Dinamica di rilassamento di un colorante quadrupolare studiata con tecniche spettroscopiche ottiche ultraveloci

2,5-Bis[1-(4-N-Methylpyridinium)ethen-2-yl)]-N-methylpyrrole (Pepep) is a quadrupolar dye studied for his linear and non linear optical absorption properties and application in NLO. The steady-state behavior of Pepep has been widely investigated while the dynamic relaxation from the electronic exited states is limited to time resolved fluorescence. In this work of thesis, measurement of dynamic relaxation from the OPA (520 nm) exited state has been carried out by using time risolved Pump-Probe techniques. For a theoretical point of view, a relation between the modulation of the intensity of Probe and microscopic features of Pepep (such as dipole moment, dephasing parameter, relaxation constant) has been developed starting from the Liouville-quantum equation and third order development of electronic polarization. The experimental part consisted in recording the modulation intensity of Probe beam keeping the orientations of polarization of Pump and Probe parallel upon a range between 475 and 1050 nm. A global fit as been performed using the experimental data and four decay constants has been found for relaxation form OPA state. The fitting results allow us to get the transient absorption spectrum which shows two typical zone of ESA at 487 nm and 1050 nm, Bleaching at 520 nm, Stimulated Emission at 612 nm. The tow shorter decay constants at 1 ps and 8 ps has been related to internal vibronic relaxation and solvation of DMSO, the constants at 379 ps related the rotational contribution of relaxation and 1.1 ns related to the life-time of exited state OPA