Gary N. Lim

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 (3) C60 * 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 (1) BODIPY* generated radical ion-pair in these triads persisted for few nanoseconds due to electron transfer/hole-shift mechanism.

Syntheses, Charge Separation, and Inverted Bulk Heterojunction Solar Cell Application of Phenothiazine–Fullerene Dyads

A series of phenothiazine-fulleropyrrolidine (PTZ-C60) dyads having fullerene either at the C-3 aromatic ring position or at the N-position of phenothiazine macrocycle were newly synthesized and characterized. Photoinduced electron transfer leading to PTZ(•+)-C60(•-) charge-separated species was established from studies involving femtosecond transient absorption spectroscopy. Because of the close proximity of the donor and acceptor entities, the C-3 ring substituted PTZ-C60 dyads revealed faster charge separation and charge recombination processes than that observed in the dyad functionalized through the N-position. Next, inverted organic bulk heterojunction (BHJ) solar cells were constructed using the dyads in place of traditionally used [6,6]-phenyl-C61- butyric acid methyl ester (PCBM) and an additional electron donor material poly(3-hexylthiophene) (P3HT). The performance of the C-3 ring substituted PTZ-C60 dyad having a polyethylene glycol substituent produced a power conversion efficiency of 3.5% under inverted bulk heterojunction (BHJ) configuration. This was attributed to optimal BHJ morphology between the polymer and the dyad, which was further promoted by the efficient intramolecular charge separation and relatively slow charge recombination promoted by the dyad within the BHJ structure. The present finding demonstrate PTZ-C60 dyads as being good prospective materials for building organic photovoltaic devices.

Charge Separation in Graphene‐Decorated Multimodular Tris(pyrene)–Subphthalocyanine–Fullerene Donor–Acceptor Hybrids

A new approach to probe the effect of graphene on photochemical charge separation in donor-acceptor conjugates is devised. For this, multimodular donor-acceptor conjugates, composed of three molecules of pyrene, a subphthalocyanine, and a fullerene C60 ((Pyr)3 SubPc-C60 ), have been synthesized and characterized. These systems were hybridized on few-layer graphene through π-π stacking interactions of the three pyrene moieties. The hybrids were characterized using Raman, HRTEM, and spectroscopic and electrochemical techniques. The energy levels of the donor-acceptor conjugates were fine-tuned upon interaction with graphene and photoinduced charge separation in the absence and presence of graphene was studied by femtosecond transient absorption spectroscopy. Accelerated charge separation and recombination was detected in these graphene-decorated conjugates suggesting that they could be used as materials for fast-responding optoelectronic devices and in light energy harvesting applications.

Synthesis and Photoinduced Electron Transfer Studies of a Tri(Phenothiazine)–Subphthalocyanine–Fullerene Pentad

A novel donor-acceptor pentad featuring subphthalocyanine and fullerene as the primary electron donor and acceptor, and three phenothiazine entities as secondary hole transferring agents, have been newly synthesized and characterized as an photosynthetic reaction center model compound. Occurrences of ultrafast photoinduced electron transfer (PET) and slower charge recombination are witnessed in the pentad from the femtosecond and nanosecond transient absorption studies.

Modulation of Energy Transfer into Sequential Electron Transfer upon Axial Coordination of Tetrathiafulvalene in an Aluminum(III) Porphyrin–Free-Base Porphyrin Dyad

Axially assembled aluminum(III) porphyrin based dyads and triads have been constructed to investigate the factors that govern the energy and electron transfer processes in a perpendicular direction to the porphyrin plane. In the aluminum(III) porphyrin-free-base porphyrin (AlPor-Ph-H2Por) dyad, the AlPor occupies the basal plane, while the free-base porphyrin (H2Por) with electron withdrawing groups resides in the axial position through a benzoate spacer. The NMR, UV-visible absorption, and steady-state fluorescence studies confirm that the coordination of pyridine appended tetrathiafulvalene (TTF) derivative (TTF-py or TTF-Ph-py) to the dyad in noncoordinating solvents afford vertically arranged supramolecular self-assembled triads (TTF-py→AlPor-Ph-H2Por and TTF-Ph-py→AlPor-Ph-H2Por). Time-resolved studies revealed that the AlPor in dyad and triads undergoes photoinduced energy and/or electron transfer processes. Interestingly, the energy and electron donating/accepting nature of AlPor can be modulated by changing the solvent polarity or by stimulating a new competing process using a TTF molecule. In modest polar solvents (dichloromethane and o-dichlorobenzene), excitation of AlPor leads singlet-singlet energy transfer from the excited singlet state of AlPor ((1)AlPor*) to H2Por with a moderate rate constant (k(EnT)) of 1.78 × 10(8) s(-1). In contrast, excitation of AlPor in the triad results in ultrafast electron transfer from TTF to (1)AlPor* with a rate constant (k(ET)) of 8.33 × 10(9)-1.25 × 10(10) s(-1), which outcompetes the energy transfer from (1)AlPor* to H2Por and yields the primary radical pair TTF(+•)-AlPor(-•)-H2Por. A subsequent electron shift to H2Por generates a spatially well-separated TTF(+•)-AlPor-H2Por(-•) radical pair.

