Langmuir-Blodgett films of a cationic zinc porphyrin-imidazole-functionalized fullerene dyad: formation and photoelectrochemical studies

Electron donor-acceptor dyad ensembles of a water-soluble cationic zinc porphyrin (viz., zinc tetrakis(N-methylpyridinium)porphyrin tetrachloride, Zn(TMPyP)) and a C60 derivative that bears an imidazole ligand (viz., 2-(phenylimidazolyl)fulleropyrrolidine, C60im) were assembled during the formation of Langmuir and then Langmuir-Blodgett (LB) films. Surface pressure versus surface area isotherms and surface pressure time profiles, as well as Brewster angle microscopic images documented that the Langmuir films formed were remarkably stable. Subsequently, these Langmuir films were transferred onto different solid substrates, by using the LB technique, for spectroscopic and photoelectrochemical characterization. The UV-vis spectroscopic investigations confirmed that the water-soluble Zn(TMPyP) was, indeed, transferred together with C60im in the LB films. Upon visible light illumination of these LB films, deposited on the ITO transparent conductive supports, a photocurrent generated in the C60im-Zn(TMPyP) system is ascribed to an efficient photoinduced electron transfer from the electron donor, porphyrin singlet excited-state to the electron acceptor, C60. Overall, internal photon-to-current efficiency, IPCE, of the photoanodic current generation (with ascorbate as a sacrificial electron donor) in the ITO/C60im-Zn(TMPyP)/ascorbate/Pt construct is over 5x larger than that of the photocathodic system (with methyl viologen, MV2+, as a sacrificial electron acceptor) in the ITO/Zn(TMPyP)-C60im/MV2+/Pt construct. Highly ordered film stacking favors vectorial electron transfer within the dyad, giving rise to the highest IPCE values of 2.5% determined for a photoanode that was composed of around 20 monolayer films.     

Control of singlet oxygen generation photosensitized by meso-anthrylporphyrin through interaction with DNA

To control the activity of photosensitized singlet oxygen ((1)O(2)) generation, the electron donor-connecting porphyrin, 5-(9'-anthryl)-10,15,20-tris(p-pyridyl)porphyrin (AnTPyP), was designed and synthesized. AnTPyP became water-soluble by the protonation of the pyridyl moieties in the presence of 5 mM trifluoroacetic acid (pH 2.3). The photoexcited state of the porphyrin ring in an AnTPyP molecule was effectively deactivated by intramolecular electron transfer from the anthracene moiety within 0.04 ns in an aqueous solution. The deactivation was suppressed by the interaction with a DNA strand, resulting in the elongation of the lifetime of the porphyrin excited state and the enhancement of the fluorescence intensity. Furthermore, it was confirmed that the interaction enabled the photoexcited AnTPyP to generate (1)O(2). Selective (1)O(2) generation by forming a complex with DNA should be the initial step to realize the target selective photodynamic therapy.     

Redox-switchable direction of photoinduced electron transfer in an Ru(bpy)3(2+)-viologen dyad

Quenching of the 3MLCT excited state of [Ru(bpy)3]2+ (bpy=bipyridine) by the reduction products (MV*+ and MV0) of methyl viologen (MV2+) was studied by a combination of electrochemistry with laser flash photolysis or femtosecond pump-probe spectroscopy. Both for the bimolecular reactions and for the reactions in an Ru(bpy)3(2+)-MVn+ dyad, quenching by MV*+ and MV0 is reductive and gives the reduced ruthenium complex [Ru(bpy)3]+, in contrast to the oxidative quenching by MV2+. Rate constants of quenching (kq), and thermal charge recombination (krec) and cage escape yields (phi(ce)) were determined for the bimolecular reactions, and rates of forward (kf) and backward (kb) electron transfer in the dyad were measured for quenching by MV2+, MV*+, and MV0. The reactions in the dyad are very rapid, with values up to kf = 1.3 x 10(12) s(-1) for *Ru(bpy)3(2+)-MV*+. In addition, a long-lived (tau = 15 ps) vibrationally excited state of MV*+ with a characteristically structured absorption spectrum was detected; this was generated by direct excitation of the MV*+ moiety both at 460 and 600 nm. The results show that the direction of photoinduced electron transfer in a Ru(bpy)3-MV molecule can be switched by an externally applied bias.     

