Recently, dye-sensitized solar cells (DSC) have attracted much attention with their low production costs of electricity and relatively high energy-conversion efficiencies. [1][2][3][4] One of the key elements in DSC is the nanoporous TiO 2 electrode, which transfers the electrons from the dye molecules to the transparent conductive-oxide (TCO) electrode and concurrently allows the electrolytes to diffuse to the anchored dyes. Typically, nanoparticles are utilized for the fabrication of the nanoporous TiO 2 layers on the TCO to obtain high surface areas and generate nanopored structures. [1,5,6] In this TiO 2 layer derived from nanoparticles, however, the electrons produced from the dye molecules have to pass through numerous grain boundaries in order to reach the TCO, and the transport of the electrolytes is not efficient due to the irregularity of the pores generated. To this point, the tailoring of TiO 2 nanostructures is a crucial aspect of increasing the current photovoltaic-conversion efficiency of DSC. [7][8][9][10][11][12][13][14][15][16][17][18][19] For the formation of an efficient DSC, a high surface area is prime and essential for the TiO 2 layer to load large amounts of dye molecules that will generate electrons by absorbing sun light. Second, the pores formed in the TiO 2 layer must be sufficiently large in size with excellent mutual connectivity for the efficient diffusion of electrolytes. Third, the defect level and the number of grain boundaries must be minimized to suppress the loss of electrons by recombination or back reaction. In general, however, these factors are not compatible with one another. For example, upon decreasing the size of the TiO 2 nanoparticles, the surface area of the fabricated nanoporous TiO 2 film is increased, and thus more dye molecules can be adsorbed. However, the average pore size is decreased simultaneously, and more defect sites and grain boundaries can be generated in the fabricated TiO 2 film. Therefore, it has been reported that the optimum particle size of TiO 2 has to be in the range of 12-20 nm. [5,6,[20][21][22] In this work, we designed a novel hierarchical pore structure that provides high surface area and large pore size at the same time. That is, nanoporous TiO 2 spheres with high surface area were synthesized and utilized to form the nanoporous TiO 2 electrode. As a result, two kinds of pores were successfully formed in the TiO 2 layer. Tiny internal pores were formed inside the TiO 2 sphere, while large external pores were generated by formation of the interstitial voids among the spherical structures. The large external pores can be used as a ''highway'' for electrolyte diffusion, as shown in Scheme 1. Therefore, it is expected that this porous spherical structure can provide both great adsorption of the dye molecules and efficient electrolyte diffusion at the same time.Sub-micrometer-sized TiO 2 spheres have often been prepared by sol-gel methods controlling the hydrolysis and condensation reactions, and their crystallized structures were formed by su...

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One- and two-photon fluorescence excitation and emission spectra of the S1↔S0 transition of trans,trans-1,3,5,7-octatetraene have been measured for the first time in free jet expansions. The one-photon excitation spectrum is the same, with the exception of significant differences in the intensities of a few lines, as the two-color, resonance-enhanced, two-photon ionization spectrum, previously assigned to the 2 1A′←1 1A′ transition of cis,trans-1,3,5,7-octatetraene. However, comparison of the one- and two-photon fluorescence excitation spectra shows clearly that the carrier of the spectrum has inversion symmetry, as expected for trans,trans-1,3,5,7-octatetraene. The one-photon spectrum is built on bu Herzberg–Teller promoting modes, which are origins of progressions in ag modes, while the two-photon spectrum is due to a single progression in ag modes starting from the 2 1Ag←1 1Ag electronic origin. The appearance of out-of-plane vibrations, possibly including torsions of the polyene framework, suggests large differences in force constants and perhaps in the geometries of the 2 1Ag and 1 1Ag potential surfaces. For 2 1Ag vibronic levels with energies ≤1000 cm−1, the fluorescence lifetimes vary between 170 and 450 ns due to the dependence of radiative and nonradiative decay rates on the vibronic state. An abrupt increase in the nonradiative decay rates at ∼2100 cm−1 excess energy is tentatively ascribed to trans→cis isomerization. This work demonstrates that the one- and two-photon cross sections of the 2 1Ag←1 1Ag transitions of all-trans linear polyenes are sufficiently large to allow the study of 2 1Ag states under isolated, unperturbed conditions.

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