Ultrafast Photoinduced Energy and Electron Transfer in Multi‐Modular Donor–Acceptor Conjugates

New multi‐modular donor–acceptor conjugates featuring zinc porphyrin (ZnP), catechol‐chelated boron dipyrrin (BDP), triphenylamine (TPA) and fullerene (C₆₀), or naphthalenediimide (NDI) have been newly designed and synthesized as photosynthetic antenna and reaction‐center mimics. The X‐ray structure of triphenylamine‐BDP is also reported. The wide‐band capturing polyad revealed ultrafast energy‐transfer (k_ENT=1.0×10¹² s^?1) from the singlet excited BDP to the covalently linked ZnP owing to close proximity and favorable orientation of the entities. Introducing either fullerene or naphthalenediimide electron acceptors to the TPA‐BDP‐ZnP triad through metal–ligand axial coordination resulted in electron donor–acceptor polyads whose structures were revealed by spectroscopic, electrochemical and computational studies. Excitation of the electron donor, zinc porphyrin resulted in rapid electron‐transfer to coordinated fullerene or naphthalenediimide yielding charge separated ion‐pair species. The measured electron transfer rate constants from femtosecond transient spectral technique in non‐polar toluene were in the range of 5.0×10⁹–3.5×10¹⁰ s^?1. Stabilization of the charge‐separated state in these multi‐modular donor–acceptor polyads is also observed to certain level.     

Supramolecular Tetrad Featuring Covalently Linked Bis(porphyrin)–Phthalocyanine Coordinated to Fullerene: Construction and Photochemical Studies

A multimodular donor–acceptor tetrad featuring a bis(zinc porphyrin)–(zinc phthalocyanine) ((ZnP–ZnP)–ZnPc) triad and bis‐pyridine‐functionalized fullerene was assembled by a “two‐point” binding strategy, and investigated as a charge‐separating photosynthetic antenna‐reaction center mimic. The spectral and computational studies suggested that the mode of binding of the bis‐pyridine‐functionalized fullerene involves either one of the zinc porphyrin and zinc phthalocyanine (Pc) entities of the triad or both zinc porphyrin entities leaving ZnPc unbound. The binding constant evaluated by constructing a Benesi–Hildebrand plot by using the optical data was found to be 1.17×10⁵ M ^?1, whereas a plot of “mole‐ratio” method revealed a 1:1 stoichiometry for the supramolecular tetrad. The mode of binding was further supported by differential pulse voltammetry studies, in which redox modulation of both zinc porphyrin and zinc phthalocyanine entities was observed. The geometry of the tetrad was deduced by B3LYP/6‐31G* optimization, whereas the energy levels for different photochemical events was established by using data from the optical absorption and emission, and electrochemical studies. Excitation of the zinc porphyrin entity of the triad and tetrad revealed ultrafast singlet–singlet energy transfer to the appended zinc phthalocyanine. The estimated rate of energy transfer (k_ENT) in the case of the triad was found to be 7.5×10¹¹ s^?1 in toluene and 6.3×10¹¹ s^?1 in o‐dichlorobenzene, respectively. As was predicted from the energy levels, photoinduced electron transfer from the energy‐transfer product, that is, singlet‐excited zinc phthalocyanine to fullerene was verified from the femtosecond‐transient spectral studies, both in o‐dichlorobenzene and toluene. Transient bands corresponding to ZnPc ^{? +} in the 850 nm range and C₆₀ ^{? ?} in the 1020 nm range were clearly observed. The rate of charge separation, k_CS, and rate of charge recombination, k_CR, for the (ZnP–ZnP)–ZnPc ^{? +}:Py₂C₆₀ ^{? ?} radical ion pair (from the time profile of 849 nm peak) were found to be 2.20×10¹¹ and 6.10×10⁸ s^?1 in toluene, and 6.82×10¹¹ and 1.20×10⁹ s^?1 in o‐dichlorobenzene, respectively. These results revealed efficient energy transfer followed by charge separation in the newly assembled supramolecular tetrad.     

