Femtosecond transient photoinduced transmission measurements on a novel conjugated zinc porphyrin system

Abstract

We present a comparative study on the ultrafast nonlinear optical response of a novel conjugated zinc porphyrin system. The linear optical absorption spectra of these molecules all show the Q-band and B-band transitions of the basic porphyrin unit. We have taken spectrally resolved ultrafast pump–probe measurements on monomer, dimer, and polymer solutions, which allows us to compare their excited state dynamics and relate these to their linear optical absorption. The spectra show several common features, but these features have markedly different decay dynamics. The bleaching is preferential in the Q band for the polymer and the B band for the monomer. The polymer Q-band bleaching shows a two-component decay, of approximately 700 fs and 170±50 ps time constants in a biexponential fit, which we attribute to both exciton–exciton annihilation and exciton diffusion to recombination centers on the polymer chain. The Q band of the dimer also has a two-component decay with 13±5 and 1250±70 ps time constants which we attribute to rotational diffusion of the excited molecule in solution, and decay to the ground state, respectively. The B-band bleaching in the monomer is long lived and has a decay constant of approximately 3.5±0.5 ns; from the absorption recovery of the B band we estimate a triplet yield of 0.8. All molecules exhibit broad π*−π* absorptions in the visible spectral region (between the Q band and the B band). In particular, we show that the monomer has potential as a broadband optical limiter in the visible region from 455 (2.72 eV) to 620 nm (2.00 eV); we estimate that its excited state absorption cross section is 8.5 times that of its ground state cross section at 532 nm (2.33 eV). There is also clear evidence of triplet transitions in the dimer and monomer; the triplet absorption feature ranging from 940 (1.32 eV) to 1000 nm (1.24 eV) rises 10 ps after excitation in the dimer which suggests a short intersystem-crossing time.