Emission energies and photophysical properties of ladder oligo(p-aniline)s

Emission properties and the photophysics of three ladder oligo(p-aniline)s; namely 5,11-diethyl-6,12-dimethylindolo3,2-b]carbazole (DIMER 2P), 14-ethyl-5,8-dihydro-diindolo3,2-b:2′,3′-h]carbazole (TRIMER 2P), and 5,8,14-triethyl-diindolo3,2-b:2′,3′-h]carbazole (TRIMER 3P) are presented. The optimization (relaxation) of the first singlet excited electronic state (S₁) has been done using the restricted configuration interaction (singles) (RCIS/6-31G*) approach. The excitation to the S₁ state does not cause important changes in the geometrical parameters of the compounds, as is experimentally corroborated by the small Stokes shifts. Emission energies from the relaxed excited states have been obtained from TDDFT calculations performed on the S₁ optimized geometries and have been correlated with the corresponding fluorescence spectra of the derivatives dissolved in dichloromethane. A good agreement has been found between TDDFT emission energies and the (0,0) fluorescence bands. As predicted from theoretical calculations, all compounds exhibit small Stokes shift, which testify the rigidity of these ladder compounds. Moreover, this theoretical approach provides a good evaluation of the bathochromic shifts caused by the increase in the conjugation length or by the presence of alkyl chains on the nitrogen atoms. Finally, the fluorescence quantum yield and lifetime of the compounds in dichloromethane have been obtained. From these data, the radiative and nonradiative rate constants of the deactivation of the S₁ state have been determined.