Photoinduced Charge Separation in Zinc–Porphyrin/Tungsten–Alkylidyne Dyads: Generation of Reactive Porphyrin and Metallo Radical States

The luminescent tungsten–alkylidyne metalloligand [WCl(≡C‐4,4′‐C₆H₄CC‐py)(dppe)₂] ( 1 ; dppe=1,2‐bis(diphenylphosphino)ethane) and the zinc–tetraarylporphyrins ZnTPP and ZnTP_ClP (TPP=tetraphenylporphyrin, TP_ClP=tetra(p‐chlorophenyl)porphyrin) self‐assemble in fluorobenzene solution to form the dyads ZnTPP( 1 ) and ZnTP_ClP( 1 ), in which the metalloligand is axially coordinated to the porphyrin. Excitation of the porphyrin‐centered S₁ excited states of these dyads initiates intramolecular energy‐transfer (ZnPor→ 1 ) and electron‐transfer ( 1 →ZnPor) processes, which together efficiently quench the S₁ state (～90 %). Transient‐absorption spectroscopy and an associated kinetic analysis reveal that the net product of the energy‐transfer process is the ³[dπ*] state of coordinated 1 , which is formed by S₁→¹[dπ*] singlet–singlet (Förster) energy transfer followed by ¹[dπ*]→³[dπ*] intersystem crossing. The data also demonstrate that coordinated 1 reductively quenches the porphyrin S₁ state to produce the [ZnPor^?][ 1⁺ ] charge‐separated state. This is a rare example of the reductive quenching of zinc porphyrin chromophores. The presence in the [ZnPor^?][ 1⁺ ] charge‐separated states of powerfully reducing zinc–porphyrin radical anions, which are capable of sensitizing a wide range of reductive electrocatalysts, and the 1⁺ ion, which can initiate the oxidation of H₂, produces an integrated photochemical system with the thermodynamic capability of driving photoredox processes that result in the transfer of renewable reducing equivalents instead of the consumption of conventional sacrificial donors.     

Regioisomeric Effects on the Electronic Features of Indenothiophene‐Bridged D–π‐A′–A DSSC Sensitizers

Two D–π‐A′–A regioisomers (A‐IDT‐D and D‐IDT‐A) featuring 4,4′‐di‐p‐tolyl‐4 H‐indeno[1,2‐b]‐thiophene as a π linker (π) between the diarylamino donor (D) and the pyrimidine–cyanoacrylic acid acceptor (A′–A) have been successfully synthesized and characterized as efficient sensitizers for the dye‐sensitized solar cells (DSSCs). The different arrangements of the D and A′–A blocks on the unsymmetrical indenothiophene (IDT) core render the dipole of IDT being along (A‐IDT‐D) or opposite (D‐IDT‐A) to the direction of intramolecular (donor‐to‐acceptor) charge transfer, and thus induce variations in the physical properties. The experimental observations correlated well with the theoretical analyses, clearly revealing the trade‐off between the molar extinction coefficient (ε) and the S₀→S₁ transition energy. As a result, a superior ε value was observed for D‐IDT‐A, whereas a bathochromic shift in the absorption occurred in A‐IDT‐D. The larger ε value of D‐IDT‐A together with its more favorable energy level relative to TiO₂ led to a higher power conversion efficiency of 7.41 % for the D‐IDT‐A‐based DSSC, retaining approximately 95 % of the N719‐based DSSC efficiency. This work manifests the clear structure–property relationship for the case of donor and acceptor components being connected by an unsymmetrical π linker and provides insights for molecular engineering of organic sensitizers.     

Synthesis and Characterization of Donor–π‐Acceptor‐Based Porphyrin Sensitizers: Potential Application of Dye‐Sensitized Solar Cells

New porphyrin sensitizers based on donor–π‐acceptor (D‐π‐A) approach have been designed, synthesized, characterized by various spectroscopic techniques and their photovoltaic properties explored. N,N′‐Diphenylamine acts as donor, the porphyrin is the π‐spacer, and either carboxylic acid or cyanoacryclic acid acts as acceptor. All compounds were characterized by using ¹H NMR spectroscopy, ESI‐MS, UV–visible emission spectroscopies as well as electrochemical methods. The presence of aromatic groups between porphyrin π‐plane and acceptor group push the absorption of both Soret and Q‐bands of porphyrin towards the red region. The electrochemical properties suggests that LUMO of these sensitizers above the TiO₂ conduction band. Finally, the device was fabricated using liquid redox electrolyte (I^?/I₃^?) and its efficiency was compared with that of a leading sensitizer.     

