Michael B. Thomas

Multichromophoric Perylenediimide–Silicon Phthalocyanine–C60 System as an Artificial Photosynthetic Analogue

Sequential photoinduced energy transfer followed by electron transfer and the formation of charge-separated states, which are primary events of natural photosynthesis, have been demonstrated in a newly synthesized multichromophoric covalently linked triad, PDI-SiPc-C60 . The triad comprises a perylenediimide (PDI), which primarily fulfils antenna and electron-acceptor functionalities, silicon phthalocyanine (SiPc) as an electron donor, and fulleropyrrolidine (C60 ) as a second electron acceptor. The multi-step convergent synthetic procedure developed here produced good yields of the triad and control dyads, PDI-SiPc and SiPc-C60 . The structures and geometries of the newly synthesized donor-acceptor systems have been established from spectral, computational, and electrochemical studies with reference to appropriate control compounds. Ultrafast energy transfer from 1 PDI* to SiPc in the case of PDI-SiPc and PDI-SiPc-C60 was witnessed. An energy-level diagram established from spectral and electrochemical data suggested the formation of two types of charge-separated states, that is, PDI-SiPc.+ -C60.- and PDI.- -SiPc.+ -C60 from the 1 SiPc* in the triad, with generation of the latter being energetically more favorable. However, photochemical studies involving femtosecond transient spectroscopy revealed the formation of PDI-SiPc.+ -C60.- as a major charge-separated product. This observation may be rationalized in terms of the closer spatial proximity to SiPc of C60 compared to PDI in the triad. The charge-separated state persisted for a few nanoseconds prior to populating the 3 SiPc* state during charge recombination.

Ultrafast Charge-Separation in Triphenylamine-BODIPY-Derived Triads Carrying Centrally Positioned, Highly Electron-Deficient, Dicyanoquinodimethane or Tetracyanobutadiene Electron-Acceptors

A series of new triphenylamine (TPA)-substituted BODIPYs 1-3 have been designed and synthesized through the Pd-catalysed Sonogashira cross-coupling and [2+2] cycloaddition-retroelectrocyclization reactions in good yields. This procedure yielded highly electron-deficient tetracyanobutadiene (TCBD) or dicyanoquinodimethane (DCNQ) electron-acceptor units centrally located at the TPA-BODIPY system. As a consequence, significant perturbation of the photonic and electronic properties was observed. The triads 2 and 3 showed red-shifted absorption, in addition to a strong charge-transfer-type absorption in the case of 3. The electrochemical studies revealed multi-redox processes involving the TPA, TCBD or DCNQ and BODIPY entities. The computational studies were performed at the B3LYP/6-31G** level to elucidate the geometry and electronic structures. An energy level diagram established for triads 2 and 3 revealed that the photoinduced charge-separation from the 1 BODIPY* is thermodynamically possible. In addition, charge transfer from TPA to TCBD in 2 and DCNQ in 3 was also possible. These charge transfer mechanisms were confirmed by photochemical studies performed using time-resolved emission and femtosecond-transient-absorption studies in solvents of varying polarity. Ultrafast charge-separation has been witnessed in these closely spaced, strongly interacting triads. The charge-separated state returned to the ground state without populating the 3 BODIPY*.

β−Functionalized Push-Pull Opp-Dibenzoporphyrins as Sensitizers for Dye-Sensitized Solar Cells

A novel class of β-functionalized push-pull zinc opp-dibenzoporphyrins were designed, synthesized, and utilized as sensitizers for dye-sensitized solar cells. Spectral, electrochemical, and computational studies were systematically performed to evaluate their spectral coverage, redox behavior, and electronic structures. These porphyrins displayed much broader spectral coverage and more facile oxidation upon extension of the π conjugation. Free-energy calculations and femtosecond transient absorption studies (charge injection rate in the range of 1011  s-1 ) suggested efficient charge injection from the excited singlet state of the porphyrin to the conduction band of TiO2 . The power conversion efficiency (η) of YH3 bearing acrylic acid linkers (η=5.9 %) was close to that of the best ruthenium dye N719 (η=7.4 %) under similar conditions. The superior photovoltaic performance of YH3 was attributed to its higher light-harvesting ability and more favorable electron injection and collection, as supported by electrochemical impedance spectral studies. This work demonstrates the exceptional potential of benzoporphyrins as sensitizers for dye-sensitized solar cells.

