Colloidal T1O2 sols with average particle size 2RP = 2.1 nm (specimen A), 13.3 nm (specimen B), and 26.7 nm (specimen C) were prepared and characterized by absorption and photoluminescence spectroscopies.Electron and X-ray diffraction examination showed the specimens contain anatase as the only crystalline phase (~20-30%) and a significant amorphous component (~70-80%). The three specimens gave identical steady-state absorption features at high loadings. Luminescence around 340-350 nm (at the absorption edge at the lower loadings) was more intense in the 13.3 nm T1O2 particles; several weaker bands were seen at longer wavelengths. Congruence of the absorption onsets for the three specimens at 15 g L-1 or at the lower loadings (0.015 and 0.30 g L-1) argues against size quantization effects for particles with Rp > 1.0 nm. Spectra at the lower loadings exhibited absorption thresholds at considerably higher energy (relative to bandgap); they are attributed to direct transitions in an otherwise indirect bandgap T1O2 semiconductor. Considerations of the effective mass model to determine particle size from spectral blue shifts of absorption edges and to ascertain size quantization in semiconductor colloidal particles suggest that it is rather premature to reach any conclusion on size quantization manifestations in the absence of a precise knowledge of the effective masses of charge carriers in small nanosized particles, and more so for anatase colloids for which inferences of effective masses of electrons and holes have been taken from rutile bulk crystals.

Colloidal T1O2 sols with mean particle diameter 2.1, 13.3, and 26.7 nm were examined by picosecond transient absorption and emission spectroscopies. Transient emission decays followed excellent single-exponential kinetics in all cases, with decay times 67 ps (2.1 nm), 405 ps (13.3 nm), and 66 ps (26.7 nm). Transient absorption spectra show that localization (trapping) of the electron as a Ti3+ species is significant in the 2.1 nm T1O2 particles. At the end of the 30 ps laser pulse, the transient spectra are fully developed and comprise spectra of trapped holes and trapped electrons. This has important consequences in heterogeneous photocatalysis: photooxidations are initiated by surface-trapped holes, h+TR (presumably as radicals)and not by valence band holes, h+VB• Absorption decay for the 2.1 nm sols is a simple first-order process, and electron/hole recombination is 100% complete by 10 ns. For the 13.3 and 26.7 nm specimens transient absorption decay follows distinct second-order biphasic kinetics; the decay times of the fast components decrease with increase in particle size. By 10 ns, about 90% or more of the photogenerated electron/hole pairs have recombined such that the quantum yield of photooxidations must be 10% or less. The faster components are due to the recombination of shallow-trapped charge carriers, whereas the slower components (r > 20 ns) reflect recombination of deep-trapped electrons and holes. It is the latter that dictate the kinetics of the photocatalyzed redox chemistries.

Solutions of free base meso-tetraphenylporphyrin (H 2 TPP), meso-tetra(4-carboxyphenyl)porphyrin (H 2 TPPC), and meso-tetra(4-pyridyl)porphyrin (H 2 TPyP) under various conditions generate aggregates whose absorption spectra are characterized by invariant Soret bands with bandwidths that are independent of the preparative method. One of the Soret bands is blue-shifted (H-aggregate) relative to the monomeric porphyrin band; other Soret bands are red-shifted (J-aggregates). The aggregates are characterized by different nonradiative rate constants for excited singlet-state decay and by different efficiencies of singlet-singlet annihilation at the high energies of laser excitation. The quantum yields of fluorescence vary between 10 -5 and 10 -2 , and the corresponding fluorescence lifetimes vary in the range from 10 -12 to 10 -9 s; they are more than 1 order of magnitude smaller than those of the corresponding monomeric porphyrins. Lifetimes (τ) correlate with the characteristic ground-state absorption recovery times of the aggregates. The sizes of the H-aggregate and aggregates that are characterized by a minimal blue shift of the Soret band range from 15 to 27 Å.

