Rate Constants Of Photoinduced Electron Transfer Research Articles

Photoinduced reactions between naphthoquinone and N,N,N′,N′-tetramethyl-p-phenylenediamine in the mixture of ionic liquid [BPy][NTf2] and acetonitrile studied by transient spectroscopy

Effects of the pyridinium ionic liquid N-butylpyridinium bis((trifluoromethyl)sulfonyl)imide ([BPy][NTf2]) on the photolysis behaviors of 1,4-naphthoquinone (NQ) in the acetonitrile solutions have been investigated by using nanosecond transient absorption techniques. When [BPy][NTf2] existed in the solution, the triplet 3NQ⁎ decayed more slowly and its half lifetime became longer. The photoinduced electron transfer rate constants of NQ and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) in the presence of [BPy][NTf2] were lower than those measured in acetonitrile, but of the same order of magnitude as estimated diffusion controlled rate constant. This finding suggests that [BPy][NTf2] could act as an appropriate solvent for electron transfer reactions. The photoinduced electron transfer reaction rate was determined not only by the viscosity but also by the microstructure of the solvent.

Binding of scandium ions to metalloporphyrin-flavin complexes for long-lived charge separation.

A porphyrin-flavin-linked dyad and its zinc and palladium complexes (MPor-Fl: 2-M, M=2 H, Zn, and Pd) were newly synthesized and the X-ray crystal structure of 2-Pd was determined. The photodynamics of 2-M were examined by femto- and nanosecond laser flash photolysis measurements. Photoinduced electron transfer (ET) in 2-H2 occurred from the singlet excited state of the porphyrin moiety (H2 Por) to the flavin (Fl) moiety to produce the singlet charge-separated (CS) state (1) (H2 Por(.+) -Fl(.-) ), which decayed through back ET (BET) to form (3) [H2 Por]*-Fl with rate constants of 1.2×10(10) and 1.2×10(9) s(-1) , respectively. Similarly, photoinduced ET in 2-Pd afforded the singlet CS state, which decayed through BET to form (3) [PdPor]*Fl with rate constants of 2.1×10(11) and 6.0×10(10) s(-1) , respectively. The rate constant of photoinduced ET and BET of 2-M were related to the ET and BET driving forces by using the Marcus theory of ET. One and two Sc(3+) ions bind to the flavin moiety to form the Fl-Sc(3+) and Fl-(Sc(3+) )2 complexes with binding constants of K1 =2.2×10(5) M(-1) and K2 =1.8×10(3) M(-1) , respectively. Other metal ions, such as Y(3+) , Zn(2+) , and Mg(2+) , form only 1:1 complexes with flavin. In contrast to 2-M and the 1:1 complexes with metal ions, which afforded the short-lived singlet CS state, photoinduced ET in 2-Pd⋅⋅⋅Sc(3+) complexes afforded the triplet CS state ((3) [PdPor(.+) -Fl(.-) (Sc(3+) )2 ]), which exhibited a remarkably long lifetime of τ=110 ms (kBET =9.1 s(-1) ).

Calculation from First Principles of Intramolecular Golden-Rule Rate Constants for Photo-Induced Electron Transfer in Molecular Donor–Acceptor Systems

The feasibility of calculating photoinduced intramolecular electron transfer rate constants in realistic molecular donor–acceptor systems via Fermi’s golden rule, using inputs obtained from state-of-the-art electronic structure techniques, is demonstrated and tested. To this end, calculations of photoinduced electron transfer rate constants were performed on two benchmark systems: (1) phenylacetylene-bridged carbazole-naphthalimide (meta and para) and (2) C60-(N,N-dimethylaniline). Intramolecular input parameters such as normal-mode frequencies, Huang–Rhys factors, and electronic coupling coefficients were obtained via ground state, time-dependent, and constrained density functional theory. Good agreement between the intramolecular Fermi’s golden rule rate constants and the experimental rate constants is found for both systems without accounting for the solvent reorganization. The relative roles of intramolecular vs intermolecular modes at promoting electron transfer and the validity of several limits of Fermi’s golden rule for describing intramolecular electron transfer are discussed.

