Photoinduced charge separation (CS) and charge recombination (CR) processes have been examined in various porphyrin−fullerene linked systems (i.e., dyads and triads) by means of time-resolved transient absorption spectroscopy and fluorescence lifetime measurements. The investigated compounds comprise a homologous series of rigidly linked, linear donor−acceptor arrays with different donor−acceptor separations and diversified donor strength:  freebase porphyrin−C60 dyad (H2P-C60), zincporphyrin−C60 dyad (ZnP-C60), ferrocene−zincporphyrin−C60 triad (Fc-ZnP-C60), ferrocene−freebase porphyrin−C60 triad (Fc-H2P-C60), and zincporphyrin−freebase porphyrin−C60 triad (ZnP-H2P-C60). Most importantly, the lowest lying charge-separated state of all the investigated systems, namely, that of ferrocenium ion (Fc+) and the C60 radical anion (C60•-) pair in the Fc-ZnP-C60 triad, has been generated with the highest quantum yields (close to unity) and reveals a lifetime as long as 16 μs. Determination of CS and CR rate const...

Modulating charge separation and charge recombination dynamics in porphyrin-fullerene linked dyads and triads: Marcus-normal versus inverted region.

Ultrafast photochemical reactions in liquids occur on similar or shorter time scales compared to the equilibration of the optically populated excited state. This equilibration involves the relaxation of intramolecular and/or solvent modes. As a consequence, the reaction dynamics are no longer exponential, cannot be quantified by rate constants, and may depend on the excitation wavelength contrary to slower photochemical processes occurring from equilibrated excited states. Such ultrafast photoinduced reactions do no longer obey the Kasha–Vavilov rule. Nonequilibrium effects are also observed in diffusion-controlled intermolecular processes directly after photoexcitation, and their proper description gives access to the intrinsic reaction dynamics that are normally hidden by diffusion. Here we discuss these topics in relation to ultrafast organic photochemical reactions in homogeneous liquids. Discussed reactions include intra- and intermolecular electron- and proton-transfer processes, as well as photochr...

Ultrafast Elementary Photochemical Processes of Organic Molecules in Liquid Solution

Both charge-transfer absorption and emission have been observed in porphyrin-linked fullerene where the C60 moiety is closely located on the porphyrin plane. Electron-transfer parameters including reorganization energies, free energy changes, and electronic coupling matrix elements were determined by analyzing the charge-transfer absorption and emission in benzene. The reorganization energy is estimated as 0.23 ± 0.11 eV, which is the smallest value among inter- and intramolecular donor−acceptor systems ever reported and is comparable to the smallest ones for the primary charge separation in the photosynthetic reaction center. The results clearly show that fullerenes combined with porphyrins are potential components for constructing artificial photosynthetic systems.

An Extremely Small Reorganization Energy of Electron Transfer in Porphyrin−Fullerene Dyad

Solvent polarity and temperature effect on the photophysical properties of two 1,2-benzopyrone dyes, namely, coumarin-152 (C152) and coumarin-481 (C481), have been investigated using steady-state and time-resolved fluorescence measurements. In nonpolar (NP) solvents (e.g., hexane, cyclohexane, methylcyclohexane, 2-methylpentane, and 3-methylpentane), the Stokes' shifts (Δν) and fluorescence lifetimes (τf) for both the dyes are unexpectedly lower. Excluding NP solvents, for all other solvents the Δν correlates linearly with the solvent polarity function, Δf = {(e − 1)/(2e + 1) − (n2 − 1)/(2n2 + 1)}, and the slopes of these plots indicate that the fluorescent states for both the dyes in these solvents are of intramolecular charge transfer (ICT) character. The unusually lower Δν and τf values in NP solvents have been rationalized considering nonpolar structures for the fluorescent states of the dyes in these solvents. The fluorescence quantum yields (Φf) and τf values of the two dyes also show an unusual ...

