Abstract
We study the excited state dynamics of polyaniline and its aromatic substituted derivatives using femtosecond and picosecond laser systems combined with techniques time correlated single photon counting and fluorescence up-conversion. Five polymers, polyaniline [Pan(1)], Benzenethiol-polyaniline [Pan-BT(2)], 2-naphthalenethiol-polyaniline [Pan-NPT(3)], 2-mercaptobenzothiazole-polyaniline [Pan-MBT(4)], 2-pyridinethiol-polyaniline [Pan-PYT(5)] are studied. Excitation at wavelength 266 nm and at 355 nm for all the molecules studied we observe four decay components. Using time correlated single photon counting and fluorescence up-conversion techniques we found that upon excitation at 266 nm 1 in NMP solution exhibit four decay processes with fitted time constants 2 ps, 22 ps, 0.6ns and 2.6 ns. The other compounds’ decay time constansts are in the range of 1.7∼3 ps, 23∼71 ps, 0.5∼0.65 ns and 2.1∼2.2 ns. According to the results we conclude that the substituents don’t affect decay lifetimes much. The photons excite the molecules to a highly unstable state followed by transfer to S1 state within 400 fs. And then cool down to the lowest vibrational state by two steps internal vibrational relaxation processes followed by fluorescence emission from S1 state. Upon excitation at 355 nm the emission wavelength is the same with excitation at 266 nm, and this means that the emission states are the same whether the excitation wavelength is 266 or 355 nm. At time zero the anisotropy value r(t＝0) equals to 0.2 means there’s a fast rotational motion within the system response time (TCSPC, 30 ps) after excitation by 355 nm light.