Controlling the rate of electron transfer between a quantum dot and a tri-ruthenium molecular cluster by tuning the chemistry of the interface

Ultrafast transient absorption measurements reveal that the rate of photoinduced electron transfer (PET) from colloidal CdSe quantum dots (QDs) to oxo-centered triruthenium clusters (Ru(3)O) depends on the structure of the chemical headgroup by which the Ru(3)O clusters adsorb to the QDs. Complexes comprising QDs and Ru(3)O clusters adsorbed through a pyridine-4-carboxylic acid ligand (nic-Ru(3)O) have an intrinsic PET rate constant of (4.9 ± 0.9) × 10(9) s(-1) whereas complexes comprising QDs and Ru(3)O clusters adsorbed through a 4-mercaptopyridine ligand (thiol-Ru(3)O) have an intrinsic PET rate constant of (36 ± 7) × 10(9) s(-1). Cyclic voltammetry measurements of nic-Ru(3)O and thiol-Ru(3)O yield reduction potentials vs. Ag/AgCl of -0.93 V for both clusters, and density functional theory calculations of the nic-Ru(3)O and thiol-Ru(3)O clusters yield internal reorganization energies for the cluster radical anion of -0.17 eV and -0.19 eV, respectively. The small differences in driving force and reorganization energy between the two complexes rule out these parameters as possible explanations for the factor-of-seven difference in the rate constants for PET. The difference in the observed rates of PET for the two complexes is therefore attributable to a difference in donor-acceptor electronic coupling, which, according to electronic structure calculations, is modulated by the torsional angle between the Ru(3)O core of the cluster and the functionalized pyridine ligand that bridges the cluster to the QD surface.

Ligand-Controlled Rates of Photoinduced Electron Transfer in Hybrid CdSe Nanocrystal/Poly(viologen) Films

This paper describes a study of the rates of photoinduced electron transfer (PET) from CdSe quantum dots (QDs) to poly(viologen) within thin films, as a function of the length of the ligands passivating the QDs. Ultrafast (<10 ps), quantitative PET occurs from CdSe QDs coated with HS-(CH(2))(n)-COOH for n = 1, 2, 5, and 7 to viologen units. The observed decrease in the magnitude of the PET rate constant with n is weaker than that expected from the decay of the electron tunneling probability across extended all-trans mercaptocarboxylic acids but well-described by electron tunneling across a collapsed ligand shell. The PET rate constants for films with n = 10 and 15 are much slower than those expected based on the trend for n = 1-7; this deviation is ascribed to the formation of bundles of ligands on the surface of the QD that make the tunneling process prohibitively slow by limiting access of the viologen units to the surfaces of the QDs. This study highlights the importance of molecular-level morphology of donor and acceptor materials in determining the rate and yield of interfacial photoinduced electron transfer in thin films.

Fluorescence Properties of Phenol-Modified Zinc Phthalocyanine that Tuned by Photoinduced Intra-Molecular Electron Transfer and pH Values

Tetra[α-(4-hydroxyphenoxy)] zinc phthalocyanine, ZnPc(α-OPhOH)(4), was synthesized and its photophysics was found to be sharply pH dependent. Dual fluorescence emission around 700 nm was observed when it is dissolved in basic solution. The fluorescence of the phthalocyanine can be sharply switched off at pH 9.1 due to the intramolecular photoinduced electron transfer (PET) in ZnPc(α-OPhONa)(4), formed by the deprotonation of ZnPc(α-OPhOH)(4). The photophysics of both ZnPc(α-OPhOH)(4) and ZnPc(α-OPhONa)(4) were studied in detail by UV-vis absorption, steady state and time-resolved fluorescence and transient absorption (TA) to reveal the fluorescence quenching mechanism. Intra-molecular PET in ZnPc(α-OPhONa)(4) from the donor, PhONa subunits, to the acceptor, ZnPc moiety, was characterized by the much smaller fluorescence quantum yield (0.003) and lifetime (<0.20 ns). PET was further evidenced by the occurrence of charge separation state (CSS) in TA spectra, i.e. the bands due to anion radical of ZnPc and phenol radical. The lifetime of the charge separation state is ca. 3 ns, the efficiency of PET is ca. 99% and the rate constant of PET is 2.3 × 10(10) s(-1).

