Mark P. Niemczyk

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.

Selectively metalated doubly cofacial porphyrin trimers. New models for the study of photoinduced intramolecular electron transfer.

Abstract The preparation of a doubly cofacial porphyrin trimer consisting of a Zn mesoporphyrin - Zn mesoporphyrin - mesoporhyrin stack is described. The optical absorption and fluorescence properties of the molecule as a function of solvent are also reported.

Ultrafast photoinduced electron transfer reactions in supramolecular arrays: From charge separation and storage to molecular switches

Photochemical electron transfer reactions on a picosecond time scale have been studied in two covalently-linked donor-acceptor systems. The first molecule is a chlorophyll-porphyrin-quinone triad that closely mimics photosynthetic charge separation by undergoing picosecond electron transfer in low temperature glasses to yield a radical ion pair that lives for 2 ms and exhibits spin-polarization. The second molecule is an electron donor-acceptor-donor molecule, consisting of two porphyrin donors rigidly attached to opposite ends of the two-electron acceptor N,N’-diphenyl3,4,9,1O-perylenebis(dicarboximide). This molecule acts as a light intensity dependent molecular switch on a picosecond time scale.

Chlorin-based supramolecular assemblies for artificial photosynthesis

Abstract Photosynthetic reaction center models consisting of zinc 9-desoxo-pyrochlorophyllide a primary electron donors, ZC, that are directly bonded at their 3-position to the 5-position of a 2,8,12,18-tetraethyl-3,7,13,17-tetramethylporphyrin, ZP, which is in turn bonded at its 15-position to 2-triptycenenaphthoquinone, 2-triptycenebenzoquinone, 1-triptycenebenzo-quinone, or N-(4-phenyl),N′-(n-octyl)-l,4,5,8-naphthalenediimide, 1, 2, 3, and 4, respectively, were prepared. Steric hindrance between adjacent substituents positions the π system of the chlorophyll perpendicular to that of the porphyrin. In turn, the π system of the chlorophyll is held about 60° to that of the O-O axis of the quinones in 1 and 2, parallel to the quinone O-O axis in 3, and parallel to the N-N axis in the diimide acceptor in 4. The resulting structures place the ZC donors in 1, 2, 3 and 4 at fixed center-to-center distances of 20, 18, 14, and 21 A from the acceptors, respectively. Photoexcitation of 1–4 in 2-methyltetrahydrofuran glass at 77K results in a single observable electron transfer reaction: 1 ZC-ZP-X→ZC+-ZP-X−, where X = benzoquinone (BQ), naphthoquinone (NQ) or naphthalenediimide (NI), that occurs with τ = 4.5, 3.3, 2.0, and 2.0 ps for 1, 2, 3, and 4, respectively. The final ZC+-ZP-X− radical pairs live for 12.7, 8.4, 2.5, and 10 ms at 77K in 1, 2, 3, and 4, respectively, and exhibit spin-polarized EPR spectra characteristic of spin-correlated radical pairs. The EPR spectra of 1–4 can be simulated using the distances and orientations of the radicals relative to one another determined from the molecular structures of 1–4. These long-lived, spin-polarized radical ion pairs closely mimic the bacteriochlorophyll cation – quinone anion radical pair produced in photosynthetic reaction centers and provide a useful tool for studying the interaction of the surrounding medium with the charge separated radical ion pair.

Effects of chemical modifications on photophysics and exciton dynamics of π-conjugation attenuated and metal-chelated photoconducting polymers

Abstract Effects of two types of chemical modifications on photoconducting polymers consisting of polyphenylene vinylene (PPV) derivatives are studied by static and ultrafast transient optical spectroscopy as well as semi-empirical ZINDO calculations. The first type of modification inserts 2,2′-bipyridyl-5-vinylene (bpyV) units in the PPV backbone, and the second type involves metal-chelation with the bpy sites. Photoluminescence (PL) and exciton dynamics of polymers 1 and 2 with PV:bpyV ratios of 1 and 3 were examined in solution, and compared with those of the homo-polymer, poly(2,5-bis(2′-ethylhexyloxy)-1,4-phenylene vinylene) (BEH-PPV). Similar studies were carried out for several metal-chelated polymers. These results can be explained by changes in π-conjugation throughout the polymer backbone. The attenuation in π-conjugation by the chemical modifications transforms a conducting polymer from one-dimensional semiconductor to molecular aggregates.