Competition between energy transfer and interligand electron transfer in porphyrin-osmium(II) bis(2,2':6',2' '-terpyridine) dyads

Rapid intramolecular energy transfer occurs from a free-base porphyrin to an attached osmium(II) bis(2,2':6',2' '-terpyridine) complex, most likely by way of the F?rster dipole-dipole mechanism. The initially formed metal-to-ligand, charge-transfer (MLCT) excited-singlet state localized on the metal complex undergoes very fast intersystem crossing to form the corresponding triplet excited state ((3)MLCT). This latter species transfers excitation energy to the (3)pi,pi* triplet state associated with the porphyrin moiety, such that the overall effect is to catalyze intersystem crossing for the porphyrin. Interligand electron transfer (ILET) to the distal terpyridine ligand, for which there is no driving force, competes poorly with triplet energy transfer from the proximal (3)MLCT to the porphyrin. Equipping the distal ligand with an ethynylene residue provides the necessary driving force for ILET and this process now competes effectively with triplet energy transfer to the porphyrin. The rate constants for all the relevant processes have been derived from laser flash photolysis studies.     

Intramolecular energy-transfer processes in a bis(porphyrin)-ruthenium(II) bis(2,2':6',2'-terpyridine) molecular array

A molecular triad has been synthesized comprising two free-base porphyrin terminals linked to a central ruthenium(II) bis(2,2':6',2'-terpyridine) subunit via meso-phenylene groups. Illumination into the ruthenium(II) complex is accompanied by rapid intramolecular energy transfer from the metal-to-ligand, charge-transfer (MLCT) triplet to the lowest-energy pi-pi* triplet state localized on one of the porphyrin subunits. Transfer takes place from a vibrationally excited level which lowers the activation energy. The electronic coupling matrix element for this process is 73 cm(-1). Selective illumination into the lowest-energy singlet excited state (S1) localized on the porphyrin leads to fast singlet-triplet energy transfer that populates the MLCT triplet state with high efficiency. This latter process occurs via Dexter-type electron exchange at room temperature, but the activation energy is high and the reaction is prohibited at low temperature. For this latter process, the electronic coupling matrix element is only 8 cm(-1).     

Photoinduced electron transfer in platinum(II) terpyridinyl acetylide complexes connected to a porphyrin unit

A series of six new dyads consisting of a zinc or magnesium porphyrin appended to a platinum terpyridine acetylide complex via a para-phenylene bisacetylene spacer are described. Different substituents on the 4' position of the terpyridinyl ligand were explored (OC7H15, PO3Et2, and H). The ground-state electronic properties of the dyads are studied by electronic absorption spectroscopy and electrochemistry, and they indicate some electronic interactions between the porphyrin subunit and the platinum complex. The photophysical properties of these dyads were investigated by steady-state, time-resolved, and femtosecond transient absorption spectroscopy in N,N-dimethylformamide solution. Excitation of the porphyrin unit leads to a very rapid electron transfer (2-20 ps) to the nearby platinum complex followed by an ultrafast charge recombination, thus preventing any observation of the charge separated state. The variation in the rate of the photoinduced electron transfer in the series of dyads is consistent with Marcus theory. The results underscore the potential of the para-phenylene bisacetylene bridge to mediate a rapid electron transfer over a long donor-acceptor distance.     

Energy transfer followed by electron transfer in a supramolecular triad composed of boron dipyrrin, zinc porphyrin, and fullerene: a model for the photosynthetic antenna-reaction center complex

The first example of a working model of the photosynthetic antenna-reaction center complex, constructed via self-assembled supramolecular methodology, is reported. For this, a supramolecular triad is assembled by axially coordinating imidazole-appended fulleropyrrolidine to the zinc center of a covalently linked zinc porphyrin-boron dipyrrin dyad. Selective excitation of the boron dipyrrin moiety in the boron dipyrrin-zinc porphyrin dyad resulted in efficient energy transfer (k(ENT)(singlet) = 9.2 x 10(9) s(-)(1); Phi(ENT)(singlet) = 0.83) creating singlet excited zinc porphyrin. Upon forming the supramolecular triad, the excited zinc porphyrin resulted in efficient electron transfer to the coordinated fullerenes, resulting in a charge-separated state (k(cs)(singlet) = 4.7 x 10(9) s(-)(1); Phi(CS)(singlet) = 0.9). The observed energy transfer followed by electron transfer in the present supramolecular triad mimics the events of natural photosynthesis. Here, the boron dipyrrin acts as antenna chlorophyll that absorbs light energy and transports spatially to the photosynthetic reaction center, while the electron transfer from the excited zinc porphyrin to fullerene mimics the primary events of the reaction center where conversion of the electronic excitation energy to chemical energy in the form of charge separation takes place. The important feature of the present model system is its relative "simplicity" because of the utilized supramolecular approach to mimic rather complex "combined antenna-reaction center" events of photosynthesis.     

