Hydrogen-Bonding Effects on the Formation and Lifetimes of Charge-Separated States in Molecular Triads

Photoinduced electron transfer in two molecular triads comprised of a triarylamine donor, a d⁶ metal diimine photosensitizer, and a 9,10-anthraquinone acceptor was investigated with particular focus on the influence of hydrogen-bonding solvents on the electron transfer kinetics. Photoexcitation of the ruthenium­(II) and osmium­(II) sensitizers of these triads leads to charge-separated states containing an oxidized triarylamine unit and a reduced anthraquinone moiety. The kinetics for formation of these charge-separated states were explored by using femtosecond transient absorption spectroscopy. Strong hydrogen bond donors such as hexafluoroisopropanol or trifluoroethanol cause a thermodynamic and kinetic stabilization of these charge-separated states that is attributed to hydrogen bonding between alcoholic solvent and reduced anthraquinone. In the ruthenium triad this effect leads to a lengthening of the lifetime of the charge-separated state from ∼750 ns in dichloromethane to ∼3000 ns in hexafluoroisopropanol while in the osmium triad the respective lifetime increases from ∼50 to ∼2000 ns between the same two solvents. In both triads the lifetime of the charge-separated state correlates with the hydrogen bond donor strength of the solvent but not with the solvent dielectric constant. These findings are relevant in the greater context of solar energy conversion in which one is interested in storing light energy in charge-separated states that are as long-lived as possible. Furthermore they are relevant for understanding proton-coupled electron transfer (PCET) reactivity of electronically excited states at a fundamental level because changes in hydrogen-bonding strength accompanying changes in redox states may be regarded as an attenuated form of PCET

Photoinduced Electron Transfer in CdSe/ZnS Quantum Dot–Fullerene Hybrids

Photoinduced electron transfer (ET) in CdSe/ZnS core–shell quantum dot (QD) – fullerene (COOH–C₆₀) hybrids was studied by the means of time-resolved emission and absorption spectroscopy techniques. A series of four QDs with emission in the range 540–630 nm was employed to investigate the dependence of the electron transfer rate on the QD size. Emission of the QDs is quenched upon hybrid formation, and the quenching mechanism is identified as photoinduced electron transfer from the QD to the fullerene moiety due to the fullerene anion signature observed in transient absorption. In order to obtain quantitative information on the ET reaction, several kinetic data analysis techniques were used, including a conventional multiexponential fitting and a maximum entropy method for emission decay analysis, as well as a distributed decay model based on the Poisson distribution of fullerenes in the hybrids. The latter gradually simplifies the interpretation of the transient absorption spectra and indicates that the spectra of QD cations are essentially similar to those of neutral QDs, differing only by a minor decrease in the intensity and broadening. Furthermore, only a minor decrease in the ET rate with the increasing QD size was observed, the time constants being in the range 100–200 ps for all studied QDs. The charge recombination is extended to 10 ns or longer for all hybrids

Quantitative Analysis of Intramolecular Exciplex and Electron Transfer in a Double-Linked Zinc Porphyrin–Fullerene Dyad

Photoinduced charge transfer in a double-linked zinc porphyrin–fullerene dyad is studied. When the dyad is excited at the absorption band of the charge-transfer complex (780 nm), an intramolecular exciplex is formed, followed by the complete charge separated (CCS) state. By analyzing the results obtained from time-resolved transient absorption and emission decay measurements in a range of solvents with different polarities, we derived a dependence between the observable lifetimes and internal parameters controlling the reaction rate constants based on the semiquantum Marcus electron-transfer theory. The critical value of the solvent polarity was found to be ε_r ≈ 6.5: in solvents with higher dielectric constants, the energy of the CCS state is lower than that of the exciplex and the relaxation takes place via the CCS state predominantly, whereas in solvents with lower polarities the energy of the CCS state is higher and the exciplex relaxes directly to the ground state. In solvents with moderate polarities the exciplex and the CCS state are in equilibrium and cannot be separated spectroscopically. The degree of the charge shift in the exciplex relative to that in the CCS state was estimated to be 0.55 ± 0.02. The electronic coupling matrix elements for the charge recombination process and for the direct relaxation of the exciplex to the ground state were found to be 0.012 ± 0.001 and 0.245 ± 0.022 eV, respectively

