Time-resolved spectroscopy of oligothiophenes using the femtosecond fluorescence upconversion technique

Abstract

Time-resolved spectroscopy of oligothiophenes using the femtosecond fluorescence upconversion technique Abstract. In the present work the electronic properties of two oligothiophenes were experimentally studied through the time-resolved fluorescence spectroscopy up-conversion technique. The effect of the substituent at the end of the main chain on the excited state dynamics was investigated. The excited state decay time (τ fl ), the reorientation time of the emission dipole (τ or ), the initial fluorescence anisotropy (r 0 ) and the time-dependence of the fluorescence anisotropy r(t) were measured. The results showed that the molecule with the bigger dipole moment has smaller excited state decay time. The other coefficients are almost the same for the two materials because of their similar chemical structure. Also, the angle (α) between the absorption and emission dipoles, µ a and µ e respectively, was calculated through the anisotropy and found to be 29 o . This is attributed to the different geometry of the molecule in its ground and excited state. IntroductionThe research in ultra fast phenomena has been intensified during the last decades. These phenomena have to do with the chemical reactions (breakage of the chemical bonds, molecules orientation, transfer energy etc) as well as the behavior of the electrons and nuclei in the molecules. For the study of such phenomena the temporal resolution of the modern electronic systems is inadequate. Thus, new techniques, based on the femtosecond laser pulses, have already developed. The time resolved fluorescence spectroscopy, in femtosecond regime, is based on the frequency mixed phenomenon and presents the best temporal resolution to-date. This technique gives the possibility of measuring the lifetime of the excited state, as well as the temporal evolution of the fluorescence spectrum of a material.In the present work the above-mentioned technique is applied for the experimental study of two oligothiophenes for a better understanding of their structure and electronics properties. The influence of the change of the substituent, been located at the edge of the main chain, on their excited state dynamics is investigated. The excited state decay time (τ fl ), the reorientation time of the emission dipole (τ or ) and the initial fluorescence anisotropy (r 0 ) in directions perpendicular and parallel to the excitation dipole were measured for both materials. The angle (α) between the absorption µa and emission µe dipoles was calculated through anisotropy.