Coupled Sensitizer–Catalyst Dyads: Electron‐Transfer Reactions in a Perylene–Polyoxometalate Conjugate

Ultrafast discharge of a single‐electron capacitor: A variety of intramolecular electron‐transfer reactions are apparent for polyoxometalates functionalized with covalently attached perylene monoimide chromophores, but these are restricted to single‐electron events. (et=electron transfer, cr=charge recombination, csr=charge‐shift reaction, PER=perylene, POM=polyoxometalate).

     

Ultrafast Photoinduced Electron Transfer in Viologen‐Linked BODIPY Dyes

New boron‐dipyrromethene (BODIPY) dyes linked to viologen are prepared and their photophysical and electrochemical properties are investigated. Both synthesized molecules have similar electronic absorption spectra with the absorption maximum localized at 517 and 501 nm for dye 1 and dye 2 , respectively. They exhibit well‐defined redox behavior, highlighting the presence of BODIPY and viologen subunits, with little perturbation of the redox potential of both subunits with respect to the parent compounds. Both dyes are heavily quenched by photoinduced electron transfer from the BODIPY to the viologen subunit. The transient absorption technique demonstrates that dye 2 forms the viologen radical within a timeframe of 7.1 ps, and that the charge‐separated species has a lifetime of 59 ps. Sustained irradiation of dye 2 in the presence of a tertiary amine allows for the accumulation of BODIPY–methyl‐4,4′‐bipyridinium (BODIPY–MV⁺), as observed by its characteristic absorption at 396 and 603 nm. However, dye 2 does not generate catalytic amounts of hydrogen under standard conditions.     

Photoinduced Energy and Electron Transfer in Phenylethynyl‐Bridged Zinc Porphyrin–Oligothienylenevinylene–C60 Ensembles

Donor–bridge–acceptor triad (Por‐2TV‐C₆₀) and tetrad molecules ((Por)₂‐2TV‐C₆₀), which incorporated C₆₀ and one or two porphyrin molecules that were covalently linked through a phenylethynyl‐oligothienylenevinylene bridge, were synthesized. Their photodynamics were investigated by fluorescence measurements, and by femto‐ and nanosecond laser flash photolysis. First, photoinduced energy transfer from the porphyrin to the C₆₀ moiety occurred rather than electron transfer, followed by electron transfer from the oligothienylenevinylene to the singlet excited state of the C₆₀ moiety to produce the radical cation of oligothienylenevinylene and the radical anion of C₆₀. Then, back‐electron transfer occurred to afford the triplet excited state of the oligothienylenevinylene moiety rather than the ground state. Thus, the porphyrin units in (Por)‐2TV‐C₆₀ and (Por)₂‐2TV‐C₆₀ acted as efficient photosensitizers for the charge separation between oligothienylenevinylene and C₆₀.     

Photoinduced Electron Transfer of Porphyrin Isomers: Impact of Molecular Structures on Electron‐Transfer Dynamics

Porphyrins have been investigated for a long time in various fields of chemistry owing to their excellent redox and optical properties. Structural isomers of porphyrins have been synthesized, namely, porphycene, hemiporphycene, and corrphycene. Although the number of studies on these structural isomers is limited, they exhibit interesting properties suitable for various applications such as photovoltaic devices, photocatalysts, and photodynamic therapy. In the present review, we summarized their photoinduced electron‐transfer processes, which are key steps of various photofunctions. Their electrochemical and photophysical properties are summarized as basic properties for the electron transfer. Furthermore, differences among these isomers in the electron‐transfer processes are clarified, and its origin has been discussed on the basis of their molecular structures.     

Porphyrin‐Ryleneimide Hybrids: Low‐Bandgap Acceptors in Energy‐Transfer Cassettes

Energy‐transfer cassettes consisting of naphthaleneimide‐fused metalloporphyrin acceptors (M=Zn and Pd) and BODIPY donors have been designed and synthesized. These systems have rigid pseudo‐tetrahedral structures with a donor‐acceptor separation of ca. 17.5 Å. Spectroscopic investigations, including femtosecond transient absorption measurements, showed efficient excitation energy transfer (EET) occurring according to the Förster mechanism. Strong fluorescence of the donor units and significant spectral overlap of the donor and acceptor subunits are prerequisites for the efficient EET in these systems.     

