A Supramolecular Tetrad Featuring Covalently Linked Ferrocene–Zinc Porphyrin–BODIPY Coordinated to Fullerene: A Charge Stabilizing,Photosynthetic Antenna–Reaction Center Mimic

A novel photosynthetic‐antenna–reaction‐center model compound, comprised of BF₂‐chelated dipyrromethene (BODIPY) as an energy‐harvesting antenna, zinc porphyrin (ZnP) as the primary electron donor, ferrocene (Fc) as a hole‐shifting agent, and phenylimidazole‐functionalized fulleropyrrolidine (C₆₀Im) as an electron acceptor, has been synthesized and characterized. Optical absorption and emission, computational structure optimization, and cyclic voltammetry studies were systematically performed to establish the role of each entity in the multistep photochemical reactions. The energy‐level diagram established from optical and redox data helped identifying different photochemical events. Selective excitation of BODIPY resulted in efficient singlet energy transfer to the ZnP entity. Ultrafast electron transfer from the ¹ZnP* (formed either as a result of singlet–singlet energy transfer or direct excitation) or ¹C₆₀* of the coordinated fullerene resulting into the formation of the Fc–(C₆₀ ^{. ?}Im:ZnP ^{. +})–BODIPY radical ion pair was witnessed by femtosecond transient absorption studies. Subsequent hole migration to the ferrocene entity resulted in the Fc⁺–(C₆₀ ^{. +}Im:ZnP)–BODIPY radical ion pair that persisted for 7–15 μs, depending upon the solvent conditions and contributions from the triplet excited states of ZnP and ImC₆₀, as revealed by the nanosecond transient spectral studies. Better utilization of light energy in generating the long‐lived charge‐separated state with the help of the present “antenna–reaction‐center” model system has been successfully demonstrated.     

Phenothiazine–BODIPY–Fullerene Triads as Photosynthetic Reaction Center Models: Substitution and Solvent Polarity Effects on Photoinduced Charge Separation and Recombination

Novel photosynthetic reaction center model compounds of the type donor₂–donor₁–acceptor, composed of phenothiazine, BF₂‐chelated dipyrromethene (BODIPY), and fullerene, respectively, have been newly synthesized using multistep synthetic methods. X‐ray structures of three of the phenothiazine‐BODIPY intermediate compounds have been solved to visualize the substitution effect caused by the phenothiazine on the BODIPY macrocycle. Optical absorption and emission, computational, and differential pulse voltammetry studies were systematically performed to establish the molecular integrity of the triads. The N‐substituted phenothiazine was found to be easier to oxidize by 60 mV compared to the C‐substituted analogue. The geometry and electronic structures were obtained by B3LYP/6‐31G(dp) calculations (for H, B, N, and O) and B3LYP/6‐31G(df) calculations (for S) in vacuum, followed by a single‐point calculation in benzonitrile utilizing the polarizable continuum model (PCM). The HOMO?1, HOMO, and LUMO were, respectively, on the BODIPY, phenothiazine and fullerene entities, which agreed well with the site of electron transfer determined from electrochemical studies. The energy‐level diagram deduced from these data helped in elucidating the mechanistic details of the photochemical events. Excitation of BODIPY resulted in ultrafast electron transfer to produce PTZ–BODIPY^.+–C₆₀^.?; subsequent hole shift resulted in PTZ^.+–BODIPY–C₆₀^.? charge‐separated species. The return of the charge‐separated species was found to be solvent dependent. In nonpolar solvents the PTZ^.+–BODIPY–C₆₀^.? species populated the ³C₆₀* prior to returning to the ground state, while in polar solvent no such process was observed due to relative positioning of the energy levels. The ¹BODIPY* generated radical ion‐pair in these triads persisted for few nanoseconds due to electron transfer/hole‐shift mechanism.     

