Disorder influenced optical properties of α-sexithiophene single crystals and thin evaporated films

Fluorescence anisotropy using highly polarized emitting dyes confined inside BNNTs

Polarized fluorescence emission of nanoscale emitters has been extensively studied for applications such as bioimaging, displays, and optical communication. Extending the polarization properties in large assemblies of compact emitters is, however, challenging because of self-aggregation processes, which can induce depolarization effects, quenching, and cancellations of molecular dipoles. Here we use α-sexithiophene (6T) molecules confined inside boron nitride nanotubes (6T@BNNTs) to induce fluorescence anisotropy in a transparent host. The experiments first indicate that individual 6T@BNNTs exhibit a high polarization extinction ratio, up to 700, at room temperature. Using aberration-corrected HRTEM, we show that the fluorescence anisotropy is consistent with a general alignment of encapsulated 6T molecules along the nanotube axis. The molecular alignment is weakly influenced by the nanotube diameter, a phenomenon ascribed to stronger molecule-to-sidewall interactions compared to intermolecular interactions. By stretching a flexible thin film made of transparent polymers mixed with 6T@BNNTs, we induce a macroscopic fluorescence anisotropy within the film. This work demonstrates that the dyes@BNNT system can be used as an easy-to-handle platform to induce fluorescence anisotropy in photonic materials.

Quasiperiodic energy dependence of exciton relaxation kinetics in the sexithiophene crystal

Femtosecond kinetics of fluorescence rise in the sexithiophene crystal is studied on a microscopic model of intraband relaxation, where exciton energy is assumed to be dissipated by phonon-accompanied scattering, with the rates calculated earlier. The temporal evolution of the exciton population is described by a set of kinetic equations, solved numerically to yield the population buildup at the band bottom. Not only the time scale but also the shape of the rise curves is found to be unusually sensitive to excitation energy, exhibiting unique quasiperiodic dependence thereon, which is rationalized in terms of the underlying model. Further simulations demonstrate that the main conclusions are robust with respect to experimental factors such as finite temperature and inherent spectral broadening of the exciting pulse, while the calculated fluorescence rise times are found to be in excellent agreement with experimental data available to date. As the rise profiles are composed of a number of exponential contributions, which varies with excitation energy, the common practice of characterizing the population buildup in the emitting state by a single value of relaxation time turns out to be an oversimplification. New experiments giving further insight into the kinetics and mechanism of intraband exciton relaxation are suggested.

Exploration of ground and excited electronic states of aromatic and quinoid S,S-dioxide terthiophenes. Complementary systems for enhanced electronic organic materials

We analyze the electronic and molecular structures for the ground and excited electronic states of aromatic terthiophene (3T), the quinodimethane 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (3Q), and isologues with the middle ring S-oxidized (3TO2, 3QO2). These represent extremes of electron rich and deficient ground states, often exhibiting complementary properties. Oxidizing the central sulfur atom affects the molecular structure, electron affinity, and photophysical properties of both pi systems. The consequences for 3T include de-aromatization of the central thiophene, red-shifting of the electronic absorption spectrum, and lowering of the reduction potential. The electron deficient quinoid 3QO2 shows an enhancement of electron affinity from reducing the electron-donor ability of sulfur, and a blue-shifting of its electronic absorption spectrum was seen. Fluorescence emission is quenched in the sulfonated terthiophene, and the contrary effect again would be expected upon sulfonation of a quinoid emitter. Raman vibrational spectroscopy, electrochemistry, and UV-vis and fluorescence spectroscopies are analyzed in conjunction with theoretical calculations.

J-aggregation in alpha-sexithiophene submonolayer films on silicon dioxide

We report on the observation of J-aggregates in submonolayer films of alpha-sexithiophene grown on silicon dioxide. Photoluminescence spectroscopy reveals that submonolayers are formed by molecules lying flat on the substrate with a head to tail configuration. Excitation energy dependence of photoluminescence shows a red-shifted absorption with respect to isolated molecules and a negligible Stokes shift between absorption and emission. The pronounced structural order of J-aggregates is reflected in the fwhm of the emission bands. From time-resolved and low-temperature photoluminescence experiments, we infer a quantum yield of the J-aggregate between 0.6 and 1. The demonstration of spontaneous formation of J-aggregates of pi-conjugated systems on amorphous silicon-based substrates can be relevant for the development of organic-inorganic hybrid photonic devices.

Enhanced Photovoltaic Response in Hydrogen-Bonded All-Organic Devices

Straightforward synthetic methodologies are reported for the functionalization of oligothiophenes with hydrogen-bonding motifs. Codeposition from a solution of symmetric melamine-terminated electron-donor oligomers with a complementary barbiturate-labeled electron-acceptor fullerene resulted in homogeneous films. Incorporation into photovoltaic devices gave a 2.5-fold enhancement in light energy to electrical energy conversion when compared to analogous systems with the non-hydrogen-bonding parent C(60). [reaction: see text]

Photoluminescence Study of Sexithiophene Thin Films

To determine the exciton diffusion length of sexithiophene (6T) thin films, quenching of the photoluminescence (PL) of vacuum-deposited 6T films on TiO2 and on quartz has been investigated. For films with a thickness of more than 22 nm and at temperatures below 100 K, additional PL lines appear in luminescence spectra. This feature is related to the structural properties of 6T films. The PL intensity is thermally activated with an activation energy of 18 meV on TiO2 and 6 meV on quartz. When 6T is applied on TiO2, exciton quenching occurs for films up to 120 nm. For 6T on quartz this value is reduced to 60 nm. By comparing the relative luminescence intensities of 6T on quartz and on TiO2 substrates, an exciton diffusion length of 60 +/- 5 nm is derived.

Intrinsic excitonic luminescence in odd and even numbered oligothiophenes

The emission properties of crystalline oligothiophenes (OTs) are investigated and related to the different selection rules arising from the number of rings. In odd OT crystals the purely electronic emission is absent since the molecular A1-1B(1) transition dipoles cancel at the bottom of the excitonic band. On the contrary, in crystals of even OTs this transition is allowed due to the constructive interference of the off-axis components of the molecular transition dipole. It possesses a polarization in the herringbone plane and exhibits superradiant behavior.