Efficient Singlet Energy Transfer Research Articles

Phase Control and Singlet Energy Transfer Enabled by Trimethylamine Modified Boron Dipyrromethene for Stable CsPbBr3 Quantum Wells

AbstractThe phase distribution and organic spacer cations play pivotal roles in determining the emission performance and stability of perovskite quantum wells (QWs). Here, we propose a universal molecular regulation strategy to tailor phase distribution and enhance the stability of CsPbBr3 QWs. The capability of sterically hindered ligands with formidable surface binding groups is underscored in directing CsPbBr3 growth and refining phase distribution. With trimethylamine modified boron dipyrromethene (BDP‐TMA) ligand as a representative, the BDP‐TMA driven can precisely control phase distribution and passivate defects of CsPbBr3. Notably, BDP‐TMA acts as a co‐spacer organic entity in obtained BDP‐TMA‐CsPbBr3, facilitating efficient singlet energy transfer and tailoring the luminescence to produce a distinctive bluish‐white emission. The BDP‐TMA‐CsPbBr3 demonstrates significant phase stability under water exposure, light irradiation, and moderate temperature. Interestingly, BDP‐TMA‐CsPbBr3 exhibits the thermally‐induced dynamic fluorescence control at elevated temperatures, which can be achieved feasible for advanced information encryption. This discovery paves the way for the exploration of perovskite QWs in applications like temperature sensing, anti‐counterfeiting, and other advanced optical smart technologies.

Phase Control and Singlet Energy Transfer Enabled by Trimethylamine Modified Boron Dipyrromethene for Stable CsPbBr3 Quantum Wells.

The phase distribution and organic spacer cations play pivotal roles in determining the emission performance and stability of perovskite quantum wells (QWs). Here, we propose a universal molecular regulation strategy to tailor phase distribution and enhance the stability of CsPbBr3 QWs. The capability of sterically hindered ligands with formidable surface binding groups is underscored in directing CsPbBr3 growth and refining phase distribution. With trimethylamine modified boron dipyrromethene (BDP-TMA) ligand as a representative, the BDP-TMA driven can precisely control phase distribution and passivate defects of CsPbBr3 . Notably, BDP-TMA acts as a co-spacer organic entity in obtained BDP-TMA-CsPbBr3 , facilitating efficient singlet energy transfer and tailoring the luminescence to produce a distinctive bluish-white emission. The BDP-TMA-CsPbBr3 demonstrates significant phase stability under water exposure, light irradiation, and moderate temperature. Interestingly, BDP-TMA-CsPbBr3 exhibits the thermally-induced dynamic fluorescence control at elevated temperatures, which can be achieved feasible for advanced information encryption. This discovery paves the way for the exploration of perovskite QWs in applications like temperature sensing, anti-counterfeiting, and other advanced optical smart technologies.

Photochromic Reaction by Red Light via Triplet Fusion Upconversion

Red or near-infrared (NIR) light responsive molecules have received much attention for biological and material applications because potentially harmful UV light for materials and cells is not required for the photochemical reactions. Although some molecular designs for photochromic molecules to increase the photosensitivity to red or NIR light have been reported, the strategies are limited to the extension of π-conjugation length and the utilization of charge transfer transition or energy and electron transfers. Triplet fusion is an attractive tool to cause chemical reactions by converting low-energy excitation light to high-energy upconversion light. However, the efficient use of the high energy of upconversion light is difficult because almost all reported triplet fusion systems rely on reabsorption of upconversion light. Here, we demonstrated red-light-driven photochromism via the triplet fusion of a phenoxyl-imidazolyl radical complex, Pery-RPIC, that has a covalently bonded perylene as an annihilator unit. The femtosecond time-resolved absorption and fluorescence spectroscopy revealed that this photochromic reaction proceeds by the highly efficient singlet energy transfer from the annihilator unit to the photochromic unit. This strategy can be applied not only to the development of visible and NIR light responsive photochromic system but also to various photochemical reactions.

A room temperature phosphorescence encoding [2]rotaxane molecular shuttle.