Porphyrin–Phthalocyanine/Pyridylfullerene Supramolecular Assemblies

The synthesis and photophysical properties of several porphyrin (P)–phthalocyanine (Pc) conjugates (P–Pc; 1 – 3 ) are described, in which the phthalocyanines are directly linked to the β‐pyrrolic position of a meso‐tetraphenylporphyrin. Photoinduced energy‐ and electron‐transfer processes were studied through the preparation of H₂P–ZnPc, ZnP–ZnPc, and PdP–ZnPc conjugates, and their assembly through metal coordination with two different pyridylfulleropyrrolidines ( 4 and 5 ). The resulting electron‐donor–acceptor hybrids, which were formed by axial coordination of compounds 4 and 5 with the corresponding phthalocyanines, mimicked the fundamental processes of photosynthesis; that is, light harvesting, the transduction of excited‐state energy, and unidirectional electron transfer. In particular, photophysical studies confirmed that intramolecular energy‐transfer resulted from the S₂ excited state as well as from the S₁ excited state of the porphyrins to the energetically lower‐lying phthalocyanines, followed by an intramolecular charge‐transfer to yield P–Pc^.+ ? C₆₀^.?. This unique sequence of processes opens the way for solar‐energy‐conversion processes.     

Photoinduced Electron Transfer within a Zinc Porphyrin–Cyclobis(paraquat‐p‐phenylene) Donor–Acceptor Dyad

Understanding the mechanism of efficient photoinduced electron‐transfer processes is essential for developing molecular systems for artificial photosynthesis. Towards this goal, we describe the synthesis of a donor–acceptor dyad comprising a zinc porphyrin donor and a tetracationic cyclobis(paraquat‐p‐phenylene) (CBPQT⁴⁺) acceptor. The X‐ray crystal structure of the dyad reveals the formation of a dimeric motif through the intermolecular coordination between the triazole nitrogen and the central Zn metal of two adjacent units of the dyad. Photoinduced electron transfer within the dyad in MeCN was investigated by femtosecond and nanosecond transient absorption spectroscopy, as well as by transient EPR spectroscopy. Photoexcitation of the dyad produced a weakly coupled ZnP^+.–CBPQT^3+. spin‐correlated radical‐ion pair having a τ=146 ns lifetime and a spin–spin exchange interaction of only 0.23 mT. The long radical‐ion‐pair lifetime results from weak donor–acceptor electronic coupling as a consequence of having nine bonds between the donor and the acceptor, and the reduction in reorganization energy for electron transfer caused by charge dispersal over both paraquat units within CBPQT^3+..     

A Supramolecular [10]CPP Junction Enables Efficient Electron Transfer in Modular Porphyrin–[10]CPP⊃Fullerene Complexes

Efficient photoinduced electron transfer was observed across a [10]cycloparaphenylene ([10]CPP) moiety that serves as a rigid non‐covalent bridge between a zinc porphyrin and a range of fullerenes. The preparation of iodo‐[10]CPP is the key to the synthesis of a porphyrin–[10]CPP conjugate, which binds C₆₀, C₇₀, (C₆₀)₂, and other fullerenes (K_A>10⁵ m ^?1). Fluorescence and pump–probe spectroscopy revealed intramolecular energy transfer between CPP and porphyrin and also efficient charge separation between porphyrin and fullerenes, affording up to 0.5 μs lifetime charge‐separated states. The advantage of this approach towards electron donor–acceptor dyads is evident in the case of dumbbell‐shaped (C₆₀)₂, which gave intricate charge‐transfer behavior in 1:1 and 2:1 complexes. These results suggest that [10]CPP and its cross‐coupled derivatives could act as supramolecular mediators of charge transport in organic electronic devices.     