Photosynthetic Antenna‐Reaction Center Mimicry with a Covalently Linked Monostyryl Boron‐Dipyrromethene–Aza‐Boron‐Dipyrromethene–C60 Triad

An efficient functional mimic of the photosynthetic antenna‐reaction center has been designed and synthesized. The model contains a near‐infrared‐absorbing aza‐boron‐dipyrromethene (ADP) that is connected to a monostyryl boron‐dipyrromethene (BDP) by a click reaction and to a fullerene (C₆₀) using the Prato reaction. The intramolecular photoinduced energy and electron‐transfer processes of this triad as well as the corresponding dyads BDP‐ADP and ADP‐C₆₀ have been studied with steady‐state and time‐resolved absorption and fluorescence spectroscopic methods in benzonitrile. Upon excitation, the BDP moiety of the triad is significantly quenched due to energy transfer to the ADP core, which subsequently transfers an electron to the fullerene unit. Cyclic and differential pulse voltammetric studies have revealed the redox states of the components, which allow estimation of the energies of the charge‐separated states. Such calculations show that electron transfer from the singlet excited ADP (¹ADP*) to C₆₀ yielding ADP^.+‐C₆₀^.? is energetically favorable. By using femtosecond laser flash photolysis, concrete evidence has been obtained for the occurrence of energy transfer from ¹BDP* to ADP in the dyad BDP‐ADP and electron transfer from ¹ADP* to C₆₀ in the dyad ADP‐C₆₀. Sequential energy and electron transfer have also been clearly observed in the triad BDP‐ADP‐C₆₀. By monitoring the rise of ADP emission, it has been found that the rate of energy transfer is fast (≈10¹¹ s^?1). The dynamics of electron transfer through ¹ADP* has also been studied by monitoring the formation of C₆₀ radical anion at 1000 nm. A fast charge‐separation process from ¹ADP* to C₆₀ has been detected, which gives the relatively long‐lived BDP‐ADP^.+C₆₀^.? with a lifetime of 1.47 ns. As shown by nanosecond transient absorption measurements, the charge‐separated state decays slowly to populate mainly the triplet state of ADP before returning to the ground state. These findings show that the dyads BDP‐ADP and ADP‐C₆₀, and the triad BDP‐ADP‐C₆₀ are interesting artificial analogues that can mimic the antenna and reaction center of the natural photosynthetic systems.     

Excitation‐Wavelength‐Dependent,Ultrafast Photoinduced Electron Transfer in Bisferrocene/BF2‐Chelated‐Azadipyrromethene/Fullerene Tetrads

Donor–acceptor distance, orientation, and photoexcitation wavelength are key factors in governing the efficiency and mechanism of electron‐transfer reactions both in natural and synthetic systems. Although distance and orientation effects have been successfully demonstrated in simple donor–acceptor dyads, revealing excitation‐wavelength‐dependent photochemical properties demands multimodular, photosynthetic‐reaction‐center model compounds. Here, we successfully demonstrate donor– acceptor excitation‐wavelength‐dependent, ultrafast charge separation and charge recombination in newly synthesized, novel tetrads featuring bisferrocene, BF₂‐chelated azadipyrromethene, and fullerene entities. The tetrads synthesized using multistep synthetic procedure revealed characteristic optical, redox, and photo reactivities of the individual components and featured “closely” and “distantly” positioned donor–acceptor systems. The near‐IR‐emitting BF₂‐chelated azadipyrromethene acted as a photosensitizing electron acceptor along with fullerene, while the ferrocene entities acted as electron donors. Both tetrads revealed excitation‐wavelength‐dependent, photoinduced, electron‐transfer events as probed by femtosecond transient absorption spectroscopy. That is, formation of the Fc⁺–ADP–C₆₀^.? charge‐separated state upon C₆₀ excitation, and Fc⁺–ADP^.?–C₆₀ formation upon ADP excitation is demonstrated.     