Monoisomeric Phthalocyanines and Phthalocyanine–Fullerene Dyads with Polar Side Chains: Synthesis, Modeling, and Photovoltage

Synthesis, characterization, molecular modeling, and photovoltage responses of four phthalocyanine–fullerene (Pc-C₆₀) dyads with two polar (−OH) side chains are described. The synthesized dyads have polar tails either on the Pc (electron donor D) side or on the fullerene (electron acceptor A) side of the dyad, providing a possibility to produce oriented donor–acceptor (D–A) monolayers with revised electron transfer direction when deposited on an aqueous subphase or on a solid surface. In the dyads, phthalocyanine and fullerene have different mutual orientations: in the trans-dyads they have face-to-face orientation, while in the cis-dyads the orientation is face-to-edge. Molecular modeling was used to examine and confirm the spatial arrangements of the synthesized dyads. The Pc-C₆₀ dyads were deposited successfully onto solid substrates as highly oriented monolayers using the Langmuir–Schäfer method. Formation of a vertically oriented monolayer and the following electron transfer from the photoexcited phthalocyanine to fullerene was demonstrated for all dyad monolayers by the time-resolved Maxwell displacement charge method. The electron transfer direction was reversed for the dyads with reversed polarity, demonstrating the ability to control charge transfer direction in the film

Excited State Intramolecular Proton Transfer in Electron-Rich and Electron-Poor Derivatives of 10-Hydroxybenzo[<i>h</i>]quinoline

Eight previously inaccessible derivatives of 10-hydroxybenzo­[<i>h</i>]­quinoline were prepared via a straightforward strategy comprising formation of the benzo­[<i>h</i>]­quinoline skeleton followed by C–H acetoxylation at position 10. The occurrence of excited state intramolecular proton transfer (ESIPT) was detected in all cases since emission was observed only from the excited keto-tautomer. Studies on derivatives bearing both electron-donating and electron-withdrawing groups adjacent to the pyridine ring allowed us to identify some design patterns giving rise to NIR emission and large Stokes shifts. For a derivative of 10-hydroxybenzo­[<i>c</i>]­acridine, emission at 745 nm was observed, one of the lowest energy fluorescence ever reported for ESIPT system. On the basis of time-resolved measurements, proton transfer was found to be extremely fast with time constants in the range (0.08–0.45 ps)

Large Stokes Shift Fluorescent Dyes Based on a Highly Substituted Terephthalic Acid Core

The synthesis of dyes based on a highly substituted terephthalic acid core is described, starting from readily available 2,5-dihydroxy-terephthalic acid diethyl ester. The dyes are highly colored, soluble in organic solvents and reasonably fluorescent in solution and in the solid state. The maxima for absorption and emission are around 402 and 502 nm, respectively. The fluorophores are readily cyclized to generate compounds which comprise the basic 6,13-dihydroxy-chromeno[2,3-<i>b</i>]xanthene-7,14-dione unit. These new derivatives are nonfluorescent

Large Stokes Shift Fluorescent Dyes Based on a Highly Substituted Terephthalic Acid Core

The synthesis of dyes based on a highly substituted terephthalic acid core is described, starting from readily available 2,5-dihydroxy-terephthalic acid diethyl ester. The dyes are highly colored, soluble in organic solvents and reasonably fluorescent in solution and in the solid state. The maxima for absorption and emission are around 402 and 502 nm, respectively. The fluorophores are readily cyclized to generate compounds which comprise the basic 6,13-dihydroxy-chromeno[2,3-<i>b</i>]xanthene-7,14-dione unit. These new derivatives are nonfluorescent

Synthesis and Photophysical Properties of Two Diazaporphyrin–Porphyrin Hetero Dimers in Polar and Nonpolar Solutions