Photocatalytic Hydrogen Evolution Based on Efficient Energy and Electron Transfers in Donor–Bridge–Acceptor Multibranched‐Porphyrin‐Functionalized Platinum Nanocomposites

Two donor–bridge–acceptor conjugates (5,10,15,20‐tetrakis[4‐(N,N‐diphenylaminobenzoate)phenyl] porphyrin (TPPZ) and 5,10,15,20‐tetrakis[4‐(N,N‐diphenylaminostyryl)phenyl] porphyrin (TPPX)) were covalently linked to triphenylamine (TPA) at the meso‐position of porphyrin ring. The triphenylamine entities were expected to act as energy donors and the porphyrins to act as an energy acceptor. In this paper, we report on the synthesis of these multibranched‐porphyrin‐functionalized Pt nanocomposites. The conjugates used here not only served as a stabilizer to prevent agglomeration of Pt nanoparticles, but also as a light‐harvesting photosensitizer. The occurrence of photoinduced electron‐transfer processes was confirmed by time‐resolved fluorescence and photoelectrochemical spectral measurements. The different efficiencies for energy and electron transfer in the two multibranched porphyrins and the functionalized Pt nanocomposites were attributed to diverse covalent linkages. Moreover, in the reduction of water to produce H₂, the photocatalytic activity of the Pt nanocomposite functionalized by TPPX, in which the triphenylamine and porphyrin moieties are bonded through an ethylene bridge, was much higher than that of the platinum nanocomposite functionalized by TPPZ, in which the two moieties are bonded through an ester. This investigation demonstrates the fundamental advantages of constructing donor–bridge–acceptor conjugates as highly efficient photosensitizers based on efficient energy and electron transfer.     

Effect of long alkyl chains of aniline donor on the photovoltaic performance of D‐π‐A zinc porphyrin for dye‐sensitized solar cells

Three novel dyes of JJ1 , JJ2 , and JJ6 featured zinc porphyrin as a basic core structure; N, N‐alkyl‐4‐(prop‐1‐yn‐1‐yl)aniline as an electron donor linked to meso‐10‐position; 4‐(prop‐1‐yn‐1‐yl)benzoic acid as an electron acceptor linked to meso‐20‐position; and 2,6‐bis(dodecyloxy)phenyl or 2,6‐bis(octyloxy)phenyl respectively linked to meso‐5 and meso‐15‐positions of zinc porphyrin have been synthesized and used for dye‐sensitized solar cells. Porphyrin JJ6 featured the shortest alkyl group (─C₄H₉) on the donor, whereas JJ2 contained the longest alkyl groups (─C₁₂H₂₅), and JJ1 has a medium length of octyl groups. With these new porphyrin sensitizers, we observed that JJ6 has 7.55% power conversion efficiency under simulated one‐sun illumination (AM 1.5 G, 100 mW/cm²) with J_SC = 18.64 mA/cm², V_OC = 0.66 V, and fill factor (FF) = 0.61, which was higher than the other two; JJ1 (7.35%) with J_SC = 18.83 mA/cm², V_OC = 0.68 V, and FF = 0.60; and JJ2 (6.33%) with J_SC = 15.69 mA/cm², V_OC = 0.62 V, and FF = 0.65. The power conversion efficiency of JJ6 and JJ1 were higher than JJ2 , demonstrating that the lengthy alkyl groups on the aniline cause a decrease in efficiency of the devices.     

A High‐Energy Charge‐Separated State of 1.70 eV from a High‐Potential Donor–Acceptor Dyad: A Catalyst for Energy‐Demanding Photochemical Reactions

A high potential donor–acceptor dyad composed of zinc porphyrin bearing three meso‐pentafluorophenyl substituents covalently linked to C₆₀, as a novel dyad capable of generating charge‐separated states of high energy (potential) has been developed. The calculated energy of the charge‐separated state was found to be 1.70 eV, the highest reported for a covalently linked porphyrin–fullerene dyad. Intramolecular photoinduced electron transfer leading to charge‐separated states of appreciable lifetimes in polar and nonpolar solvents has been established from studies involving femto‐ to nanosecond transient absorption techniques. The high energy stored in the form of charge‐separated states along with its persistence of about 50–60 ns makes this dyad a potential electron‐transporting catalyst to carry out energy‐demanding photochemical reactions. This type of high‐energy harvesting dyad is expected to open new research in the areas of artificial photosynthesis especially producing energy (potential) demanding light‐to‐fuel products.     