Model Reactions for Photosynthesis—Photoinduced Charge and Energy Transfer between Covalently Linked Porphyrin and Quinone Units

Photosynthesis is one of the fascinating fields of current interdisciplinary research. It seems miraculous that nature, in the process of evolution, has managed to bring about the process of photosynthesis. The first step involves a charge separation at the reaction centers, which proceeds with 100% quantum yield from the photoexcited singlet state of the bacteriochlorophyll donor, despite the fact that the wasteful deactivation of the electron into the ground state should be highly favored. Biomimetic model compounds (that is, those which resemble the pigments nature has developed) have been constructed from porphyrins and quinones. These model systems have allowed the study of the factors contributing to the highly efficient charge separation. This report focuses on recent developments in the study of electron transfer in porphyrinoquinones. Some of the results of these investigations may not be not fully understood and are often the subject of controversial discussions.     

Energy Transfer in Dye‐Coupled Lanthanide‐Doped Nanoparticles: From Design to Application

Surface modification with organic dye molecules is a useful strategy to manipulate the optical properties of lanthanide‐doped nanoparticles (LnNPs). It enables energy transfer between dyes and LnNPs, which provides unprecedented possibilities to gain new optical phenomena from the dye–LnNPs composite systems. This has led to a wide range of emerging applications, such as biosensing, drug delivery, gene targeting, information storage, and photon energy conversion. Herein, the mechanism of energy transfer and the structural‐dependent energy‐transfer properties in dye‐coupled LnNPs are reviewed. The design strategies for achieving effective dye–LnNP functionalization are presented. Recent advances in these composite nanomaterials in biomedicine and energy conversion applications are highlighted.     

Unusual Photoinduced Electron Transfer from a Zinc Porphyrin to a Tetrapyridyl Free‐Base Porphyrin in a Noncovalent Multiporphyrin Array

Excitation of the peripheral Zn porphyrin units in a noncovalent five‐porphyrin array, formed by gable‐like zinc(II) bisporphyrins and a central free‐base meso‐tetrakis(4‐pyridyl)porphyrin in a 2:1 ratio, ( ZnP₂ )₂? ( TPyP ), does not lead to a quantitative sensitization of the luminescence of the free‐base porphyrin acceptor, even though there is an effective energy transfer. Time resolution of the luminescence evidences a quenching of TPyP upon sensitization by the peripheral ZnP₂ . The time evolution of the TPyP fluorescence in the complex can be described by a bi‐exponential fitting with a major component of 180 ps and a minor one of 5 ns, compared to an isolated TPyP lifetime of 9.4 ns. The two quenched lifetimes are shown to be correlated to the presence of 2:1 and 1:1 complexes, respectively. No quenching of TPyP fluorescence occurs in ( ZnP₂ )₂?( TPyP ) at 77 K in a rigid solvent for which only an energy‐transfer process (τ=150±10 ps) from peripheral ZnP₂ to the central TPyP is observed. An unusual HOMO–HOMO electron‐transfer reaction from ZnP₂ to the excited TPyP units, responsible for the observed phenomena, is detected. The resulting charge‐separated state, ( ZnP₂ )⁺₂?( TPyP )^? is found to recombine to the ground state with a lifetime of 11 ns.     

Dynamics of Photoinduced Electron‐Transfer Processes in Fullerene‐Based Dyads: Effects of Varying the Donor Strength

Two classes of fullerene‐based donor–bridge–acceptor (D–B–A) systems containing donors of varying oxidation potentials have been synthesized. These systems include fullerenes linked to heteroaromatic donor groups (phenothiazine/phenoxazine) as well as substituted anilines (p‐anisidine/p‐toluidine). In contrast to the model compound, an efficient intramolecular electron transfer is observed from the fullerene singlet excited state in polar solvents. An increase in the rate constant and quantum yield of charge separation (k_cs and Φ_cs) has been observed for both classes of dyads, with decrease in the oxidation potentials of the donor groups. This observation indicates that the rates of the forward electron transfer fall in the normal region of the Marcus curve. The long‐lived charge separation enabled the characterization of electron transfer products, namely, the radical cation of the donor and radical anion of the pyrrolidinofullerene, by using nanosecond transient absorption spectroscopy. The small reorganization energy (λ) of C₆₀ coupled with large negative free energy changes (‐ΔG°) for the back electron transfer places the back electron process in the inverted region of Marcus curve, thereby stabilizing the electron transfer products.     