Directly Attached Bisdonor‐BF2 Chelated Azadipyrromethene‐Fullerene Tetrads for Promoting Ground and Excited State Charge Transfer

The efficiency and mechanism of electron- and energy-transfer events occurring in both natural and synthetic donor-acceptor systems depend on their distance, relative orientation, and the nature of the surrounding media. Fundamental knowledge gained from model studies is key to building efficient energy harvesting and optoelectronic devices. Faster charge separation and slower charge recombination in donor-acceptor systems is often sought out. In our continued effort to build donor-acceptor systems using near-IR sensitizers, in the present study, we report ground and excited-state charge transfer in newly synthesized, directly linked tetrads featuring bisdonor (donor=phenothiazine and ferrocene), BF2 -chelated azadipyrromethane (azaBODIPY) and C60 entities. The tetrads synthesized using multi-step synthetic procedure revealed strong charge-transfer interactions in the ground state involving the donor and azaBODIPY entities. The near-IR emitting azaBODIPY acted as a photosensitizing electron acceptor along with fullerene whereas the phenothiazine and ferrocene entities acted as electron donors. The triads (bisdonor-azaBODIPY) and tetrads revealed ultrafast photoinduced charge separation leading to D.+ -azaBODIPY.- -C60 and D.+ -azaBODIPY-C60.- (D=phenothiazine or ferrocene) charge separated states from the femtosecond transient absorption spectral studies in both polar and nonpolar solvent media. The charge-separated states populated the triplet excited state of azaBODIPY prior returning to the ground state.

Competitive Energy and Electron Transfer in β-Functionalized Free-Base Porphyrin–Zinc Porphyrin Dimer Axially Coordinated to C60: Synthesis, Supramolecular Formation and Excited-State Processes

Simultaneous occurrence of energy and electron transfer events involving different acceptor sites in a newly assembled supramolecular triad comprised of covalently linked free-base porphyrin-zinc porphyrin dyad, H2 P-ZnP axially coordinated to electron acceptor fullerene, has been successfully demonstrated. The dyad was connected through the β-pyrrole positions of the porphyrin macrocycle instead of the traditionally used meso-positions for better electronic communication. Interestingly, the β-pyrrole functionalization modulated the optical properties to such an extent that it was possible to almost exclusively excite the zinc porphyrin entity in the supramolecular triad. The measured binding constant for the complex with 1:1 molecular stoichiometry was in the order of 104  m-1 revealing moderately stable complex formation. An energy level diagram constructed using optical, electrochemical and computational results suggested that both the anticipated energy and electron events are thermodynamically feasible in the triad. Consequently, it was possible to demonstrate occurrence of excited state energy transfer to the covalently linked H2 P, and electron transfer to the coordinated ImC60 from studies involving steady-state and time-resolved emission, and femto- and nanosecond transient absorption studies. The estimated energy transfer was around 67 % in the dyad with a rate constant of 1.1×109  s-1 . In the supramolecular triad, the charge separated state was rather long-lived although it was difficult to arrive the exact lifetime of charge separated state from nanosecond transient spectral studies due to overlap of strong triplet excited signals of porphyrin in the monitoring wavelength window. Nevertheless, simultaneous occurrence of energy and electron transfer in the appropriately positioned energy and electron acceptor entities in a supramolecular triad was possible to demonstrate in the present study, a step forward to unraveling the complex photochemical events occurring in natural photosynthesis and its implications in building light energy harvesting devices.

C3-Symmetric Positional Isomers of BODIPY Substituted Triazines: Synthesis and Excited State Properties

A series of meso- O-aryl functionalized BODIPY trimers positioned along the C3-symmetric axis of triazine ring have newly been synthesized to probe the ground and excited state intramolecular type interactions between the BODIPY entities within the trimer. The developed synthetic strategy resulted in BODIPY trimers in good yields. The electron rich, meso- O-aryl functionalized BODIPYs revealed larger HOMO-LUMO gap and higher Stokes shift and fluorescence lifetimes compared to the traditional BODIPY derivatives having an aryl group attached at the meso position. The optical absorption, steady-state fluorescence, and electrochemical studies revealed weak, if any, intramolecular type interactions among the BODIPY entities within the trimer and the central triazine unit to be both photo- and redox-salient. The possibility of singlet-singlet energy migration among the BODIPY entities was investigated using time-resolved emission and femtosecond transient absorption studies. Excitation of a BODIPY entity in the trimers led to successful formation of 1BODIPY*, which populated the 3BODIPY* via intersystem crossing. Among the three trimers, although very weak, only trimer 8 revealed excitation transfer to some extent. The present findings suggest that the meso- O-aryl functionalized BODIPYs due to their superior fluorescence properties are better probes to build light energy harvesting supramolecular oligomeric systems and for other applications such as sensing and imaging.