Ken T. Ngo

Engineering of Ruthenium(II) Photosensitizers with Non-Innocent Oxyquinolate and Carboxyamidoquinolate Ligands for Dye-Sensitized Solar Cells

An alternative approach to replacing the isothiocyantate ligands of the N3 photosensitizer with light-harvesting bidentate ligands is investigated for application in dye-sensitized solar cells (DSSCs). An in-depth theoretical analysis has been applied to investigate the optical and redox properties of four non-innocent ligand platforms, which is then corroborated with experiment. Taking advantage of the 5- and 7-positions of 8-oxyquinolate, or the carboxyaryl ring system of the N-arylcarboxy-8-amidoquinolate ligand, fluorinated aryl substituents are demonstrated as an effective means of tuning complex redox potentials and light-harvesting properties. The non-innocent character, resulting from mixing of both the central metal-dπ and ligand-π manifolds, generates hybrid metal-ligand frontier orbitals. These play a major role by contributing to the redox properties and visible electronic transitions, and promoting an improved power conversion efficiency in a Ru DSSC device featuring non-innocent ligands.

Rigid triarylamine donor–π–acceptor porphyrin dyes and their application in dye-sensitized solar cells

Three donor–π–acceptor porphyrin dyes bearing a variety of rigid triarylamine donor groups were synthesized for application as photosensitizers in TiO2 based dye-sensitized solar cells (DSSCs). Compared with the “naked” porphyrin ZnP, i.e. having no triarylamine moiety, broadened and red-shifted spectral features were exhibited by the triaylamine porphyrin sensitizers ISB-ZnP, CZ-ZnP, and IDB-ZnP where ISB = 5-phenyliminostilbene, CZ = 5-phenylcarbazole, and IDB = 5-phenyliminodibenzyl. Percentage power conversion efficiencies (η) and incident photon-to-current conversion efficiencies (%IPCE) in DSSC devices show the trend ISB-ZnP < ZnP < CZ-ZnP < IDB-ZnP. Inferior performance of the ISB-ZnP dye is attributed to its weaker adsorption to the TiO2 film, which is roughly half that of ZnP. In contrast, introduction of the CZ and IDB electron donors is demonstrated to promote a better performance than the “naked” ZnP porphyrin. The best performance was observed for the IDB-ZnP device reaching a power conversion efficiency of η = 3.62% under AM 1.5 irradiation conditions with a corresponding %IPCE maximizing at 48% for both Soret band (450 nm) and Q band (570 nm) photoexcitation.

Probing the Noninnocent π-Bonding Influence of N-Carboxyamidoquinolate Ligands on the Light Harvesting and Redox Properties of Ruthenium Polypyridyl Complexes

Electronic and photophysical characterization is presented for a series of bis-heteroleptic [Ru(bpy)2(R-CAQN)](+) complexes where CAQN is a bidentate N-(carboxyaryl)amidoquinolate ligand and the aryl substituent R = p-tolyl, p-fluorobenzene, p-trifluoromethylbenzene, 3,5-bis(trifluoromethyl)benzene, or 4-methoxy-2,3,5,6-tetrafluorobenzene. Characterized by a strong noninnocent Ru(dπ)-CAQN(π) bonding interaction, density functional theory (DFT) analysis is used to estimate the contribution of both atomic Ru(dπ) and ligand CAQN(π) manifolds to the frontier molecular orbitals of these complexes. UV-vis absorption and emission studies are presented where the noninnocent Ru(dπ)-CAQN(π) bonding scheme plays a major role in defining complex electronic and photophysical properties. Oxidation potentials are tuned over a range of 0.92 V with respect to the [Ru(bpy)3](2+) reference system, hereafter referred to as 1(2+), by varying the degree of R-CAQN fluorination while maintaining consistently strong and panchromatic visible absorption properties. Electron paramagnetic resonance (EPR) spectroscopy is employed to experimentally map delocalization of the unpaired electron/electron-hole within the delocalized Ru(dπ)-CAQN(π) singly occupied valence molecular orbital of the one-electron oxidized complexes. EPR data is complemented experimentally by UV-vis-NIR spectroelectrochemistry, and computationally by molecular orbital Mulliken contributions and spin-density analysis. It is ultimately demonstrated that the CAQN ligand framework provides a simple yet broad synthetic platform in the design of redox-active transition metal chromophores with a range of electronic and spectroscopic characteristics hinting at the diversity and potential of these complexes toward photochemical and catalytic applications.

Principles of Photochemical Activation Toward Artificial Photosynthesis and Organic Transformations

Abstract The basic principles of photocatalysis are introduced from the perspective of a homogeneous system using a molecular photosensitizer. Model photophysical and redox properties of a molecular photosensitizer are discussed. Generic reaction schemes are presented for established photocatalytic protocols via photooxidative and photoreductive mechanisms with sacrificial electron donating or accepting agents. Inspired by natural photosynthesis, prototypical examples of photocatalytic CO2 reduction and photocatalytic H2O oxidation are also included. Finally, an introduction to the growing field of photocatalytic organic transformations is provided.