Douglas G. Johnson

Ultrafast photoinduced electron transfer in rigid donor-spacer-acceptor molecules: modification of spacer energetics as a probe for superexchange

Abstract Four fixed-distance porphyrin-quinone molecules, 1-syn, 1-anti, 2-syn, and 2-anti, were synthesized. These molecules possess a zinc 5-phenyl-10,15,20-tripentylporphyrin electron donor attached to a naphthoquinone via a rigid pentiptycene spacer. The central benzene ring of the spacer is unsubstituted in 1 and possesses p-dimethoxy substituents in 2. The naphthoquinone is oriented either syn or anti to the porphyrin across the spacer. These molecules provide information concerning the orientation dependence of electron transfer between the porphyrin and the quinone, and the dependence of this transfer on low-lying ionic states of the spacer. The rate constants for the oxidation of the porphyrin lowest excited singlet state by the naphthoquinone are 1-syn: 8.2 x 109 s−1; 1-anti: 1.7 x 1010 s−1; 2-syn: 8.5 x 109 s−1; 2-anti: 1.9 x 1010 S−1. The corresponding rate constants for the porphyrin cation - naphthoquinone anion recombination reaction are 1-syn: 1.4 x 1010 s−1; 1-anti: 2.5 x 1010 s−1; 2-syn: 5.0 x 1010 s−1; 2-anti: 8.2 x 1010 s-1. The rate constants for the syn isomers are uniformly a factor of about 2 slower than those of the anti isomers. The charge separation reaction rates for 1 and 2 are similar, while the ion pair recombination reactions are about 3-4 x faster in 2 than in 1. The conformational effect is attributed to better overlap of the spacer wave functions in the anti vs the syn conformation, while the increase in recombination rate for 2 over 1 is attributed to a superexchange interaction involving an electronic configuration of the spacer in which the dimethoxybenzene cation contributes.

Temperature dependence of the lowest excited singlet‐state lifetime of all‐trans‐β‐carotene and fully deuterated all‐trans‐β‐carotene

A 4 ps, 450 nm laser pulse was used to electronically excite all‐trans‐β‐carotene and all‐trans‐β‐carotene‐d56 in 3‐methylpentane. The transient absorption spectra of these molecules were measured as a function of temperature down to 20 K. In all cases the 400–500 nm electronic absorption band of each carotene bleaches and a new absorption band near 560 nm appears immediately upon excitation. These bands recover with single exponential kinetics: τ=8.1±0.5 ps for all‐trans‐β‐carotene, and τ=10.5±0.6 ps for all‐trans‐β‐carotene‐d56 at 294 K. These recovery times increase by about a factor of 2 in glassy 3‐methylpentane, and are nearly independent of temperature from 100 to 20 K. The weak dependencies of the lowest excited single‐state lifetime of all‐trans‐β‐carotene on deuteration and temperature are discussed in terms of nonradiative decay mechanisms within carotenoids.

Determination of the primary charge separation rate in Photosystem II reaction centers at 15 K

We have measured the rate constant for the formation of the oxidized chlorophyll a electron donor (P680+) and the reduced electron acceptor pheophytin a− (Pheo a−) following excitation of isolated Photosystem II reaction centers (PS II RC) at 15 K. This PS II RC complex consists of D1, D2, and cytochrome b-559 proteins and was prepared by a procedure which stabilizes the protein complex. Transient absorption difference spectra were measured from 450–840 nm as a function of time with 500fs resolution following 610 nm laser excitation. The formation of P680+-Pheo a− is indicated by the appearance of a band due to P680+ at 820 nm and corresponding absorbance changes at 490, 515 and 546 nm due to the formation of Pheo a−. The appearance of the 490 nm and 820 nm bands is monoexponenital with τ=1.4±0.2 ps. Treatment of the PS II RC with sodium dithionite and methyl viologen followed by exposure to laser excitation results in accumulation of Pheo a−. Laser excitation of these prereduced RCs at 15 K results in formation of a transient absorption spectrum assigned to 1*P680. We observe wavelength-dependent kinetics for the recovery of the transient bleach of the Qy absorption bands of the pigments in both untreated and pre-reduced PS II RCs at 15K. This result is attributed to an energy transfer process within the PS II RC at low temperature that is not connected with charge separation.

Sub-picosecond photoinduced electron transfer in water-soluble porphyrin dimers

Abstract Picosecond as well as femtosecond transient absorption measurements have been performed on eight water-soluble heterodimers consisting of combinations of oppositely charged free base and metal porphyrins which have been electrochemically characterized. The ultrafast non-exponential absorption increase, attributed to solvent-controlled, intradimer charge separation, is energy-gap independent. By contrast, the much slower charge recombination is energy-gap dependent.