Substituent effects on the site of electron transfer during the first reduction for gold(III) porphyrins

Gold(III) porphyrins of the type (P-R)AuPF(6), where P = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)porphyrin and R is equal to H (1), NO(2) (2), or NH(2) (3) which is substituted at one of the eight beta-pyrrolic positions of the macrocycle, were investigated as to their electrochemistry and spectroelectrochemistry in nonaqueous media. Each compound undergoes three reductions, the first of which involves the central metal ion to give a Au(II) porphyrin or a Au(III) porphyrin pi-anion radical depending upon the nature of the porphyrin ring substituent. A similar metal-centered reduction also occurs for compounds 1, 3, and Au(III) quinoxalinoporphyrin, (PQ)AuPF(6) (4), where PQ = 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin, and these results on the three Au(III) porphyrins overturn the long held assumption that reductions of such complexes only occur at the macrocycle. In contrast, when a NO(2) group is introduced on the porphyrin ring to give (P-NO(2))AuPF(6) (2), the site of electron transfer is changed from the gold metal to the macrocycle to give a porphyrin pi-anion radical in the first reduction step. This change in the site of electron transfer was examined by electrochemistry combined with thin-layer UV-vis spectroelectrochemistry and ESR spectroscopy of the singly reduced compound produced by chemical reduction. The reorganization energy (lambda) of the metal-centered electron transfer reduction for (P-H)AuPF(6) (1) in benzonitrile was determined as lambda = 1.23 eV by analyzing the rates of photoinduced electron transfer from the triplet excited states of an organic electron donor to 1 in light of the Marcus theory of electron transfer. The lambda value of the metal-centered electron transfer of gold porphyrin (1) is significantly larger than lambda values of ligand-centered electron transfer reactions of metalloporphyrins.     

卟啉-噁二唑二元体系光诱导能量及电子传递研究

以5-对氨基苯基-10,15,20-三苯基卟啉及2-苯基-5-(对氨基苯基)-1,3,4-噁二唑为原料合成了系列卟啉-噁二唑二元化合物, 其结构通过¹H NMR, ESI-MS, IR, UV-Vis确定. 对合成化合物进行光谱性能测定, 结果表明, 在卟啉与噁二唑混合体系中, 存在着卟啉激发态分子向噁二唑基态分子的分子间电子传递过程, 导致卟啉激发态的荧光猝灭; 在卟啉-噁二唑二元体系中, 315 nm激发下发生了由激发态噁二唑基团至卟啉基团的能量传递, 导致噁二唑基团荧光猝灭, 卟啉基团荧光增强. 420 nm激发下不存在分子内卟啉基团向噁二唑基团的电子回传竞争; 电化学性能测定进一步表明从噁二唑基团向卟啉基团的电子传递是可能的. 因此卟啉-噁二唑二元化合物可能作为一种模型, 模拟光合作用中电子给体至叶绿素之间的电子传递过程.     

Photoinduced electron transfer in multiporphyrinic interlocked structures: the effect of copper(I) coordination in the central site

