Singlet-singlet Energy Transfer Research Articles

Panchromatic Light-Capturing Bis-styryl BODIPY-Perylenediimide Donor-Acceptor Constructs: Occurrence of Sequential Energy Transfer Followed by Electron Transfer.

Two wide-band-capturing donor-acceptor conjugates featuring bis-styrylBODIPY and perylenediimide (PDI) have been newly synthesized, and the occurrence of ultrafast excitation transfer from the 1 PDI* to BODIPY, and a subsequent electron transfer from the 1 BODIPY* to PDI have been demonstrated. Optical absorption studies revealed panchromatic light capture but offered no evidence of ground-state interactions between the donor and acceptor entities. Steady-state fluorescence and excitation spectral recordings provided evidence of singlet-singlet energy transfer in these dyads, and quenched fluorescence of bis-styrylBODIPY emission in the dyads suggested additional photo-events. The facile oxidation of bis-styrylBODIPY and facile reduction of PDI, establishing their relative roles of electron donor and acceptor, were borne out by electrochemical studies. The electrostatic potential surfaces of the S1 and S2 states, derived from time-dependent DFT calculations, supported excited charge transfer in these dyads. Spectro-electrochemical studies on one-electron-oxidized and one-electron-reduced dyads and the monomeric precursor compounds were also performed in a thin-layer optical cell under corresponding applied potentials. From this study, both bis-styrylBODIPY⋅+ and PDI⋅- could be spectrally characterizes and were subsequently used in characterizing the electron-transfer products. Finally, pump-probe spectral studies were performed in dichlorobenzene under selective PDI and bis-styrylBODIPY excitation to secure energy and electron-transfer evidence. The measured rate constants for energy transfer, kENT , were in the range of 1011 s-1 , while the electron transfer rate constants, kET , were in the range of 1010 s-1 , thus highlighting their potential use in solar energy harvesting and optoelectronic applications.

Assembling Artificial Photosynthetic Models in Water Using β-Cyclodextrin-Conjugated Phthalocyanines as Building Blocks.

Two water-soluble zinc(II) phthalocyanines substituted with two or four permethylated β-cyclodextrin (β-CD) moieties at the α positions have been utilized as building blocks for the construction of artificial photosynthetic models in water. The hydrophilic and bulky β-CD moieties not only can increase the water solubility of the phthalocyanine core and prevent its stacking in water but can also bind with a tetrasulfonated zinc(II) porphyrin (ZnTPPS) and/or sodium 2-anthraquinonesulfonate (AQ) in water through host-guest interactions. The binding interactions of these species have been studied spectroscopically, while the photoinduced processes of the resulting complexes have been investigated using steady-state and time-resolved spectroscopic methods. In the ternary complexes, the ZnTPPS units serve as light-harvesting antennas to capture the light energy and transfer it to the phthalocyanine core via efficient excitation energy transfer. The excited phthalocyanine is subsequently quenched by the electron-deficient AQ units through electron transfer. Femtosecond transient absorption spectroscopy provides clear evidence for the singlet-singlet energy transfer from the photo-excited ZnTPPS to the phthalocyanine core with a rate constant (kENT ) in the order of 109 s-1 . The population of phthalocyanine radical cations indicates the occurrence of electron transfer from the excited phthalocyanine to the AQ moieties, forming a charge-separated state.

Panchromatic Light-Absorbing [70]Fullerene-Perylene-BODIPY Triad with Cascade of Energy Transfer as an Efficient Singlet Oxygen Sensitizer.

A panchromatic light-absorbing [70]fullerene-perylene-BODIPY triad (C70-P-B) was synthesized and applied as a heavy atom-free organic triplet photosensitizer for photooxidation. The photophysical processes were comprehensively investigated by the methods of steady-state spectroscopy, time-resolved spectroscopy, as well as theoretical calculations. C70-P-B shows a strong absorption ability from 300-620 nm. Efficient cascading intramolecular singlet-singlet energy transfer in C70-P-B was confirmed by the luminescence study. The backward triplet excited state energy transfer from C70 moiety to perylene then occurs to populate 3perylene*. Thus, the triplet excited states of C70-P-B are distributed on both C70 and perylene moiety with lifetimes of 23 ± 1 μs and 175 ± 17 μs, respectively. C70-P-B exhibits excellent photooxidation capacity, and its yield of singlet oxygen reaches 0.82. The photooxidation rate constant of C70-P-B is 3.70 times that of C70-Boc and 1.58 times that of MB, respectively. The results in this paper are useful for designing efficient heavy atom-free organic triplet photosensitizers for practical application in photovoltaics, photodynamic therapy, etc.

