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

Abstract

As eries of chlorin-bacteriochlorin dyads (derived from naturallyo ccurring chlorophyll-a and bacteriochlorophyll-a), covalently connected either through the meso-aryl or b-pyrrolep osition (position-3) via an ester linkageh ave been synthesized and characterized as an ew class of far-red emittingf luorescence resonance energy transfer (FRET) imaging, and heavy atom-lacking singlet oxygen-producing agents.F rom systematic absorption, fluorescence, electrochemical, and computational studies, the role of chlorina s an energy donora nd bacteriochlorin as an energy acceptor in these wide-band-capturing dyads was established. Efficiency of FRET evaluated from spectralo verlap was found to be 95 and 98 %f or the meso-linked and b-pyrrole-linked dyads, respectively.F urthermore, evidence fort he occurrence of FRET from singlet-excited chlorint ob acteriochlorin was secured from studies involving femtosecond transient absorption studies in toluene. The measuredF RET rate constants, k FRET ,w ere in the order of 10 11 s À1 ,s uggesting the occurrenceo fu ltrafaste nergy transfer in these dyads. Nanosecondt ransienta bsorption studies confirmed relaxation of the energyt ransfer product, 1 BChl*, to its triplets tate, 3 Bchl*. The 3 Bchl* thus generated was capable of producing singleto xygen with quantumy ields comparablet ot heir monomeric entities. The occurrenceo fe fficient FRET emitting in the far-red region and the ability to produces inglet oxygen make the present series of dyads useful for photonic, imaging and therapy applications.

Meso‐Biphenyl‐Linked, Near‐ and Far‐Infrared Emitting, Chlorin and Bacteriochlorin Dimers: Synthesis, Excitation Transfer, and Singlet Oxygen Production

“…Such analysis yielded τ h values of 1.05×10 11 s −1 , 0.89×10 11 s −1 and 0.12×10 11 s −1 (error=±10 %), respectively, for 2 , 4 and 6 . The magnitude of these τ h values are in good agreement with earlier reported rate constants of energy transfer ( k ENT ) values in Chl−BChl dyads connected by a biphenyl spacer, [14] indicating occurrence of efficient excitation transfer in the present dimers.…”

“…With this goal in mind, the dimers were investigated using both femto‐ (fs‐TA) and nanosecond transient absorption (ns‐TA) spectroscopic techniques. The fs‐TA and ns‐TA characterization of monomeric chlorin and bacteriochlorin, used as control in the present study, is reported by us earlier [14] …”

“…Recently, we reported on the occurrence of efficient Förster type resonance energy transfer (FRET) in a series of chlorin‐bacteriochlorin, Chl−BChl, dyads (hetero‐dimers) [14] . The measured rate constant for excitation transfer from 1 Chl* to Bchl in these dyads was on the order of 10 11 s −1 , revealing ultrafast excitation transfer.…”

“…Please note that dimer 2 was isolated as a stereoisomeric mixture of RR/SS and R/S from monomer 1 , an epimeric mixture, due to the presence of a chiral center at position 3(1’). Synthesis of meso‐phenyl derivatized chlorin and bacteriochlorin monomers, (Phen‐HPPHMe, Phen‐Pyro, and Phen‐BacPyro) is detailed in our previous publication [14] while their structure and abbreviations are given in Figure 1.…”

“…In the reaction center, the primary electron donor is either a chlorophyll or bacteriochlorophyll dimer arranged in a cofacial arrangement [12] . To mimic the antenna/reaction center photo‐events, a few examples of covalently linked chlorin and bacteriochlorin dimers have appeared in the literature [13–14] . Recently, we reported on the synthesis and occurrence of efficient singlet‐singlet excitation transfer in a series of covalently linked chlorin‐bacteriochlorin hetero‐dimers [14] .…”