Photoinduced Energy Transfer from Poly(N‐vinylcarbazole) to Tricarbonylchloro‐(2,2′‐bipyridyl)rhenium(I)

This work investigates the photoinduced energy transfer from poly(N‐vinylcarbazole) (PVK), as a donor material, to fac‐(2,2′‐bipyridyl)Re(CO)₃Cl, as a catalyst acceptor, for its potential application towards CO₂ reduction. Photoluminescence quenching experiments reveal dynamic quenching through resonance energy transfer in solid donor/acceptor mixtures and in solid/liquid systems. The bimolecular reaction rate constant at solution–film interfaces for the elementary reaction of the excited state with the quencher material could be determined as 8.8(±1.4)×10¹¹ L mol^?1 s^?1 by using Stern–Volmer analysis. This work shows that PVK is an effective and cheap absorber material that can act efficiently as a redox photosensitizer in combination with fac‐(2,2′‐bipyridyl)Re(CO)₃Cl as a catalyst acceptor, which might lead to possible applications in photocatalytic CO₂ reduction.     

Controlled self‐assembly of porphyrin/fullerene donor–acceptor complex in a polymer thin film

Thin films composed of polycyclohexane (PCHE), zinc(II)‐5,10,15,20‐tetra‐(2‐naphthyl)porphyrin (ZnTNpP), and [6,6]‐phenyl‐C₆₁‐butyric acid methyl ester (PCBM) blends are prepared to investigate their potential for the controlled self‐assembly of a porphyrin/fullerene donor–acceptor complex in a polymer thin film. The compatibilities of PCHE/PCBM (p), PCHE/ZnTNpP (q), and ZnTNpP/PCBM (r) in these blends have a significant effect on the dispersion of the ZnTNpP/PCBM donor–acceptor complex in the PCHE thin film. When the compatibilities are p << q, r, and q ≈ r, the ZnTNpP/PCBM donor–acceptor complex is formed between the PCHE and PCBM phases. This concept to form a controlled self‐assembly of the ZnTNpP/PCBM donor–acceptor complex may be applied to various combinations of porphyrin/fullerene systems in polymer thin film solar cells to achieve excellent performance. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 743–746     

Diameter‐Sorted SWCNT–Porphyrin and SWCNT–Phthalocyanine Conjugates for Light‐Energy Harvesting

A non‐covalent double‐decker binding strategy is employed to construct functional supramolecular single‐wall carbon nanotubes (SWCNT)–tetrapyrrole hybrids capable of undergoing photoinduced electron transfer and performing direct conversion of light into electricity. To accomplish this, two semiconducting SWCNTs of different diameters (6,5 and 7,6) were modified via π–π stacking of pyrene functionalized with an alkyl ammonium cation (PyrNH₃⁺). Such modified nanotubes were subsequently assembled via dipole–cation binding of zinc porphyrin with one ( 1 ) or four benzo‐18‐crown‐6 cavities ( 2 ) or phthalocyanine with four benzo‐18‐crown‐6 cavities at the ring periphery ( 3 ), employed as visible‐light photosensitizers. Upon charactering the conjugates using TEM and optical techniques, electron transfer via photoexcited zinc porphyrin and phthalocyanine was investigated using time‐resolved emission and transient absorption techniques. Higher charge‐separation efficiency is established for SWCNT(7,6) with a narrow band gap than the thin SWCNT(6,5) with a wide band gap. Photoelectrochemical studies using FTO/SnO₂ electrodes modified with these donor–acceptor conjugates unanimously demonstrated the ability of these conjugates to convert light energy into electricity. The photocurrent generation followed the trend observed for charge separation, that is, incident‐photon‐to‐current efficiency (IPCE) of a maximum of 12 % is achieved for photocells with FTO/SnO₂/SWCNT(7,6)/PyrNH₃⁺: 1 .     

