H.-J. Egelhaaf

Optical spectra of oligothiophenes: vibronic states, torsional motions, and solvent shifts

Abstract Low temperature fluorescence and absorption spectra of α,α′-quinquethiophene (5T) are almost mirror symmetrical due to small differences in the nuclear displacements of the vibronic modes in the electronic S 0 and S 1 states. The spectra are in good accordance with Franck–Condon calculations based on ab initio Hartree–Fock (HF/6-311G ∗ ) and restricted configuration singles (RCIS/6-311G ∗ ) quantum chemical calculations. With increasing temperature the spectra are broadened and blue-shifted, and the mirror symmetry between fluorescence and absorption is lost. The spectral shifts can be quantitatively correlated to the changes in the polarizabiltity of the environment. Inhomogeneous broadening is simulated by convolution of the spectra with appropriate distribution functions. The loss of mirror symmetry, which occurs exactly at the melting point of the solvent, is well described by the excitation of torsional modes. Measurements of the oligothiophenes in different solvents allow the determination of the adiabatic transition energies in vacuo, which are compared to the quantum chemical calculations.

Orientation and mobility in ultrathin oligothiophene films: UV-Vis, IR and fluorescence studies

Abstract Thin films of oligothiophenes αn Ts are prepared by vapour deposition on fused silica and crystalline sapphire substrates, and characterized with respect to molecular orientation by polarized fluorescence, UV-Vis and IR absorption spectroscopy. Isolated αn Ts are adsorbed in the submonolayer region with their long molecular axes parallel to the surface plane. In the monolayer region, the αn Ts organize with their long axes perpendicular to the surface plane forming aggregates of the pin cushion type with a mean tilt angle of θ = 15 ± 3° relative to the normal of the macroscopic surface. Terminal alkylation of αn T or multilayer coverage results in a loss of orientation with respect to the surface. Heating the substrate during film deposition either reduces or enhances the molecular order depending on temperature and molecular chain length.

UV/Vis and IR spectroscopic studies on molecular orientation in ultrathin films of polythiophene model compounds

Abstract The characteristic differences shown by optical spectra of oligothiophenes (OTs) in films on silica surfaces and in solution are explained by the Molecular Exciton Model. Thus unsubstituted OTs in monolayers are arranged in a side-by-side manner, whereas substitution with terminal alkyl groups destroys this alignment, thereby reducing the π-overlap of adjacent molecules. With the orientation of the transition moments within the molecular framework known from stretched sheet experiments polarized UV/Vis and IR spectra reveal that the long molecular axis of the unsubstituted OTs in monolayers on silica is directed perpendicularly to the surface, while alkyl substituted oligomers do not show any anisotropy. This influence of the alkyl groups and the reduced order in multilayers of unsubstituted OTs both suggest a decisive role of the interaction between surface and π-electron-system for the molecular arrangement in thin films.

Fluorescence yields and molecular orientation of thin organic films: Vapor-Deposited Oligothiophenes α3T-α8T.

The fluorescence quantum yields of vapor-deposited (VD) films of α-oligothiophenes,nT, with ring numbers ofn=3–8 and layer thicknesses ofd=3–50 nm were determined at room temperature andT=77 K and compared to the yields of dilute solutions and small (5T)x clusters. The yields of highly oriented ultrathin films are of the order of ΦF=5*10−5-1*10−4. The yields increase strongly with the layer thickness and also upon cooling, but do not reach the values in dilute solution. The main nonradiative deactivation step S1 → T1 in solution was quantified by1O2 production, the yields of which systematically decrease withn from ΦF (3T) to 0.36 (6T), in contrast to the fluorescence yields, which increase from ΦF=0.01 (2T) to 0.40 (6T). In films or clusters the S1 → T1 deactivation step must be a very unimportant side reaction: neither1O2 nor any signal of triplet-triplet absorption could be positively identified.

Polarizability effects and energy transfer in quinquethiophene doped bithiophene and OPV films

Abstract Oligothiophene films ( n T) and oligophenylenevinylene (OPV) nanoparticles are doped with quinquethiophene (5T) and investigated by steady-state and time-resolved fluorescence spectroscopy. In 2T films and highly ordered nanoparticles of unsubstituted OPVs, the fluorescence excitation spectra of 5T show vibrational structure, indicating a small distribution of ground-state geometries. The anisotropic effective polarizabilities of the host materials are obtained from the spectral positions of the fluorescence maxima of 5T. In the highly disordered nanoparticles of tert -butyl substituted OPVs, the vibrational structure of 5T excitation spectra is lost and the polarizability is isotropic. Efficient host–guest energy transfer is observed upon excitation of the host. The overall fluorescence quantum yield φ F increases fivefold upon doping 2T films at doping levels of x 5T =5×10 −4 . In OPV films, φ F is almost independent of the doping level, probably due to the competition of energy and electron transfer between host and dopant.