Photoinduced processes have been determined in a [2]catenane containing a zinc(II) porphyrin, a gold(III) porphyrin, and two free phenanthroline binding sites, Zn-Au(+), and in the corresponding copper(I) phenanthroline complex, Zn-Cu(+)-Au(+). In acetonitrile solution Zn-Au(+) is present in two different conformations: an extended one, L, which accounts for 40 % of the total, and a compact one, S. In the L conformation, the electron transfer from the excited state of the Zn porphyrin to the gold-porphyrin unit (k = 1.3x10(9) s(-1)) is followed by a slow recombination (k = 8.3x10(7) s(-1)) to the ground state. The processes in the S conformation cannot be clearly resolved but a charge-separated (CS) state is rapidly formed and decays with a lifetime on the order of fifty picoseconds. In the catenate Zn-Cu(+)-Au(+), the zinc-porphyrin excited state initially transfers energy to the Cu(I)-phenantholine unit, producing a metal-to-ligand charge-transfer (MLCT) excited state localized on the copper complex with a rate k = 1.4x10(9) s(-1). From this excited state the transfer of an electron to the gold-porphyrin unit takes place, producing the CS state Zn-Cu(2+)-Au(.), which decays with a lifetime of 10 ns. The results are discussed in comparison with the closely related [2]rotaxane, in which a further charge shift from the copper center to the zinc-porphyrin unit leads to the fully CS state. Even in the absence of such full charge separation, it is shown that the lifetimes of the CS states are increased by a factor of about 2-2.5 over those of the corresponding rotaxanes.     

Stepwise charge separation and charge recombination in ferrocene-meso,meso-linked porphyrin dimer-fullerene triad

A meso,meso-linked porphyrin dimer [(ZnP)(2)] as a light-harvesting chromophore has been incorporated into a photosynthetic multistep electron-transfer model for the first time, including ferrocene (Fc), as an electron donor and fullerene (C(60)) as an electron acceptor to construct the ferrocene-meso,meso-linked porphyrin dimer-fullerene system (Fc-(ZnP)(2)-C(60)). Photoirradiation of Fc-(ZnP)(2)-C(60) results in photoinduced electron transfer from the singlet excited state of the porphyrin dimer [(1)(ZnP)(2)] to the C(60) moiety to produce the porphyrin dimer radical cation-C(60) radical anion pair, Fc-(ZnP)(2)(*+)-C(60)(*-). In competition with the back electron transfer from C(60)(*-) to (ZnP)(2)(*+) to the ground state, an electron transfer from Fc to (ZnP)(2)(*+) occurs to give the final charge-separated (CS) state, that is, Fc(+)-(ZnP)(2)-C(60)(*-), which is detected as the transient absorption spectra by the laser flash photolysis. The quantum yield of formation of the final CS state is determined as 0.80 in benzonitrile. The final CS state decays obeying first-order kinetics with a lifetime of 19 micros in benzonitrile at 295 K. The activation energy for the charge recombination (CR) process is determined as 0.15 eV in benzonitrile, which is much larger than the value expected from the direct CR process to the ground state. This value is rather comparable to the energy difference between the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)) and the final CS state (Fc(+)-(ZnP)(2)-C(60)(*-)). This indicates that the back electron transfer to the ground state occurs via the reversed stepwise processes,that is, a rate-limiting electron transfer from (ZnP)(2) to Fc(+) to give the initial CS state (Fc-(ZnP)(2)(*+)-C(60)(*-)), followed by a fast electron transfer from C(60)(*-) to (ZnP)(2)(*+) to regenerate the ground state, Fc-(ZnP)(2)-C(60). This is in sharp contrast with the extremely slow direct CR process of bacteriochlorophyll dimer radical cation-quinone radical anion pair in bacterial reaction centers.     

Convenient scaffold for forming heteroporphyrin arrays in aqueous media

A new methodology for preparing heteroporphyrin arrays in aqueous solution has been presented. The present method is based on the extremely strong ability of heptakis(2,3,6-tri-O-methyl)-beta-cyclodextrin (TMe-beta-CD) to include 5,10,15,20-tetrakis(p-substituted-phenyl)porphyrins (Por) affording trans-type 1:2 complexes of the porphyrins and TMe-beta-CD. Two different Por-per-O-methylated beta-CD (per-Me-beta-CD) conjugates were synthesized. Conjugate 2 was prepared by an S(N)2 reaction of 5,10,15,20-tetrakis(p-hydroxyphenyl)porphyrin and per-O-methylated beta-cyclodextrin having one primary OTs group. Four per-Me-beta-CD moieties are attached to the meso positions of 2. Conjugate 3, synthesized from 5-(p-hydroxyphenyl)-10,15,20-tris(3,5-dicarboxyphenyl)porphyrin and monotosylated per-O-methylated beta-cyclodextrin, has one per-Me-beta-CD moiety at the periphery of the porphyrin. Conjugate 2 yields a stable 1:4 complex with the zinc complex of 5-phenyl-10,15,20-tris(3,5-dicarboxyphenyl)porphyrin (8) in the dissociated form. In this system, the energy transfer from photoexcited Zn-8 to free base 2 occurs with 85% efficiency. Conjugate 3 forms a very stable 1:1 complex with Zn-8 (K = (7.0 +/- 0.3) x 10(5) dm(3) mol(-1)) with an energy transfer efficiency (93%) larger than that obtained in the case of 2. The structure of the 3-Zn-8 complex, which can account for the efficient energy transfer, was deduced from (1)H NMR spectroscopy. Intramolecular fluorescence quenching of 2 and 3 by Fe(III)-8 also occurred through an electron-transfer process as the main quenching mechanism. The present method is a very simple and convenient means to construct various heteroporphyrin arrays in aqueous solution.     