“Two‐Point” Self‐Assembly and Photoinduced Electron Transfer in meso‐Donor‐Carrying Bis(styryl crown ether)‐BODIPY–Bis(alkylammonium)fullerene Donor–Acceptor Conjugates

BF₂‐chelated dipyrromethene, BODIPY, was functionalized to carry two styryl crown ether tails and a secondary electron donor at the meso position. By using a “two‐point” self‐assembly strategy, a bis‐alkylammonium‐functionalized fullerene (C₆₀) was allowed to self‐assemble the crown ether voids of BODIPY to obtain multimodular donor–acceptor conjugates. As a consequence of the two‐point binding, the 1:1 stoichiometric complexes formed yielded complexes of higher stability in which fluorescence of BODIPY was found to be quenched; this suggested the occurrence of excited‐state processes. The geometry and electronic structure of the self‐assembled complexes were derived from B3LYP/3‐21G(*) methods in which no steric constraints between the entities was observed. An energy‐level diagram was established by using spectral, electrochemical, and computational results to help understand the mechanistic details of excited‐state processes originating from ¹bis‐styryl‐BODIPY*. Femtosecond transient absorbance studies were indicative of the formation of an exciplex state prior to the charge‐separation process to yield a bis‐styryl‐BODIPY ^{. +}–C₆₀ ^{. −} radical ion pair. The time constants for charge separation were generally lower than charge‐recombination processes. The present studies bring out the importance of multimode binding strategies to obtain stable self‐assembled donor–acceptor conjugates capable of undergoing photoinduced charge separation needed in artificial photosynthetic applications.     

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.     

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.     

Energy‐Funneling‐Based Broadband Visible‐Light‐Absorbing Bodipy–C60 Triads and Tetrads as Dual Functional Heavy‐Atom‐Free Organic Triplet Photosensitizers for Photocatalytic Organic Reactions

C₆₀–bodipy triads and tetrads based on the energy‐funneling effect that show broadband absorption in the visible region have been prepared as novel triplet photosensitizers. The new photosensitizers contain two or three different light‐harvesting antennae associated with different absorption wavelengths, resulting in a broad absorption band (450–650 nm). The panchromatic excitation energy harvested by the bodipy moieties is funneled into a spin converter (C₆₀), thus ensuring intersystem crossing and population of the triplet state. Nanosecond time‐resolved transient absorption and spin density analysis indicated that the T₁ state is localized on either C₆₀ or the antennae, depending on the T₁ energy levels of the two entities. The antenna‐localized T₁ state shows a longer lifetime (τ_T=132.9 μs) than the C₆₀‐localized T₁ state (ca. 27.4 μs). We found that the C₆₀ triads and tetrads can be used as dual functional photocatalysts, that is, singlet oxygen (¹O₂) and superoxide radical anion (O₂ ^{. ?}) photosensitizers. In the photooxidation of naphthol to juglone, the ¹O₂ photosensitizing ability of the C₆₀ triad is a factor of 8.9 greater than the conventional triplet photosensitizers tetraphenylporphyrin and methylene blue. The C₆₀ dyads and triads were also used as photocatalysts for O₂ ^{. ?}‐mediated aerobic oxidation of aromatic boronic acids to produce phenols. The reaction times were greatly reduced compared with when [Ru(bpy)₃Cl₂] was used as photocatalyst. Our study of triplet photosensitizers has shown that broadband absorption in the visible spectral region and long‐lived triplet excited states can be useful for the design of new heavy‐atom‐free organic triplet photosensitizers and for the application of these triplet photosensitizers in photo‐organocatalysis.     

Near‐IR Excitation Transfer and Electron Transfer in a BF2‐Chelated Dipyrromethane–Azadipyrromethane Dyad and Triad

