Expanding π-Conjugation in Chlorins Using Ethenyl Linker

Abstract

A series of chlorin monomers and dyads has been prepared to probe the effect of ethenyl vs ethynyl linkers on the electronic conjugation and optical properties in resulting derivatives. Styryl-substituted chlorins have been prepared either by a Heck reaction or by microwave-assisted olefin metathesis, while β-β ethenyl-linked dyads have been synthesized from the corresponding vinyl-substituted chlorin monomer using microwave-assisted olefin metathesis. It has been found that when an ethenyl linker is connected at the β-position of chlorin it provides stronger electronic conjugation than an ethynyl one, which is manifested by a greater bathochromic shift of the longest wavelength absorption (Q ) and emission bands. Stronger electronic coupling is particularly evident for dyads, where ethenyl-linked dyad exhibits a strong near-IR absorption band emission (λ = 707 nm, λ = 712 nm, Φ = 0.45), compared to the deep-red absorption and emission of a corresponding ethynyl-linked dyad (λ = 689 nm, λ = 691 nm, Φ = 0.48). The reactivity of ethenyl-linked dyads with singlet oxygen is discussed as well. The results reported here provide further guidelines for molecular design of deep-red and near-IR absorbing and intensely emitting chlorin derivatives and chlorins with extended π-electronic conjugation for a variety of photonic applications.

Photoisomerization of Enediynyl Linker Leads to Slipped Cofacial Hydroporphyrin Dyads with Strong Through-Bond and Through-Space Electronic Interactions

“…These emission features differ significantly from those observed for monomers or previously reported ethynyl-, butadienyl-, or vinyl-linked, strongly conjugated hydroporphyrin arrays, which all exhibit relatively narrow (FWHM < 400 cm −1 ) emission with Stokes shift of < 200 cm −1 . 55,57 In addition, the fluorescence quantum yield (Φ f ) is substantially reduced in comparison to respective monomers (4-fold for cis-2C compared to that of C-PE and 6fold for cis-2BC compared to that of BC-2PE), which also differs from that observed for previously reported strongly conjugated hydroporphyrin dyads, which typically exhibit enhanced Φ f , compared to the corresponding monomers. 55,57 Broad emission spectra with a relatively large Stokes shift and low Φ f observed for cis-2C, cis-2ZnC, and cis-2BC are characteristic for excimers.…”

“…The Q y -type band in trans-2C (λ Q y = 701 nm) is broadened and significantly shifted bathochromically, compared to that in C-PE (λ Q y = 656 nm). 57 The maximum of Q y band in trans-2C is located between ethynyl-linked dyad (λ Q y = 689 nm) and vinyl-linked dyad (λ Q y = 707 nm) reported previously. 56,57 Absorption spectrum of cis-2C contains a split Q y -type band, with the maximum of the more intensive band centered at 659 nm and a second maximum centered at 704 nm (the second maximum coincidentally overlaps with the maximum of Q y band in trans-2C; therefore, we cannot rule out that a trace amount of trans-2C is present even in a sample purified under rigorous exclusion of light).…”

“…69−71 As a reference, we also prepared dyad phen-2C, where two chlorins are connected by o-diethynylphenylene linker. As benchmarks, we included known monomers: C-PE 57 and BC-2PE 55,56 (Chart 1). We found that enediyne-linked hydroporphyrin dyads exhibit trans−cis photoisomerization, leading to thermally stable cis isomers, with a slipped cofacial arrangement of macrocycles, in which both strong through-bond and through-space interactions occur.…”

Influence of Quinoidal Distortion on the Electronic Properties of Oxidized Divinylarylene-Bridged Diruthenium Complexes

“…As is often observed, the vinyl linker provides a more efficient conjugation with the attached aryl chromophores (for a recent example, see Meares et al). 97 Considering that the length of the π-conjugated bridge almost doubles, the red-shifts compared to Ru 2 BTD are rather modest. This indicates that the underlying transition is primarily due to intraligand charge transfer from the conjugated phenyl ring and metal-to-ligand charge transfer (MLCT) from the electron-rich vinylruthenium appendices to the corresponding [2,1,3]-thiadiazole acceptor units.…”

“…As is often observed, the vinyl linker provides a more efficient conjugation with the attached aryl chromophores (for a recent example, see Meares et al.) . Considering that the length of the π-conjugated bridge almost doubles, the red-shifts compared to Ru 2 BTD are rather modest.…”