Design, synthesis and assembly of new thiophene-based molecular functional units with controlled properties

Abstract Individual aspects in the field of ‘molecular electronics’ are addressed using new materials which are based on conjugated thiophenes as key building blocks. Among then, functionalized polythiophenes represent prototype compounds in which the three-dimensional network of ‘molecular wires’ can interact with the specific functional groups. Corresponding structurally defined oligothiophenes serve as model compounds and allow the correlation of various properties with structure. In addition, novel properties and superior processibility of these oligomers lead to interesting applications. Light emitting diodes and metal/organic/metal nanostructures are constructed by evaporation of thin oligomer films. A clear dependence of electroluminescence and electrical transport properties on chain length is found. Finally, the supramolecular assembly of functionalized (oligo)thiophenes is achieved by oriented adsorption on single crystal surfaces and self-organization in highly ordered LB films.

Electronic deactivation and energy transfer in doped oligophenylenevinylene nanoparticles

Suspensions of oligophenylenevinylene (nPV) nanoparticles withn = 2 vinylene units are doped with nPVs of longer chainlengths,n = 3–5. Absorption and fluorescence spectroscopy and steady-state and time-resolved fluorescence anisotropy measurements are used to determine the photo-physical properties of the suspensions. Undoped nanoparticles form highly oriented H-aggregates with low fluorescence quantum yields (ΦF ≈ 0.1). Introduction of bulky substituents into the particle constituting molecules perturbs the intermolecular orientation. Upon doping, efficient energy transfer to the dopants is found, changing the color and leading to enhancement of the fluorescence quantum yields up to ΦF = 0.6. The intermolecular orientation is not changed upon doping.

Photophysical properties of β-alkylated quater-, octa-, dodeca- and hexadecatiophenes

Abstract The order and deactivation of excited electronic states of β-alkylated oligothiophenes consisting of n = 4, 8, 12 and 16 thiophene rings are investigated by absorption spectroscopy as well as by steady state and time resolved fluorescence spectroscopy. Transition energies in oligothiophenes are linearly dependent on reciprocal chain-length only for n n > 8, chain-length dependence is correctly described by calculations which take alternating bond-lengths into account. β-Alkylation leads to enhanced nonplanarity in the ground state and thus to blue-shifted absorption maxima. Due to the quinoid structure of the excited state, which leads to planarization, fluorescence maxima are not blue-shifted versus unsubstituted oligothiophenes. In solution, up to the longest chain under investigation, n = 16, the 1B u state is energetically below the 2A g state, as inferred from comparison of radiative rate constants of fluorescence and absorption according to the Strickler-Berg relationship. 113, is also below 2A g in the solid state, as concluded from the coincidence of the 0-0-transitions of absorption and fluorescence.

Highly luminescent oligo(phenylenevinylene) films : the stereochemical approach

Unsubstituted OPVs are bright fluorophores in solution but poor ones in films, due to unfavorable side-by-side inter-molecular arrangement. To overcome this morphology, the complete isomeric series of di- and tetracyano trimeric PVs and (RO) 2 PVs are synthesized and investigated as films with time-resolved polarized fluorescence spectroscopy. Depending on intermolecular arrangement and intramolecular torsional degrees of freedom, the fluorescence yields in the films range from 0.01 to 0.8. Highest values are obtained with head-to-tail intermolecular arrangement. Depending on the substitution pattern, excimer or monomer like fluorescence dominates.

Evolution of optical absorption from small oligomers to ideally conjugated PPV and MEH-PPV polymers

Oligomeric series of p-phenylenevinylene and p-(dialkoxyphenylene)vinylene with n = 1 - 11 repeating units are characterized in solution by polarized fluorescence and absorption spectroscopy. Assignments of the six lowest singlet-singlet transitions are given as a function of n and extrapolated to the perfectly conjugated polymer. Experiments are compared with semi-empirical quantum mechanical calculations. Especially the role of the symmetry breaking alkoxy-substituents is discussed.