Physicochemical and photophysical studies on porphyrin-based donor-acceptor systems: effect of redox potentials on ultrafast electron-transfer dynamics

We report new polychromophoric complexes, where different porphyrin (P) derivatives are covalently coupled to a redox active Mo center, MoL*(NO)Cl(X) (L* is the face-capping tridentate ligand tris(3,5-dimethylpyrazolyl) hydroborate and X is a phenoxide/pyridyl/amido derivative of porphyrin). The luminescence quantum yields of the bichromophoric systems (1, 2, and 5) were found to be an order of magnitude less than those of their respective porphyrin precursors. Transient absorption measurements revealed the formation of the porphyrin radical cation species (P(.)(+)) and photoinduced electron transfer from the porphyrin moiety to the respective Mo center in 1, 2, and 5. Electrochemical studies showed that the reduction potentials of the acceptor Mo centers in a newly synthesized pyridyl derivative (2; E(1/2)[Mo(I/0)] = approximately -1.4 V vs Ag/AgCl) and previously reported phenoxy- (1; E(1/2)[Mo(II/I)] = approximately -0.3 V vs Ag/AgCl) and amido- (3; E(1/2)[Mo(II/I)] = approximately -0.82 V vs Ag/AgCl) derivatives were varied over a wide range. Thus, studies with these complexes permitted us to correlate the probable effect of this potential gradient on the electron-transfer dynamics. Time-resolved absorption studies, following excitation at the Soret band of the porphyrin fragment in complexes 1, 2, and 5, established that forward electron transfer took place biexponentially from both S2 and S1 states of the porphyrin center to the Mo moiety with time constants 150-250 fs and 8-20 ps, respectively. In the case of MoL*(NO)ClX (where X is pyridine derivative 2), the high reduction potential for the MoI/0 couple allowed electron transfer solely from the S2 state of the porphyrin center. Time constants for the charge recombination process for all complexes were found to be 150-300 ps. Further, electrochemical and EPR studies with the trichromophoric complexes (3 and 4) revealed that the orthogonal orientation of the peripheral phenoxy/pyridyl rings negated the possibility of any electronic interaction between two paramagnetic Mo centers in the ground state and thereby the spin exchange, which otherwise was observed for related Mo complexes when two Mo centers are separated by a polyene system with comparable or larger separation distances.     

Photoinduced intermolecular and intramolecular actions between eosin and porphyrin

Two dyads of eosin and porphyrin linked with a semi-rigid (-CH2phCH2-) or flexible (-(CH2)4-) bridge and their reference model compounds were synthesized and characterized The intermoleccular interaction and intramolecular photoinduced singlet energy transfer and electron transfer were studied by their absorp tion spectra,fluorescence emission,excitation spectra and fluorescence lifetime The model compounds,ethyl ester of eosm (EoEt) and porphyrin (PorEt),could form complexes in the ground state.When the eosin moieties in dyads were excited,they could transfer some singlet energy to the porphyrins; in the meantime,they could also ndsce electron transfer between two chromophores.Exciting the porphyrin moieties in dyads could induce electron transfer from eosin moieties to porphyrin moieties.The efficiencies (EnT,ET) and rate constants (kEnT,kET) were related to the polarity of solvents and mutual orientation of the two chromophores in dyads.     