Highly Coupled Heterobicycle-Fused Porphyrin Dimers: Excitonic Coupling and Charge Separation with Coordinated Fullerene, C60.

Porphyrin dimers have been widely explored and studied owing to their importance in photosynthetic systems. A vast variety of dimers linked by different groups and at different angles have been synthesized and studied; however, the means by which to synthesize rigidly fused porphyrins with direct conjugation of the chromophores remains limited. Such a class of porphyrins may possess interesting properties that unconjugated or stacked dimers may not exhibit. In this study, bisbenzimidazole-fused porphyrin dimers and their mono- and bis-zinc derivatives are synthesized and characterized. As a consequence of excitonic coupling, these dimers exhibit a split Soret band irrespective of the metal ion in the porphyrin cavity. Steady-state fluorescence and excitation spectra followed by femtosecond transient absorption spectral studies of the heterometallated dimer, (free-base and zinc porphyrin) reveals the occurrence of efficient singlet-singlet energy transfer (>95 % efficiency and rate constant >1012 s-1 ) within the dyad. Further, donor-acceptor conjugates were formed by metal-ligand axial coordination of phenyl imidazole functionalized C60 and were characterized by a variety of physicochemical techniques. Excited state charge separation from both singlet and triplet excited states of ZnP in the conjugates has been established. The lifetime of the final charge-separated state was in the 30-40 μs range revealing charge stabilization. Interestingly, no charge separation in the conjugate derived from the heterometallated dimer was observed wherein excitation transfer dominated the process. The present study brings out the importance of the rigid π-spacer connecting porphyrin dimers in governing the energy and electron transfer events when coupled with an electron acceptor.

Selective Triplet–Singlet Förster‐Resonance Energy Transfer for Bright Red Afterglow Emission (Adv. Funct. Mater. 11/2023)

Selective Förster-Resonance Energy Transfer In article number 2211604, Shuzo Hirata and co-workers demonstrate the efficient triplet-singlet Förster-resonance energy transfer from the N-fused chromophore to an acceptor leads to the bright red afterglow emission. The selective triplet-singlet energy transfer is due to the high radiation rate from the lowest triplet state facilitated by the out-of-plane molecular vibrations in the N-fused chromophore and near-zero fluorescence suppressing the singlet-singlet energy transfer pathways.

Plasmon effect on simultaneous singlet-singlet and triplet-singlet energy transfer

The influence of plasmonic effect of Ag nanoparticles (NPs) on singlet-singlet (S–S) and triplet-singlet (T–S) energy transfer in the same donor–acceptor pair of organic molecules was studied. It was established that a 7-fold increase in the fluorescence and phosphorescence intensity of the energy donor (Rose bengal) and a 4-fold increase in the fluorescence intensity of the acceptor (polymethine dye) are observed on silver island films. The changes are associated with an increase in the emission rate of dye molecules, as evidenced by a decrease in the lifetime of their luminescence in the presence of Ag NPs. An increase in the efficiency of both types of energy transfer was recorded in the plasmon field of Ag NPs. The plasmon effect has almost the same influence on both S–S and T–S energy transfer. Estimation of the energy transfer rates using of the model of electric dipole interaction showed a good correlation between the experimental and calculated data for the S–S energy transfer.

Proton-coupled energy transfer in molecular triads.

We experimentally discovered and theoretically analyzed a photochemical mechanism, which we term proton-coupled energy transfer (PCEnT). A series of anthracene-phenol-pyridine triads formed a local excited anthracene state after light excitation at a wavelength of ~400 nanometers (nm), which led to fluorescence around 550 nm from the phenol-pyridine unit. Direct excitation of phenol-pyridine would have required ~330-nm light, but the coupled proton transfer within the phenol-pyridine unit lowered its excited-state energy so that it could accept excitation energy from anthracene. Singlet-singlet energy transfer thus occurred despite the lack of spectral overlap between the anthracene fluorescence and the phenol-pyridine absorption. Moreover, theoretical calculations indicated negligible charge transfer between the anthracene and phenol-pyridine units. We construe PCEnT as an elementary reaction of possible relevance to biological systems and future photonic devices.

