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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.

A model photosynthetic antenna system consisting of a carotenoid moiety covalently linked to a purpurin has been prepared to study singlet-singlet energy transfer from a carotenoid to a cyclic tetrapyrrole. Ultrafast fluorescence upconversion measurements of the carotenopurpurin dyad and an unlinked reference carotenoid demonstrate that the fluorescent S 2 excited state of the carotenoid model has a lifetime of 150 ′ 3 fs, whereas the corresponding excited state of the carotenoid in the carotenopurpurin dyad is quenched to 40 ′ 3 fs. This quenching is assigned to energy transfer from the S 2 state to the purpurin with a 73 ′ 6% efficiency, which is in accord with the 67 ′ 4% quantum yield obtained by steady-state fluorescence excitation measurements. Concomitant with the decay of the carotenoid S 2 excited state, a single-exponential rise of the excited S 1 state of the purpurin moiety was observed at 699 nm with a time constant of 64 fs. However, the decay of the fluorescence anisotropy was faster at this wavelength (40 fs) and isotropic rise times as short as 44 fs were determined at other emission wavelengths. The lifetime of the Si state of the carotenoid (7.8 ps) was the same in both the carotenoid model and the dyad. Taken together, these results unequivocally demonstrate that the S 2 state of the carotenoid moiety is the sole donor state in this efficient singlet-singlet energy transfer process. The simple dyad described in this work mimics the ultrafast energy transfer kinetics found in certain naturally occurring pigment protein complexes and is thus able to reproduce the high electronic coupling needed for efficient energy transfer from an extremely short-lived energy donor state.

The solvent dependence of the steady-state and time-resolved fluorescence emission from the S2 (11Bu) state of all-trans-β-carotene was investigated in a range of polar and nonpolar solvents and in hexane/carbon disulfide mixtures. The steady-state absorption and fluorescence emission maxima in both polar and nonpolar solvents showed parallel shifts with increasing solvent polarizability, indicating that the emitting state is the S2 (11Bu) state. The lifetime of the S2 state was determined using the fluorescence upconversion technique, and lifetimes varying from 120 fs in quinoline to 177 fs in hexane were found. An intramolecular relaxation process occurring on a similar time scale was observed as a broadening in the reconstructed time-dependent emission spectra in hexane and as emission wavelength-dependent lifetimes in all the solvents studied. Correlations were observed between the S2 lifetimes, the π* solvatochromic parameter, and the ratio of the absorption fine structure. A correlation between the ...

Two closely related, rigidly linked porphyrin−naphthoquinone dyads have been prepared and studied using time-resolved fluorescence and absorption methods. Dyad 1, whose quinone carbonyl groups are relatively close to the porphyrin macrocycle, exhibits photoinduced electron-transfer rate constants that are virtually independent of solvent dielectric constant and temperature within the range 77−295 K. Dyad 2, which has a similar donor−acceptor linkage but whose quinone carbonyl groups are ∼2 A farther from the porphyrin, features photoinduced electron-transfer rate constants that decrease with decreasing solvent dielectric constant. Electron transfer in this molecule ceases at low temperatures. Photoinduced electron transfer in dyad 2 exhibits the usual dependence on free energy change and solvent reorganization observed in many similar porphyrin−quinone systems. The behavior of 1 may be attributed at least in part to the smaller separation of the porphyrin radical cation and the quinone radical anion, whic...

Porphyrin-C60 dyads in which the two chromophores are linked by a bicyclic bridge have been synthesized using the Diels-Alder reaction. The porphyin singlet lifetimes of both the zinc (Pzn-C60) and free base (P-C60) dyads, determined by time-resolved fluorescence measurements, are ≦17 ps in toluene. This substantial quenching is due to singlet-singlet energy transfer to C60 The lifetime of Pzn-1C60 is -5 ps in toluene, whereas the singlet lifetime of an appropriate C60 model compound is 1.2 ns. This quenching is attributed to electron transfer to yield Pznbull;+-C60bull;-. In toluene, P-1C60 is unquenched; the lack of electron transfer is due to unfavorable thermodynamics. In this solvent, a transient state with an absorption maximum at 700 ran and a lifetime of-10 μs was detected using transient absorption methods. This state was quenched by oxygen, and is assigned to the C60 triplet. In the more polar benzonitrile, P-1C60 underoes photoinduced electron transfer to give P•+-C60bull;-. The electron transfer rate constant is −2 × 1011 s−1.