Photoinduced electron transfer in a hexaphenylbenzene-based self-assembled porphyrin-fullerene triad

A hexaphenylbenzene-based zinc porphyrin dyad forms a 1:1 complex with a fullerene bearing two pyridyl groups via coordination of the pyridyl nitrogens with the zinc atoms. The fullerene is symmetrically located between the two zinc porphyrins. The binding constant for the complex is 7.3 x 10(4) M(-1) in 1,2-difluorobenzene. Photoinduced electron transfer from a porphyrin first excited singlet state to the fullerene occurs with a time constant of 3 ps, and the resulting charge-separated state has a lifetime of 230 ps. This self-assembled construct should form a basis for the construction of more elaborate model photosynthetic antenna-reaction center systems.     

Regiospecifically alpha-13C-labeled porphyrins for studies of ground-state hole transfer in multiporphyrin arrays

Insight into the electronic communication between the individual constituents of multicomponent molecular architectures is essential for the rational design of molecular electronic and/or photonic devices. To clock the ground-state hole/electron-transfer process in oxidized multiporphyrin architectures, a p-diphenylethyne-linked zinc porphyrin dyad was prepared wherein one porphyrin bears two (13)C atoms and the other porphyrin is unlabeled. The (13)C atoms are located at the 1- and 9-positions (alpha-carbons symmetrically disposed to the position of linker attachment), which are sites of electron/spin density in the a(1u) HOMO of the porphyrin. The (13)C labels were introduced by reaction of KS(13)CN with allyl bromide to give the allyl isothiocyanate, which upon Trofimov pyrrole synthesis followed by methylation gave 2-(methylthio)pyrrole-2-(13)C. Reaction of the latter with paraformaldehyde followed by hydrodesulfurization gave dipyrromethane-1,9-(13)C, which upon condensation with a dipyrromethane-1,9-dicarbinol bearing three pentafluorophenyl groups gave the tris(pentafluorophenyl)porphyrin bearing (13)C labels at the 1,9-positions and an unsubstituted meso (5-) position. Zinc insertion, bromination at the 5-position, and Suzuki coupling with an unlabeled porphyrin bearing a suitably functionalized diphenylethyne linker gave the regiospecifically labeled zinc porphyrin dyad. Examination of the monocation of the isotopically labeled dyad via electron paramagnetic resonance (EPR) spectroscopy (and comparison with the monocations of benchmark monomers, where hole transfer cannot occur) showed that the hole transfer between porphyrin constituents of the dyad is slow (<10(6) s(-1)) on the EPR time scale at room temperature. The slow rate stems from the a(1u) HOMO of the electron-deficient porphyrins, which has a node at the site of linker connection. In contrast, analogous dyads of electron-rich porphyrins (wherein the HOMO is a(2u) and has a lobe at the site of linker connection) studied previously exhibit rates of hole transfer that are fast (>5 x 10(7) s(-1)) on the EPR time scale at room temperature.     

Effect of surface immobilization on intramolecular and intermolecular electron transfer in a chromophore-donor-acceptor assembly

A chromophore-donor-acceptor assembly [Ru(bpyCOOH)(bpyCH(2)MV(2+)) (bpyCH(2)PTZ)](4+)(1) (where bpyCOOH = 4-carboxylic acid-4'-methyl-2,2'-bipyridine, bpyCH(2)MV(2+) = 1-[(4'-methyl-2,2'-bipyridin-4-yl)methyl]-1'-methyl-4,4'-bipyridinediium, and bpyCH(2)PTZ = 10-[(4'-methyl-2,2'-bipyridin-4-yl)methyl]phenothiazine) has been adsorbed on the surface of nanocrystalline ZrO(2) and its excited state properties studied by emission and transient absorption spectroscopy. In deaerated acetonitrile solution, the complex emits weakly with an emission quantum yield of phi(em) approximately equal to 0.01 with an excited-state lifetime of tau approximately equal to 20 ps. Emission from the surface-adsorbed complex is intense, with phi(em) approximately equal to 0.4 and tau approximately equal to 40 ns. The increase in emission on the surface is likely due to a significant inhibition to the electron-transfer quenching of the metal-to-ligand charge transfer (MLCT) excited state caused by surface adsorption-induced changes in the redox potentials. Transient (nanosecond time scale) absorption monitoring, following laser flash photolysis, reveals the presence of a transient or transients that are formed during the flash. Transient spectral changes that occur during and after the flash are consistent with the formation and decay of the intermediate ZrO(2)-[Ru(bpyCOOH)(bpyCH(2)MV(+*))(bpyCH(2)PTZ(+*))](4+). It returns to the ground state by both intramolecular and intermolecular processes. Intramolecular electron transfer occurs with k(BET) = 6.3 x 10(6) s(-1) (tau = 160 ns), which is comparable to the rate constant for back-electron transfer in solution. The back-electron transfer is a second-order process and is much slower, with k(BET) = 390 M(-1) s(-1) (tau = 2.6 ms).     

