Intramolecular excitation energy transfer in diarylurea-linked zinc porphyrin–anthracene dyads

Abstract

Structurally controlled zinc porphyrin–anthracene dyads, syn-arranged 1 and anti-arranged 2, were newly synthesized employing a diarylurea linkage, and the excitation energy transfer (EET) from the anthracene to the zinc porphyrin chromophore was investigated by steady-state fluorescence emission spectroscopy as well as fluorescence lifetime measurement, especially focusing on the effect of the chromophoric orientation on the EET. In both of the dyads, intramolecular EET was facilitated upon excitation of the anthracene chromophore (λ_ex = 401 nm), and the zinc porphyrin S₁–S₀ emission (580–720 nm) was enhanced. The EET in the syn-arranged dyad 1 was more efficient than in the anti-arranged 2: the S₁–S₀ emission in 1 was 1.8 times larger than that in the zinc porphyrin reference compound 3, whereas that in 2 was enhanced by 1.6 times, compared to that in 3. In the fluorescence lifetime measurement, the quiet short-lived component assignable to the EET was observed for the dyads 1 and 2 beyond the analysis limit (<25 ps). The EET rate constants in the dyads 1 and 2 were estimated as not less than 4.0 × 10¹⁰ s⁻¹. However, in the case of 2, the residual long-lived component assigned to the anthracene emission was also observed at 425 nm. These results showed that the syn-arrangement of the zinc porphyrin and anthracene chromophores was more preferred for intramolecular EET to the anti-arrangement.