Luminescence Properties of C‐Diazaborolyl‐ortho‐Carboranes as Donor–Acceptor Systems

Seven derivatives of 1,2‐dicarbadodecaborane (ortho‐carborane, 1,2‐C₂B₁₀H₁₂) with a 1,3‐diethyl‐ or 1,3‐diphenyl‐1,3,2‐benzodiazaborolyl group on one cage carbon atom were synthesized and structurally characterized. Six of these compounds showed remarkable low‐energy fluorescence emissions with large Stokes shifts of 15100–20260 cm^?1 and quantum yields (Φ_F) of up to 65 % in the solid state. The low‐energy fluorescence emission, which was assigned to a charge‐transfer (CT) transition between the cage and the heterocyclic unit, depended on the orientation (torsion angle, ψ) of the diazaborolyl group with respect to the cage C? C bond. In cyclohexane, two compounds exhibited very weak dual fluorescence emissions with Stokes shifts of 15660–18090 cm^?1 for the CT bands and 1960–5540 cm^?1 for the high‐energy bands, which were assigned to local transitions within the benzodiazaborole units (local excitation, LE), whereas four compounds showed only CT bands with Φ_F values between 8–32 %. Two distinct excited singlet‐state (S₁) geometries, denoted S₁(LE) and S₁(CT), were observed computationally for the benzodiazaborolyl‐ortho‐carboranes, the population of which depended on their orientation (ψ). TD‐DFT calculations on these excited state geometries were in accord with their CT and LE emissions. These C‐diazaborolyl‐ortho‐carboranes were viewed as donor–acceptor systems with the diazaborolyl group as the donor and the ortho‐carboranyl group as the acceptor.     

Synthesis,Photophysical Properties and Computational Studies of beta‐Substituted Porphyrin Dyads

Beta‐pyrrole‐substituted porphyrin dyads connected by ethynyl linkage to N‐butylcarbazole or triphenylamine donors are reported. Donor‐π‐acceptor type beta‐substituted porphyrin dyads and their Zn(II) and Pd(II) complexes were characterized by MALDI‐MS, NMR, UV‐vis absorption, fluorescence and cyclic voltammetry techniques. The S₁ emission dynamics were analyzed by time‐resolved spectroscopy (TCSPC); dyads exhibited efficient energy transfer up to 93% from beta‐donors (N‐butylcarbazole or triphenylamine group) to the porphyrin core. The efficiency of energy transfer for the beta‐substituted porphyrin dyads were much higher than those of the corresponding meso‐substituted porphyrin dyads, reflecting enhanced communications between the beta‐donors and the porphyrin core. The Pd(II) dyads, showed characteristic phosphorescence in the near IR region and very efficient singlet oxygen quantum yields (53–60%); these dyads are promising candidates for photocatalytic oxidations of organic compounds. The donor‐acceptor interaction between the porphyrin core and the beta‐donors was supported by the DFT studies in the porphyrin dyads.     

Synthesis and Aggregation Behavior of meso‐Sulfinylporphyrins: Evaluation of S‐Chirality Effects on the Self‐Organization to S–Oxo‐Tethered Cofacial Porphyrin Dimers

The synthesis and aggregation behavior of meso‐sulfinylporphyrins are described. The copper‐catalyzed C–S cross‐coupling reaction of a meso‐iodoporphyrin with benzenethiol and n‐octanethiol has proved to be an efficient method for the synthesis of meso‐sulfanylporphyrins, which are oxygenated by m‐chloroperbenzoic acid to produce the corresponding meso‐sulfinylporphyrins. Optically active zinc meso‐sulfinylporphyrins were successfully isolated by means of optical resolution of the racemates on a chiral HPLC column. Zinc sulfinylporphyrins readily undergo self‐organization through S–oxo–zinc coordination to form cofacial porphyrin dimers in solution, in which the hetero‐ and homodimers are present as a diastereomeric mixture. The aggregation modes of the S–oxo‐tethered porphyrin dimers were fully characterized by ¹H NMR, IR, and UV/Vis spectroscopy as well as DFT calculations on their model compounds, thus revealing that the self‐aggregation behavior depends on the combination of S chirality. The absolute configurations at the sulfur center can be determined by the exciton‐coupled CD method. The observed self‐association constant for the S–oxo‐tethered dimerization of (S)‐phenylsulfinylporphyrin in toluene is larger than that in dichloromethane, which reflects the difference in dipole moments between the homodimer and the monomer. In cyclic and differential pulse voltammetry, the first oxidation process of the cofacial dimers is split into two reversible steps, which indicates that the initially produced π radical cations are delocalized efficiently between the two porphyrin rings. The present findings demonstrate the potential utility of meso‐sulfinyl groups as promising ligands for investigating the effects of peripheral chirality on the structures and optical and electrochemical properties of metal‐assisted porphyrin self‐assemblies.     