The effect of phenyl substitution on the fluorescence characteristics of fluorescein derivatives via intramolecular photoinduced electron transfer

UV-vis absorption, steady state fluorescence emission, time-correlated single photon counting and laser flash photolysis methods were employed to examine the excited state properties of fluorescein derivatives to understand the mechanism that controls their fluorescence efficiency. The fluorescein derivatives contain amino, t-butyl, carboxyl or nitro on their phenyl moieties, respectively. These substituents are not directly connected to the fluorophore but still showed a very remarkable effect on the fluorescence properties. Compared to fluorescein, the introduction of nitro, a strong electron withdrawing group, or amino, a strong electron donating group, caused a substantial quenching of both the fluorescence quantum yield and lifetime. The presence of a t-butyl or carboxyl, on the other hand, caused a smaller decrease. The mechanism for the substituent effect is due to the involvement of an additional de-excitation process, i.e. intramolecular photoinduced electron transfer (PET). The thermodynamics and kinetics of PET were analyzed. Depending on the nature of the substituent, the xanthenic ring acts as an electron acceptor (or donor), while the phenyl moiety is the corresponding electron donor (or acceptor) in PET. The rate constant of PET for the amino case is larger than 4.79 x 10(9) s(-1), while for nitro substitution it is 0.67 x 10(9) s(-1). Both values are much larger than the radiation rate constant of 0.20 x 10(9) s(-1), meaning that PET plays important roles in the deactivation of S(1) for the two dyes. The charge transfer state generated by PET was observed by laser flash photolysis.

Tetra(β-phenothiazinyl) zinc phthalocyanine: An easily prepared D 4–A system for efficient photoinduced electron transfer

Four identical electron donor (D) moieties, phenothiazines (PTZs), were covalently attached onto the same acceptor (A), zinc phthalocyanine (ZnPc), to form ZnPc(β-PTZ) 4, i.e. D 4–A. The symmetrical D 4–A was synthesized by the condensation method to examine intra-molecular photoinduced electron transfer (PET). The common electron donor-spacer–acceptor (D–A) photosynthetic models, which involve the asymmetrical synthesis of a mono-substituted porphyrin or its analogs, suffer from a low yield and arduous isolation, since D–A is only one of several products (D n –A or A n –D, n = 0, 2–4). The D 4–A preparation, however, can be carried out without the problem. The steady state and time-resolved fluorescence of the D 4–A were measured and compared with that of ZnPc(β-R) 4 (R = H, OPh). The result showed that the excited singlet state of phthalocyanine moiety in the D 4–A molecule was efficiently quenched by phenothiazine units owing to intra-molecular PET. The rate constant of PET ( k et) was calculated and the value is much higher than the rate constant of fluorescence emission, intersystem crossing and internal conversion of ZnPc moiety. The laser flash photolysis study revealed the presence of a long-lived charge-separated state due to PET. The results suggest that a D 4–A system can be a more efficient artificial photosynthetic model than D–A towards the practical commercial use.