One-Photon Excitation Followed by a Three-Step Sequential Energy-Energy-Electron Transfer Leading to a Charge-Separated State in a Supramolecular Tetrad Featuring Benzothiazole-Boron-Dipyrromethene-Zinc Porphyrin-C60.

A panchromatic triad, consisting of benzothiazole (BTZ) and BF2 -chelated boron-dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using 1 H NMR spectroscopy, ESI-MS, UV-visible, steady-state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ-BODIPY-ZnP, and dyads, BTZ-BODIPY and BODIPY-ZnP, along with the reference compounds BTZ-OMe, BODIPY-OMe, and ZnP-OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ-BODIPY-ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ-BODIPY-ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from 1 BTZ* to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the 1 BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two-step singlet-singlet energy transfer. Further, a supramolecular tetrad, BTZ-BODIPY-ZnP:ImC60 , was formed by axially coordinating the triad with imidazole-appended fulleropyrrolidine (ImC60 ), and parallel steady-state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from 1 ZnP* to ImC60 . Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely, 1 BTZ* -BODIPY-ZnP:ImC60 →BTZ-1 BODIPY* -ZnP:ImC60 →BTZ-BODIPY-1 ZnP* :ImC60 →BTZ-BODIPY-ZnP.+ :ImC60 .- in the supramolecular tetrad. The evaluated rate of energy transfer, kEnT , was found to be 3-5×1010 s-1 , which was slightly faster than that observed in the case of BODIPY-ZnP and BTZ-BODIPY-ZnP, lacking the coordinated ImC60 . The rate constants for charge separation and recombination, kCS and kCR , respectively, calculated by monitoring the rise and decay of C60 .- were found to be 5.5×1010 and 4.4×108 s-1 , respectively, for the BODIPY-ZnP:ImC60 triad, and 3.1×1010 and 4.9×108 s-1 , respectively, for the BTZ-BODIPY-ZnP:ImC60 tetrad. Initial excitation of the tetrad, promoting two-step energy transfer and a final electron-transfer event, has been successfully demonstrated in the present study.

Efficient energy transfer in phenyl-ethynyl-linked asymmetric BODIPY dimers

Abstract Three asymmetric boron dipyrromethene (BODIPY) dimers were synthesized via coupling of a BODIPY monomer with a meso-phenyl ethynyl group substituent to the different monomers with an iodine atom at the second position of the BODIPY core. The dimers were characterized by 1H, 13C, and 11B, 19F NMR spectroscopy and high-resolution mass spectrometry. Density functional theory calculations show minimal conjugation between the two BODIPY units in the dimers due to a large dihedral angle between the BODIPYs and the linker. Absorption and fluorescence measurements of monomeric and dimeric compounds in dichloromethane indicated highly efficient singlet-singlet energy transfer between two BODIPY units (in which the meso-phenyl containing BODIPY unit act as the donor while the other BODIPY unit is the acceptor) and large Stokes shifts. In particular, the dimer with the styryl BODIPY unit absorbs light in a wide range of the spectrum (325 nm–690 nm) with emission in the near IR region. The ethynyl thiophene linked dimer showed the highest fluorescence quantum yield (76%) of all three dimers investigated in this study.

Some peculiarities of electronic and vibronic excitations transfer in organic media and hybrid nanosystems

The peculiarities of electronic and vibronic excitations transfer processes in polymers and hybrid nanosystems were examined. It is shown, that at distances less 15–25 Å the probability of electronic singlet and triplet excitation energy transfer is rather high and is close to unity. For the singlet-singlet energy transfer between neighbor π-electron-containing macromolecular cells the phenomenon is explained by essential contribution of an exchange mechanism. A similar effect for hybrid functional nanosystems (that were designed for photodynamic therapy) was observed. The manifestations of vibronic excitation transfer along polymer chain were also studied. The critical distance for vibronic excitation transfer jump was estimated as 3–6 Å.