A novel bistable molecular shuttle, composed of a Pt(ii) porphyrin-containing dibenzo[24]crown-8 macrocycle threaded onto two different recognition sites (secondary dialkylammonium (NH2+) and 4,4'-bipyridinium (Bpym2+) units), and the anthracene (Anth) moiety as one terminal stopper, was synthesized by click chemistry. Its acid-base switchable shuttling could be addressed by both the room temperature phosphorescence (RTP) emission signals of the Pt(ii) porphyrin moiety and the fluorescence emission of the Anth unit, as well as their lifetime changes. When the macrocycle was switched to be located on the NH2+ site close to Anth, the Pt(ii) porphyrin moiety exhibited strong RTP emission, excited in the Anth band at 370 nm. This was due to the distance-dependent efficient singlet energy transfer between the Anth unit and the porphyrin moiety, followed by intersystem crossing from a singlet to a triplet state in Pt(ii) porphyrin, while its RTP emission dramatically decreased when located on the Bpym2+ site far from the Anth unit. When excited in the porphyrin band at 402 nm, the RTP emission lifetimes changed obviously. This is the first rotaxane-type molecular shuttle whose shuttling has been encoded by RTP signals.

A supramolecular tetrad featuring covalently linked ferrocene-zinc porphyrin-BODIPY coordinated to fullerene: a charge stabilizing, photosynthetic antenna-reaction center mimic.

A novel photosynthetic-antenna-reaction-center model compound, comprised of BF2 -chelated dipyrromethene (BODIPY) as an energy-harvesting antenna, zinc porphyrin (ZnP) as the primary electron donor, ferrocene (Fc) as a hole-shifting agent, and phenylimidazole-functionalized fulleropyrrolidine (C60 Im) as an electron acceptor, has been synthesized and characterized. Optical absorption and emission, computational structure optimization, and cyclic voltammetry studies were systematically performed to establish the role of each entity in the multistep photochemical reactions. The energy-level diagram established from optical and redox data helped identifying different photochemical events. Selective excitation of BODIPY resulted in efficient singlet energy transfer to the ZnP entity. Ultrafast electron transfer from the (1) ZnP* (formed either as a result of singlet-singlet energy transfer or direct excitation) or (1) C60 * of the coordinated fullerene resulting into the formation of the Fc-(C60 (.) (-) Im:ZnP(.) (+) )-BODIPY radical ion pair was witnessed by femtosecond transient absorption studies. Subsequent hole migration to the ferrocene entity resulted in the Fc(+) -(C60 (.) (+) Im:ZnP)-BODIPY radical ion pair that persisted for 7-15 μs, depending upon the solvent conditions and contributions from the triplet excited states of ZnP and ImC60 , as revealed by the nanosecond transient spectral studies. Better utilization of light energy in generating the long-lived charge-separated state with the help of the present "antenna-reaction-center" model system has been successfully demonstrated.

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.

Shape-persistent poly-porphyrins assembled by a central truxene: synthesis, structure, and singlet energy transfer behaviors

Four dyad systems composed of a central truxene and either one or three β-substituted zinc(II) porphyrins (ZnP: TruZnP (7) and TruTriZnP (9)) or free-bases (H2P: TruP (6) and TruTriP (8)) have been prepared. The presence of β-methyl groups minimizes π-conjugation through the quasi right angle made by the porphyrin and the truxene planes, and renders these dyads relatively rigid. The position of the absorption and emission 0–0 peaks confirms the role of the truxene and porphyrin as the energy donor and acceptor, respectively. Selective excitation of the truxene results in an efficient singlet energy transfer (S1 ET) from the truxene to the porphyrin unit. The rates for S1 ET (k ET ) are extracted from the change in the fluorescence lifetime of truxene in the presence and absence of the acceptor, and are temperature independent, (TruP (6), TruTriP (8), TruZnP (7) and TruTriZnP (9) are 5.0, 1.4, 1.0 and 1.4 at 298 K and 5.9, 1.3, 2.6, and 0.86 (ns)-1 at 77 K, respectively), consistent with their relative rigidity. These k ET 's are similar to other related but more flexible systems reported by one of us (Inorg. Chem.2011, 50, 11493–11505). The k ET 's time scale was assumed, based on modeling, to be related with hindered rotations about the truxene-porphyrin C–C bonds due to steric hexyl–hexyl interactions. This work confirms this earlier conclusion was correct.

