S. Rentsch

Time-resolved fluorescence studies on thiophene oligomers in solution

Abstract Fluorescence lifetime τ and fluorescence quantum yield Φ F of thiophene oligomers with two to six rings were measured in solution. Radiative and nonradiative constants were estimated. The τ and Φ F values increased with ring number, n , mainly caused by a decrease of nonradiative decay.

Theoretical studies and spectroscopic investigations of ground and excited electronic states of thiophene oligomers

Abstract CNDO/S calculations including single and double excited configurations were performed on thiophene oligomers n T with 2–6 monomer units. In contrast to the results on linear polyenes only single excited configurations are necessary to explain the experimental results. The theoretical energies for the lowest 1 1 B states are in good agreement with the low energy absorption origins and the high energy fluorescence maxima and with the 1 1 B state energies observed at low temperature. The results show that the 2 1 A state lies above the 1 1 B state for 2T–6T. Transient absorption bands measured by picosecond spectroscopy could be assigned to transitions between 1 1 B and higher calculated 1 A states of all studied oligomers n T.

Determination of real and imaginary part of χ(3) of thiophene oligomers using the z-scan technique

Abstract Using the single beam z -scan technique with 30 ps pulses at 532 nm (close to the linear absorption region) the values of real and imaginary part of third-order optical nonlinear susceptibility χ (3) in solutions of thiophene oligomers have been determined for different numbers of repeat units ( n =2–6). The values of the real part of χ (3) were found to be negative, the contribution of each repeat unit to this value increases with increasing chain length. In solutions of dioxane (10 −3 mol/l) we measured n 2 values in the order of 10 −13 esu, whereas two-photon absorption coefficients of about 0.1 cm/GW were found.

Measurement of strong nonlinear absorption in stilbene-chloroform solutions, explained by the superposition of two-photon absorption and one-photon absorption from the excited state

Abstract A strong nonlinear absorption is measured when ruby laser light passes through a solution of stilbene in chloroform. The experimental results are interpreted by the use of the following model: two-photon absorption (TPA) is followed by one-photon absorption (OPA) by the excited molecules. The solution of the corresponding system of rate equations and fitting of parameters to the experimental curve give the TPA cross section σ (2) = 8 × 10 −49 cm 4 sec/photon and the product σ (1) τ 10 = 3 × 10 −26 cm 2 sec. Measurements at other frequencies are in agreement with this model.

Size-dependent transient behaviour of thiophene oligomers studied by picosecond absorption spectroscopy

Abstract Picosecond time-resolved excite-and-probe experiments are applied to thiopene oligomers for the first time. The oligomers n T with n = 3, 4 and 5 are investigated in solution. The time-resolved spectra in the visible range show bands shifted in wavelength with picosecond and nanosecond components. These bands shift to longer wavelengths with increasing size n . Whereas the fast components because of their kinetic behaviour could be assigned to excited state absorptions, the longer-living bands may be due to charged particles. The band position shift of the transient absorption with n corresponds to the band shift of radical ions reported on doped thiophene oligomers. Especially for 5T an additional absorption is found which corresponds to the transition between intragap states expected for polarons.

Femtosecond-spectroscopic investigations on bithiophene, terthiophene and tetrathiophene in solution

Abstract Thiophene oligomers, n T ( n =2, 3 and 4), were studied by femtosecond excite and probe spectroscopy. Solvated n T were excited with 350 fs pulses of 308 nm wavelength and probed between 450 and 750 nm. The emergence of induced fluorescence and absorption bands was observed. These spectra shift to longer wavelengths during a 1000 fs pulse. After this the transient spectra remain stable in form and spectral position; they agree with spectra observed by earlier ps experiments. The fast spectra shift is interpreted as an adjustment of the equilibrium between excited n T and its solvate shell.

Femtosecond investigations of photophysics of ultrafast intersystem crossing in terthiophene by wavelength dependent excitation

Abstract Ultrafast intersystem crossing (ISC) and triplet generation in terthiophene (3T) was investigated by femtosecond time-resolved spectroscopy in dependence of excitation wavelengths. Triplet yield was monitored by examining transient spectra of the triplet absorption band. We find that triplets are generated by different processes: (i) ISC starting from the lowest vibrational S1 state and (ii) ultrafast ISC from an intermediate state. We discuss the probable formation mechanism and the nature of the intermediate state. The results suggest that a planarisation process of excited twisted conformers, which proceeds on a 2 ps time scale, is responsible for the highly effective triplet generation.

Femtosecond Spectroscopic Studies on the Red Light-Absorbing Form of Oat Phytochrome and 2,3-Dihydrobiliverdin

Abstract— The excited state behavior of the red light‐absorbing form of phytochrome (Pr) was studied on the femtosecond time scale. After excitation of Pr with 75 fs laser pulses at 616 nm the kinetics of the transient absorption changes was recorded at selected wavelengths probing mainly the bleaching of the Pr ground‐state absorption and the stimulated emission. The kinetic data obtained indicate the population of an excited state with a 3 ps lifetime immediately after excitation. This state precedes the formation of another excited state with a 32 ps lifetime. The decay of the latter state is followed by the appearance of a first product state that is assumed to represent lunii‐R. In addition, 2,3‐dihydrobiliverdin, which is considered to be an adequate model of the Pr chro‐mophore, was included in the femtosecond studies. The absorption difference spectra recorded at various delay times show an immediate bleaching of the ground‐state absorption. Simultaneously with bleaching a broad transient absorption appears between 410 and 525 nm. The data analysis yields similar kinetic components as they were observed in the decay of Pr. It is suggested from this finding that within the first tens of picoseconds after excitation the excited‐state properties of Pr are mainly determined by the properties of the chromophore itself.

Femtosecond time-resolved measurements of terthiophene: fast singlet–triplet intersystem crossing

Abstract Femtosecond time-resolved studies using pump–probe spectroscopy were performed on 2,2′:5′,2″-terthiophene in solution. Optical density changes Δ D were observed in the bleaching region at different excitation wavelengths. The decay curves could be described using a bi-exponential fitting procedure. A fast decay time of 2 ps and a slower one varying with excitation wavelengths (400 nm: 126 ps; 385 nm: 74 ps; 374 nm: 51 ps) were determined. To explain this time behavior we suggest a model with two channels for intersystem crossing starting from different vibronic states in the singlet system into suitable states of the triplet manifold. Experimental data, as well as the applied model, indicate that we observed a fast two-channel intersystem crossing process.

Ultrafast intersystem crossing in thiophene oligomers investigated by fs-pump-probe spectroscopy

Exciting the shortest members of thiophene oligomers, we observed ultrafast triplet formation on a time scale of 1.5-3 ps. From the time resolved transient spectra in terthiophene at different excitation wavelengths, we concluded the existence of an intermediate state within the S 1 manifold. This state is populated during the relaxation of the structure of the excited molecule to a planar conformation and can be the key for understanding the high triplet of about 0.95 in dioxane. In bithiophene, using two-photon excitation, we found excited state absorption, triplet-triplet absorption and radical cation absorption. The time constant for triplet formation was estimated to amount to 1.5 ps.