Photophysics of Reduced Silicon Tetraphenylporphyrin

Abstract

The contrasting photophysical properties of two silicon (IV) tetraphenylporphyrins, Si(TPP)(py)2 and Si(TPP)Cl2, have been investigated using static absorption and fluorescence spectroscopy and ultrafast transient absorption measurements. The parent Si(TPP)Cl2, in which the porphyrin macrocycle has its normal 2- oxidation state, has a fluorescence yield of 0.027, and a lifetime of 1.8 ns for the lowest excited singlet state. In marked contrast, the reduced, anti-aromatic complex Si(TPP)(py)2, with the macrocycle in the 4- oxidation state, has an extremely low fluorescence yield (< or =0.0004) and a 750-fold shorter excited-state lifetime (2.4 ps) in the same solvent (pyridine). The rapid deactivation of photoexcited Si(TPP)(py)2 to the ground state is likely associated with its ruffled structure and the presence of low-energy excited states in its electronic manifold.

Facile Redox‐Induced Aromatic–Antiaromatic Interconversion of a β‐Tetracyano‐21,23‐Dithiaporphyrin under Ambient Conditions

“…The spectrum exhibited a sharp singlet signal for the β protons at δ =4.99 ppm (Figure c), which were, compared to 1 , considerably upfield shifted. Comparison of the chemical shifts of selected isophlorins with antiaromatic and nonaromatic characters, other relevant antiaromatic porphyrinoids, and quinoidal thiophene compounds is summarized in Table S16. The peak of [ 2 ] 2− is shifted downfield compared to those of the strong antiaromatic analogues (from 3 to −1 ppm), but shifted upfield compared to those of the distorted nonaromatic analogues and quinoidal thiophene compounds (from 8 to 6 ppm).…”

“…The second type of isophlorins contain tetravalent elements, such as Si or Ge or their respective analogues . They complement isophlorins in terms of their charge valance because isophlorins serve as tetravalent ligands.…”

Chemo‐ and Regioselective Reduction of 5,15‐Diazaporphyrins Providing Antiaromatic Azaporphyrinoids

“…[4] In 2005, Vaid and co-workers reported the synthesis of an antiaromatic 20 p silicon(IV) porphyrinw ith ah ighly ruffled structure. [5] Since the isophlorin macrocycle works as at etravalent ligand,t he choice of central metals is limited. [6] The synthesis of 20 p porphyrinoids with both high planarity and distinct antiaromaticity is still challenging.…”

“…We have now succeeded in the development of reagentcontrolled chemo-and regioselective reduction of 5,. In this process, the selectiver eduction of carbon-carbon double bonds provided the 18 p aromatic isobacteriochlorin-type product 2M,w hereas the reduction of carbon-nitrogen double bonds yielded N,N'-dihydrodiazaporphyrins 3M selectively.T he planar structure and 20 p conjugation,i ncluding lone pairs on the nitrogen atoms at the meso positions, induce distinct antiaromaticc haracter of 3M.…”

Investigation of Aromaticity and Photophysical Properties in [18]/[20]π Porphycene Derivatives

“…Although these [4n+2]p porphyrinoids have been intensively synthesized and characterized, porphyrinoids with [4n]p electrons in their delocalized pathways have been little explored on account of their synthetic difficulties and instabilities under ambient conditions. [9][10][11][12][13][14] As a consequence, a majority of studies on [4n]p porphyrinoids have dealt with expanded porphyrins, because the difference in resonance stabilization energies between [4n]p and [4n+2]p electronic systems decreases for larger values of n. [5,15] In this regard, the isolation and stability of [4n]p porphyrinoids are the essential requirements for the reliable photophysical characterization. Chen et al [16] reported in 2007 that trifluoromethyl groups on pyrrole rings give rise to relatively air-stable [20]p porphyrin.…”