabricated electroluminescent devices, reaching 17%, is comparable to that achieved in phosphorescence-based organic light-emitting diodes.

Evidence for reverse pathways and equilibrium in singlet energy transfers between an artificial special pair and an antenna

A dyad, 1, built on an artificial special pair (bis(meso-nonyl)zinc(II)porphyrin), [Zn2], a spacer (biphenylene), a bridge (1,4-benzene), and an antenna (di-meso-(3,5-di(t-butyl)phenyl)porphyrin free base), FB, is prepared by Suzuki coupling and is analyzed by absorption and steady state, and time-resolved emission spectroscopy at 298 and 77 K. Using bases from the Förster theory, evidence for two pathways for S 1 energy transfer, FB* → [Zn2], and [Zn2]* → FB, along with their respective rates, k ET ( S 1)1 and k ET ( S 1)-1, are extracted from the comparison of the fluorescence decays monitored at the emission maximum. At 77 K, the unquenched (1.79 ([Zn2]) and 10.6 ns (FB)) and quenched components (<100 ps; i.e. k ET ( S 1) > 10 (ns)-1), are observed, hence, demonstrating the bidirectional paths with no back energy transfer. A 298 K, only two components are detected (0.44 ([Zn2]) and 2.64 ns (FB)) and the resulting reduced τFs indicates back energy transfer, therefore cycling and equilibrium. Their global rates are 0.31 and 1.8 (ns)-1 for k ET ( S 1)1 and k ET ( S 1)-1 at 298 K. This large temperature dependence on k ET ( S 1) is fully consistent with the participation of thermal activation. Finally, DFT calculations (B3LYP) were used to illustrate a clear correlation between the relative k ET ( S 1) s and the amplitude of the MO couplings between the artificial special pair and the antenna.

Highly Efficient Singlet–Singlet Energy Transfer in Light‐Harvesting [60,70]Fullerene–4‐Amino‐1,8‐naphthalimide Dyads

New C60 and C70 fullerene dyads formed with 4-amino-1,8-naphthalimide chromophores have been prepared by the Bingel cyclopropanation reaction. The resulting monoadducts were investigated with respect to their fluorescence properties (quantum yields and lifetimes) to unravel the role of the charge-transfer naphthalimide chromophore as a light-absorbing antenna and excited-singlet-state sensitizer of fullerenes. The underlying intramolecular singlet-singlet energy transfer (EnT) process was fully characterized and found to proceed quantitatively (Φ(EnT)≈1) for all dyads. Thus, these conjugates are of considerable interest for applications in which fullerene excited states have to be created and photonic energy loss should be minimized. In polar solvents (tetrahydrofuran and benzonitrile), fluorescence quenching of the fullerene by electron transfer from the ground-state aminonaphthalimide was postulated as an additional path.

Intramolecular RET Enhanced Visible Light-Absorbing Bodipy Organic Triplet Photosensitizers and Application in Photooxidation and Triplet–Triplet Annihilation Upconversion

Resonance energy transfer (RET) was used for the first time to enhance the visible light absorption of triplet photosensitizers. The intramolecular energy donor (boron-dipyrromethene, Bodipy) and acceptor (iodo-Bodipy) show different absorption bands in visible region, thus the visible absorption was enhanced as compared to the monochromophore triplet photosensitizers (e.g., iodo-Bodipy). Fluorescence quenching and excitation spectra indicate that the singlet energy transfer is efficient for the dyad triplet photosensitizers. Nanosecond time-resolved transient absorption spectroscopy has confirmed that the triplet excited states of the dyads are distributed on both the energy donor and acceptor, which is the result of forward singlet energy transfer from the energy donor to the energy acceptor and in turn the backward triplet energy transfer. This 'ping-pong' energy transfer was never reported for organic molecular arrays, and so it is useful to study the energy level of organic chromophores. The triplet photosensitizers were used for singlet oxygen ((1)O2) mediated photooxidation of 1,5-dihydroxylnaphthalene to produce juglone. The visible light absorption of the new visible light-absorbing triplet photosensitizers are higher than the conventional monochromophore based triplet photosensitizers, as a result, the (1)O2 photosensitizing ability is improved with the new triplet photosensitizers. Triplet-triplet annihilation upconversion with these triplet photosensitizers was also studied. Our results are useful to design the triplet photosensitizers showing strong visible light absorbance and for their applications in photocatalysis and photodynamic therapy.

