Transient infrared spectroscopy: a new approach to investigate valence tautomerism

Abstract

In this work we present, to our knowledge for the first time, the results of a transient infrared spectroscopic study of the photoinduced valence tautomerism process in cobalt-dioxolene complexes with sub-picosecond time resolution. The molecular systems investigated were [Co(tpa)(diox)]PF(6) (1) and [Co(Me(3)tpa)(diox)]PF(6) (2), where diox = 3,5-di-tert-butyl-1,2-dioxolene; tpa = tris(2-pyridylmethyl)amine and Me(3)tpa its 6-methylated analogue. Complex (1) is present in solution as ls-Co(III)(catecholate) (1-CAT), while (2) as hs-Co(II)(semiquinonate) (2-SQ). DFT calculation of the harmonic frequencies for (1) and (2) allowed us to identify the vibrational markers of catecholate and semiquinonate redox isomers. Irradiation with 405 and 810 nm pulses (~35 fs) of (1-CAT) induces the formation of an intermediate excited species from which the ground state population is recovered with a time constant of 1.5 ± 0.3 ns. Comparing the 1 ns transient infrared spectrum with the experimental difference spectrum FTIR(2-SQ)-FTIR(1-CAT) and with the calculated difference spectrum IR(c)(1-SQ)-IR(c)(1-CAT) we are able to unequivocally identify the long lived species as the semiquinonate redox isomer of (1). On the other hand, no evidence of photoconversion is observed upon irradiation of (2) with 405 nm. Temporal evolution of transient spectra was analyzed with the combined approach consisting of singular values decomposition and global fitting (global analysis). After 405 and 810 nm excitation of (1-CAT), the semiquinonate excited species is formed on an ultrafast time scale (<200 fs) and cools down within the first 50 ps. Excitation of (2-SQ) with 405 nm wavelength produces a short lived excited state in which the semiquinonate nature of dioxolene is preserved and the ground state recovery is completed within 30 ps.

Redox Ladder of Ni3 Complexes with Closed‐Shell, Mono‐, and Diradical Triphenylene Units: Molecular Models for Conductive 2D MOFs

“…[27] In the mid-IR region, ab lue-shift of the n(C À O)-based stretching band from 1310, 1331, to 1341 cm À1 ,again suggests increased quinoidal character and CÀObond order in HOTP upon oxidation from 1 to 2 and 3 (Figure S7). [28] Altogether, CV and UV/Visible-IR measurements confirm 1, 2,a nd 3 as ar edox ladder of three successive oxidation states for [Ni 3 (HOTP nÀ )] (6Àn)+ (n = 4, 3, and 2) with electron transfer events all ligand-based.…”

Redox Ladder of Ni3 Complexes with Closed‐Shell, Mono‐, and Diradical Triphenylene Units: Molecular Models for Conductive 2D MOFs

“…A blue‐shift of the lowest‐energy absorption in the near‐IR region has been associated with an increase of quinoidal character in related trinuclear HOTP complexes [27] . In the mid‐IR region, a blue‐shift of the ν(C−O)‐based stretching band from 1310, 1331, to 1341 cm −1 , again suggests increased quinoidal character and C−O bond order in HOTP upon oxidation from 1 to 2 and 3 (Figure S7) [28] . Altogether, CV and UV/Visible‐IR measurements confirm 1 , 2 , and 3 as a redox ladder of three successive oxidation states for [Ni 3 (HOTP n − )] (6− n )+ ( n =4, 3, and 2) with electron transfer events all ligand‐based.…”

Valence Tautomeric Transitions of Three One-Dimensional Cobalt Complexes

“…Because complexes 1·S and 2·S are analogues, only 1·S is described here. At room temperature, the bands at 1437, 1411, and 1284 cm −1 are characteristic features of the catecholate ligand 3,5-DBCat 2− , while those at 1458 and 1355 cm −1 can be assigned to the semiquinonate ligand 3,5-DBSq • , 38 which are coincident with the ls-Co III −Cat 2 -Sq • species of 1·S at 300 K. In the desolvated sample 1, the ligand 3,5-DBCat 2− has converted to ligand 3,5-DBSq • at 300 K; accompanying this conversion, the characteristic bands of catecholate (1437, 1411 and 1284) disappear, and the relative intensity of semiquinonate bands increases. So, although 1·S itself can not undergo thermally induced VT transition near room temperature, a solvent-induced VT transition between 1·S and 1 can be established.…”