Nondestructive Photoluminescence Read‐Out by Intramolecular Electron Transfer in a Perylene Bisimide‐Diarylethene Dyad

A novel, highly stable photochromic dyad 3 based on a perylene bisimide (PBI) fluorophore and a diarylethene (DAE) photochrome was synthesized and the optical and photophysical properties of this dyad were studied in detail by steady‐state and time‐resolved ultrafast spectroscopy. This photochromic dyad can be switched reversibly by UV‐light irradiation of its ring‐open form 3 o leading to the ring‐closed form 3 c , and back reaction of 3 c to 3 o by irradiation with visible light. Solvent‐dependent fluorescence studies revealed that the emission of ring‐closed form 3 c is drastically quenched in solvents of medium (e.g., chloroform) to high (e.g., acetone) polarities, while the emission of the ring‐open form 3 o is appreciably quenched only in highly polar solvents like DMF. The strong fluorescence quenching of 3 c is attributed to a photoinduced electron‐transfer (PET) process from the excited PBI unit to ring‐closed DAE moiety, as this process is thermodynamically highly favorable with a Gibbs free energy value of ?0.34 eV in dichloromethane. The electron‐transfer mechanism for the fluorescence quenching of ring‐closed 3 c is substantiated by ultrafast transient measurements in dichloromethane and acetone, revealing stabilization of charge‐separated states of 3 c in these solvents. Our results reported here show that the new photochromic dyad 3 has potential for nondestructive read‐out in write/read/erase fluorescent memory systems.     

High‐Potential Perfluorinated Phthalocyanine–Fullerene Dyads for Generation of High‐Energy Charge‐Separated States: Formation and Photoinduced Electron‐Transfer Studies

High oxidation potential perfluorinated zinc phthalocyanines (ZnF_nPcs) are synthesised and their spectroscopic, redox, and light‐induced electron‐transfer properties investigated systematically by forming donor–acceptor dyads through metal–ligand axial coordination of fullerene (C₆₀) derivatives. Absorption and fluorescence spectral studies reveal efficient binding of the pyridine‐ (Py) and phenylimidazole‐functionalised fullerene (C₆₀Im) derivatives to the zinc centre of the F_nPcs. The determined binding constants, K, in o‐dichlorobenzene for the 1:1 complexes are in the order of 10⁴ to 10⁵ M ^?1; nearly an order of magnitude higher than that observed for the dyad formed from zinc phthalocyanine (ZnPc) lacking fluorine substituents. The geometry and electronic structure of the dyads are determined by using the B3LYP/6‐31G* method. The HOMO and LUMO levels are located on the Pc and C₆₀ entities, respectively; this suggests the formation of ZnF_nPc^.+–C₆₀Im^.? and ZnF_nPc^.+–C₆₀Py^.? (n=0, 8 or 16) intra‐supramolecular charge‐separated states during electron transfer. Electrochemical studies on the ZnPc–C₆₀ dyads enable accurate determination of their oxidation and reduction potentials and the energy of the charge‐separated states. The energy of the charge‐separated state for dyads composed of ZnF_nPc is higher than that of normal ZnPc–C₆₀ dyads and reveals their significance in harvesting higher amounts of light energy. Evidence for charge separation in the dyads is secured from femtosecond transient absorption studies in nonpolar toluene. Kinetic evaluation of the cation and anion radical ion peaks reveals ultrafast charge separation and charge recombination in dyads composed of perfluorinated phthalocyanine and fullerene; this implies their significance in solar‐energy harvesting and optoelectronic device building applications.     