A molecular dyad and triad, comprised of a known photosensitizer, BF₂‐chelated dipyrromethane (BDP), covalently linked to its structural analog and near‐IR emitting sensitizer, BF₂‐chelated tetraarylazadipyrromethane (ADP), have been newly synthesized and the photoinduced energy and electron transfer were examined by femtosecond and nanosecond laser flash photolysis. The structural integrity of the newly synthesized compounds has been established by spectroscopic, electrochemical, and computational methods. The DFT calculations revealed a molecular‐clip‐type structure for the triad, in which the BDP and ADP entities are separated by about 14 Å with a dihedral angle between the fluorophores of around 70°. Differential pulse voltammetry studies have revealed the redox states, allowing estimation of the energies of the charge‐separated states. Such calculations revealed a charge separation from the singlet excited BDP (¹BDP*) to ADP (BDP^.+‐ADP^.?) to be energetically favorable in nonpolar toluene and in polar benzonitrile. In addition, the excitation transfer from the singlet BDP to ADP is also envisioned due to good spectral overlap of the BDP emission and ADP absorption spectra. Femtosecond laser flash photolysis studies provided concrete evidence for the occurrence of energy transfer from ¹BDP* to ADP (in benzonitrile and toluene) and electron transfer from BDP to ¹ADP* (in benzonitrile, but not in toluene). The kinetic study of energy transfer was measured by monitoring the rise of the ADP emission and revealed fast energy transfer (ca. 10¹¹ s^?1) in these molecular systems. The kinetics of electron transfer via ¹ADP*, measured by monitoring the decay of the singlet ADP at λ=820 nm, revealed a relatively fast charge‐separation process from BDP to ¹ADP*. These findings suggest the potential of the examined ADP–BDP molecules to be efficient photosynthetic antenna and reaction center models.     

Ultrafast Photoinduced Electron Transfer and Charge Stabilization in Donor–Acceptor Dyads Capable of Harvesting Near‐Infrared Light

To harvest energy from the near‐infrared (near‐IR) and infrared (IR) regions of the electromagnetic spectrum, which constitutes nearly 70 % of the solar radiation, there is a great demand for near‐IR and IR light‐absorbing sensitizers that are capable of undergoing ultrafast photoinduced electron transfer when connected to a suitable electron acceptor. Towards achieving this goal, in the present study, we report multistep syntheses of dyads derived from structurally modified BF₂‐chelated azadipyrromethene (ADP; to extend absorption and emission into the near‐IR region) and fullerene as electron‐donor and electron‐acceptor entities, respectively. The newly synthesized dyads were fully characterized based on optical absorbance, fluorescence, geometry optimization, and electrochemical studies. The established energy level diagram revealed the possibility of electron transfer either from the singlet excited near‐IR sensitizer or singlet excited fullerene. Femtosecond and nanosecond transient absorption studies were performed to gather evidence of excited state electron transfer and to evaluate the kinetics of charge separation and charge recombination processes. These studies revealed the occurrence of ultrafast photoinduced electron transfer leading to charge stabilization in the dyads, and populating the triplet states of ADP, benzanulated‐ADP and benzanulated thiophene‐ADP in the respective dyads, and triplet state of C₆₀ in the case of BF₂‐chelated dipyrromethene derived dyad during charge recombination. The present findings reveal that these sensitizers are suitable for harvesting light energy from the near‐IR region of the solar spectrum and for building fast‐responding optoelectronic devices operating under near‐IR radiation input.     

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.     

Distance Matters: Effect of the Spacer Length on the Photophysical Properties of Multimodular Perylenediimide–Silicon Phthalocyanine–Fullerene Triads

A multimodular donor–acceptor conjugate featuring silicon phthalocyanine (SiPc) as the electron donor, and two electron acceptors, namely tetrachloroperylenediimide (PDI) and C₆₀, placed at the opposite ends of the SiPc axial positions, was newly designed and synthesized, and the results were compared to the earlier reported PDI-SiPc-C₆₀ triad. Minimal intramolecular interactions between the entities was observed. Absorption, fluorescence, computational and electrochemical studies were performed to evaluate the excitation energy, geometry and electronic structure, and energy levels of different photoevents. Steady-state absorption, fluorescence and excitation spectral studies revealed efficient singlet–singlet energy transfer from ¹PDI* to SiPc in the PDI-SiPc dyad and the PDI-SiPc-C₆₀ triad. The measured rates for these photochemical events were found to be much higher than those reported earlier for the triad, due to closer proximity between the PDI and SiPc entities. The distance also affected the charge separation path in which involvement of PDI, and not C₆₀, in charge separation in the present triad was witnessed. The present investigation brings out the importance of donor–acceptor distances in channeling photochemical events in a multimodular system.     