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 (1) corrole* 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, kCS , was found to be about 10(11)  s(-1), suggesting the occurrence of an ultrafast charge-separation process. Interestingly, although an order of magnitude slower than kCS , the rate of charge recombination, kCR , was also found to be rapid (kCR ≈10(10)  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 (3) corrole* was observed, thus implying that the energy of the charge-separated state in a nonpolar solvent is higher than the energy of (3) corrole* 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.

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.

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 10(10)  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.

Charge separation in supramolecular ferrocene(s)-zinc porphyrin-fullerene triads: A femtosecond transient absorption study

Mechanistic aspects of photoinduced charge separation in supramolecular triads, constructed using covalently linked zinc porphyrin-ferrocene(s) dyads — self-assembled via axial coordination to either pyridine or phenylimidazole appended fulleropyrrolidine (Fcx-ZnP:PyC60 or Fcx-ZnP:ImC60; x = 1 or 2), has been investigated using femtosecond pump-probe transient spectroscopy. Upon photoexcitation of ZnP, charge separation from ferrocene to 1ZnP* to yield the initial Fc+-ZnP•-:C60 radical ion-pair or charge separation from 1ZnP* to C60 to yield the initial Fc-ZnP•+:C60•- radical ion-pair, depending upon the ferrocene-zinc porphyrin intermolecular distance, was observed. These radical ion-pairs resulted in the formation of ultimate distantly separated Fc+-ZnP:C60•- radical ion-pairs either via an electron migration (former case) or hole shift (latter case) process. Kinetics of charge separation as a function of spacer connecting the ferrocene and porphyrin, and spacer between the porphyrin and fullerene is reported. In agreement with our earlier study (J. Phys. Chem. B 2004; 108: 11333–11343), the Fc+-ZnP:C60•- radical ion-pair persisted beyond the monitoring time window of our instrument, suggesting charge stabilization in these supramolecular triads.

Effect of Spacer Connecting the Secondary Electron Donor Phenothiazine in Subphthalocyanine-Fullerene Conjugates in Promoting Electron Transfer Followed by Hole Shift Process.

Sequential electron/hole transfer between energetically well-positioned entities of photosynthetic reaction center models is one of the commonly employed mechanisms to generate long-lived charge-separated states. A wealth of information, applicable towards light energy harvesting and building optoelectronic devices, has been acquired from such studies. In the present study, we report on the effect of spacer (direct or via phenoxy linkage) connecting the hole shifting agent, phenothiazine (PTZ), on photoinduced charge stabilization in subphthalocyanine-fullerene donor-acceptor conjugates. In these conjugates, the subphthalocyanine (SubPc) and fullerene (C60 ) served as primary electron donor and acceptor, respectively, while the phenothiazine entities act as hole shifting agents. The newly synthesized compounds were characterized by optical absorption and emission, computational, and electrochemical methods. The redox potentials measured using differential pulse voltammetry were used to estimate free-energy changes for charge separation, hole migration, and charge recombination processes. Using femto- and nanosecond transient absorption techniques, evidence for charge separation, and kinetics of charge separation and recombination were obtained in polar benzonitrile and nonpolar toluene solvents. In the conjugate where the phenothiazine entities are directly linked to SubPc, evidence for sequential electron transfer followed by hole shift leading to long-lived charge separated state was weak, primarily due to the delocalization of HOMO on both SubPc and PTZ entities. However, in case of the conjugate where the PTZ and SubPc are linked via phenoxy spacers, sequential electron transfer/hole shift was observed leading to the formation of long-lived charge-separated states. The present study brings out the importance of the spacer group connecting the hole shifting agent in model donor-acceptor conjugates to generate long-lived charge-separated states.