Probing the Electronic State of a Single Coronene Molecule by the Emission from Proximate Fluorophores

We measured electronic transitions of the 2D graphene-type molecule hexa-peri-hexabenzocoronene (HBC) at the single-molecule level. The large intersystem crossing rate and long triplet state lifetime in the range of seconds are prohibitive for direct single-molecule observation. By covalently coupling fluorescent acceptor molecules (perylenecarboximide, PMI) to HBC, efficient singlet energy transfer gives rise to strong PMI fluorescence. Confocal single-molecule fluorescence microscopy with two excitation colours matching the HBC and PMI transition frequencies, respectively, was conducted. Single HBC-6PMI molecules were readily observed via the PMI emission. It was found that after selective excitation of the HBC the PMI emission is interrupted by dark intervals whose length of several seconds is in agreement with the triplet state lifetime of HBC. Accordingly, the presence/absence of PMI emission permits to read out the spin state of a single HBC molecule. Moreover, due to spectral overlap, the HBC triplet state acts as an energy acceptor for PMI in the excited singlet state, thus leading to efficient singlet-triplet annihilation (STA) during its lifetime. Hence, intersystem crossing into the HBC triplet state serves as a collective fluorescence switch for individual multichromophores.

Molecular design of efficient white‐light‐emitting fluorene‐based copolymers by mixing singlet and triplet emission

AbstractA new strategy to realize efficient white‐light emission from a binary fluorene‐based copolymer (PF‐Phq) with the fluorene segment as a blue emitter and the iridium complex, 9‐iridium(III)bis(2‐(2‐phenyl‐quinoline‐N,C3′)(11,13‐tetradecanedionate))‐3,6‐carbazole (Phq), as a red emitter has been proposed and demonstrated. The photo‐ and electroluminescence properties of the PF‐Phq copolymers were investigated. White‐light emission with two bands of blue and red was achieved from the binary copolymers. The efficiency increased with increasing concentration of iridium complex, which resulted from its efficient phosphorescence emission and the weak phosphorescent quenching due to its lower triplet energy level than that of polyfluorene. In comparison with the binary copolymer, the efficiency and color purity of the ternary copolymers (PF‐Phq‐BT) were improved by introducing fluorescent green benzothiadiazole (BT) unit into polyfluorene backbone. This was ascribed to the exciton confinement of the benzothiadiazole unit, which allowed efficient singlet energy transfer from fluorene segment to BT unit and avoided the triplet quenching resulted from the higher triplet energy levels of phosphorescent green emitters than that of polyfluorene. The phosphorescence quenching is a key factor in the design of white light‐emitting polyfluorene with triplet emitter. It is shown that using singlet green and triplet red emitters is an efficient approach to reduce and even avoid the phosphorescence quenching in the fluorene‐based copolymers. The strategy to incorporate singlet green emitter to polyfluorene backbone and to attach triplet red species to the side chain is promising for white polymer light‐emitting diodes. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 453–463, 2008

Intramolecular Triplet Energy Transfer via Higher Triplet Excited State during Stepwise Two-Color Two-Laser Irradiation

We studied the energy transfer processes in the molecular array consisting of pyrene (Py), biphenyl (Ph2), and bisphthalimidethiophene (ImT), (Py-Ph2)2-ImT, during two-color two-laser flash photolysis (2-LFP). The first laser irradiation predominantly generates ImT in the lowest triplet excited state (ImT(T1)) because of the efficient singlet energy transfer from Py in the lowest singlet excited state to ImT and, then, intersystem crossing of ImT. ImT(T1) was excited to the higher triplet excited state (Tn) with the second laser irradiation. Then, the triplet energy was rapidly transferred to Py via a two-step triplet energy transfer (TET) process through Ph2. The efficient generation of Py(T1) was suggested from the nanosecond-picosecond 2-LFP. The back-TET from Py(T1) to ImT was observed for several tens of microseconds after the second laser irradiation. The estimated intramolecular TET rate from Py(T1) to ImT was as slow as 3.1 x 104 s-1. Hence, long-lived Py(T1) was selectively and efficiently produced during the 2-LFP.

Geometry and Solvent-Polarity Dependences of the Relaxation Dynamics of the Singlet Excited State of Center-to-Edge Phosphorus(V) Porphyrin Heterodimers