Broadband Visible‐Light‐Harvesting trans‐Bis(alkylphosphine) Platinum(II)‐Alkynyl Complexes with Singlet Energy Transfer between BODIPY and Naphthalene Diimide Ligands

A heteroleptic bis(tributylphosphine) platinum(II)‐alkynyl complex ( Pt‐1 ) showing broadband visible‐light absorption was prepared. Two different visible‐light‐absorbing ligands, that is, ethynylated boron‐dipyrromethene (BODIPY) and a functionalized naphthalene diimide (NDI) were used in the molecule. Two reference complexes, Pt‐2 and Pt‐3 , which contain only the NDI or BODIPY ligand, respectively, were also prepared. The coordinated BODIPY ligand shows absorption at 503 nm and fluorescence at 516 nm, whereas the coordinated NDI ligand absorbs at 594 nm; the spectral overlap between the two ligands ensures intramolecular resonance energy transfer in Pt‐1 , with BODIPY as the singlet energy donor and NDI as the energy acceptor. The complex shows strong absorption in the region 450 nm–640 nm, with molar absorption coefficient up to 88 000 M ^?1 cm^?1. Long‐lived triplet excited states lifetimes were observed for Pt‐1 – Pt‐3 (36.9 μs, 28.3 μs, and 818.6 μs, respectively). Singlet and triplet energy transfer processes were studied by the fluorescence/phosphorescence excitation spectra, steady‐state and time‐resolved UV/Vis absorption and luminescence spectra, as well as nanosecond time‐resolved transient difference absorption spectra. A triplet‐state equilibrium was observed for Pt‐1 . The complexes were used as triplet photosensitizers for triplet–triplet annihilation upconversion, with upconversion quantum yields up to 18.4 % being observed for Pt‐1 .     

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.     

One-Photon Excitation Followed by a Three-Step Sequential Energy–Energy–Electron Transfer Leading to a Charge-Separated State in a Supramolecular Tetrad Featuring Benzothiazole–Boron-Dipyrromethene–Zinc Porphyrin–C60

A panchromatic triad, consisting of benzothiazole (BTZ) and BF₂-chelated boron-dipyrromethene (BODIPY) moieties covalently linked to a zinc porphyrin (ZnP) core, has been synthesized and systematically characterized by using ¹H NMR spectroscopy, ESI-MS, UV-visible, steady-state fluorescence, electrochemical, and femtosecond transient absorption techniques. The absorption band of the triad, BTZ-BODIPY-ZnP, and dyads, BTZ-BODIPY and BODIPY-ZnP, along with the reference compounds BTZ-OMe, BODIPY-OMe, and ZnP-OMe exhibited characteristic bands corresponding to individual chromophores. Electrochemical measurements on BTZ-BODIPY-ZnP exhibited redox behavior similar to that of the reference compounds. Upon selective excitation of BTZ (≈290 nm) in the BTZ-BODIPY-ZnP triad, the fluorescence of the BTZ moiety is quenched, due to photoinduced energy transfer (PEnT) from ¹BTZ^* to the BODIPY moiety, followed by quenching of the BODIPY emission due to sequential PEnT from the ¹BODIPY* moiety to ZnP, resulting in the appearance of the ZnP emission, indicating the occurrence of a two-step singlet–singlet energy transfer. Further, a supramolecular tetrad, BTZ-BODIPY-ZnP:ImC₆₀, was formed by axially coordinating the triad with imidazole-appended fulleropyrrolidine (ImC₆₀), and parallel steady-state measurements displayed the diminished emission of ZnP, which clearly indicated the occurrence of photoinduced electron transfer (PET) from ¹ZnP* to ImC₆₀. Finally, femtosecond transient absorption spectral studies provided evidence for the sequential occurrence of PEnT and PET events, namely, ¹BTZ^*-BODIPY-ZnP:ImC₆₀→BTZ-¹BODIPY^*-ZnP:ImC₆₀→BTZ-BODIPY-¹ZnP^*:ImC₆₀→BTZ-BODIPY-ZnP^.+:ImC₆₀^.− in the supramolecular tetrad. The evaluated rate of energy transfer, k_EnT, was found to be 3–5×10¹⁰ s⁻¹, which was slightly faster than that observed in the case of BODIPY-ZnP and BTZ-BODIPY-ZnP, lacking the coordinated ImC₆₀. The rate constants for charge separation and recombination, k_CS and k_CR, respectively, calculated by monitoring the rise and decay of C₆₀^.− were found to be 5.5×10¹⁰ and 4.4×10⁸ s⁻¹, respectively, for the BODIPY-ZnP:ImC₆₀ triad, and 3.1×10¹⁰ and 4.9×10⁸ s⁻¹, respectively, for the BTZ-BODIPY-ZnP:ImC₆₀ tetrad. Initial excitation of the tetrad, promoting two-step energy transfer and a final electron-transfer event, has been successfully demonstrated in the present study.     