A molecular tetrad consisting of a free-base porphyrin (P) linked to a carotenoid polyene (C) and a diquinone moiety (Q A –Q B ) has been synthesized, and its photochemistry has been investigated using time-resolved techniques. Excitation of the porphyrin moiety of the tetrad in dichloromethane solution is followed by photoinduced electron transfer to yield an initial C-P + -Q − A –Q B state, which is formed with arate constant of 2.3×10 9 s −1 and a quantum yield of 0.87. In chloroform, the rate is 4.1×10 9 s −1 and the quantum yield is 0.94. Transient absorption studies show that this state evolves by subsequent electron transfer pathways to a final C + -P-Q A –Q − B charge-separated state whose lifetime is 7.4 μs in dichloromethane and 740 ns in chloroform. The quantum yield of the final state is 0.49 in dichloromethane and 0.57 in chloroform. The yield of the final state is substantially higher than that in a related, previously-reported tetrad in spite of the fact that the quantum yield of the initial C-P + -Q − A –Q B species is lower. This fact is interpreted in terms of the rates of charge-separation reactions relative to those of charge recombination. It is shown that yields of charge separation in multicomponent molecules may be altered in a predictable fashion using the basic tenets of electron transfer theory.

The Z–E-isomerisation of bilirubin upon excitation with visible light is a fundamental step in phototherapy of newborns with neonatal jaundice. Here we report results of an ultrafast optical spectr ...

The emission peaks and solvent-induced spectral shifts of the singlet oxygen transitions to the ground and first vibrational level of the ground triplet state of oxygen have been determined in a large number of solvents. The spectra were measured with a substantially higher accuracy than previously reported, by use of an FT spectrometer and a filtered white-light source.The results were interpreted in terms of the formation of van der Waals (vdW) complexes of singlet oxygen with four or six solvent molecules and existing theory satisfactorily accounts for the observed shifts in a number of solvents. For perfluorocarbons, no vdW complex is formed and it is suggested that weak long-range repulsive forces dominate, leading to small blue shifts in the emission peaks. The theory as used takes no account of specific interactions such as charge transfer, which could explain why the shifts predicted for the dispersion forces are smaller than the observed shifts in solvents of low ionisation potential. In solvents such as benzene, pyridine, toluene, hexachlorobutadiene, tetrachloroethene and iodopentafluorobenzene, charge-transfer interactions between all the solvent molecules and singlet oxygen are apparently important and lead to a further stabilisation of the complex.The bandwidths of the transitions did not correlate particularly well with the radiative rates, but did show a good correlation with the solvent spectral shifts. A number of other subtle solvent perturbations were observed. The ratio of the intensities of the (0–0) to (0–1) transitions showed a solvent dependence, as did the vibrational spacing of the ground triplet state of oxygen. These results indicate the existence of weak ground-state complexes and a relaxation of the poential-energy surfaces describing the complex states.

Ground-state, fluorescence, triplet-state and singlet-oxygen luminescence studies are reported for two purpurins, a metallo (SnIV) purpurin and a porphyrin tautomer, NTW, containing an unsaturated meso-substituent. Fluorescence lifetimes of the purpurins are short (ca. 2 ns) and the fluorescence yields are low ( 0.9) for NTW is suggested to be free rotation/isomerisation of the unsaturated meso-substituent. These results are discussed with respect to the possible role of purpurins, the tautomeric porphyrin, NTW, and the photo-oxidation products as sensitisers for photodynamic therapy of cancer.