Exciplex‐Forming Co‐host for Organic Light‐Emitting Diodes with Ultimate Efficiency

AbstractPhosphorescent organic light‐emitting diodes (OLEDs) with ultimate efficiency in terms of the external quantum efficiency (EQE), driving voltage, and efficiency roll‐off are reported, making use of an exciplex‐forming co‐host. This exciplex‐forming co‐host system enables efficient singlet and triplet energy transfers from the host exciplex to the phosphorescent dopant because the singlet and triplet energies of the exciplex are almost identical. In addition, the system has low probability of direct trapping of charges at the dopant molecules and no charge‐injection barrier from the charge‐transport layers to the emitting layer. By combining all these factors, the OLEDs achieve a low turn‐on voltage of 2.4 V, a very high EQE of 29.1% and a very high power efficiency of 124 lm W−1. In addition, the OLEDs achieve an extremely low efficiency roll‐off. The EQE of the optimized OLED is maintained at more than 27.8%, up to 10 000 cd m−2.

Stilbene Molecular Probes as Potential Materials for Bioengineering: Real Time Analysis of Antioxidants and Nitric Oxide, Immunoassay in Solution and Biomembranes Fluidity

A series of fluorescent methods of analysis and investigation of system based on the use of stilbenes and potentially important in biochemistry, biophysics, biotechnology, and biomedicine were proposed and developed. In these methods, two new types of stilbene molecular probes have been used: (i) fluorescent photochrome molecules and (ii) super molecules containing fluorescent and fluorescent quenching segments. These methods utilize the following photochemical and photophysical phenomena: the fluorescence and phosphorescence quenching, photochrome photoisomerization, and triplet–triplet and singlet–singlet energy transfer. The fluorescence properties of the new probes were intensively exploited as the basis of several methodologies that include a real-time analysis of nitric oxide, immunoassay in solution, investigation of molecular dynamics of biomembranes in a wide range characteristic times, and characterization of sensors for antibodies. These techniques may be adapted to fibro-optic sensoring.

ChemInform Abstract: Optical Switches with Biplanemers Obtained by Intramolecular Photocycloaddition Reactions of Tethered Arenes

AbstractReview: 68 refs.

Artificial Photosynthetic Reaction Center with a Coumarin-Based Antenna System

In photosynthesis, sunlight is absorbed mainly by antenna chromophores that transfer singlet excitation energy to reaction centers for conversion to useful electrochemical energy. Antennas may likewise be useful in artificial photosynthetic systems that use sunlight to make fuels or electricity. Here, we report the synthesis and spectroscopic properties of a molecular hexad comprising two porphyrin moieties and four coumarin antenna chromophores, all organized by a central hexaphenylbenzene core. Light absorbed by any of the coumarins is transferred to a porphyrin on the 1-10 ps time scale, depending on the site of initial excitation. The quantum yield of singlet energy transfer is 1.0. The energy transfer rate constants are consistent with transfer by the Förster dipole-dipole mechanism. A pyridyl-bearing fullerene moiety self-assembles to the form of the hexad containing zinc porphyrins to yield an antenna-reaction center complex. In the resulting heptad, energy transfer to the porphyrins is followed by photoinduced electron transfer to the fullerene with a time constant of 3 ps. The resulting P(•+)-C60(•-) charge-separated state is formed with an overall quantum yield of 1.0 and decays with a time constant of 230 ps in 1,2-difluorobenzene as the solvent.

Photo-induced energy transfer in ruthenium-centred polymers prepared by a RAFT approach

Triplet and singlet excitation energy transfer processes are investigated in two novel di- and hexa-functionalised ruthenium centred polymers prepared by RAFT methods. The polymers consist of a ruthenium trisbipyridyl ligand complex (Ru(bpy)3) core and pendant polymer ‘arms’ of 9,10-diphenylanthracene (DPA). Steady state and transient emission and absorption experiments are used to investigate intramolecular energy transfer processes following photoexcitation of Ru(bpy)3 and DPA. Triplet energy transfer from the Ru(bpy)3 core to pendant DPA chromophores in the polymer arms is found to be more efficient than the reverse singlet energy transfer process, attributed in part to the long triplet lifetime of Ru(bpy)3 and the polymer structure. A consequence is that intra-polymer chain triplet energy transfer from Ru(bpy)3 to DPA chromophores followed by triplet–triplet annihilation and delayed fluorescence from the DPA can be observed.