The photophysics of three cyanine dyes (i) 1,1‘-diethyl-2,2‘-cyanine iodide (pseudoisocyanine, PIC), (ii) 3,3‘-didodecyldithia-2,2‘-carbocyanine bromide (dye 1), and (iii) 3,3‘-diethyldithia-2,2‘-carbocyanine iodide (dye 2) have been examined by picosecond-laser photolysis in aqueous and methanolic-aqueous media. At moderately high concentration, solutions of PIC in 5 M NaCl/water contain monomers, H-aggregates, and J-aggregates; dye 1 water/methanol solutions consist mostly of monomers and H-aggregates (dimers and higher n-mers); aqueous dye 2 solutions contain only monomers and dimers. Photolysis of H- and/or J-aggregates in PIC and dye 1 cyanine solutions leads to photobleaching of the respective aggregate absorption bands and subsequently decays by biphasic kinetics. Two mechanisms are discussed for the deactivation of excited aggregates. In the first, nonradiative decay of the excited singlet states results in considerable heating of the aggregates together with their surrounding solvent shells causing the probe laser light in a pump−probe experiment to be strongly attenuated after excitation (see, e.g., J. Phys. Chem. 1995, 99, 11952). This heating, which subsequently leads to partial dissolution of the aggregates (deaggregation), later reformed slowly on cooling, can also arise from another mechanism. That is, relaxation of singlet excited states of H-aggregates can also occur, in part, by exciton−exciton annihilation as occurs in J-aggregates at high laser pump intensities. In this case, the longer lived excited singlet states of H-aggregates, relative to those of J-aggregates, are likely due to a less efficient (slower) exciton−exciton annihilation process in H-aggregates which would infer a weaker exciton coupling in comparison to the strong coupling known to prevail in J-aggregates. For dye monomers, singlet excited state lifetimes increase (i) with an increase in the length of the aliphatic residues attached to the dye molecule and (ii) with dimerization of the dye molecules. Dimerization restricts torsional dynamics along the dyes polymethine chain and diminishes the nonradiative deactivation channel. An important conclusion from this work is that the thermal energy stored in the aggregates in a short time after picosecond-laser excitation, and the subsequent heating of the surrounding solvent shells, leading to dissolution to smaller aggregates and monomers, provides another pathway to S1−S1 annihilation in the deactivation of excited dye aggregates. This could provide an added/alternative path for the decrease of net efficiency (i.e., decrease in charge injection efficiency) in J-aggregate sensitization of silver halide grains (see, e.g., Lanzafame et al. Chem. Phys. 1996, 210, 79) in laser-imaging technologies.

The photochromic behavior of several spirooxazines (SO) containing phenanthrene or phenanthroline moieties in the oxazine part of molecules has been investigated in several solvents and phosphatidylcholine (PC) liposomes. The solvatochromic properties of the merocyanine (MC) forms of these dyes were used to probe their location within the PC membrane. Transient spectroscopic measurements revealed that, when first formed by photoexcitation, the MC forms of phenanthroline-containing spirooxazines were located at relatively nonpolar sites within the membrane, but they subsequently moved to a more polar environment typical of the aqueous-organic interface. The characteristic time for this intersite movement was τ ≈ 10 -3 s, corresponding to a diffusion coefficient of D ≈ 10 -11 cm 2 s -1 . In contrast, these spectral shifts were not observed when PC liposome-bound SO containing the phenanthrene moiety were photoexcited, suggesting that either intersite diffusion was more rapid for these compounds or the initially formed MC (and its spiro precursor) were located in a more polar microenvironment. The rate of thermal ring-closing following UV photoexcitation decreased modestly when either an electron-withdrawing group was present on the MC oxazine ring or an electrondonating group was present on the MC indoline ring. A dramatic increase in the ring-closing rate was observed for an o-phenanthroline-containing SO coordinated to a Ru(bpy) 2 2+ metal center, an effect attributable to strong donation of electron density from the Ru(II) d-orbitals into the ligand π*-orbitals.