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.     

Effects of Light Energy and Reducing Agents on C60‐Mediated Photosensitizing Reactions

Many biomolecules contain photoactive reducing agents, such as reduced nicotinamide adenine dinucleotide (NADH) and 6‐thioguanine (6‐TG) incorporated into DNA through drug metabolism. These reducing agents may produce reactive oxygen species under UVA irradiation or act as electron donors in various media. The interactions of C₆₀ fullerenes with biological reductants and light energy, especially via the Type‐I electron‐transfer mechanism, are not fully understood although these factors are often involved in toxicity assessments. The two reductants employed in this work were NADH for aqueous solutions and 6‐TG for organic solvents. Using steady‐state photolysis and electrochemical techniques, we showed that under visible light irradiation, the presence of reducing agents enhanced C₆₀‐mediated Type‐I reactions that generate superoxide anion (O₂^.?) at the expense of singlet oxygen (¹O₂) production. The quantum yield of O₂^.? production upon visible light irradiation of C₆₀ is estimated below 0.2 in dipolar aprotic media, indicating that the majority of triplet C₆₀ deactivate via Type‐II pathway. Upon UVA irradiation, however, both C₆₀ and NADH undergo photochemical reactions to produce O₂^.?, which could lead to a possible synergistic toxicity effects. C₆₀ photosensitization via Type‐I pathway is not observed in the absence of reducing agents.     

Remarkably Efficient Photocurrent Generation Based on a [60]Fullerene–Triosmium Cluster/Zn–Porphyrin/Boron–Dipyrrin Triad SAM

A new artificial photosynthetic triad array, a [60]fullerene–triosmium cluster/zinc–porphyrin/boron–dipyrrin complex ( 1 , Os₃C₆₀/ZnP/Bodipy), has been prepared by decarbonylation of Os₃(CO)₈(CN(CH₂)₃Si(OEt)₃)(μ₃‐η²:η²:η²‐C₆₀) ( 6 ) with Me₃NO/MeCN and subsequent reaction with the isocyanide ligand CNZnP/Bodipy ( 5 ) containing zinc porphyrin (ZnP) and boron dipyrrin (Bodipy) moieties. Triad 1 has been characterized by various spectroscopic methods (MS, NMR, IR, UV/Vis, photoluminescence, and transient absorption spectroscopy). The electrochemical properties of 1 in chlorobenzene (CB) have been examined by cyclic voltammetry; the general feature of the cyclic voltammogram of 1 is nine reversible one‐electron redox couples, that is, the sum of those of 5 and 6 . DFT has been applied to study the molecular and electronic structures of 1 . On the basis of fluorescence‐lifetime measurements and transient absorption spectroscopic data, 1 undergoes an efficient energy transfer from Bodipy to ZnP and a fast electron transfer from ZnP to C₆₀; the detailed kinetics involved in both events have been elucidated. The SAM of triad 1 ( 1 /ITO; ITO=indium–tin oxide) has been prepared by immersion of an ITO electrode in a CB solution of 1 and diazabicyclo‐octane (2:1 equiv), and characterized by UV/Vis absorption spectroscopy, water contact angle, X‐ray photoelectron spectroscopy, and cyclic voltammetry. The photoelectrochemical properties of 1 /ITO have been investigated by a standard three‐electrode system in the presence of an ascorbic acid sacrificial electron donor. The quantum yield of the photoelectrochemical cell has been estimated to be 29 % based on the number of photons absorbed by the chromophores. Our triad 1 is unique when compared to previously reported photoinduced electron‐transfer arrays, in that C₆₀ is linked by π bonding with little perturbation of the C₆₀ electron delocalization.     

