Asymmetrically Crowded "Push-Pull" Octaphenylporphyrins with Modulated Frontier Orbitals: Syntheses, Photophysical, and Electrochemical Redox Properties

作者：Grover Nitika; Sankar Muniappan^*; Song Yang; Kadish Karl M^*

来源：Inorganic Chemistry, 2016, 55(2): 584-597.

DOI：10.1021/acs.inorgchem.5b01339

A new series of b-substituted octaphenylporphyrins were synthesized and the influence of unsymmetrical substitution on the photophysical and electrochemical properties of the compounds is elucidated. The examined compounds are represented as MOPP(R)X-2 where OPP = the dianion of octaphenylporphyrin, R = NO2, CHO, or CH2OH, X = Br or CN, and M is Co(II), Cu(II), Ni(II), or Zn(II). Routes to the trifunctionalized b-octaphenylporphyrins begin with the synthesis of MOPP(R) (R = NO2, CHO, and CH2OH) and the conversion of MOPP(NO2) to MOPP(NO2)X-2 (X = Br and CN). These pushpull octaphenylporphyrins exhibit high dipole moments, tunable redox properties, and red-shifted electronic spectral features due to asymmetric b-substitution. Photophysical data on the series of MOPP(R)X-2 compounds also reflect the nonplanar conformation of these porphyrins. Quantum yield and lifetime data are invariably lower than what has been reported in the literature for related b-substituted porphyrins. The spectroscopic properties and electrochemical redox potentials of the porphyrins are influenced by both the peripheral substituents and nature of the core metal ion. A decrease in the HOMOLUMO gap and increase in b1 is observed as the number of electron withdrawing groups on the molecule was increased. In addition, a tuning of the redox potentials could be achieved by introducing both electron donating (CH2OH) and withdrawing (CN, NO2, CHO, and Br) substituents into the MOPP skeleton which led to a cross-polarized pushpull effect of the b-substituents and a nonplanarity of the molecule. Metal-centered oxidations were exhibited for all of the Co(II) porphyrins and an M(II)/M(III) process was also observed to occur for NiOPP(R) (R = CH2OH, H, CHO, and NO2) and CuOPP(NO2)(CN)(2). These electrode reactions for the latter two series of compounds occur after an initial conversion of the neutral porphyrin to its dicationic form under the electrochemical conditions. Evidence for the site of electron transfer is given in part by comparison with data in the literature for related compounds and in part by theoretical calculations and thin-layer spectroelectrochemical data carried out in the current study.

• 出版日期2016-1-18