Bodipy−Corrole dyad with truxene bridge: Photophysical Properties and Application in Triplet−Triplet Annihilation upconversion

Free base corrole can generate triplet state without relying on the heavy-atom effect, however, its absorption in the visible spectral range (430–650 nm) is very weak which makes corrole not an ideal triplet photosensitizer. Herein, we prepared a novel truxene-bridged Bodipy−corrole dyad (B−T−Co) which have strong, broad absorption in Uv–visible spectral range (300 nm − 676 nm, the maximum molar absorption coefficient is up to 1.09 − 105 M−1 cm−1 at 426 nm). The dyad showed fast singlet-singlet energy transfer from Bodipy moiety to corrole moiety (2.16 − 108 s−1), high strong singlet-oxygen (1O2) photosensitizing ability (70%), prolonged triplet state lifetimes (149 μs) and high triplet state energy levels (1.41 eV) as compared to corrole (singlet-oxygen photosensitizing ability: 58%; triplet lifetimes: 116 μs; triplet energy levels: 1.32 eV). Moreover, the dyad used as triplet photosensitizer for triplet−triplet annihilation upconversion, with red excitation at 589 nm, the upconversion emission intensity is higher than the corrole itselef and the upconversion quantum yield (2.57%) increases by ca. 2-folds.

Supramolecular complex of a fused zinc phthalocyanine-zinc porphyrin dyad assembled by two imidazole-C60 units: ultrafast photoevents.

A new zinc phthalocyanine-zinc porphyrin dyad (ZnPc-ZnP) fused through a pyrazine ring has been synthesized as a receptor for imidazole-substituted C60 (C60Im) electron acceptor. Self-assembly via metal-ligand axial coordination and the pertinent association constants in solution were determined by 1H-NMR, UV-Vis and fluorescence titration experiments at room temperature. The designed host was able to bind up to two C60Im electron acceptor guest molecules to yield C60Im:ZnPc-ZnP:ImC60 donor-acceptor supramolecular complex. The spectral data showed that the two binding sites behave independently with binding constants similar in magnitude. Steady-state fluorescence studies were indicative of an efficient singlet-singlet energy transfer from zinc porphyrin to zinc phthalocyanine within the fused dyad. Accordingly, the transient absorption studies covering a wide timescale of femto-to-milli seconds revealed ultrafast energy transfer from 1ZnP* to ZnPc (kEnT ∼ 1012 s-1) in the fused dyad. Further, a photo induced electron transfer was observed in the supramolecularly assembled C60Im:ZnPc-ZnP:ImC60 donor-acceptor complex leading to charge separated states, which persisted for about 200 ns.

A Rotaxane Scaffold for the Construction of Multiporphyrinic Light-Harvesting Devices.

A sophisticated photoactive molecular device has been prepared by combining recent concepts for the preparation of multifunctional nanomolecules (click chemistry on multifunctional scaffolds) with supramolecular chemistry (self-assembly to prepare rotaxanes). Specifically, a clickable [2]rotaxane scaffold incorporating a free-base porphyrin stopper has been prepared and functionalized with ten peripheral Zn(II)-porphyrin moieties. Electrochemical investigations of the final compound revealed a peculiar behavior resulting from the intramolecular coordination of the Zn(II) porphyrin moieties to 1,2,3-triazole units. Finally, steady state investigations of the compound combining Zn(II) and free-base porphyrin moieties have shown that this compound is a light-harvesting device capable of channeling the light energy from the peripheral Zn(II)-porphyrin subunits to the core by singlet-singlet energy transfer.

Corrole-phenothiazine and porphyrin-phenothiazine dyads connected at β-position: Synthesis and photophysical properties

Two novel donor-acceptor dyads, in which phenothiazine (PTZ) connected at β-pyrrolic position of either freebase corrole (TPC) or freebase porphyrin (TPP) via vinylic spacer have been synthesized. Both the dyads were characterized by ESI-MS, IR, 1H NMR (1D and 2D 1H-1H COSY and J-Resolved), UV–Vis, Study state Fluorescence, Time-resolved fluorescence (Time-correlated single photon counting (TCSPC), Streak Camera) as well as electrochemical methods. In the absorption spectra of the dyads, both Soret and Q-bands were red shifted by 8–20 nm indicating weak electronic communication between the two chromophores. However, the fluorescence emission from the PTZ of the dyad was efficiently quenched (96–99%) as compared to pristine PTZ where the dyad was excited at 310 nm, which is attributed to singlet-singlet excited energy transfer. Fluorescence emission from porphyrin part of the TPP-PTZ dyad also quenched when excite the dyad at 420 nm, which is ascribed to photoinduced electron transfer from ground state of PTZ to excited state of porphyrin. In contrast, when excite the corrole at 420 nm in TPC-PTZ dyad, we found there are no significant changes in photophysical properties. In both the cases the solvent dependence of the rate of energy and electron transfer was observed.