Ultrafast Photoinduced Charge Separation in Wide‐Band‐Capturing Self‐Assembled Supramolecular Bis(donor styryl)BODIPY–Fullerene Conjugates

A new series of self‐assembled supramolecular donor–acceptor conjugates capable of wide‐band capture, and exhibiting photoinduced charge separation have been designed, synthesized and characterized using various techniques as artificial photosynthetic mimics. The donor host systems comprise of a 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) containing a crown ether entity at the meso‐position and two styryl entities on the pyrrole rings. The styryl end groups also carried additional donor (triphenylamine or phenothiazine) entities. The acceptor host system was a fulleropyrrolidine comprised of an ethylammonium cation. Owing to the presence of extended conjugation and multiple chromophore entities, the BODIPY host revealed absorbance and emission well into the near‐IR region covering the 300–850 nm spectral range. The donor–acceptor conjugates formed by crown ether–alkyl ammonium cation binding of the host–guest system was characterized by optical absorbance and emission, computational, and electrochemical techniques. Experimentally determined binding constants were in the range of 1–2×10⁵ M ^?1. An energy‐level diagram to visualize different photochemical events was established using redox, computational, absorbance, and emission data. Spectral evidence for the occurrence of photoinduced charge separation in these conjugates was established from femtosecond transient absorption studies. The measured rates indicated ultrafast charge separation and relatively slow charge recombination revealing their usefulness in light‐energy harvesting and optoelectronic device applications. The bis(donor styryl)BODIPY‐derived conjugates populated their triplet excited states during charge recombination.     

Improved Spectral Coverage and Fluorescence Quenching in Donor–acceptor Systems Involving Indolo[3‐2‐b]carbazole and Boron‐dipyrromethene or Diketopyrrolopyrrole

A novel π‐conjugated triad and a polymer incorporating indolo[3,2‐b]‐carbazole (ICZ) and 4,4‐difluoro‐4‐bora‐3a,4a‐diaza‐s‐indacene (BODIPY) were synthesized via a Sonogashira coupling. Compared to the parent BODIPY the absorption and fluorescence spectrum were for both compounds broader and redshifted. The redshift of the fluorescence and the decrease of the fluorescence quantum yield and decay time upon increasing solvent polarity were attributed to the formation of a partial charge‐transfer state. Upon excitation in the ICZ absorption band the ICZ fluorescence was quenched in both compounds mainly due to energy transfer to the BODIPY moiety. In a similar ICZ–π–DPP polymer (where DPP is diketopyrrolopyrrole), a smaller redshift of the absorption and fluorescence spectra compared to the parent DPP was observed. A less efficient quenching of the ICZ fluorescence in the ICZ–π–DPP polymer could be related to the unfavorable orientation of the transition dipoles of ICZ and DPP. The rate constant for energy transfer was for all compounds an order of magnitude smaller than predicted by Förster theory. While in a solid film of the triad a further redshift of the absorption maximum of nearly 100 nm was observed, no such shift was observed for the ICZ–π–BODIPY polymer.