Effects of DNA on driving force dependence of photoinduced electron transfer from the excited state of tris(2,2-bipyridine)ruthenium(II) to intercalators in DNA: Distinction between intra- and inter-DNA pathways

Photoinduced electron transfer (ET) dynamics from the excited state of a ruthenium complex [Ru(bpy) 3 2+ (bpy = 2,2′-bipyridine)] to a series of intercalators in DNA, 9-substituted-10-methylacridinium ions (AcrR +, R = H, CH 2Ph, Pr i and Ph), 3-substituted-1-methylquinolinium ions (RQuH +, R = H, Me, CN and Br) and 4- and 5-methylphenanthridinium ions (4- and 5-MePhen +), were examined from the emission decay profiles of Ru(bpy) 3 2+ in the absence and presence of DNA in an aqueous solution. Intercalation of AcrH + to DNA is found to result in inhibition of hydride transfer from an NADH model compound, 1-benzyl-1,4-dihydronicotinamide, to AcrH +. In contrast, the rate constants of photoinduced ET of intercalated molecules to DNA become much larger than those of free intercalators in solution due to the positive shift in the one-electron reduction potentials by the intercalation into DNA. The intramolecular pathway of photoinduced ET from Ru(bpy) 3 2+* bound electrostatically to DNA to intercalators bound to the same DNA molecule has been distinguished from the intermolecular pathway of photoinduced ET of intercalators bound to a different DNA molecule. The occurrence of photoinduced ET is examined by laser flash photolysis experiments which show the transient absorption spectra of the one-electron reduced intercalator when the ET is exergonic. The resulting data were analyzed in light of the Marcus theory of ET to determine reorganization energies of ET in DNA as well as in an aqueous solution.

Protic solvent effects on the photophysical properties of O-TiIV TSPP: photoinduced electron transfer

The photophysical properties of oxotitanium(IV)meso-tetra(4-sulfonatophenyl) porphyrin (O=Ti(IV)TSPP) have been investigated in water and methanol by laser spectroscopic techniques. The fluorescence emission spectrum of O=Ti(IV)TSPP in methanol exhibits two strong emission bands at 610 and 670 nm at room temperature with the decay time of ca. 310 +/- 10 ps and the rise time shorter than 30 ps, in contrast to the extremely weak emission with the decay time of ca. 27 +/- 4 ps in water, indicating that the fluorescence emissive states are different in the two solvents as supported by the solvent dependences of the excitation spectrum. The transient Raman spectra of O=Ti(IV)TSPP in water has been observed to exhibit a remarkable enhancement of phenyl-related mode at 1599 cm(-1), while in methanol, the Raman frequencies of the porphyrin skeletal modes (upsilon2 and upsilon4) are down-shifted without any apparent enhancement of the phenyl-related mode, indicating different interactions of the two solvents with the excited O=Ti(IV)TSPP. These Raman studies reveal that methanol molecule interacts with the photoexcited O=Ti(IV)TSPP more strongly than water, forming the exciplex, O=Ti(IV)TSPP(MeOH)*, suggesting that the two different emissive states are the singlet Franck-Condon state and the exciplex state in methanol and water, respectively. A broad triplet transient absorption of O=Ti(IV)TSPP has been also observed at 480 nm in water as well as in methanol, which is decreased upon addition of methyl viologen (MV2+) with appearance of a new absorption band at 620 nm. This indicates that the photoinduced electron transfer (PET) takes place from the porphyrin to MV2+ in both solvents. The kinetic analysis of the transient absorption band exhibits the PET rate constants of 4.76 x 10(5) s(-1) and 3,03 x 10(4) s(-1) in methanol and water, respectively. All these results infer that the PET takes place from the (d,pi) CT state and the triplet state of the excited porphyrin in methanol and water, respectively.

Linkage Dependent Charge Separation and Charge Recombination in Porphyrin-Pyromellitimide-Fullerene Triads