Bridge-dependent electron transfer in porphyrin-based donor-bridge-acceptor systems.

Photoinduced electron transfer in donor-bridge-acceptor systems with zinc porphyrin (or its pyridine complex) as the donor and gold(III) porphyrin as the acceptor has been studied. The porphyrin moieties were covalently linked with geometrically similar bridging chromophores which vary only in electronic structure. Three of the bridges are fully conjugated pi-systems and in a fourth, the conjugation is broken. For systems with this bridge, the quenching rate of the singlet excited state of the donor was independent of solvent and corresponded to the rate of singlet energy transfer expected for a F?rster mechanism. In contrast, systems with a pi-conjugated bridging chromophore show a solvent-dependent quenching rate that suggests electron transfer in the Marcus normal region. This is supported by picosecond transient absorption measurements, which showed formation of the zinc porphyrin radical cation only in systems with pi-conjugated bridging chromophores. On the basis of the Marcus and Rehm-Weller equations, an electronic coupling of 5-20 cm(-)(1) between the donor and acceptor is estimated for these systems. The largest coupling is found for the systems with the smallest energy gap between the donor and bridge singlet excited states. This is in good agreement with the coupling calculated with quantum mechanical methods, as is the prediction of an almost zero coupling in the systems with a nonconjugated bridging chromophore.     

Photoinduced Electron and Energy Transfer in Dyads of Porphyrin Dimer and Perylene Tetracarboxylic Diimide

Two compounds containing a porphyrin dimer and a perylene tetracarboxylic diimide (PDI) linked by phenyl ( 1 ) or ethylene groups ( 2 ) are prepared. The photophysical properties of these two compounds are investigated by steady state electronic absorption and fluorescence spectra and lifetime measurements. The ground state absorption spectra reveal intense interactions between the porphyrin units within the porphyrin dimer, but no interactions between the porphyirn dimer and PDI. The fluorescence spectra suggest efficient energy transfer from PDI to porphyrin accompanied by less efficient electron transfer from porphyrin to PDI. The energy transfer is not affected by the dimeric structure of porphyrin or the linkage between the porphyrin dimer and PDI. However, the electron transfer from porphyrin to PDI is significantly affected by either the linkage between the donor and the acceptor or the polarity of the solvents. The dimeric structure of the porphyrin units in these compounds significantly promotes electron transfer in nonpolar, but not in polar solvents.     

Electronic and fluorescence spectral studies of a novel porphyrin-polypyridyl ruthenium(II) hybrid linked by a butyl chain

The electronic and fluorescence spectroscopic properties of a novel porphyrin-polypyridyl ruthenium(II) hybrid, [C(4)-TPP-(ip)Ru(phen)(2)](ClO(4))(2) (TPP=5,10,15,20-tetraphenylporphyrin, ip=imidazo[4,5-f][1,10]phenanthroline and phen=1,10-Phenanthroline), in which a polypyridyl ruthenium(II) moiety is linked to a porphyrin moiety by a butyl chain have been investigated and compared to its corresponding reference compounds. The studies of electronic absorption spectra have shown that there is an electronic interaction between the porphyrin moiety and the polypyridyl ruthenium(II) moiety in the hybrid. It can be found that intramolecular photoinduced electron and energy transfer processes may occur in the hybrid from the fluorescence spectra. When exciting in Soret band and Q band of porphyrin, the fluorescence quenching of the porphyrin moiety of the hybrid takes place due to electron transfer from the lowest singlet excited state (S(1)) to the appended polypyridyl rutherium(II) moiety, while the decay of S(2) (the second-excited singlet state) of the porphyrin moiety is mainly contributed to internal conversion to S(1). When exciting in MLCT band of the polypyridyl ruthenium(II) moiety, fluorescence corresponding to the polypyridyl ruthenium(II) moiety is quenched by intramolecular energy transfer from (3)MLCT of the ruthenium moiety to the lowest-energy triplet state localized on the porphyrin moiety.