http://nathan.instras.com/documentDB/paper-439.pdf

Photophysical Properties of Coumarin-152 and Coumarin-481 Dyes: Unusual Behavior in Nonpolar and in Higher Polarity Solvents

The broad varieties of organic and organometallic reactions merge into a common unifying mechanism by considering all nucleophiles and electrophiles as electron donors (D) and electron acceptors (A), respectively. Comparison of outer-sphere and inner-sphere electron transfers with the aid of Marcus theory provides the thermochemical basis for the generalized free energy relationship for electron transfer (FERET) in Equation (37) and its corollaries in Equations (43) and (44) that have wide predictive applicability to electrophilic aromatic substitutions, olefin additions, organometallic cleavages, etc. The FERET is based on the conversion of the weak nucleophile–electrophile interactions extant in the ubiquitous electron donor—acceptor (EDA) precursor complex [D, A] to the radical ion pair [D⊕, A⊖], for which the free energy change can be evaluated from the charge-transfer absorption spectra according to Mulliken theory. FERET analysis thus indicates that the charge-transfer ion pairs [D⊕, A⊖] are energetically equivalent to the transition states for nucleophile/electrophile transformations. The behavior of such ion pairs can be directly observed immediately following the irradiation of the charge-transfer bands of various EDA complexes with a 25-ps laser pulse. Such studies confirm the radical ion pair [Arene⊕, NO2] as a viable intermediate in electrophilic aromatic nitration, as presented in the electron-transfer mechanism between arenes and the nitryl cation (NO) electrophile.

Electron Transfer and Charge Transfer: Twin Themes in Unifying the Mechanisms of Organic and Organometallic Reactions

The bell-shaped energy gap dependence of the charge recombination reaction of geminate radical ion pairs produced by fluorescence quenching reaction in acetonitrile solution

Picosecond laser photolysis and transient photocurrent studies of the ionic dissociation mechanism of heteroexcimers: pyrene-n,n-dimethylaniline and pyrene-p-dicyanobenzene systems in polar solvents

In relation to the fact that there are no clear-cut experimental results indicating the “inverted region” in the strongly exothermic charge separation (CS) in the fluorescence quenching reaction, the possibilities of (a) formation of a nonfluorescent charge transfer complex in the course of quenching and (b) participation of excited electronic states of the ion pair in the course of CS at the encounter of fluorescer and quencher are examined for aromatic hydrocarbon-tetracyanoethylene and aromatic hydrocarbon-pyromellitic dianhydride systems in acetonitrile with the picosecond laser photolysis method. Both (a) and (b) are shown improbable as mechanisms for the lack of an inverted region in the photoinduced CS reaction. Discussions on these results are given on the basis of a new theoretical treatment.

Mechanisms of the strongly exothermic charge separation reaction in the excited singlet state. Picosecond laser photolysis studies on aromatic hydrocarbon-tetracyanoethylene and aromatic hydrocarbon-pyromellitic dianhydride systems in polar solutions

Dynamics of aromatic hydrocarbon cation-tetracyanoethylene anion geminate ion pairs in acetonitrile solution with implications to the mechanism of strongly exothermic charge separation reaction in the excited singlet state

A porphyrin-tetraviologen supramolecule P-V{sub 4}, in which a viologen molecule is appended to each of the porphyrin meso positions via a 1,3-propanoxy-4-phenyl chain, has been studied by picosecond and nanosecond laser flash photolysis techniques. In DMSO solution, rapid charge separation (CS) occurs from the first excited singlet state of the porphyrin, giving rise to long-lived redox products. These products recombine via first-order kinetics ({tau} = 6.4 {plus minus} 0.7 {mu}s) to restore the ground-state reactants. Similar, but much slower, CS takes place from the porphyrin triplet excited state, which is formed in competition to CS from the singlet state. Quantum yields for formation of redox products and rates of both CS and charge recombination (CR) are solvent dependent, protic solvents favoring rapid CR.

Photoinduced charge separation in a porphyrin-tetraviologen supramolecular array

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