Sushanta K. Das

Thieno-Pyrrole-Fused 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene–Fullerene Dyads: Utilization of Near-Infrared Sensitizers for Ultrafast Charge Separation in Donor–Acceptor Systems

Donor-acceptor dyads featuring near-IR sensitizers derived from thieno-pyrrole-fused BODIPY (abbreviated as SBDPiR) and fullerene, C60 have been newly synthesized and characterized. Occurrence of ultrafast photoinduced electron transfer (PET) leading to the formation of charge-separated state in these dyads, capable of harvesting light energy from the near-IR region, is established from femtosecond transient absorption studies.

Dual Functioning Thieno-Pyrrole Fused BODIPY Dyes for NIR Optical Imaging and Photodynamic Therapy: Singlet Oxygen Generation without Heavy Halogen Atom Assistance.

We discovered a rare phenomenon wherein a thieno-pyrrole fused BODIPY dye (SBDPiR690) generates singlet oxygen without heavy halogen atom substituents. SBDPiR690 generates both singlet oxygen and fluorescence. To our knowledge, this is the first example of such a finding. To establish a structure-photophysical property relationship, we prepared SBDPiR analogs with electron-withdrawing groups at the para-position of the phenyl groups. The electron-withdrawing groups increased the HOMO-LUMO energy gap and singlet oxygen generation. Among the analogs, SBDPiR688, a CF3 analog, had an excellent dual functionality of brightness (82290 m(-1)  cm(-1) ) and phototoxic power (99170 m(-1)  cm(-1) ) comparable to those of Pc 4, due to a high extinction coefficient (211 000 m(-1)  cm(-1) ) and balanced decay (Φflu =0.39 and ΦΔ =0.47). The dual functionality of the lead compound SBDPiR690 was successfully applied to preclinical optical imaging and for PDT to effectively control a subcutaneous tumor.

Functionalization of Diameter-Sorted Semiconductive SWCNTs with Photosensitizing Porphyrins: Syntheses and Photoinduced Electron Transfer

Covalent functionalization of diameter sorted SWCNTs with porphyrins (MP), and photochemistry to establish nanotube diameter-dependent charge separation efficiencies are reported. The MP-SWCNT(n,m) [M=2H or Zn, and (n,m)=(7,6) or (6,5)] nanohybrids are characterized by a variety of spectroscopic, thermogravimetric, TEM imaging techniques, and also by DFT MO calculations. The thermogravimetric, Raman and fluorescence studies reveal the presence of a moderate number of porphyrins on the SWCNT surface. The MO results suggest charge separation (CS) via the excited state of MP. Time-resolved fluorescence studies reveal quenching of the singlet excited state of the MP with SWCNT(n,m), giving the rate constants of charge separation (k(CS)) in the range of (4-5)×10(9) s(-1). Nanosecond transient absorption measurements confirm the charge-separated radical cation and the radical anion as [MP(.+)-SWCNT(.-)] with their characteristic absorption bands in the visible and near-IR regions. The charge separated states persist for about 70-100 ns thus giving an opportunity to utilize them to build photoelectrochemical cells, which allowed us to derive the structure-reactivity relationship between the nature of porphyrin and diameter of the employed nanotubes.

Thieno-pyrrole-fused BODIPY intermediate as a platform to multifunctional NIR agents.

We report the synthesis, photophysical and electrochemical properties, and in vivo fluorescence imaging of a series of new thieno-pyrrole-fused near-infrared (NIR) BODIPY agents by using a versatile intermediate as a building block. The versatile thieno-pyrrole-fused BODIPY intermediate was rationally designed to bear bromo-substituents and absorb in the mid-red region (635 nm) to act as an organic electrophile for the development of NIR multifunctional agents. The use of subsequent palladium-catalyzed and nucleophilic substitution reactions afforded highly conjugated NIR BODIPYs. The novel BODIPYs exhibit long-wavelength absorptions in the NIR region (650-840 nm). The agents produce sharp fluorescence bands, and most of them display respectable quantum yields of fluorescence (0.05-0.87) useful for biomedical imaging, as demonstrated by in vivo imaging with SBDPiR740. Interestingly, a number of agents in the series that are non-halogenated were reactive to O2 at the triplet photo-excited state coupled with a favorable redox potential and decent fluorescence, and hence could be potential candidates for use as photosensitizers in fluorescence-guided photodynamic therapy. Furthermore, the synthetic approach allows further functionalization of the highly conjugated NIR BODIPYs to tune the excited states (PET, ICT) and to conjugate targeting moieties for enhanced biological applications.

