Ultrafast Electron Transfer Dynamics in a Series of Porphyrin/Viologen Complexes: Involvement of Electronically Excited Radical Pair Products

Recyclable and Stable Porphyrin-Based Self-Assemblies by Electrostatic Force for Efficient Photocatalytic Organic Transformation

Development of efficient, stable, and recyclable photocatalysts for organic synthesis is vital for transformation of traditional thermal organic chemistry into green sustainable organic chemistry. In this work, the study reports an electrostatic approach to assemble meso-tetra (4-sulfonate phenyl) porphyrin (TPPS)tetra (4-sulfonate phenyl) porphyrin (TPPS) as a donor and benzyl viologen (BV) as an acceptor into stable and recyclable photocatalyst for an efficient organic transformation reaction - aryl sulfide oxidation. By use of the electrostatic TPPS-BV photocatalysts, 0.1 mmol aryl sulfide with electron-donating group can be completely transformed into aryl sulfoxide in 60 min without overoxidation into sulfone, rendering near 100% yield and selectivity. The photocatalyst can be recycled up to 95% when 10 mg amount is used. Mechanistic study reveals that efficient charge separation between TPPS and BV results in sufficient formation of superoxide which further reacts with the oxidized sulfide by the photocatalyst to produce the sulfoxide. This mechanistic pathway differs significantly from the previously proposed singlet oxygen-dominated process in homogeneous TPPS photocatalysis.

Semiclassical Theory of Multistage Nonequilibrium Electron Transfer in Macromolecular Compounds in Polar Media with Several Relaxation Timescales

Many specific features of ultrafast electron transfer (ET) reactions in macromolecular compounds can be attributed to nonequilibrium configurations of intramolecular vibrational degrees of freedom and the environment. In photoinduced ET, nonequilibrium nuclear configurations are often produced at the stage of optical excitation, but they can also be the result of electron tunneling itself, i.e., fast redistribution of charges within the macromolecule. A consistent theoretical description of ultrafast ET requires an explicit consideration of the nuclear subsystem, including its evolution between electron jumps. In this paper, the effect of the multi-timescale nuclear reorganization on ET transitions in macromolecular compounds is studied, and a general theory of ultrafast ET in non-Debye polar environments with a multi-component relaxation function is developed. Particular attention is paid to designing the multidimensional space of nonequilibrium nuclear configurations, as well as constructing the diabatic free energy surfaces for the ET states. The reorganization energies of individual ET transitions, the equilibrium energies of ET states, and the relaxation properties of the environment are used as input data for the theory. The effect of the system-environment interaction on the ET kinetics is discussed, and mechanisms for enhancing the efficiency of charge separation in macromolecular compounds are analyzed.

Ultrafast Electron Transfer in a Self-Assembling Sulfonated Aluminum Corrole-Methylviologen Complex

Photoinduced electron transfer systems can mimic certain features of natural photosynthetic reaction centers, which are crucial for solar energy production. Among other tetra-pyrroles, the versatile chemical and photophysical properties of corroles make them very promising donors applicable in donor-acceptor complexes. Here, we present a first comprehensive study of ultrafast photoinduced electron transfer in a self-assembling sulfonated aluminum corrole-methylviologen complex combining visible and mid-IR transient absorption spectroscopy. The noncovalent D-A association of the corrole-methylviologen complex has the great advantage that photoinduced charge separation becomes possible even though the back electron transfer (BET) rate is large. Initial forward electron transfer from corrole to methylviologen is observed on an ∼130 fs time scale. Subsequent back electron transfer takes place with τ_BET = (1.8 ± 0.5) ps, revealing very complex relaxation dynamics. Direct probing in the mid-IR allows us to unravel the back electron transfer and cooling dynamics/electronic reorganization. Upon tracing the dynamics of the methylviologen-radical marker band at 1640 cm^-1 and the C═C stretching of corrole at around 1500 cm^-1, we observe that large amounts of excess energy survive the back transfer, leading to the formation of hot ground state absorption. A closer examination of the signal after 300 ps, surviving the back transfer, exhibits a charge-separation yield of 10-15%.