Two diazaporphyrin (DAP)-porphyrin hetero dimers, in β–<i>meso</i> and β–β configurations, were prepared to study their photoinduced intramolecular electron transfer properties. The two <i>meso</i> nitrogen atoms in the porphyrin ring of DAP change its redox potential, making DAP more easily reduced, compared to its porphyrin counterpart. A charge-transfer from porphyrin to DAP in both hetero dimers was verified by versatile optical spectroscopic methods. The steady-state fluorescence spectra indicated an efficient intramolecular exciplex formation for both dimers. For the β–<i>meso</i> dimer, ultrafast time-resolved spectroscopic methods revealed the subpicosecond formation of two types of primary short-living (1–18 ps) intramolecular exciplexes, which relaxed in toluene to form a long-living final exciplex (1.4 ns) followed by a longer-living charge transfer complex (>5 ns). However, in benzonitrile, the lifetime of the final exciplex was longer (660 ps) as was that of the charge transfer complex (180 ps). The β–β analogue formed similar short-living exciplexes in both solvents, but the final exciplex and the charge transfer state had significantly shorter lifetimes. The electrochemical redox potential measurements and density functional theory calculations supported the proposed mechanism

Syntheses and Excitation Transfer Studies of Near-Orthogonal Free-Base Porphyrin–Ruthenium Phthalocyanine Dyads and Pentad

A new series of molecular dyads and pentad featuring free-base porphyrin and ruthenium phthalocyanine have been synthesized and characterized. The synthetic strategy involved reacting free-base porphyrin functionalized with one or four entities of phenylimidazole at the meso position of the porphyrin ring with ruthenium carbonyl phthalocyanine followed by chromatographic separation and purification of the products. Excitation transfer in these donor–acceptor polyads (dyad and pentad) is investigated in nonpolar toluene and polar benzonitrile solvents using both steady-state and time-resolved emission techniques. Electrochemical and computational studies suggested that the photoinduced electron transfer is a thermodynamically unfavorable process in nonpolar media but may take place in a polar environment. Selective excitation of the donor, free-base porphyrin entity, resulted in efficient excitation transfer to the acceptor, ruthenium phthalocyanine, and the position of imidazole linkage on the free-base porphyrin could be used to tune the rates of excitation transfer. The singlet excited Ru phthalocyanine thus formed instantly relaxed to the triplet state via intersystem crossing prior to returning to the ground state. Kinetics of energy transfer (<i>k</i>_ENT) was monitored by performing transient absorption and emission measurements using pump–probe and up-conversion techniques in toluene, respectively, and modeled using a Förster-type energy transfer mechanism. Such studies revealed the experimental <i>k</i>_ENT values on the order of 10¹⁰–10¹¹ s^–1, which readily agreed with the theoretically estimated values. Interestingly, in polar benzonitrile solvent, additional charge transfer interactions in the case of dyads but not in the case of pentad, presumably due to the geometry/orientation consideration, were observed

Sequential Photoinduced Energy and Electron Transfer Directed Improved Performance of the Supramolecular Solar Cell of a Zinc Porphyrin–Zinc Phthalocyanine Conjugate Modified TiO₂ Surface

Improved performance of a photosynthetic antenna–reaction center mimicking supramolecular solar cell is demonstrated. Toward this, porphyrin–phthalocyanine conjugates connected by amide linkers, as wide-band capturing solar energy harvesting materials, have been newly synthesized and characterized. Efficient singlet–singlet energy transfer from the zinc or free-base porphyrin to phthalocyanine is evidenced from steady-state emission and transient absorption studies in nonpolar and polar solvents. Further, the dyad was immobilized via axial coordination of zinc porphyrin of the dyad on semiconducting TiO₂ surface modified with axial coordinating ligand functionality, phenylimidazole. Photoelectrochemical studies revealed improved performance of this cell compared to either zinc porphyrin or zinc phthalocyanine only modified electrodes under similar experimental conditions. Transient absorption studies performed on the dyad immobilized on glass/TiO₂ surface suggested that upon excitation of the axially coordinated zinc porphyrin of the dyad singlet–singlet energy transfer to zinc phthalocyanine occurs within 0.2 ps instead of a competing charge injection reaction from the singlet excited zinc porphyrin to TiO₂. Further, sequential photoinduced electron transfer from the newly formed singlet excited zinc phthalocyanine to zinc porphyrin producing ZnP^•––ZnPc^•+ with a 2 ps time constant and followed by electron injection from the ZnP^•– to TiO₂ within 30 ps has been proposed as a mechanism of photocurrent generation in the biomimetic supramolecular photocell