Photochemistry of polychromophoric arylamines

Abstract Rates and yields of intramolecular exciplex formation and electrontransfer in polychromophoric arylamines may be modified dramatically by varying the nature of the aryl or the amino group as well as the nature and length of the chain between the two groups. In trichromophoric systems 1 containing one aryl and two amino groups, photoinduced electron transfer may be affected between the aryl group and one of the amino groups in fairly non-polar solvents.

Picosecond charge separation and triplet formation in a closely spaced photosynthetic model system

Abstract Intramolecular electron transfer has been investigated for three diphenyletioporphyrins covalently linked to an anthraquinone via a sulfonyloxy bridge at the ortho, meta or para substitution position of the phenyl groups of the porphyrin. The ortho-linked porphyrin is strongly folded and exhibits extremely fast charge separation (CS) and recombination (CR) between porphyrin and anthraquinone (kCS > 3 × 1011 s−1 and kCR = 5.9 × 1010 s−1 in CH2Cl2). The less-folded meta- and para-linked porphyrins have smaller electron transfer rate constants. For the ortho-linked compound in toluene a biphasic charge separation is observed. The triplet state is formed in high yield via fast intersystem crossing in the charge-separated state.

Photoinduced Electron Transfer in Fixed Distance Chlorophyll-Quinone Donor-Acceptor Molecules

A series of fixed distance chlorophyll-quinone donor-acceptor molecules have been prepared. The donor consists of either methyl pyropheophorbide a or methyl pyrochlorophyllide a, while the acceptor is either benzoquinone or naphthoquinone. The acceptors are fused to a triptycene spacer group, which in turn is attached to the donors at their vinyl groups. Picosecond transient absorption measurements have been used to identify electron transfer from the lowest excited singlet state of the donor to the acceptor as the mechanism of fluorescence quenching in these molecules. The charge separation rate constants increase from 2 × 1010 s−1 to 4 × 1011 s−1 as the free energy of charge separation increases, while the radical pair recombination rate constants decrease from 1.2 × 1011 s−1 to 2 × 109 s−1 as the free energy of recombination increases. The resulting total reorganization energy λ = 0.9 eV.

The Primary Charge-Separation Rate in Isolated Photosystem II Reaction Center Complex

The primary charge-separation act in isolated bacterial reaction center (RC) complex occurs in 2.8 ps at room temperature and 0.7–1.2 ps at 10K [see refs. in 1]. Because of similarities between the bacterial and photosystem II (PSII) RC’s, it is of considerable interest to obtain analogous charge-separat ion rates in the higher plant system. Although the original isolation of the PSII RC was an important advance [2], the complex was too unstable [3–5] for femtosecond transient absorption spectroscopy. Recently, we have reported several procedures that stabilize the PSII RC complex in both the dark and the light [3,6]. One of these procedures produces material that is sufficiently stable to survive the extensive signal averaging required to measure the primary charge-separation act [1,7].

Photosynthetic Model Systems That Address the Role of Superexchange in Electron Transfer Reactions

Four fixed-distance porphyrin-quinone molecules, 1–syn, 1–anti, 2–syn, and 2–anti, were synthesized. These molecules possess a zinc 5–phenyl-10,15,20–tripentylporphyrin electron donor attached to a naphthoquinone via a rigid pentiptycene spacer. The central benzene ring of the spacer is unsubstituted in 1 and possesses p-dimethoxy substituents in 2. The naphthoquinone is oriented either syn or anti to the porphyrin across the spacer. These molecules provide information concerning the orientation dependence of electron transfer between the porphyrin and the quinone, and the dependence of this transfer on low-lying ionic states of the spacer. The rate constants for the oxidation of the porphyrin lowest excited singlet state by the naphthoquinone are 1–syn: 8.2 × 109 s-1; 1–anti: 1.7 × 1010 s-1; 2–syn: 8.5 × 109 s-1; 2–anti: 1.9 × 1010 s-1. The corresponding rate constants for the porphyrin cation — naphthoquinone anion recombination reaction are 1–syn: 1.4 × 1010 s-1; 1–anti: 2.5 × 1010 s-1; 2–syn: 5.0 × 1010 s-1; 2–anti: 8.2 × 1010 s-1. The rate constants for the syn isomers are uniformly a factor of about 2 slower than those of the anti isomers. The charge separation reaction rates for 1 and 2 are similar, while the ion pair recombination reactions are about 3–4 × faster in 2 than in 1. The conformational effect is attributed to better overlap of the spacer wave functions in the anti vs the syn conformation, while the increase in recombination rate for 2 over 1 is attributed to a superexchange interaction involving an electronic configuration of the spacer in which the dimethoxybenzene cation contributes.