Para- and meta-isomers of center-to-edge phosphorus(V) porphyrin heterodimers (p- and m-Pm-PCl2) composed of a phosphorus(V) tetraphenylporphyrin (P) and a phosphorus(V) tetrakis(4-methoxyphenyl)porphyrin (Pm) were synthesized to investigate the geometry and solvent-polarity dependences of the relaxation dynamics of the lowest singlet excited state (S1) of the porphyrin dimer. The geometrical difference between m- and p-Pm-PCl2 in solution was confirmed by 1H NMR on the basis of the porphyrin ring current model. By the photoexcitation of the dimers, only the fluorescence from 1Pm*-P was observed in both dimers because of the efficient singlet energy transfer from the higher energy 1P* to the lower energy 1Pm*. However, the 1Pm*-P fluorescence had features that differ from the typical fluorescence from monomeric 1Pm*, especially for the solvent-polarity dependence of the quantum yields. Furthermore, all the fluorescence decay curves of 1Pm*-P were double-exponential. The special fluorescence features of 1Pm*-P are due to the equilibrium between 1Pm*-P and the charge-transfer (CT) excited state of the dimer ((Pm-P)CT). By the excited-state equilibrium, 1Pm*-P is efficiently quenched through (Pm-P)CT in highly polar solvents. The contribution of (Pm-P)CT to the decay process of 1Pm*-P is more remarkable in m-Pm-PCl2 than in p-Pm-PCl2, depending on the interaction between the two porphyrin -systems in the heterodimer.

The 2-Norbornyl Cation is not a Single Minimum Energy System

Simple and dual variable linear regression equations are presented which can estimate the sensitivity constants ρ for the solvolysis reactions of hundreds of bicyclic and tricyclic compounds. These are the first QSARs which utilize dihedral angles to estimate/predict ρ constants. These QSARs and other analyses herein support the new theory that the bridgehead hydrogen bond orbital assists the displacement of the exo leaving group in the solvolyses of 2-norbornyl derivatives. Alternative interpretions of the 1H NMR and 13C NMR spectra indicate that C1 is hypercoordinated rather than C6. Consequently, in normal hydroxylic solvents the 2-norbornyl cation is not symmetrical, does not require C—C σ-bond bridging and is not a single minimum energy system, i.e. it is a pair of rapidly equilibrating cations, eq 1, structure 2b. At ≤ −158°C, the 2-norbornyl cation is proposed to be an H—C1—H σ-bond delocalized resonance hybrid structure 16/16’. Hypotheses are presented which suggest that the norbornane system can be viewed as a saturated counterpart of a conjugated π system, e.g. benzene. This new delocalization and bond concept, sigma aromaticity, can also help to explain the preferential “exo'’ reactions of norbornane and norbornene systems, the unusual stability of the 2-norbornyl cation, and perhaps provide new insight concerning the ubiquitousness of six-membered rings. Sigma aromaticity may also help account for the greater efficiency of singlet energy transfer between chromophores when the molecular spacer group is rigid rather than flexible, and electron transfer in DNA.

High-efficiency energy transfer from carotenoids to a phthalocyanine in an artificial photosynthetic antenna.

Two carotenoid pigments have been linked as axial ligands to the central silicon atom of a phthalocyanine derivative, forming molecular triad 1. Laser flash studies on the femtosecond and picosecond time scales show that both the carotenoid S1 and S2 excited states act as donor states in 1, resulting in highly efficient singlet energy transfer from the carotenoids to the phthalocyanine. Triplet energy transfer in the opposite direction was also observed. In polar solvents efficient electron transfer from a carotenoid to the phthalocyanine excited singlet state yields a charge-separated state that recombines to the ground state of 1.

Enhanced intersystem crossing in donor/acceptor systems based on zinc/iron or free-base/iron porphyrins.

The deactivation pathways of the singlet excited state of a series of zinc or free-base donor porphyrins covalently linked by a bridge to a paramagnetic iron(III) chloride porphyrin acceptor have been studied. These donor-bridge-acceptor systems all share a similar geometry (25 A donor-acceptor center-to-center distance), but the bridges vary in electronic structure. In previously reported investigations of zinc/iron porphyrin systems, the fluorescence quenching of the donor has predominantly been assigned to electron transfer. However, for the porphyrin systems studied in this paper, we show that the dominant deactivation channels are enhanced intersystem crossing and singlet energy transfer. In both series, the intersystem crossing rate (S1-->T1) of the donor moiety is almost doubled in the presence of a paramagnetic high-spin metal-porphyrin acceptor. The significant spectral overlap of the donor fluorescence and acceptor absorption in both series allows for efficient singlet energy transfer (Forster mechanism). Furthermore, the bridging chromophores mediate energy transfer and the enhancement is inversely dependent upon the energy gap between the donor and bridge excited states. Although Marcus theory predicts thermodynamically favorable electron transfer to occur in the systems investigated, the quenching rate constants were found to be independent of solvent polarity, and no charge-separated state could be detected, indicating very small electronic coupling for electron transfer.

