Excited state relaxation of the poly(spirobifluorene-co-benzothiadiazole) has been investigated bymeans of the time resolved fluorescence and femtosecond transient absorption. The experimentalresults and their comparison with the similar data obtained for poly(fluorene-co-benzothiadiazole)show that the long-lived excited state with the reduced dipole moment of the transition to the groundstate is formed on a subpicosecond time scale due to the exciton localization on a polymer backboneand spirobifuorene side group. Excited state stabilization reduces the transition dipole moment stillfurther, causing fluorescence relaxation faster than the relaxation of the excited state.
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