2- and 2,7-Substituted para-N-Methylpyridinium Pyrenes: Syntheses, Molecular and Electronic Structures, Photophysical, Electrochemical, and Spectroelectrochemical Properties and Binding to Double-Stranded (ds) DNA

Two N-methylpyridinium compounds and analogous N-protonated salts of 2- and 2,7-substituted 4-pyridyl-pyrene compounds were synthesised and their crystal structures, photophysical properties both in solution and in the solid state, electrochemical and spectroelectrochemical properties were studied. Upon methylation or protonation, the emission maxima are significantly bathochromically shifted compared to the neutral compounds, although the absorption maxima remain almost unchanged. As a result, the cationic compounds show very large apparent Stokes shifts of up to 7200 cm-1 . The N-methylpyridinium compounds have a single reduction at ca. -1.5 V vs. Fc/Fc+ in MeCN. While the reduction process was reversible for the 2,7-disubstituted compound, it was irreversible for the mono-substituted one. Experimental findings are complemented by DFT and TD-DFT calculations. Furthermore, the N-methylpyridinium compounds show strong interactions with calf thymus (ct)-DNA, presumably by intercalation, which paves the way for further applications of these multi-functional compounds as potential DNA-bioactive agents.

The Efficiency of Photoinduced Intramolecular Charge Separation from the Second Excited State: What Factors Can Control It?

The efficiency of photoinduced charge separation (CS) in electron donor-acceptor compounds is commonly limited due to fast deactivation processes, such as the excited-state internal conversion and ultrafast hot reverse electron transfer to the acceptor, charge recombination (CR). A traditional way to avoid undesired energy losses due to CR is to put the reverse electron transfer into the Marcus inverted region, thus effectively suppressing it. This method, however, is not generally applicable when considering CS from the second locally excited state because the driving force of CR to the first excited state is small, and thus charge recombination is ultrafast and efficient. In this paper, we study the kinetic features of CS/CR from the second locally excited state of the donor using a semiclassical stochastic model of electron transfer. Particular attention is paid to the CS efficiency as well as the influence of the polar environment and intramolecular high-frequency vibrational modes on the kinetics of the charge-separated state. The influence of a number of model parameters on the CS yield and the energy efficiency has been analyzed using the results of numerical simulations. Several simple practical recipes for creating molecular compounds with high CS yields have been suggested. Simulations have also revealed a strong and non-monotonous (double-humped) dependence of both the yield and energy efficiency of CS on the driving force.

Quantum yield and energy efficiency of photoinduced intramolecular charge separation

Kinetics of photoinduced intramolecular charge separation (CS) and the ensuing ultrafast charge recombination (CR) in electron-donor-acceptor dyads are studied numerically, taking into account the excitation of charge-transfer active intramolecular vibrations and multiple relaxation time scales of the surrounding polar solvent. Both energetic and dynamic properties of intramolecular and solvent reorganization are considered, and their influence on the CS/CR kinetics and quantum yield of ultrafast CS is explored. Particular attention is paid to the energy efficiency of CS, as one of the most important parameters indicating the promise of using a molecular compound as a basis for emerging optoelectronic devices. The CS quantum yield and the energy efficiency of CS are shown to depend differently on the key model parameters. Necessary conditions for the highly efficient CS are evaluated using analytic formulae for the electron transfer rates and derived from numerical simulation data. The reasons why low-exergonic CS taking place in the Marcus normal region can be much slower than CR in the deep inverted region are discussed.

Bimolecular photoinduced electron transfer in non-polar solvents beyond the diffusion limit

Electron transfer (ET) quenching dynamics in non-polar solvents are investigated using ultrafast spectroscopy with a series of six fluorophore/quencher pairs, covering a driving force range of more than 1.3 eV. The intrinsic ET rate constants, k₀, deduced from the quenching dynamics in the static regime, are of the order of 10¹²-10¹³ M^-1 s^-1, i.e., at least as large as in acetonitrile, and do not exhibit any marked dependence on the driving force. A combination of transient electronic and vibrational absorption spectroscopy measurements reveals that the primary product of static quenching is a strongly coupled exciplex that decays within a few picoseconds. More weakly coupled exciplexes with a longer lifetime are generated subsequently, during the dynamic, diffusion-controlled, stage of the quenching. The results suggest that static ET quenching in non-polar solvents should be viewed as an internal conversion from a locally excited state to a charge-transfer state of a supermolecule rather than as a non-adiabatic ET process.

