Fine Tuning of Photophysical Properties of meso–meso‐Linked Zn^II–Diporphyrins by Dihedral Angle Control

Abstract

A series of meso-meso-linked diporphyrins S(n) strapped with a dioxymethylene group of various length were synthesized by intramolecular Ag(I)-promoted coupling of dioxymethylene-bridged diporphyrins B(n), for n=10, 8, 6, 5, 4, 3, 2, and 1. Shortening of the strap length causes a gradual decrease in the dihedral angle between the porphyrins and increasing distortion of porphyrin ring, as suggested by MM2 calculations and (1)H NMR studies. This trend has been also suggested by X-ray crystallographic studies on the corresponding Cu(II) complexes of nonstrapped diporphyrin 2 Cu, and strapped diporphyrins S(8)Cu, S(4)Cu, and S(2)Cu. The absorption spectrum of relatively unconstrained diporphyrins S(10) strapped with a long chain exhibits split Soret bands at 414 and 447 nm and weak Q(0,0)- and prominent Q(1,0)-bands, both of which are similar to those of nonstrapped diporphyrin 2. Shortening of the strap length causes systematic changes in the absorption spectra, in which the intensities of the split Soret bands decrease, the absorption bands at about 400 nm and > 460 nm increase in intensity, and a prominent one-band feature of a Q-band is changed to a distinct two-band feature with concurrent progressive red-shifts of the lowest Q(0,0)-band. The fluorescence spectra also exhibit systematic changes, roughly reflecting the changes of the absorption spectra. The strapped diporphyrins S(n) are all chiral and have been separated into enantiomers over a chiral column. The CD spectra of the optically active S(n) display two Cotton effects at 430-450 and at about 400 nm with the opposite signs. The latter effect can be explained in terms of oblique arrangement of m( perpendicular 1) and m( perpendicular 2) dipole moments, while the former effect cannot be accounted for within a framework of the excition coupling theory. The resonance Raman (RR) spectra taken for excitation at 457.9 nm are variable among S(n), while the RR spectra taken for excitation at 488.0 nm are constant throughout the S(n) series. These photophysical properties can be explained in terms of INDO/S-SCI calculations, which have revealed charge transfer (CT) transitions accidentally located close in energy to the excitonic Soret transitions. This feature arises from a close proximity of the two porphyrins in meso-meso-linked diporphyrins. In addition to the gradual red-shift of the exciton split Soret band, the calculations predict that the high-energy absorption band at about 400 nm, the lower energy Cotton effect, and the RR spectra taken for excitation at 457.9 nm are due to the CT states which are intensified upon a decrease in the dihedral angle.

Supramolecular Chirality in Porphyrin Chemistry

“…The situation with the chiroptical activity was considerably improved upon employing an additional strapping between the two porphyrins in the case of the bis-porphyrins, 34-41 ( Figure 12) [40]. This was a result of a severe constraint imposed by the alkyl bridge, yielding further conformational fixation.…”

Subporphyrins: A Legitimate Ring-Contracted Porphyrin with Versatile Electronic and Optical Properties

“…6 It is amazing that extremely long porphyrin arrays (even 128-mer and 256-mer) smoothly undergo the coupling reaction, although they bear only two edge free meso-positions that can be used for the coupling. On the basis of this coupling reaction, we prepared extremely long porphyrin arrays up to linear 1024-mer, 6 three-dimensionally extending windmill porphyrin arrays, 79 dihedral angle controlled mesomeso-linked diporphyrins, 10,11 large porphyrin wheels, 1214 helical porphyrin arrays held by intermolecular hydrogen bonding hostguest interaction, 15 and directly linked porphyrin rings. 16 mesomeso-Linked diporphyrin motifs are suitable for supramolecular assembling.…”

Synthesis and Crystal Structure of a Meso-Meso Directly Linked Bisporphyrin

“…As shown by the crystal structure of the free base (vide supra) the two porphyrin rings lie almost orthogonal to each other, which results in a weak ground state interaction between the two chromophores [16].…”