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.     

Engaging Copper(III) Corrole as an Electron Acceptor: Photoinduced Charge Separation in Zinc Porphyrin–Copper Corrole Donor–Acceptor Conjugates

An electron‐deficient copper(III) corrole was utilized for the construction of donor–acceptor conjugates with zinc(II) porphyrin (ZnP) as a singlet excited state electron donor, and the occurrence of photoinduced charge separation was demonstrated by using transient pump–probe spectroscopic techniques. In these conjugates, the number of copper corrole units was varied from 1 to 2 or 4 units while maintaining a single ZnP entity to observe the effect of corrole multiplicity in facilitating the charge‐separation process. The conjugates and control compounds were electrochemically and spectroelectrochemically characterized. Computational studies revealed ground state geometries of the compounds and the electron‐deficient nature of the copper(III) corrole. An energy level diagram was established to predict the photochemical events by using optical, emission, electrochemical, and computational data. The occurrence of charge separation from singlet excited zinc porphyrin and charge recombination to yield directly the ground state species were evident from the diagram. Femtosecond transient absorption spectroscopy studies provided spectral evidence of charge separation in the form of the zinc porphyrin radical cation and copper(II) corrole species as products. Rates of charge separation in the conjugates were found to be of the order of 10¹⁰ s^?1 and increased with increasing multiplicity of copper(III) corrole entities. The present study demonstrates the importance of copper(III) corrole as an electron acceptor in building model photosynthetic systems.     

Ultrafast Photoinduced Charge Separation Leading to High‐Energy Radical Ion‐Pairs in Directly Linked Corrole–C60 and Triphenylamine–Corrole‐C60 Donor–Acceptor Conjugates

Closely positioned donor–acceptor pairs facilitate electron‐ and energy‐transfer events, relevant to light energy conversion. Here, a triad system TPACor‐C₆₀ , possessing a free‐base corrole as central unit that linked the energy donor triphenylamine ( TPA ) at the meso position and an electron acceptor fullerene (C₆₀) at the β‐pyrrole position was newly synthesized, as were the component dyads TPA‐Cor and Cor‐C₆₀ . Spectroscopic, electrochemical, and DFT studies confirmed the molecular integrity and existence of a moderate level of intramolecular interactions between the components. Steady‐state fluorescence studies showed efficient energy transfer from ¹ TPA* to the corrole and subsequent electron transfer from ¹corrole* to fullerene. Further studies involving femtosecond and nanosecond laser flash photolysis confirmed electron transfer to be the quenching mechanism of corrole emission, in which the electron‐transfer products, the corrole radical cation ( Cor^?+ in Cor‐C₆₀ and TPA‐Cor^?+ in TPACor‐C₆₀ ) and fullerene radical anion (C₆₀^??), could be spectrally characterized. Owing to the close proximity of the donor and acceptor entities in the dyad and triad, the rate of charge separation, k_CS, was found to be about 10¹¹ s^?1, suggesting the occurrence of an ultrafast charge‐separation process. Interestingly, although an order of magnitude slower than k_CS, the rate of charge recombination, k_CR, was also found to be rapid (k_CR≈10¹⁰ s^?1), and both processes followed the solvent polarity trend DMF>benzonitrile>THF>toluene. The charge‐separated species relaxed directly to the ground state in polar solvents while in toluene, formation of ³corrole* was observed, thus implying that the energy of the charge‐separated state in a nonpolar solvent is higher than the energy of ³corrole* being about 1.52 eV. That is, ultrafast formation of a high‐energy charge‐separated state in toluene has been achieved in these closely spaced corrole–fullerene donor–acceptor conjugates.