Excited State Charge Separation in Solution and in Electropolymerized Films of Terthiophene-Fullerene Dyad and Phenothiazine-Terthiophene-Fullerene Triad

A covalently linked molecular dyad comprised of terthiophene and C60, and a molecular triad comprised of phenothiazine, terthiophene and C60 have been newly synthesized to probe photoexcited events in solution and at the electropolymerized film. The X-ray structure of the dyad revealed spatial disposition of the donor-acceptor entities without significant intermolecular type interactions. Optical absorbance, fluorescence, and the electrochemical studies were consistent with the structural integrity of the dyad and triad. Further, the TT-C60 dyad (TT = terthiophene) was found to electropolymerize during multi-cycling of voltammograms. The frontier HOMO and LUMO orbitals of the phenothiazine-terthiophene-C60 triad (PTZ-TT-C60, PTZ = phenothiazine) were found to be located respectively on the phenothiazine-terthiophene and C60 entities. Free-energy calculations revealed that the excited state charge transfer resulting into the formation of TT•+-C60•− in the case of the dyad, and (PTZ-TT)•+-C60•− charge separated state in the case of triad to be thermodynamically feasible. Evidence for the occurrence of excited state singlet-singlet energy transfer from 1TT* to form 1C60* followed by charge transfer was secured from studies involving femtosecond transient absorption studies in polar benzonitrile. Charge stabilization to some extent in the triad compared to that in the dyad was obtained. Attempts were also made to secure evidence of charge separation in the terthiophene-fullerene drop-casted and electropolymerized films on FTO surface.

Light-Harvesting Phthalocyanine-Diketopyrrolopyrrole Derivatives: Synthesis, Spectroscopic, Electrochemical, and Photochemical Studies.

A new family of light-harvesting zinc phthalocyanine (ZnPc)-diketopyrrolopyrrole (DPP) hybrids have been synthesized and characterized. The absorption spectral measurements showed that the major absorptions of DPP (450-600 nm) are complementary to those of zinc phthalocyanine (300-400 and 600-700 nm). Therefore, the designed hybrids absorb over a broad range in the visible region. The geometric and electronic structures of the dyads were probed by initio B3LYP/6-311G methods. The majority of the HOMOs were found to be located on the ZnPc, while the majority of the LUMOs were on the DPP units. The DPP units serve as the antenna, which upon excitation undergo efficient singlet-singlet energy transfer to the attached ZnPc units. The formed singlet ZnPc, in turn, donates its electron to the electron-deficient DPP forming the low-lying radical ion pairs ZnPc.+ -DPP.- (energy=1.44-1.56 eV as calculated from the electrochemical measurements). The excited-state events were confirmed by using a transient absorption technique in the picosecond-microsecond time range, as well as a time-resolved emission technique. The rates of energy transfer from the singlet DPP to ZnPc were found to be extremely fast >1010 s-1 , while the rates of electron transfer from the singlet excited state of ZnPc to DPP were found to be 3.7-6.6×109 s-1 .

Panchromatic Light Capture and Efficient Excitation Transfer Leading to Near-IR Emission of BODIPY Oligomers.

All-BODIPY-based (BODIPY=boron-dipyrromethene) donor-acceptor systems capable of wide-band absorbance leading to efficient energy transfer in the near-IR region are reported. A covalently linked 3-pyrrolyl BODIPY-BODIPY dimer building block bearing an ethynyl group at the meso-aryl position is synthesized and coupled with three different monomeric BODIPY/pyrrolyl BODIPY building blocks with a bromo/iodo group under Pd(0) coupling conditions to obtain three covalently linked 3-pyrrolyl-BODIPY-based donor-acceptor oligomers in 19-29 % yield. The oligomers are characterized in detail by 1D and 2D NMR spectroscopy, high-resolution mass spectrometry, and optical spectroscopy. Due to the presence of different functionalized BODIPY derivatives in the oligomers, panchromatic light capture (300-725 nm) is witnessed. Fluorescence studies reveal singlet-singlet energy transfer from BODIPY monomer to BODIPY dimer leading to emission in the 700-800 nm range. Theoretical modeling according to the Förster mechanism predicts ultrafast energy transfer due to good spectral overlap of the donor and acceptor entities. Femtosecond transient absorption studies confirm this to be the case and thus show the relevance of the currently developed all-BODIPY-based energy-funneling supramolecular sytems with near-IR emission to solar-energy harvesting applications.

Supramolecular Surface Photochemistry: Cascade Energy Transfer between Encapsulated Dyes Aligned on a Clay Nanosheet Surface.

