Stepwise photoconversion of an artificial light-harvesting array built from extended BODIPY units

Abstract

A molecular dyad, comprising two disparate extended boron dipyrromethene (BODIPY) units, has been identified as a potential component of artificial light-harvesting arrays. Highly efficient, intramolecular electronic energy transfer takes place under illumination but there is some competition from light-induced electron transfer along the molecular axis. The primary energy acceptor has a somewhat shortened excited-state lifetime and reduced emission quantum yield due to charge transfer from a terminal amine residue, the latter being required for the molecular system to operate in organic solar cells. Under continuous illumination with simulated solar light, the dyad undergoes very slow decomposition. In a protic solvent, both BODIPY units degrade at the same rate via an autocatalytic process. The products, one of which is a protonated analogue of the donor, degrade further by independent routes. In aprotic solvents or thin plastic films, the acceptor BODIPY dye absorbing at lowest energy undergoes photochemical degradation as above but the donor is much more stable under these conditions. At each stage of degradation, the molecule retains the ability to sensitize an amorphous silicon solar cell and the overall turnover number with respect to absorbed photons exceeds 10 million. The optical properties of the target compound nicely complement those of the solar cell and sensitization helps to avoid Staebler-Wronski photo-degradation.

Inhibition of the Photobleaching of Methylene Blue by Association with Urea

“…In certain cases, a cascade of well‐defined intermediate species develops during the reaction so that the bleaching kinetics can be considered in some detail. In general, quantum yields for photofading tend to be very low but recent observations of autocatalysis mean that such measurements could be misleading. The most common method by which to follow chromophore photobleaching is via absorption spectrophotometry and numerous sophisticated protocols have been reported .…”

Bio-Inspired Artificial Light-Harvesting Arrays Based on Boron(III)-Chelates

“…Only the terminal dye is susceptible to photobleaching and only if the deactivation of this unit is slow -for example, when the device is inoperable. This situation has been described by Stachelek et al using a multi-component BODIPY-based array [8] (Figure 6). Here, the molecule undergoes stepwise photobleaching leading to successive loss of individual chromophores but retains photoactivity at all stages.…”

Photofading of an Extended BOPHY Chromophore Dispersed in Poly(methyl methacrylate) as a Chemical Actinometer

“…The primary product, which dominates at short illumination periods, displays the characteristic BOPHY absorption spectrum, thereby indicating to us that the core of the molecule remains intact. The blue‐shifted absorption spectrum can be used to argue that one or both of the styryl arms have been damaged so as to shorten the overall π‐conjugation length . It might be noted that the films were stored in the dark for a week prior to photobleaching studies but were found to be completely stable.…”

“…This provides us with a good appreciation of the absorption spectrum for the primary product (Figure ). The absorption maximum lies between those of TM‐BOPHY and PEG‐BOPHY, suggesting that only one of the styryl arms has been damaged to the point where it no longer contributes to the π‐conjugation length. Absorbance attributable to this product builds up quickly but reaches a plateau.…”