Singlet Excited-state Energy Transfer Research Articles

Conformational Impact on Energy Storage Efficiency of Subphthalocyanine-Fullerene Hybrids.

Hybrid molecules involving subphthalocyanine and Buckminsterfullerene derivatives are interesting candidates as heavy metal free triplet sensitizers. Subphthalocyanine efficiently absorbs visible photons and transfer the singlet excited state energy to the Buckminsterfullerene where intersystem crossing produces triplet states in high yield. Thus, far the efficiency of the triplet-generating photophysics in these systems has been hampered by back energy transfer to the subphthalocyanine triplet state resulting in loss of excitation energy. Herein an efficient strategy is realized to avoid loss of triplet energy by back energy transfer. A hybrid molecule based on subphthalocyanine and Buckminsterfullerene is presented in which dispersion-induced π-π interactions result in a molecular geometry where highly efficient through-space singlet excited state energy transfer takes place in one direction, whereas energy flow in the opposite direction via the triplet manifold is blocked by lack of orbital overlap. The approach opens for a new class of heavy-metal-free triplet sensitizers of particular relevance to the fields of photodynamic therapy and noncoherent photon upconversion.

Mechanism of Carotenoid Singlet Excited State Energy Transfer in Modified Bacterial Reaction Centers

Ultrafast transient laser spectroscopy has been used to investigate carotenoid singlet excited state energy transfer in various Rhodobacter (Rb.) sphaeroides reaction centers (RCs) modified either genetically or chemically. The pathway and efficiency of energy transfer were examined as a function of the structures and energies of the donor and acceptor molecules. On the donor side, carotenoids with various extents of pi-electron conjugation were examined. RCs studied include those from the anaerobically grown wild-type strain containing the carotenoid spheroidene, which has 10 conjugated carbon-carbon double bonds; the GA strain containing neurosporene, which has nine conjugated double bonds; and aerobically grown wild-type cells, as well as aerobically grown H(M182)L mutant, both containing the carbonyl-containing carotenoid spheroidenone, which has 11 conjugated double bonds. By varying the structure of the carotenoid, we observed the effect of altering the energies of the carotenoid excited states on the rate of energy transfer. Both S(1)- and S(2)-mediated carotenoid-to-bacteriochlorophyll energy transfer processes were observed. The highest transfer efficiency, from both the S(1) and S(2) states, was observed using the carotenoid with the shortest chain. The S(1)-mediated carotenoid-to- bacteriochlorophyll energy transfer efficiencies were determined to be 96%, 84%, and 73% for neurosporene, spheroidene, and spheroidenone, respectively. The S(2)-mediated energy transfer efficiencies follow the same trend but could not be determined quantitatively because of limitations in the time resolution of the instrumentation. The dependence of the energy transfer rate on the energetics of the energy transfer acceptor was verified by performing measurements with RCs from the H(M182)L mutant. In this mutant, the bacteriochlorophyll (denoted B(B)) located between the carotenoid and the RC special pair (P) is replaced by a bacteriopheophytin (denoted phi(B)), where the Q(X) and Q(Y) bands of phi(B) are 1830 and 1290 cm(-1), respectively, higher in energy than those of B(B). These band shifts associated with phi(B) in the H(M182)L mutant significantly alter the spectral overlap between the carotenoid and phi(B), resulting in a significant decrease of the transfer efficiency from the carotenoid S(1) state to phi(B). This leaves energy transfer from the carotenoid S(2) state to phi(B) as the dominant channel. Largely because of this change in mechanism, the overall efficiency of energy transfer from the carotenoid to P decreases to less than 50% in this mutant. Because the spectral signature of phi(B) is different from that of B(A) in this mutant, we were able to demonstrate clearly that the carotenoid-to-P energy transfer is via phi(B). This finding supports the concept that, in wild-type RCs, the carotenoid-to-P energy transfer occurs through the cofactor located at the B(B) position.

Excited State Energy Transfer Pathways in Photosynthetic Reaction Centers. 4. Asymmetric Energy Transfer in the Heterodimer Mutant

