The ground- and excited-state electronic structures of the photosensitizer bis(4,4‘-dicarboxylato-2,2‘-bipyridine)-bis(isothiocyanato)ruthenium(II), [RuL‘2(NCS)2]4- (where L‘ = 4,4‘-dicarboxylato-2,2‘-bipyridine), have been examined computationally in an effort to better understand this molecule's effectiveness in TiO2-based photoelectrochemical cells. Density functional theory (DFT) calculations of the compound's ground state indicate that occupied molecular orbitals (MOs) localized on carboxylate groups of the bipyridyl ligands (through which the compound binds to the TiO2 nanoparticles) energetically match the semiconductor valence band; the lowest unoccupied MOs lie above the conduction band edge and are bipyridine π* in character. These results suggest that the compound is well-positioned to bind strongly to TiO2 and engage in electron transfer from excited states associated with the bipyridyl groups. Various excited states of the chromophore were identified using time-dependent density functional th...

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Ground- and Excited-State Electronic Structures of the Solar Cell Sensitizer Bis(4,4'-dicarboxylato-2,2'-bipyridine)bis(isothiocyanato)ruthenium(II)

The binding interactions between two cyanine dyes, pseudoisocyanine (PIC) and pinacyanol (PIN), and the cucurbit[n]uril hosts, cucurbit[7]uril (CB7) and cucurbit[6]uril (CB6), were investigated by electronic absorption spectroscopy and DFT computational methods. The CB7 host forms more stable complexes with both dyes than CB6 and the computational studies suggest that the cavity of the smaller host CB6 is not threaded by the dyes. The equilibrium association constants (K) for complexation by CB7 were measured and found to be 2.05 x 10(4) and 3.84 x 10(5) M(-1) for PIC and PIN, respectively, in aqueous media at 23 degrees C. CB7 complexation was found to effectively disrupt the intermolecular forces responsible for the aggregation of both dyes. Thus, CB7 completely disrupts the J-aggregates formed by PIC and the H-aggregates (as well as lower concentrations of J-aggregates) formed by PIN. In both cases a competing guest, 1-aminoadamantane (AD), could be used to adjust the extent of aggregation of the cyanine dye. AD regulates aggregate formation because it forms an extremely stable complex with CB7 (K approximately = 10(12) M(-1)) and exerts a tight control on the CB7 concentration available to interact and bind with the dye.

Control of H- and J-aggregate formation via host-guest complexation using cucurbituril hosts.

Chalcogenoxanthylium dyes were characterized as sensitizers of nanocrystalline titania in dye-sensitized solar cells (DSSCs). Four series of dyes were characterized: 2,7-bis(dimethylamino)-9-(2-thienyl-5-carboxy)chalcogenoxanthylium dyes (1-E, where E = O, S, Se); 2,7-bis(dimethylamino)-9-(3-thienyl-2-carboxy)chalcogenoxanthylium dyes (2-E, where E = S, Se); a 2,7-bis(dimethylamino)-9-(2-thienyl)selenoxanthylium dye (3-Se); 4-Se, a constrained analog of 1-Se. The orientation and aggregation state of the dyes were controlled by varying the position of the surface-attachment group relative to the xanthylium core. Series 1 dyes and 4-Se underwent H-aggregation on titania surfaces, whereas series 2 dyes adsorbed in amorphous monolayers. (3-Se did not adsorb appreciably to titania films, due to the lack of a tethering group.) The H-aggregated dyes exhibited broader absorption bands, increased light-harvesting efficiencies, and improved photoelectrochemical performance compared to the dyes which adsorbed in amo...

Optimizing the Photocurrent Efficiency of Dye-Sensitized Solar Cells through the Controlled Aggregation of Chalcogenoxanthylium Dyes on Nanocrystalline Titania Films

Au nanoparticles (particle diameter ∼2 nm) prepared by thiocyanate reduction method are too small to exhibit the characteristic surface plasmon band. Addition of Rhodamine 6G (Rh 6G) to the colloidal gold solution brings about significant changes in the absorption spectrum. Two distinct peaks appear at 507 and 537 nm. Close packing of the cationic dye molecules on the gold surface induces intermolecular and intercluster interactions. Furthermore, the adsorption of the cationic dye on the gold surface results in surface charge neutralization causing the Au/dye assembly to aggregate. When the Au/Rh-6G solution was subjected to 532 nm laser pulse irradiation for long time intervals, we observed distinct changes in the absorption spectrum due to morphological changes. A growth in the particle size (5−20 nm) is observed as a result of melting and fusion of gold nanoparticles. The photoinduced morphological changes have been elucidated using picosecond laser flash photolysis. The multiphoton process leading to ...

Dye-Capped Gold Nanoclusters: Photoinduced Morphological Changes in Gold/Rhodamine 6G Nanoassemblies

A series of squaraine dyes are found to be in H- or J-aggregates and are almost nonfluorescent in aqueous solution. Upon addition of bovine serum albumin (BSA), however, the fluorescence intensity (at λem ≈ 690 nm) increases by a factor of up to 200. Transformation of the dye molecules from the aggregates to the monomeric species appears to be responsible for the large fluorescence turn-on. While both H- and J-aggregates contribute to the observed fluorescence turn-on, the former appears to play a more important role. Electrophoresis imaging shows that these probes are good BSA indicators.

Aggregation Control of Squaraines and Their Use as Near-Infrared Fluorescent Sensors for Protein

The singlet and triplet excited-state behavior of a symmetric carbocyanine dye, 1,1‘-diethyl-2,2‘-carbocyanine (commonly referred as pinacyanol), adsorbed on SiO2 and SnO2 nanocrystallites has been investigated using transient absorption spectroscopy. The adsorption of the dye molecules on the negatively charged SiO2 or SnO2 colloids results in H-type aggregation. When excited with a 532 nm laser pulse we observe a short-lived (τ 5 × 1010 and 7 × 10...

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and Surat Hotchandani

Papers

Photosensitization Aspects of Pinacyanol H-Aggregates. Charge Injection from Singlet and Triplet Excited States into SnO2 Nanocrystallites