Spin-Polarized Radical Ion Pair Formation Resulting from Two-step Electron Transfer from the Lowest Excited Singlet State of a Fixed-Distance Photosynthetic Model System at 5 K

Abstract A photosynthetic reaction center model consisting of a zinc porphyrin primary electron donor, ZP, positioned between a naphthoquinone electron acceptor, NQ, and an N,N,N,N–tetraalkyl–p–phenylenediamine secondary electron donor, TAPD, was synthesized. The resulting rigid structure places the TAPD donor at a fixed 23 A center–to–center distance from the NQ acceptor. Excitation of ZP with 540 nm light in butyronitrile glass at 5 K results in two–step sequential electron transfer: TAPD–1*ZP–NQ > TAPD–ZP+−NQ− −> TAPD+−ZP–NQ−. The final TAPD+−ZP–NQ− radical pair lives for 4 ms and exhibits a spin–polarized EPR spectrum characteristic of a spin–correlated radical pair. The EPR spectrum of this long–lived, spin–polarized radical ion pair closely mimics the bacteriochlorophyll cation – quinone anion radical pair produced in photosynthetic reaction centers.

Photorefractive Conjugated Polymer-Liquid Crystal Composites

A new mechanism for space-charge field formation in photorefractive liquid crystal composites containing poly(2,5-bis(2{prime}-ethylhexyloxy)-1,4-phenylenevinylene) (BEH-PPV) and the electron acceptor N,N{prime}-dioctyl-1,4:5,8-naphthalenediimide, NI, is observed. Using asymmetric energy transfer (beam coupling) measurements that are diagnostic for the photorefractive effect, the direction of beam coupling as a function of grating fringe spacing inverts at a spacing of 5.5 {micro}m. The authors show that the inversion is due to a change in the dominant mechanism for space-charge field formation. At small fringe spacings, the space-charge field is formed by ion diffusion in which the photogenerated anion is the more mobile species. At larger fringe spacings, the polarity of the space charge field inverts due to dominance of a charge transport mechanism in which photogenerated holes are the most mobile species due to hole migration along the BEH-PEV chains coupled with interchain hole hopping. Control experiments are presented, which use composites that can access only one of the two charge transport mechanisms. The results show that charge migration over long distances leading to enhanced photorefractive effects can be obtained using conjugated polymers dissolved in liquid crystals.

Low Temperature Ultrafast Charge Separation: Rate Versus Free Energy

The rate of electron transfer from the lowest excited singlet state of a porphyrin to an acceptor was measured in 2-MTHF glass at 77 K for a series of molecules. Analysis of the rate data as a function of free energy of reaction reveals that the ion-pair state of the oxidized donor and reduced acceptor is destabilized in solid solution by about 0.7 eV relative to its energy in polar liquids.

Multi-Step Electron Transfer in Rigid Photo-synthetic Models at Low Temperature: Requirements for Charge Separation and Spin-Polarized Radical Ion Pair Formation

Photoinduced, multi-step charge separation in bacterial photosynthetic reaction centers proceeds from the lowest excited singlet state of the dimeric bacteriochlorophyll electron donor in two steps to yield a weakly interacting dimer cation — quinone anion radical pair, P+-Q−, separated by 28 A.1 The chromophores within the reaction center are positioned at precise distances and orientations to insure that the electronic coupling between P+ and Q− is sufficiently weak to allow P+-Q− to live for about 100 ms.2 At long distances the electron-electron exchange interaction, J, between radicals within a charge separated ion pair is sufficiently weak that differences in local magnetic fields surrounding each radical result in S-T0 mixing of the radical pair spin sublevels.3 This mixing produces a non-Boltzmann population of the spin sublevels of the radical pair and may result in the appearance of spin-polarized EPR spectra. Such spectra have been reported extensively for both bacterial and green plant reaction centers,4–6 but have not been observed previously in rigid model systems.

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.