Carboxylated pillar[n]arene (n = 5-7) host molecules: high affinity and selective binding in water

Water-soluble carboxylated pillar[n]arenes (n = 5-7) or WPns were discovered to be high affinity host molecules with selective binding for different guests based on a systematic investigation. We chose 22 dyes or guests and determined the value of K_a for 51 supramolecular complexes. It was discovered that the electrostatic interactions, π-π stacking and hydrophobic effect were the driving force for high affinity supramolecular encapsulation. WPns had selective binding toward suitable guests based on their sizes and molecular structures. Based on the above discovery, a guest (guest 21) was designed, which bound with WP7 3.3-fold tighter compared to methyl viologen.

The effect of solvent relaxation time constants on free energy gap law for ultrafast charge recombination following photoinduced charge separation

Ultrafast low exergonic charge recombination following photoinduced charge separation proceeds in a non-equilibrium mode and its rate constant is nearly independent of the free energy gap.

Bimolecular Photoinduced Electron Transfer in Static Quenching Regime: Illustration of Marcus Inversion in Micelle

Ultrafast bimolecular photoinduced electron transfer (PET) between six coumarin dyes and four viologen molecules in the stern layer of sodium dodecyl sulfate micelle have been studied using femtosecond broadband transient absorption spectroscopy and femtosecond fluorescence up-conversion spectroscopy over a broad reaction exergonicity (ΔG⁰). Emanating the formation of radical cation intermediates of viologen molecules using the transient absorption and the fast decay component of coumarins using the fluorescence up-conversion studies the forward bimolecular electron transfer rate (k_et) have been measured with high accuracy. The relationship of k_et with ΔG⁰ found to follow a Marcus type bell-shaped dependence with an inversion at -1.10 eV. In this report, we have studied PET reaction using ultrafast spectroscopy at the quencher concentration where static quenching regime prevails. Moreover, the incompetency of Stern-Volmer experiments in studying ultrafast PET has been revealed. In contrary to previous claims, here we found that the k_et is lower for lower lifetime coumarins, indicating that static, nonstationary and stationary regime of quenching have the minimal role to play to in the bimolecular electron transfer process. By far, this report is believed to be the most efficient and immaculate way of approaching Marcus inverted region problem in the case of bimolecular PET and settles the long-lasting debate of whether the same can be observed in micellar systems.

Synthesis of benzo[6]urils and their selective interactions with bipyridines

Benzo[6]urils bearing hydroxymethyl or methyl groups have been synthesized with a facile method, and their supramolecular interactions with bipyridine guests have been investigated.

Decoupling diffusion from the bimolecular photoinduced electron transfer reaction: a combined ultrafast spectroscopic and kinetic analysis

We have studied the bimolecular photoinduced electron transfer (PET) reaction between benzophenone (Bp) and DABCO using femtosecond broadband transient absorption spectroscopy in different compositions of acetonitrile/1-butanol binary solvent mixtures.

Heterogeneous Electron-Transfer Dynamics through Dipole-Bridge Groups

Heterogeneous electron transfer (HET) between photoexcited molecules and colloidal TiO2 has been investigated for a set of Zn-porphyrin chromophores attached to the semiconductor via linkers that allow to change level alignment by 200 meV by reorientation of the dipole moment. These unique dye molecules have been studied by femtosecond transient absorption spectroscopy in solution and adsorbed on the TiO2 colloidal film in vacuum. In solution energy transfer from the excited chromophore to the dipole group has been identified as a slow relaxation pathway competing with S2-S1 internal conversion. On the film heterogeneous electron transfer occurred in 80 fs, much faster compared to all intramolecular pathways. Despite a difference of 200 meV in level alignment of the excited state with respect to the semiconductor conduction band, identical electron transfer times were measured for different linkers. The measurements are compared to a quantum-mechanical model that accounts for electronic-vibronic coupling and finite band width for the acceptor states. We conclude that HET occurs into a distribution of transition states that differs from regular surface states or bridge mediated states.