The effects of the detergent LDAO on the carotenoid metabolism and growth of Rhodovulum sulfidophilum

Carotenoids play an important role in the purple non-sulfur bacteria. In this work we show that addition of the zwitterionic detergent LDAO (lauryldimethylamine N-oxide) to the growth media of Rhodovulum sulfidophilum results in a drastic alteration of the carotenoid biosynthesis, leading to the accumulation of the precursor neurosporene instead of carotenoids of the spheroidene series. This is probably due to a specific inhibition of the hydroxylation of neurosporene either by direct inhibition of the hydroxylase or by disruption of a presumed carotenoid synthesis multienzyme complex coordinated by the crtK encoded protein. Neurosporene can be integrated into peripheral light harvesting complexes (LHII), and is capable of efficient singlet energy transfer, but the resulting LHII complexes show a reduced stability relative to those containing normal carotenoids.

Effect of excitation wavelength on the photophsics of fluorescein-anthracenemethyl ester

Fluorescein-anthracenemethyl ester was synthesized. Its quantum yields and lifetimes of fluorescence on excitation at various wavelengths were measured in methanol. Various photophysical parameters of the singlet state decay were determined and calculated. A ground state complex is formed between the moieties in the diad. When the fluorescein is excited, the efficiency of electron transfer from methylanthracene to the dye is 0.14. The equilibrium constant for the formation of the complex between the moieties is 2.68. When methylanthracene is excited, the efficiency of singlet energy transfer is 0.38. The maximum efficiency of electron transfer can reach 0.19 when the two moieties in the diad are excited simultaneously.

Ultrafast carotenoid to pheophorbide energy transfer in a biomimetic model for antenna function in photosynthesis

Carotenoids serve as light-harvesting pigments and as photoprotective agents in photosynthetic organisms1–9. Their role as antenna pigments involves absorption of photons in the blue-green spectral region followed by highly efficient singlet–singlet energy transfer to a neighbouring chlorophyll. The dependence of both the rate and mechanism of energy transfer on carotenoid–chlorophyll distance and orientation is Unknown. Here, we have directly measured both the rate and efficiency of singlet energy transfer from a carotenoid covalently linked to pyropheophorbide a (PPheo a) in two model compounds, using picosecond transient absorption spectroscopy. In one model the π systems of the carotenoid and PPsheo a possess a maximum edge-to-edge distance of 5 A, while in the other model this distance is only 2A. Energy transfer occurs from the carotenoid to PPheo a at the 2-A distance with a rate constant of 7 ± 2 × 1010 s−1 and 53±5% efficiency, while energy transfer at the 5-A distance occurs at a rate constant of <3 ×109 s−1 and with <5% efficiency. These results provide evidence that short distances and strong electronic interactions between carotenoids and chlorophylls are necessary to achieve the high energy transfer efficiencies observed in vivo.

Singlet energy transfer from the charge-transfer excited state of tris(2,2'-bipyridine)ruthenium(II) to laser dyes

Efficient singlet energy transfer from the charge-transfer (CT) excited state of (R~(bpy)~)~+ (bpy = 2,2'-bipyridine) to several laser dyes has been demonstrated. The limiting efficiencies of dye singlet state formation are 1.00 which shows the efficiency of formation of the sensitizing state of (Ru(bpy)J2+ is unity. For most of the dyes in methanol, energy transfer is shown to be predominantly by diffusion-assisted Forster transfer. For Rhodamine 101, however, energy transfer is predominantly by a contact exchange transfer. This result shows that the CT state of the sensitizer has a great deal of singlet character and should be referred to as a spin-orbit state.

Singlet energy transfer between aromatic amino acids and nucleic acid bases. Theoretical calculations.

Abstract— Critical Forster distances for excitation energy transfer at the singlet level between the tyrosyl or tryptophyl residues of proteins and nucleic acid bases have been calculated. Efficient singlet energy transfer can be expected from tyrosine to nucleic acid bases both at room temperature (10 &lt;R0 &lt; 17 A) and at low temperature (20 &lt; R0 &lt; 23 Å). At low temperature nucleic acid bases should be able to transfer their singlet excitation energy to the indole ring of tryptophan with a reasonable probability (9 &lt; R0 &lt; 15 A).

Energy transfer in polymers—VI: Singlet energy transfer in solid polystyrene and methylacrylate styrene copolymer

The efficiency of singlet energy transfer to tetraphenylbutadiene has been measured as a function of acceptor concentration for polystyrene at 77 K and for a copolymer (74:26) styrene-methylacrylate at 77 K and room temperature. It has been shown that energy transfer from polymer and copolymer to this additive occurs from both excimer and isolated excited chromophore.