A homologous series of zincporphyrin (ZnP)-pyromellitimide (Im)-C60 linked triads where the pyromellitimide (Im) moiety is incorporated as an intermediate acceptor between the above two chromophores with a linkage of different spacers, ZnP-Im-CH2-C60, ZnP-Im-C60, and ZnP-CH2-Im-C60 as well as the reference dyads (ZnP-Im-CH2-ref, ZnP-Im-ref, and ZnP-CH2-Im-ref) have been prepared to investigate linkage dependence of photoinduced electron transfer (ET) and back ET to the ground state in the triads. Time-resolved transient absorption spectra of the triads measured by picosecond laser photolysis as well as the fluorescence lifetimes in THF reveal the occurrence of photoinduced ET from the singlet excited state of the ZnP to the Im moiety to give the initial charge-separated state, i.e., the zincporphyrin radical cation (ZnP•+)-imide radical anion (Im•-) pair, followed by a charge shift (CSH) to produce the final charge-separated state, the ZnP•+-C60•- pair. The rate constants of photoinduced ETs in ZnP-Im-C60 (1.8 × 1010 s-1) and ZnP-Im-CH2-C60 (1.3 × 1010 s-1) in THF are much larger than those in ZnP-CH2-Im-C60 (2.9 × 109 s-1) and ZnP-CH2-Im-ref (1.9 × 109 s-1). The larger charge separation (CS) rates in the former case are ascribed to the relatively strong electronic coupling because of the absence of a methylene linkage between the ZnP and the Im moieties in ZnP-Im-C60 and ZnP-Im-CH2-C60 as compared to the triad and dyad with the methylene linkage. The transient absorption spectra of the final charge-separated state, the ZnP•+-C60•- pair, have been also measured by nanosecond laser photolysis. It has been found that the rate constants of charge recombination (CR) of ZnP•+-Im-C60•- are temperature independent, but that the CR rate constants of ZnP•+-Im-CH2-C60•-exhibit an Arrhenius-like temperature dependence with an activation energy of 0.13 eV which corresponds to the energy difference between ZnP•+-Im•--CH2-C60 and ZnP•+-Im-CH2-C60•-. This indicates that the relatively strong electronic coupling without methylene linkage in ZnP-Im-C60 results in the preference of the tunneling superexchange ET over the sequential ET in the CR process which requires the thermal activation to reach the higher energy state (i.e., ZnP•+-Im•--C60), whereas the sequential ET predominates in the triads with the methylene linkage.

Distance Dependence of Photoinduced Electron Transfer in Metalloporphyrin Dimers

To study the dynamics and mechanism of intramolecular photoinduced electron transfer (PET) reactions, a series of (ZnII−FeIII) meso-tetraarylmetalloporphyrin dimers were synthesized and the kinetics of their PET reactivity was measured. Molecular building blocks were prepared by selective nucleophilic aromatic substitution of a para fluorine on tetraarylporphyrins containing a single pentafluorophenyl group. This synthetic approach allows a wide variety of systematic modifications such as type and length of spacer, metal center, and redox-potential difference between donor and acceptor. The edge-to-edge distance between the two porphyrins varies from 14.4 to 27.3 Å. Into a symmetric dimer, with two identical porphyrins covalently linked by a rigid partly saturated bridge, one zinc(II) and one iron(III) can be inserted. From measurements of fluorescence lifetimes the rate constants for PET from the electronically excited state of the zinc porphyrin to the bis(imidazole)iron porphyrin cation were evaluated. The electron-transfer rate decreases by a factor of only 165 when the distance increases by 13 Å. This small decrease is indicative of a surprisingly weak attenuation of the electronic coupling with distance.

Distance Dependence of Electron Transfer along Artificial β-Strands at 298 and 77 K

Photoinduced electron-transfer rate constants were measured for a series of binuclear metallopeptides consisting of a [Ru(bpy)2(cmbpy)]2+ electron donor tethered to a CoIII(NH3)5 electron acceptor ...

Direct evaluation of electronic coupling mediated by hydrogen bonds: implications for biological electron transfer.