Three coumarin derivatives (7-propoxy coumarin, coumarin-480, and coumarin-540a, 2, 3, and 4, respectively) having different absorption and emission spectra were encapsulated within a water-soluble organic capsule formed by the two positively charged ammonium-functionalized cavitand octaamine (OAm, 1). Guests 2, 3, and 4 absorb in ultraviolet, violet, and blue regions and emit in violet, blue, and green regions, respectively. Energy transfer between the above three coumarin@(OAm)2 complexes assembled on the surface of a saponite clay nanosheet was investigated by steady-state and time-resolved emission techniques. Judging from their emission and excitation spectra, we concluded that the singlet-singlet energy transfer proceeded from 2 to 3, from 2 to 4, and from 3 to 4 when OAm-encapsulated 2, 3, and 4 were aligned on a clay surface as two-component systems. Under such conditions, the energy transfer efficiencies for the paths 2* to 3, 2* to 4, and 3* to 4 were calculated to be 33, 36, and 50% in two-component systems. When all three coumarins were assembled on the surface and 2 was excited, the energy transfer efficiencies for the paths 2* to 3, 2* to 4, and 3* to 4 were estimated to be 32, 34, and 33%. A comparison of energy transfer efficiencies of the two-component and three-component systems revealed that excitation of 2 leads to emission from 4. Successful merging of supramolecular chemistry and surface chemistry by demonstrating novel multi-step energy transfer in a three-component dye encapsulated system on a clay surface opens up newer opportunities for exploring such systems in an artificial light-harvesting phenomenon.

Benzophenone and zinc(II) phthalocyanine dichromophores-labeled poly (aryl ether) dendrimer: synthesis, characterization and photoinduced energy transfer

A series of benzophenone chromospheres and zinc(II) phthalocyanine dichromophores labeled poly (aryl benzyl ether) dendrimer (Gn-DZnPc(BP)8n, n = 1−2) were synthesized. Their structures were characterized by elemental analysis, 1H NMR, IR, UV–vis and matrix-assisted laser desorption/ionization time-of-flight spectrometry (MALDI-TOF MS). Their photophysical properties were examined by steady-state and time-resolved fluorescence methods. Both the poly (aryl benzyl ether) dendrimer and BP terminal chromophores had a significant effect on photophysical properties of the zinc(II) phthalocyanine core. Time-resolved spectroscopic measurements indicated that the lifetime of benzophenone (donor) chromophore was longer than that of the zinc(II) phthalocyanine (acceptor). The fluorescence of the peripheral benzophenone chromophores was quenched by the phthalocyanine group attached to the focal point. All of these observations suggest that an intramolecular singlet energy transfer occurs in Gn-DZnPc(BP)8n molecules. The light-harvesting abilities of these molecules increased with generations due to an increase in the number of benzophenone chromophores. The energy transfer efficiencies were ca. 0.49 and 0.68 for generations 1 and 2, respectively, and the rate constants of the singlet-singlet energy transfer were ca. 108 s−1. The rate constants changed inconspicuously with increase of dendron generations. The intramolecular singlet-singlet energy transfer is proposed to proceed mainly via a Förster-type interaction mechanism involving the dendrimer backbone as a scaffold to hold the peripheral benzophenone chromophores and the phthalocyanine core together. This dendrimer was an effective new energy transmission complex with high efficiency and could be used as a potential light-harvesting system.

Carotenoids and Photosynthesis.

Carotenoids are ubiquitous and essential pigments in photosynthesis. They absorb in the blue-green region of the solar spectrum and transfer the absorbed energy to (bacterio-)chlorophylls, and so expand the wavelength range of light that is able to drive photosynthesis. This is an example of singlet-singlet energy transfer, and so carotenoids serve to enhance the overall efficiency of photosynthetic light reactions. Carotenoids also act to protect photosynthetic organisms from the harmful effects of excess exposure to light. Triplet-triplet energy transfer from chlorophylls to carotenoids plays a key role in this photoprotective reaction. In the light-harvesting pigment-protein complexes from purple photosynthetic bacteria and chlorophytes, carotenoids have an additional role of structural stabilization of those complexes. In this article we review what is currently known about how carotenoids discharge these functions. The molecular architecture of photosynthetic systems will be outlined first to provide a basis from which to describe carotenoid photochemistry, which underlies most of their important functions in photosynthesis.