meso‐ and β‐Pyrrole‐Linked Chlorin‐Bacteriochlorin Dyads for Promoting Far‐Red FRET and Singlet Oxygen Production

A series of chlorin‐bacteriochlorin dyads (derived from naturally occurring chlorophyll‐a and bacteriochlorophyll‐a), covalently connected either through the meso‐aryl or β‐pyrrole position (position‐3) via an ester linkage have been synthesized and characterized as a new class of far‐red emitting fluorescence resonance energy transfer (FRET) imaging, and heavy atom‐lacking singlet oxygen‐producing agents. From systematic absorption, fluorescence, electrochemical, and computational studies, the role of chlorin as an energy donor and bacteriochlorin as an energy acceptor in these wide‐band‐capturing dyads was established. Efficiency of FRET evaluated from spectral overlap was found to be 95 and 98 % for the meso‐linked and β‐pyrrole‐linked dyads, respectively. Furthermore, evidence for the occurrence of FRET from singlet‐excited chlorin to bacteriochlorin was secured from studies involving femtosecond transient absorption studies in toluene. The measured FRET rate constants, kFRET, were in the order of 1011 s−1, suggesting the occurrence of ultrafast energy transfer in these dyads. Nanosecond transient absorption studies confirmed relaxation of the energy transfer product, 1BChl*, to its triplet state, 3Bchl*. The 3Bchl* thus generated was capable of producing singlet oxygen with quantum yields comparable to their monomeric entities. The occurrence of efficient FRET emitting in the far‐red region and the ability to produce singlet oxygen make the present series of dyads useful for photonic, imaging and therapy applications.