Syntheses of Chalcone‐Type Chlorophyll Derivatives Possessing a Bacteriochlorin, Chlorin or Porphyrin π‐System and Their Optical Properties

Abstract

C3‐(Trans‐2‐arylethenyl)carbonylated chlorophyll derivatives possessing a bacteriochlorin or chlorin π‐system were synthesized by cross‐aldol (Claisen–Schmidt) condensation of methyl pyrobacteriopheophorbide‐a or 3‐acetyl‐3‐devinyl‐pyropheophorbide‐a bearing the C3‐acetyl group with p‐(un)substituted benzaldehydes under basic conditions. The corresponding porphyrin‐type chlorophyll derivatives were prepared by the oxidation (17,18‐didehydrogenation) of the chlorin‐type. Their Qy absorption and fluorescence emission maxima in dichloromethane correlated well with Hammett substituent constants of the p‐substituents. Several electron‐withdrawing p‐substituents suppressed the emission due to photoinduced electron transfer quenching in a molecule. The substitution sensitivities for their maxima and fluorescence quantum yields decreased in the order of bacteriochlorin‐, chlorin‐ and porphyrin‐type derivatives.

Synthesis of 3‐arylated chlorophyll‐a derivatives via Diels–Alder reaction and their atropisomerism

“… 15,16 Similar 3‐heteroaryl‐chlorins were previously reported in several papers. 3‐Pyrazolyl‐, 15 triazolyl‐, 15,17,18 and oxazolyl‐chlorins 19 were obtained by 1,3‐dipolar cycloadditions including Huisgen and van Leusen reactions, while multicondensations led to 3‐pyrazolyl‐, 20,21 thienyl‐ (via Gewald thiophene synthesis), 22,23 indolyl‐ (Fischer indole synthesis), 22,23 quinolinyl‐ (Friedläder quinoline synthesis), 24 and quinoxalinyl‐chlorins 22,23,25 . These (hetero)aryl groups (R) at the 3‐position are more sterically demanded than the 3‐vinyl group, so their larger interaction with the 2‐methyl group and the 5‐hydrogen atom restricted their conformation.…”

“…These (hetero)aryl groups (R) at the 3‐position are more sterically demanded than the 3‐vinyl group, so their larger interaction with the 2‐methyl group and the 5‐hydrogen atom restricted their conformation. The π‐plane of the 3‐(hetero)aryl groups was apt to rotate away from the chlorin π‐system to give fairly perpendicular conformers with less π‐conjugation 15,16,19,21,26 . Here, we report the synthesis of methyl 3‐( mono ‐substituted phenyl)pyropheophorbides‐ a using the Diels–Alder reaction of a 3‐dienyl‐chlorin with a mono ‐substituted acetylene and their atropisomerism around the C3–C3 1 bond in a solution.…”