Planar and Nonplanar Conformations of (meso-Tetraphenylporphinato)nickel(II) in Solution As Inferred from Solution and Solid-State Raman Spectroscopy

Abstract

The coexistence in solution of at least two conformers of (meso-tetraphenylporphinato)nickel(II) [Ni(TPP)] is inferred from solution and single-crystal resonance Raman spectra obtained at different temperatures (170 − 297 K) and excitation wavelengths (413.1 and 457.9 nm). The shapes of the structure-sensitive Raman lines ν8 and ν2 are clearly asymmetric and change with temperature. These broad lines can be decomposed into at least two sublines, a low-frequency (LF) and a high-frequency (HF) component. In contrast, the corresponding single-crystal Raman lines of the nonplanar structure of Ni(TPP) in the crystal are narrow and symmetric. For the line ν2, the broad LF subline results from nonplanar conformers and the narrow HF subline arises from a more planar conformer. This assignment is consistent with the observation that the LF subline of ν2 is more enhanced upon changing the excitation wavelength from 413.1 to 457.9 nm. The selective resonance enhancement is caused by the red shifts of the UV−visible absorption bands and Raman excitation profiles of the nonplanar form. The frequency assignment for the sublines of ν8 is reversed from that of ν2 (i.e., the HF subline of ν8 arises from nonplanar conformers and the LF subline results from a more planar macrocycle). This assignment is based on subline broadness and enhancement behavior using an excitation wavelength located on the red side of the B band. The assignments of the sublines to the nonplanar conformer are also in agreement with the Raman spectra of single crystals in which Ni(TPP) is known from X-ray crystallography to have a predominantly ruffled nonplanar conformation. Specifically, the frequencies of the sublines of ν8 and ν2 that are assigned to the nonplanar form in solution closely match the frequencies of Ni(TPP) in the single crystal. We propose two thermodynamic models for the interpretation of the temperature dependence of the intensity ratios of the sublines. A two-state model assumes one planar and one nonplanar conformer in solution. From the slopes in the van't Hoff plots, the nonplanar conformer is energetically favored by about 1.8 kJ mol-1 in this model. Since molecular mechanics calculations predict three conformers of Ni(TPP) in solution, we also consider a three-state model. The three structures are two nonplanar structures of purely ruffled (ruf) and purely saddled (sad) macrocyclic distortions and a planar conformer. In the calculations, the ruf conformation is the lowest-energy structure with the planar and sad conformers having almost equal higher energies. Assuming this relationship between the energies, but allowing the actual energy separation to vary, the three-state analysis gives the similar result that the ruf conformation is stabilized by about 2.3 kJ mol-1 with respect to the planar and sad conformers.

A New Method for Evaluating the Conformations and Normal Modes of Macromolecule Vibrations with a Reduced Force Field. 2. Application to Nonplanar Distorted Metal Porphyrins

“…These findings provide strongly the notion that the subbands arise from coexisting conformers. 12,13 As expected, 2ν 8 appears as a single line in the spectrum of CuTPP.…”

“…24 For NiTMeP, Table 3 lists the shifts for the d 12 derivative, in which all hydrogens of the methyl groups are replaced by deuterium. Most of the calculated values are in good agreement with the calculated ones.…”

“…Meso-substituted copper(II) tetraphenylporphyrin (CuTPP), for instance, crystallizes in a strongly ruffled and slightly saddled conformation, 10 as does the corresponding nickel(II) compound (NiTPP). 11 In organic solution, however, CuTPP was shown to be (quasi)-planar, 12 while NiTPP coexists in two conformations, from which the nonplanar conformation, which is very likely identical or similar to that obtained in the crystal, is the more stabilized one. 13 For porphyrins with more (bulky) substituents the structure is probably less dependent on environmental parameters and the core size.…”

Supramolecular Dye Inclusion Single Crystals Created from 2,3,6‐Trimethyl‐β‐cyclodextrin and Porphyrins

“…It is known that certain metalloporphyrin planes, such as Ni-TPP, are more distorted than free-base porphyrins. [19] This trend can be further emphasized in the DISC.…”

Charge-induced distortion and stabilization of surface transfer doped porphyrin films

“…Porphyrin molecules have been demonstrated to possess a high degree of conformational freedom, as the macrocycles exhibit both planar and nonplanar (saddled and ruffled) structures in solution. 34 Hence, in response to the 30% fraction of distorted ZnTPP molecules the nonplanar nature of the remaining molecules is enhanced and distinctly saddled and ruffled structures are favoured. It is furthermore conceivable that part of the charge on the contact porphyrin is shared with two or three immediate neighbours that then undergo similar distortions.…”