Fine Tuning of Photophysical Properties of meso–meso‐Linked ZnII–Diporphyrins by Dihedral Angle Control

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Abstract
A series of meso–meso‐linked diporphyrins Sn strapped with a dioxymethylene group of various length were synthesized by intramolecular AgI‐promoted coupling of dioxymethylene‐bridged diporphyrins Bn, 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 1H NMR studies. This trend has been also suggested by X‐ray crystallographic studies on the corresponding CuII complexes of nonstrapped diporphyrin 2 Cu, and strapped diporphyrins S8Cu, S4Cu, and S2Cu. The absorption spectrum of relatively unconstrained diporphyrins S10 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 Sn are all chiral and have been separated into enantiomers over a chiral column. The CD spectra of the optically active Sn 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⊥1 and m⊥2 dipole moments, while the former effect cannot be accunted for within a framework of the exction coupling theory. The resonance Raman (RR) spectra taken for excitation at 457.9 nm are variable among Sn, while the RR spectra taken for excitation at 488.0 nm are constant throughout the Sn 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.