Singlet Oxygen Generation and Photoinduced Charge Separation of Tetra Polyethyleneglycol Functionalized Zinc Phthalocyanine-Fullerene Dyad

Utilization of polyethyleneglycol （PEG） functionalized zinc phthalocyanine, （PEG）4ZnPc in （i） singlet oxygen generation, and （ii） in building energy harvesting donor-acceptor systems using fullerene, C60 as an acceptor via the well-known metal-ligand axial coordination approach is reported. The （PEG）4ZnPe was found to be capable of producing singlet oxygen with a quantum yield, ooa of 0.77 in toluene, a value higher than that obtained for pristine （t-bu）4ZnPc （φ△=0.54） carrying no PEG groups, revealing its usefulness in photodynamic therapy applications. Spectroscopic studies revealed efficient binding of phenylimidazole functionalized fullerene, C60 Im with l ： 1 stoichiometry to （PEG）4ZnPc. Binding constant K for the formation of （PEG）4ZnPc：ImC60 dyad was found to be 6 × 103 M 1 revealing moderate stability. Geometric and electronic studies of the dyad was arrived by B3LYP/3-21G（＊） method. The HOMO level was found to be on zinc phthalocyanine entity while the LUMO level was found to be on the C60 entity suggesting formation of （PEG）4ZnPc＊＋：ImC60＊ charge separated state during the process of electron transfer reaction. Redox studies on the （PEG）4ZnPc：ImC60 dyad enabled accurate determination of the oxidation and reduction potentials of the donor-acceptor system, and to evaluate free-energy changes associated for the charge separation process. Kinetics of photoinduced charge separation and recombination in the （PEG）4ZnPc：ImC60 dyad was investigated using femtosecond transient absorption studies. Relatively long-lived charge separated states were confirmed for the dyad suggesting their potential usefulness in energy harvesting applications.

Absorption and emission modulation in a MoS2–GaN（0001） heterostructure by interface phonon–exciton coupling

Semiconductor heterostructures based on layered two-dimensional transition metal dichalcogenides(TMDs)interfaced to gallium nitride(Ga N)are excellent material systems to realize broadband light absorbers and emitters due to their close proximity in the lattice constants.The surface properties of a polar semiconductor such as Ga N are dominated by interface phonons,and thus the optical properties of the vertical heterostructure are influenced by the coupling of these carriers with phonons.The activation of different Raman modes in the heterostructure caused by the coupling between interfacial phonons and optically generated carriers in a monolayer MoS_2–Ga N(0001)heterostructure is observed.Different excitonic states in MoS_2 are close to the interband energy state of intraband defect state of Ga N.Density functional theory(DFT)calculations are performed to determine the band alignment of the interface and revealed a type-I heterostructure.The close proximity of the energy levels and the excitonic states in the semiconductors and the coupling of the electronic states with phonons result in the modification of carrier relaxation rates.Modulation of the excitonic absorption states in MoS_2 is measured by transient optical pump-probe spectroscopy and the change in emission properties of both semiconductors is measured by steady-state photoluminescence(PL)emission spectroscopy.There is significant red-shift of the C excitonic band and faster dephasing of carriers in MoS_2.However,optical excitation at energy higher than the bandgap of both semiconductors slows down the dephasing of carriers and energy exchange at the interface.Enhanced and blue-shifted PL emission is observed in MoS_2.Ga N band-edge emission is reduced in intensity at room temperature due to increased phonon-induced scattering of carriers in the Ga N layer.Our results demonstrate the relevance of interface coupling between the semiconductors for the development of optical and electronic applications.