Conjugated-linker dependence of the photophysical properties and electronic structure of chlorin dyads

Abstract

The synthesis, photophysical properties and electronic structure of seven new chlorin dyads and associated benchmark monomers are described. Each dyad contains two identical chlorins linked at the macrocycle [Formula: see text]-pyrrole 13-position. The extent of electronic communication between chlorin constituents depends on the nature of the conjugated linker. The communication is assessed by modification of prominent ground-state absorption and redox properties, rate constants and yields of excited-state decay processes, and molecular-orbital characteristics. Relative to the benchmark monomers, the chlorin dyads in toluene exhibit a substantial bathochromic shift of the long-wavelength absorption band (30 nm average), two-fold increased radiative rate constant [average (10 ns)[Formula: see text] vs. (22 ns)[Formula: see text]], reduced singlet excited-state lifetimes (average 5.0 ns vs. 8.2 ns), and increased fluorescence quantum yields (average 0.56 vs. 0.42). The excited-state lifetime and fluorescence yield for the chlorin dyad with a benzothiadiazole linker are reduced substantially in benzonitrile vs. toluene due largely to [Formula: see text]25-fold accelerated internal conversion. The results aid design strategies for molecular architectures that may find utility in solar-energy conversion and photomedicine.

Photophysical Properties and Electronic Structure of Hydroporphyrin Dyads Exhibiting Strong Through-Space and Through-Bond Electronic Interactions

“…The consequence is a larger-energy dominator and thus less mixing between the LUMO of the linker and the LUMO+1 (−0.68 eV) or LUMO (−1.43 eV) of the chlorins in C2 versus cis- C1 . To aid comparisons, Table lists the frontier MO energies for C0 and BC0 and the energies of the HOMO and LUMO of the linkers used in cis- C1 and BC1 (L1) and C2 (L2) and for the ethynyl and butadiynyl linkers used previously. − …”

“…The relative intensities of the S 0 → S 1 and S 0 → S 2 features are governed by the spatial relationship of the two macrocycles and the resultant symmetry of the dyad. The slipped cofacial disposition of the two macrocycles for cis- C1 (and the other cis- dyads) gives intensity in both the S 0 → S 1 and S 0 → S 2 , while the effective edge-on arrangement for trans -C1 places all of the oscillator strength into S 0 → S 1 . ,, …”

“…Such properties demonstrate that the electronic characteristics of the cis -type dyads are more complex than can be captured by an extension of the four-orbital model. Such an extension provided a reasonable first-order description of the chlorin and bacteriochlorin dyads that utilized a vinylene, ethynyl, or butadiynyl linker and possess an edge-to-edge arrangement, − more similar to that for trans- C1 .…”

“…Another would involve vibronic coupling in which motions that alter the relative positions of the two macrocycles would modulate the relative energies of the CR and exciton configurations and thus the electronic character of S 1 . Such considerations have been employed previously to analyze the enhanced internal conversion in covalently linked chlorin–chlorin and bacteriochlorin dyads , and the bacteriochlorophyll special pair in the bacterial photosynthetic reaction center …”

“…Examples of other porphyrinic systems include dyads with directly linked units , or systems that have cofacial, − slipped cofacial, , or edge-to-edge arrangements that employ ethynyl or butadienyl linkers . Synthetic arrays have been prepared in which hydroporphyrins (chlorins or bacteriochlorins) are directly attached , or held in a cofacial arrangement via various linkers. − A few examples of dyads from our prior studies that involve the joining of two chlorins or two bacteriochlorins via a vinylene (i.e., 1,2-ethenediyl), ethynyl, or butadiynyl linker are shown in Chart . − Also shown is the dyad denoted trans- C1 in which two chlorins are joined via a linker comprised of a 3,4-dimethoxycarbonyl-hexa-3-en-1,5-diyne linker that contains a triple–double–triple-bond (enediyne) motif . Such enediyne linkers have been employed to construct a variety of arrays. − The dyads shown in Chart that employ a vinylene, ethynyl, or butadiynyl linker show split frontier MOs, split redox waves, split optical bands, and altered excited-state properties compared to the monomeric chlorins or bacteriochlorins. − The subunits have TB interactions and potentially also short-range and long-range TS interactions depending on the tetrapyrrole, linker type, and attachment sites.…”