Decorating single layer graphene oxide with electron donor and acceptor molecules for the study of photoinduced electron transfer

Graphene oxide decorated with an electron donor, zinc(II) phthalocyanine, and an electron acceptor, fullerene, has been synthesized, and photoinduced electron transfer leading to charge-separation is reported.

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(n)Pcs) 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(n)Pcs. The determined binding constants, K, in o-dichlorobenzene for the 1:1 complexes are in the order of 10(4) to 10(5) 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(n)Pc(.+)-C60Im(.-) and ZnF(n)Pc(.+)-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(n)Pc 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.

Photoinduced charge separation in three-layer supramolecular nanohybrids: fullerene–porphyrin–SWCNT

Photoinduced charge separation processes of three-layer supramolecular hybrids, fullerene-porphyrin-SWCNT, which are constructed from semiconducting (7,6)- and (6,5)-enriched SWCNTs and self-assembled via π-π interacting long alkyl chain substituted porphyrins (tetrakis(4-dodecyloxyphenyl)porphyrins; abbreviated as MP(alkyl)(4)) (M = Zn and H(2)), to which imidazole functionalized fullerene[60] (C(60)Im) is coordinated, have been investigated in organic solvents. The intermolecular alkyl-π and π-π interactions between the MP(alkyl)(4) and SWCNTs, in addition, coordination between C(60)Im and Zn ion in the porphyrin cavity are visualized using DFT calculations at the B3LYP/3-21G(*) level, predicting donor-acceptor interactions between them in the ground and excited states. The donor-acceptor nanohybrids thus formed are characterized by TEM imaging, steady-state absorption and fluorescence spectra. The time-resolved fluorescence studies of MP(alkyl)(4) in two-layered nanohybrids (MP(alkyl)(4)/SWCNT) revealed efficient quenching of the singlet excited states of MP(alkyl)(4) ((1)MP*(alkyl)(4)) with the rate constants of charge separation (k(CS)) in the range of (1-9) × 10(9) s(-1). A nanosecond transient absorption technique confirmed the electron transfer products, MP˙(+)(alkyl)(4)/SWCNT˙(-) and/or MP˙(-)(alkyl)(4)/SWCNT˙(+) for the two-layer nanohybrids. Upon further coordination of C(60)Im to ZnP, acceleration of charge separation via(1)ZnP* in C(60)Im→ZnP(alkyl)(4)/SWCNT is observed to form C(60)˙(-)Im→ZnP˙(+)(alkyl)(4)/SWCNT and C(60)˙(-)Im→ZnP(alkyl)(4)/SWCNT˙(+) charge separated states as supported by the transient absorption spectra. These characteristic absorptions decay with rate constants due to charge recombination (k(CR)) in the range of (6-10) × 10(6) s(-1), corresponding to the lifetimes of the radical ion-pairs of 100-170 ns. The electron transfer in the nanohybrids has further been utilized for light-to-electricity conversion by the construction of proof-of-concept photoelectrochemical solar cells.

Studies on the Photocatalytic Electron Pooling of Graphene Oxide Hybrids Decorated with Electron Donor and Electron Acceptor Molecules

Photocatalytic behavior of a recently synthesized, single layer graphene oxide (SLGO) decorated with an electron donor, zinc phthalocyanine (ZnPc) and an electron acceptor, fulleropyrrolidine (C60) donor-acceptor hybrid is demonstrated. Electron accumulation in the form of one-electron reduced product of methyl viologen was obtained in high yields in an electron pooling experiment involving the ZnPc-SLGO-C60 hybrid and a sacrificial electron donor compared with control hybrids involving either ZnPc-SLGO or SLGO-C60 hybrids. This novel property of ZnPc-SLGO-C60 hybrid has been ascribed to the proximity effect offered by GO with covalently linked donor and acceptor entities on its surface. The present studies reveal that the ZnPc-SLGO-C60 hybrid is a suitable catalyst for solar fuel production.