A "sense and shoot" approach has been developed for the photocatalytic degradation of organic contaminants from aqueous solutions. The presence of aromatic compounds such as chlorinated phenols is readily detected from the quenching of visible emission of ZnO semiconductor film. The emission quenching is quantitative and can be analyzed on the basis of adsorption equilibrium between ZnO and organic molecules. High apparent association constant values (K a ) (1-5) × 10 4 M -1 ) make possible the detection of these organic molecules at very low concentration levels. For example, 1 ppm of 4-chlorocatechol in water results in a 15% decrease of ZnO emission. Interestingly, the same ZnO film is capable of degrading aromatic compounds present in water under UV irradiation. The concept of adopting a strategy of simultaneous sensing and degradation in photocatalysis is illustrated in this paper.

The photoresponse of quantum dot solar cells (QDSCs) has been successfully extended to the near-IR (NIR) region by sensitizing nanostructured TiO(2)-CdS films with a squaraine dye (JK-216). CdS nanoparticles anchored on mesoscopic TiO(2) films obtained by successive ionic layer adsorption and reaction (SILAR) exhibit limited absorption below 500 nm with a net power conversion efficiency of ~1% when employed as a photoanode in QDSC. By depositing a thin barrier layer of Al(2)O(3), the TiO(2)-CdS films were further modified with a NIR absorbing squaraine dye. Quantum dot sensitized solar cells supersensitized with a squariand dye (JK-216) showed good stability during illumination with standard global AM 1.5 solar conditions, delivering a maximum overall power conversion efficiency (η) of 3.14%. Transient absorption and pulse radiolysis measurements provide further insight into the excited state interactions of squaraine dye with SiO(2), TiO(2), and TiO(2)/CdS/Al(2)O(3) films and interfacial electron transfer processes. The synergy of combining semiconductor quantum dots and NIR absorbing dye provides new opportunities to harvest photons from different regions of the solar spectrum.

Polythiophene-based polymers are an important class of organic semiconductors that serve as the building blocks for polymer-based hybrid solar cells. These polymers are susceptible to oxidative photodegradation in air. A model water-soluble polymer, the sodium salt of poly[2-(3-thienyl)ethoxy-4-butylsulfonate], was employed to investigate the role of excited-state and singlet-state properties in the photodegradation of the polymer. The singlet excited state produces characteristic absorption at 770 nm with a lifetime of 22 ps. The triplet state exhibits a broad absorption in the 650−800 nm region and has a lifetime of 18.7 μs in deaerated water. The excited triplets are readily quenched by oxygen with a rate constant of 1.9 × 109 M−1 s−1 via both electron and energy transfer pathways. Pulse radiolysis experiments have been conducted to verify the identity of the cation radical and hydroxyl adduct of the polymer. It is evident from the photochemical experiments that the high photochemical reactivity of triplets with oxygen is responsible for the photodegradation.

The photochemical and redox properties of two newly synthesized tetrahydroquinoxaline-based squaraine dyes (SQ) are investigated using femto- and nanosecond laser flash photolysis, pulse radiolysis, and cyclic voltammetry. In acetonitrile and dichloromethane, these squaraines exist as monomers in the zwitterionic form (lambda(max) approximately 715 nm, epsilon(max) approximately 1.66 x 10(5) M(-1) cm(-1) in acetonitrile). Their excited singlet states ((1)SQ*) exhibit a broad absorption band at 480 nm, with singlet lifetimes of 44 and 123 ps for the two dyes. Both squaraines exhibit poor intersystem crossing efficiency (Phi(ISC) < 0.001). Their excited triplet states ((3)SQ*), however, are efficiently generated by triplet-triplet energy transfer using triplet excited 9,10-dibromoanthracene. The excited triplet states of the squaraine dyes exhibit a broad absorption band at ca. 560 nm (epsilon(triplet) approximately 4.2 x 10(4) M(-1) cm(-1)) and undergo deactivation via triplet-triplet annihilation and ground-state quenching processes. The oxidized forms of the investigated squaraines (SQ(*+)) exhibit absorption maxima at 510 and 610 nm.

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