Efficient Singlet–Singlet Energy Transfer in a Novel Host–Guest Assembly Composed of an Organic Cavitand, Aromatic Molecules, and a Clay Nanosheet

A supramolecular host-guest assembly composed of a cationic organic cavitand (host), neutral aromatic molecules (guests), and an anionic clay nanosheet has been prepared and demonstrated that in this arrangement efficient singlet-singlet energy transfer could take place. The novelty of this system is the use of a cationic organic cavitand that enabled neutral organic molecules to be placed on an anionic saponite nanosheet. Efficient singlet-singlet energy transfer between neutral pyrene and 2-acetylanthracene enclosed within a cationic organic cavitand (octa amine) arranged on a saponite nanosheet was demonstrated through steady-state and time-resolved emission studies. The high efficiency was realized from the suppression of aggregation, segregation, and self-fluorescence quenching. We believe that the studies presented here using a novel supramolecular assembly have expanded the types of molecules that could serve as candidates for efficient energy-transfer systems, such as in an artificial light-harvesting system.

Time-Resolved EPR of Photoinduced Excited States in a Semiconducting Polymer/PCBM Blend

We studied by means of electron spin resonance (EPR) techniques the photoinduced processes in a poly[(9,9-bis(octyl)fluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol-4,8-diyl) (F8BT) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61 (PCBM) blend. Using light-induced EPR (LEPR), we observed the generation of charge-separated states in the blend upon visible light excitation, and we detected the formation of the PCBM excited triplet state by using time-resolved EPR (TREPR) with microsecond time resolution. From spectral simulation of the TREPR spectrum we were able to identify a double pathway for the PCBM triplet generation: the normal intersystem crossing from PCBM excited singlet state (either directly produced by light absorption or populated by singlet–singlet energy transfer from the polymer) and a generation pathway via polaron pairs recombination. The experimental evidence of this process allows to assign the relative order of the energies of the polaron pairs and the triplet states and provides a mor...

Sequential Photoinduced Energy and Electron Transfer Directed Improved Performance of the Supramolecular Solar Cell of a Zinc Porphyrin–Zinc Phthalocyanine Conjugate Modified TiO2Surface

Improved performance of a photosynthetic antenna–reaction center mimicking supramolecular solar cell is demonstrated. Toward this, porphyrin–phthalocyanine conjugates connected by amide linkers, as wide-band capturing solar energy harvesting materials, have been newly synthesized and characterized. Efficient singlet–singlet energy transfer from the zinc or free-base porphyrin to phthalocyanine is evidenced from steady-state emission and transient absorption studies in nonpolar and polar solvents. Further, the dyad was immobilized via axial coordination of zinc porphyrin of the dyad on semiconducting TiO2 surface modified with axial coordinating ligand functionality, phenylimidazole. Photoelectrochemical studies revealed improved performance of this cell compared to either zinc porphyrin or zinc phthalocyanine only modified electrodes under similar experimental conditions. Transient absorption studies performed on the dyad immobilized on glass/TiO2 surface suggested that upon excitation of the axially coor...

Optical switches with biplanemers obtained by intramolecular photocycloaddition reactions of tethered arenes

The dimerization of anthracene by a [4π + 4π] cycloaddition is one of the oldest and best known reactions in photochemistry. In the series of tethered bichromophoric arenes, this reaction type could be extended to anthracene-naphthalene, naphthalene-naphthalene and recently even to anthracene-benzene and naphthalene-benzene systems. Cyclophanes, which can be regarded as twofold or multiple tethered systems, are not discussed here. The cycloisomerizations are performed by irradiation at the long-wavelength absorption (λ > 270 nm), whereas shorter wavelengths (λ < 270 nm) lead to cycloreversions, which can be also achieved by a thermal route. The systems represent therefore a P- and T-type photochromism, which can be used for optical or chiroptical switches. An acceleration of the switch is possible by a singlet energy transfer (light harvesting antenna effect) in dendritic compounds. In the past 5 to 10 years many applications of these switches were studied in the context of photonic devices, sensor techniques, lithographic processes, imaging techniques, data processing and data storage.