D. Oelkrug

Fluorescence and absorption spectra of oligophenylenevinylenes: Vibronic coupling, band shapes, and solvatochromism

Fluorescence emission and excitation spectra of para-phenylene vinylenes nPV with n=1–4 styryl units are investigated experimentally and theoretically as a function of the temperature and the polarizability of the solvent. At low temperatures, the vibronic structures of the S0↔S1 emission and excitation bands are mirror symmetrical with negligible 0–0 energy gaps. The frequencies of the prominent vibrational modes are assigned to the second longitudinal acoustic phonon modes of the entire molecules and to localized carbon–carbon stretching vibrations. The complete vibronic structures of the spectra are calculated at the ab initio Hartree–Fock (HF/6-311G*) and restricted configuration interaction singles (RCIS/6-311G*) levels of theory assuming planar C2h molecular symmetry. The theoretically predicted spectra are in good agreement with the experiments. At room temperature, a 0–0 energy gap between the first band maxima opens, and the mirror symmetry between absorption and emission is lost. The vibronic ba...

Towards highly luminescent phenylene vinylene films

Abstract Fluorescence and electronic absorption spectra, fluorescence decay curves and fluorescence quantum yields of a series of oligo (p-phenylene vinylenes) are investigated in solution, nanoaggregates and vapour-deposited or cast ultrathin films. The film constituting molecules are varied in chain length and modified by electron donating and withdrawing substituents and bulky alkyl spacers. PPP-MO calculations serve to rationalize the resulting spectral changes. In dilute solutions, fluorescence yields of the short oligomers with alkyl or oxyalkyl substituents approach the region of unity. The yields decrease with chain length, reaching a long-chain limit of ΦF=0.4–0.7. Introduction of electron withdrawing -CN or -SO2CF3 groups can reduce the yields to almost zero, due to facilitated excited-state torsions around the vinylene double bonds. In films, the situation changes drastically. Fluorescence yields of the parent compounds become very low because of molecular excition coupling which reduces the radiative rates and increases the nonradiative rates of charge separation or internal conversion. Introduction of bulky or polar substituents reduces excitonic coupling, but keeps the molecular environment rigid enough to suppress nonradiative torsional deactivation, so that finally the substituted oligophenylene vinylenes produce highly luminescent films with a present maximum of ΦF≈0.6 for 1,4-bis(α-cyanostyryl)-2,5-di-n-hexylbenzene (HTCo).

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.

Electronic spectra of self-organized oligothiophene films with “standing” and “lying” molecular units

Abstract The molecular orientations of vapor-deposited α-quinque- and α-sexithiophene films are investigated with angular resolved polarized electronic absorption spectroscopy. On stretched polyethylene substrates, the film constituting molecules are “lying” with their long axes parallel to the surface and to the stretching direction, forming a biaxially anisotropic layer. On fused silica, the molecules are “standing” perpendicular to the surface, even at film thicknesses of d = 100 nm, forming a uniaxially anisotropic layer. The distinct differences in the absorption spectra of the two film types are modeled with the Fresnel equations. Within this model, the strong spectral blue-shifts observed in H-aggregates with standing oscillators, are not a consequence of excitonic coupling but of the optical anisotropy of the film.

Characterization of oriented oligo(phenylenevinylene) films and nano-aggregates by UV/Vis-absorption and fluorescence spectroscopy

Abstract Absorption maxima in spectra of vapor-deposited oligo(phenylenevinylenes) (OPVs) are blue shifted versus solution, revealing strong exciton coupling due to parallel alignment of the film-consitituting molecules. The films are anisotropic, as shown by polarized angular-resolved absorption spectroscopy. OPV nano-aggregates precipitated from solution are also strongly anisotropic, as shown by steady-state and time-resolved fluorescence anisotropy measurements. Introducing bulky substituents into the OPVs reduces the degree of molecular alignment. Fluorescence spectra of OPV films and aggregates consist of several distinct components, which are due to emission from traps below the exciton band. The overall fluorescence quantum yield is shown to depend mainly on the concentration of traps, which in turn is determined by the degree of molecular alignment.

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.

Absorption, fluorescence and light scattering of oligothiophene and oligophenylenevinylene nanoaggregates

Abstract Molecular orientations in oligothiophene (Tn) - and oligo- (p-phenylenevinylene) (Pn) — nanoaggregates are investigated by time-resolved fluorescence anisotropies and polarized light scattering. Tn- and unsubstituted Pn-molecules align parallel to each other, but not the substituted Pns. Electronic absorption and fluorescence spectra of the oriented systems are strongly altered against the monomers and the fluorescence quantum yields are strongly reduced.

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.