Three supramolecular bischromophoric systems featuring zinc(II) and iron(III) porphyrins have been synthesized to evaluate the relative magnitudes of electronic coupling provided by hydrogen, sigma, and pi bonds. Laser flash excitation generates the highly reducing singlet excited state of the (porphinato)zinc chromophore that can subsequently be electron transfer quenched by the (porphinato)iron(III) chloride moiety. Measurement of the photoinduced electron transfer rate constants enables a direct comparison of how well these three types of chemical interactions facilitate electron tunneling. In contrast to generally accepted theory, electronic coupling modulated by a hydrogen-bond interface is greater than that provided by an analogous interface composed entirely of carbon-carbon sigma bonds. These results bear considerably on the analysis of through-protein electron transfer rate data as well as on the power of theory to predict the path traversed by the tunneling electron in a biological matrix; moreover, they underscore the cardinal role played by hydrogen bonds in biological electron transfer processes.

Photophysical behavior of poly(L‐lysine) carrying porphyrin and naphthyl chromophores

AbstractThe photophysical behavior of protoporphyrin IX (P) and 1‐naphthylacetic acid (N), covalently bound to ε‐amino groups of poly (L‐lysine) (PL), was investigated by steady‐state and time‐resolved fluorescence as a function of pH. Within the whole range of pH explored, i.e., 7–11, exciplex emission is minor and nearly pH independent. Fluorescence quantum yields, decay time measurements, and transient absorption spectra suggest that quenching of the excited naphthyl chromophore chiefly occurs by interconversion to the triplet state when the sample is randomly coiled and by intramolecular electron transfer (ET) from ground‐state porphyrin when the polypeptide is in α‐helical conformation. The kinetic law, based on a two‐state model for the polymeric matrix, is presented. The specific rate constant of photoinduced ET is 3.1 · 107 s−1 (25°C), in excellent agreement with that obtained by taking simply into account the lifetimes of naphthalene fluorescence in α‐helical PNPL and NPL (pH 11). The relaxation time of the helix‐coil transition was found to be definitely shorter than 20 ns. © 1994 John Wiley &amp; Sons, Inc.

Quenching of fluorescence in a series of covalently linked porphyrin-dinitrobenzene compounds

Isomeric donor-acceptor (D—A) compounds were synthesized by linking a 3,5-dinitroaromatic acceptor subunit, via an ester bridge, to the para, meta or ortho position of one of the aryl groups of meso-5,10,15,20-(tetraphenyl)porphyrin (H 2TPP) and its zinc (II) analogue (ZnTPP). UV-visible and proton nuclear magnetic resonance ( 1H NMR) spectra suggest the existence of weak intramolecular π-π interactions in these systems. Low fluorescence quantum yields and short lifetimes were obtained for all the ZnTPP-based D—A pairs in comparison with the corresponding values for ZnTPP. Multiexponential fluorescence decays obtained in five different solvents for these systems have been interpreted in terms of the coexistence of “closed” and “extended” forms of D—A structures that do not equilibrate within the singlet state lifetime of the donor porphyrin. A photoinduced electron transfer (PET) from the ZnTPP donor to the appended dinitroaromatic acceptor subunit of “closed” forms has been suggested. The apparent PET rate constants obtained from time-resolved ( k et TR) and steady state ( K et SS) fluorescence data vary as meta<para<ortho. This observation is inconsistent with the known distance dependence of k et and indicates an orientation influence on the PET reactions.

Luminescence and charge transfer. Part 2. Aminomethyl anthracene derivatives as fluorescent PET (photoinduced electron transfer) sensors for protons

The importance of the modular structure ‘fluor-spacer-amine ’is pointed out for the design of fluorescent molecular sensors for pH according to the principle of photoinduced electron transfer (PET). Anthracen-9-yl methylamines (24) and some azacrown ether analogues (15 and 23) are examined in this context. They show pH-dependent fluorescence quantum yields describable by eqn. (5) while all other electronic spectral parameters remain essentially pH-invariant. The range of pKa values of these sensors are understandable in terms of macrocyclic effects and the transmission of electric fields across the anthracene short axis. Phase-shift fluorometric determination of the fluorescence lifetimes of these sensors allows the calculation of the rate constant of PET in their proton-free form to be 1010–1011 s–1, with the diamines 23 and 24b exhibiting the faster rates.