Ultrafast Energy Transfer in Triptycene‐Grafted Bodipy Scaffoldings

A series of new compounds in which various Bodipy dyes are grafted logically on triptycene rigid structures are synthesized and characterized, and their absorption spectra and photophysical properties are studied, also by pump‐probe transient absorption spectroscopy. The studied compounds are: the mono‐Bodipy species TA, TB, and TC (where A, B, and C identify different Bodipy subunits absorbing and emitting at different wavelengths), the multichromophore species TA₃, which bears three identical A subunits, and the three multichromophoric species TAB, TBC, and TABC, all of them containing at least two different types of Bodipy subunits. The triptycene moiety plays the role of a rigid scaffold, keeping the various dyes at predetermined distances and allowing for a three‐dimensional structural arrangement of the multichromophoric species. The absorption spectra of the multichromophoric Bodipy species are essentially additive, indicating that negligible inter‐chromophoric interaction takes place at the ground state. Luminescence properties and transient absorption spectroscopy indicate that a very fast (on the picosecond time scale) and efficient photoinduced energy transfer occurs in all the multi‐Bodipy species, with the lower‐energy Bodipy subunits of each multi‐Bodipy compounds playing the role of an electronic energy collector. In TAB, an energy transfer from the A‐type Bodipy subunit to the B‐type one takes place with a rate constant of 1.6×10¹⁰ s^?1, whereas in TBC an energy transfer from the B‐type Bodipy subunit to the C‐type subunit is bi‐exponential, exhibiting rate constants of 1.7×10¹¹ and 1.9×10¹⁰ s^?1; the possible presence of different conformers with different donor–acceptor distances in this bichromophoric species is proposed to cause the bi‐exponential energy‐transfer process. Interpretation of the intricate energy‐transfer pathways occurring in TABC is made with the help of the processes identified in the bichromophoric compounds. In all cases, the measured energy‐transfer rate constants agree with a Förster mechanism for the energy‐transfer processes.     

On the Role of Hydrogen Bonds in Photoinduced Electron‐Transfer Dynamics between 9‐Fluorenone and Amine Solvents

Using ultrafast fluorescence upconversion and mid‐infrared spectroscopy, we explore the role of hydrogen bonds in the photoinduced electron transfer (ET) between 9‐fluorenone (FLU) and the solvents trimethylamine (TEA) and dimethylamine (DEA). FLU shows hydrogen‐bond dynamics in the methanol solvent upon photoexcitation, and similar effects may be anticipated when using DEA, whereas no hydrogen bonds can occur in TEA. Photoexcitation of the electron‐acceptor dye molecule FLU with a 400 nm pump pulse induces ultrafast ET from the amine solvents, which is followed by 100 fs IR probe pulses as well as fluorescence upconversion, monitoring the time evolution of marker bands of the FLU S₁ state and the FLU radical anion, and an overtone band of the amine solvent, marking the transient generation of the amine radical cation. A comparison of the experimentally determined forward charge‐separation and backward charge‐recombination rates for the FLU‐TEA and FLU‐DEA reaction systems with the driving‐force dependencies calculated for the forward and backward ET rates reveals that additional degrees of freedom determine the ET reaction dynamics for the FLU‐DEA system. We suggest that hydrogen bonding between the DEA molecules plays a key role in this behaviour.     

Electrografting of Diazonium‐Functionalized Polyoxometalates: Synthesis,Immobilisation and Electron‐Transfer Characterisation from Glassy Carbon

Polyoxometalates (POMs) are attractive candidates for the rational design of multi‐level charge‐storage materials because they display reversible multi‐step reduction processes in a narrow range of potentials. The functionalization of POMs allows for their integration in hybrid complementary metal oxide semiconductor (CMOS)/molecular devices, provided that fine control of their immobilisation on various substrates can be achieved. Owing to the wide applicability of the diazonium route to surface modification, a functionalized Keggin‐type POM [PW₁₁O₃₉{Ge(p‐C₆H₄‐C?C‐C₆H₄‐${{\rm N}{{+\hfill \atop 2\hfill}}}$ )}]^3? bearing a pending diazonium group was prepared and subsequently covalently anchored onto a glassy carbon electrode. Electron transfer with the immobilised POM was thoroughly investigated and compared to that of the free POM in solution.     

Sensitised LnIII Emission and Excited‐State Dynamics of Cofacial ‘Pacman’ Porphyrin Terpyridine Complexes

An asymmetric ‘Pacman’ metalloligand, [Zn(PXT)], which features a cofacial Zn^II–porphyrin unit (P) covalently attached to a terpyridine (T) chelating group via a rigid xanthene (X) moiety has been prepared, and its interactions with several different trivalent Ln^III cations (Nd^III, Gd^III, Yb^III and Lu^III) have been examined. The formation of 1:1 metal–ligand complexes was monitored by ¹H NMR spectroscopy and corroborated by HRMS data. Solution‐stability constants were determined by UV/Vis titration, and the resulting complexes with Nd^III or Yb^III demonstrated sensitised emission in the NIR region due to energy transfer from the Zn^II–porphyrin donor to Ln^III acceptor. The energy transfer was investigated by transient absorption techniques, which provided insight into the kinetics and efficiency of the antenna effect.