Spectroscopic study of electron and energy transfer in novel silicon phthalocyanine—boron dipyrromethene triads

Abstract

Phthalocyanines (Pcs) and boron dipyrromethenes (BDPs) are two versatile classes of functional dyes suitable for design of artificial light harvesting and charge separation systems. In the present work, we report the results of photophysical investigations of two novel non-sandwich-type BDP-Pc heterotriads, in which two BDP or mono-styryl BDP moieties are linked to the central atom of a silicon(iv) phthalocyanine core (triad 4 and 5, respectively). It was found that the photophysical properties of the triads in toluene and N,N-dimethylformamide (DMF) are strongly affected by two different types of interaction between the BDP and the Pc parts, namely excitation energy transfer (EET) and photoinduced charge transfer (CT). The first process delivers the excitation to the first excited singlet state of the Pc-part upon initial BDP-part excitation. The probability of EET supersedes that of CT in toluene, whereas the latter transfer process dominates in BDP-part depopulation when triads are dissolved in polar DMF. The direct or indirect (via EET) population of Pc moiety is followed by the hole transfer to the charge-separated state in 4 (in DMF) and in 5 (in DMF and toluene). At the same time, it was found that CT is energetically unfavorable for the triad 4 in toluene upon excitation of the Pc-part. The charge-recombination in DMF occurs very fast with a decay time of 40 and 30 ps for 4 and 5, respectively, whereas toluene stabilizes the charge-separated state, prolonging the lifetime to 4.5 and 1.7 ns, respectively.