Mechanisms of IR amplification in radical cation polarons

Abstract

Break down of the Born-Oppenheimer approximation is caused by mixing of electronic and vibrational transitions in the radical cations of some conjugated polymers, resulting in unusually intense vibrational bands known as infrared active vibrations (IRAVs). Here, we investigate the mechanism of this amplification, and show that it provides insights into intramolecular charge migration.Spectroelectrochemical time-resolved infrared (TRIR) and two-dimensional infrared (2D-IR) spectroscopies were used to investigate the radical cations of two butadiyne-linked conjugated porphyrin oligomers, a linear dimer and a cyclic hexamer. The 2D-IR spectra reveal strong coupling between all the IRAVs and the electronic p-p* polaron band. Intramolecular vibrational energy redistribution (IVR) and vibrational relaxation occur within $0.1-7 ps. TRIR spectra show that the transient ground state bleach (GSB) and excited state absorption (ESA) signals have anisotropies of 0.31 AE 0.07 and 0.08 AE 0.04 for the linear dimer and cyclic hexamer cations, respectively. The small TRIR anisotropy for the cyclic hexamer radical cation indicates that the vibrationally excited polaron migrates round the nanoring on a time scale faster than the measurement, i.e. within 0.5 ps, at 298 K. Density functional theory (DFT) calculations qualitatively reproduce the emergence of the IRAVs. The first singlet (S 1 ) excited states of the neutral porphyrin oligomers exhibit similar IRAVs to the radical cations, implying that the excitons have similar electronic structures to polarons. Our results show that IRAVs originate from the strong coupling of charge redistribution to nuclear motion, and from the similar energies of electronic and vibrational transitions.

Three dimensional polyurethane/ hydroxyapatite bioactive scaffolds: The role of hydroxyapatite on pore generation

“…On the other hand, HA would be covalently bonded to PMDI through a chemical interaction between PMDI's isocyanate group and HA's hydroxyl group, and consequently grafting of polymer chains to HA surface may occur. Such chemical interactions between HA and isocyanate‐terminated molecules have been reported by Liu et al 33,52–54 It should be noticed that the mentioned interactions affect the interface strength of the two phases, which is an important concern in polymer/mineral composites 44,45,49 The hydroxyl group presented in HA can react with the terminal NCO group of the PMDI molecule, which results in urethane linkage formation and PU grafting on the mineral particle surface. Water molecules adsorbed onto the surface of HA particles can react with the terminal NCO groups of the PMDI molecules and produce CO 2 gas.…”

“…The band intensity at 1071 cm −1 and 3401 cm −1 change from 34% and 43% in PU foam to 15% and 13% in PU-HA scaffolds, respectively. The shifting of the peaks at 1012, 1223, and 3401 cm −1 in PU foam up to 1026, 1228 and 3409 cm −1 in PU-HA scaffolds, respectively, can be assigned to physical and/or chemical interactions [48][49][50] between HA and PU/ PU monomers ( Figure 5). Physical interaction may include hydrogen bonding between ─NH in urethane linkage, 17,21 and phosphate group and hydroxyl group in HA, hydrogen bonding between ether oxygen 51 in the soft segment F I G U R E 4 FTIR spectra of HA powder, PU foam and PU-HA scaffolds for a wavenumber region of (a) 400-4000 cm −1 , (b) 400-2000 cm −1 .…”

“…Such chemical interactions between HA and isocyanate-terminated molecules have been reported by Liu et al 33,[52][53][54] It should be noticed that the mentioned interactions affect the interface strength of the two phases, which is an important concern in polymer/mineral composites. 44,45,49 1. The hydroxyl group presented in HA can react with the terminal NCO group of the PMDI molecule, which results in urethane linkage formation and PU grafting on the mineral particle surface.…”

Addressing the Elusive Polaronic Nature of Multiple Redox States in a π‐Conjugated Ladder‐Type Polymer

“…The electron polaron P−(D) for both cis and trans conformers, shows four peculiar IRAV bands [ 42 ] in the spectral regions 1650, 1500, 1280, and 1200 cm −1 (labeled with * in Figure 4; also see Supporting Information for a spectroscopic assignment). The 1650 cm −1 band is associated with the antisymmetric stretching mode of the two carbonyl groups involved in the polaron structural relaxation (see spin density map, Figure 3).…”

IR linewidth and intensity amplifications of nitrile vibrations report nuclear-electronic couplings and associated structural heterogeneity in radical anions

“… 34 Similar huge IR bands were observed in porphyrin oligomer radical cations. 35 The origin of these IR enhancements is attributed to two processes: coupling between charge distribution and vibrational motion (large dipole derivatives), or mixing between low-lying electronic and vibrational transitions. 34 Such significant enhancements were not previously reported for C N vibrations, and it is not clear if the absorption of a “localized” C N vibration can be similarly enhanced in charged species.…”