In bacterial photosynthetic reaction centers, ultrafast singlet excited state energy transfer occurs from the monomeric bacteriochlorophylls, B, and bacteriopheophytins, H, to the homodimer special pair, a pair of strongly interacting bacteriochlorophylls. In the M202HL mutant, one of the bacteriochlorophylls comprising the special pair is replaced by a bacteriopheophytin, and this is called the heterodimer special pair or D. We report the direct observation of spontaneous fluorescence from 1 B in the heterodimer mutant. In contrast to results for the homodimer special pair where 1 B decays with a rate constant of (160 fs) -1 (King, B. A.; McAnaney, T. B.; de Winter, A.; Boxer, S. G. J. Phys. Chem. B2000, 104, 8895-8902), 1 B decay in M202HL exhibits two components with rate constants (700 fs) -1 and (190 fs) -1 ; these are similar to what we reported earlier for the rise of 1 D spontaneous fluorescence (King, B. A.; Stanley, R. J.; Boxer, S. G. J. Phys. Chem. B 1997, 101, 3644-3648). In the double mutant M202HL/M182HL, where the accessory bacteriochlorophyll on the M side is replaced by a bacteriopheophytin, the absorption bands corresponding to the chromophores in the BL and BM binding sites are quite well resolved, and it is possible to preferentially excite the chromophore on either the L or the M side. Analysis of the rise of 1 D fluorescence in the double mutant supports the earlier assignment of the slower 700 fs energy transfer component to 1 BL f D, while the faster 190 fs energy transfer component is assigned to 1 BM f D. Replacement of bacteriochlorophyll by bacteriopheophytin in the BM binding site does not alter the time constants of the two energy transfer pathways. Excited state energy transfer to D is the same in QA-depleted and QA-reduced reaction centers, suggesting that electron transfer processes that be might sensitive to a charge on Q A, such as 1 BL f BL + HL - , do not compete with relatively slow 1 BL f D energy transfer. The results support earlier findings that singlet energy transfer from the monomeric chromophores along the L and M branches to the heterodimer special pair is asymmetric and is faster along the M side, in contrast to the homodimer special pair in wild type where the energy transfer rates along the two branches are very similar. Thus, conversion of the special pair homodimer to a heterodimer breaks the symmetry of ultrafast energy transfer along the two branches of chromophores. These findings may provide information on differences in the electronic interactions on the L vs M sides of the RC that is relevant to unidirectional electron transfer.

Excited State Energy Transfer Pathways in Photosynthetic Reaction Centers. 3. Ultrafast Emission from the Monomeric Bacteriochlorophylls

Ultrafast singlet excited state energy transfer occurs from the monomeric bacteriopheophytin (H) and bacteriochlorophyll (B) chromophores to the primary electron donor or special pair (P) in bacterial photosynthetic reaction centers. Because of rapid quenching of the singlet excited state of B by energy transfer to P, 1B emission has not previously been observed in functional reaction centers. Using fluorescence upconversion, spontaneous fluorescence associated with the monomeric bacteriochlorophylls is observed for excitation of the monomeric bacteriochlorophylls and bacteriopheophytins at 85 K. The decay kinetics of the fluorescence match the kinetics of the rise of emission from 1P, the ultimate acceptor of singlet energy. Together with measurements of the time-resolved fluorescence anisotropy, the data suggest that 1B is populated in the energy transfer pathway from 1H to P. By exciting H in wild-type and in the reaction center mutant M182HL, where contributions from the chromophores in the B sites on...

Excited State Energy Transfer Pathways in Photosynthetic Reaction Centers. 2. Heterodimer Special Pair

Ultrafast singlet excited state energy transfer occurs from the monomeric chromophores to the primary electron donor or special pair in photosynthetic reaction centers. The mechanism of this proces...

Soluble Synthetic Multiporphyrin Arrays. 2. Photodynamics of Energy-Transfer Processes

Soluble ethyne-linked tetraarylporphyrin arrays that mimic natural light-harvesting complexes by absorbing light and directing excited-state energy have been investigated by static and time-resolved absorption and fluorescence spectroscopies. Of particular interest is the role of the diarylethyne linkers in mediating energy transfer. The major conclusions from this study, which is limited to the examination of arrays containing Zn and free-base (Fb) porphyrins, include the following: (1) Singlet excited-state energy transfer from the Zn porphyrin to the Fb porphyrin is extremely efficient (95−99%). Competitive electron-transfer reactions are not observed. (2) The rate of energy transfer is slowed up to 4-fold by the addition of groups to the linker that limit the ability of the linker and porphyrin to adopt geometries tending toward coplanarity. Thus, the mechanism of energy transfer predominantly involves through-bond communication via the linker. Consistent with this notion, the measured lifetimes of the Zn porphyrin in the dimers at room temperature yield energy-transfer rates ((88 ps)-1 < ktrans < (24 ps)-1) that are significantly faster than those predicted by the Förster (through-space) mechanism ((720 ps)-1). Nevertheless, the electronic communication is weak and the individual porphyrins appear to retain their intrinsic radiative and non-radiative rates upon incorporation into the arrays. (3) Transient absorption data indicate that the energy-transfer rate between two isoenergetic Zn porphyrins in a linear trimeric array terminated by a Fb porphyrin is (52 ± 19 ps)-1 in toluene at room temperature, while the time-resolved fluorescence data suggest that it may be significantly faster. Accordingly, incorporation of multiple isoenergetic pigments in extended linear or two-dimensional arrays will permit efficient overall energy transfer. (4) Medium effects, including variations in solvent polarity, temperature, viscosity, and axial solvent ligation, only very weakly alter (≤2.5-fold) the energy-transfer rates. However, the Fb porphyrin fluorescence in the Zn−Fb dimers is quenched in the polar solvent dimethyl sulfoxide (but not in toluene, castor oil, or acetone), which is attributed to charge-transfer with the neighboring Zn porphyrin following energy transfer. Collectively, the studies demonstrate that extended multiporphyrin arrays can be designed in a rational manner with predictable photophysical features and efficient light-harvesting properties through use of the diarylethyne-linked porphyrin motif.