Direct evidence of a bipolaron charge carrier in conducting polyazines from carbon-13 and nitrogen-15 solid-state NMR spectroscopy: detection of a nitrenium cation by natural abundance 15N solid-state NMR spectroscopy

Abstract

6831cooperative complex calculated with the parameters tb = -4500 cal/mol, So = 1.6 X IO4, and u = 3.2 X For illustrative purposes, we may define an entropy of nucleation, AS, = R In u = -1 1.4 eV and an entropy of bond formation, ASb R In So = -17.3 eu. The model with N = 5 and these parameters obviously generates a cooperative melting curve that closely mimics the transition reported here (Figures 7,8, and 10). Within the spirit of this model, the parameters do not violate reasonable ideas of the strength of the entropy change for localizing opposing segments of the interface. (58) The fraction of broken bonds is defined as ( ( N ( 0 ) ) -( N ( T ) ) ) / ( N -( 0 ) ) , where (N(T)) is the average number of bonds and is given by eqs 16-93 of ref 57 and N(0) 5 N -1 in the model as formulated (ref 57). We have explored a range of N and find this makes no difference to the qualitative features of this approach. However, the magnitudes of cb, ASSb, and ASn decrease with increasing N .Future work will address such features as the structural differences that cause the high-temperature forms of the complex to exhibit quenching whereas the low-temperature form@) do not. It will further seek a realistic picture of the interfacial interactions in both types of forms, as well as a kinetic mechanism that embodies the ideas proposed here. Acknowledgment.We acknowledge research support from the NSF (Grant DMB 8907559 to B.M.H.), the N I H (Grants HL13531 to B.M.H.; GM 33806 to M.K.J.; GM 29001 and G M 19121 to E.M.), and the MRC of Canada (Grant MT-1706 to M.S.). We are indebted to Profs. John Torkelson and Laszlo Lorand for the gracious use of their spectrofluorimeters and to Drs. Sten Wallin, Eric Garber, and Thomas Luntz for helpful discussions. Abstract: Natural abundance 13C and I5N solid-state NMR experiments were performed on undoped and doped samples of polyazine, (-N=C(R)-C(R)=N-),,The topology of the polymer requires that doping-induced charge carriers must be either polarons (radical cations) or bipolarons (dications). Magnetic susceptibility measurements shows that the iodine-doped polymers are diamagnetic, so the charge carriers must be bipolarons. The specific bipolaron charge carrier in polyazines has been identified; the nitrogens bear the charge primarily in the form of a nitrenium cation. This is the first report of a nitrenium cation detected by I5N solid-state NMR spectroscopy. Independent of the substitution of the polyazine, R = H or CH3, the same charge carrier is present.

End-Group Effects on the Structure and Spectroscopy of Oligoazines

“…Such a simple pattern is not found for compounds II- n in the methyl region, although as the chain length increases the methyl peaks are becoming more shielded. In long-chain polyazines, in the solid state, the limiting methyl resonance is found at 12.6 ppm,6b in agreement with the solution values found here. This implies that interactions between chains in the solid state are small or, coincidentally, are remarkably similar to chloroform solvation.…”

“…No specific assignments can be made, but clearly a large number of conformers are present in solution. This was somewhat surprising since previous work on the 13 C solid-state NMR did not show evidence of multiple conformers . This could be attributed to the larger line width found in solid-state spectra that hide the number of contributions to each observed resonance or the possibility that only single structures arise in the solid materials (compared to multiple conformations in solution).…”

Ab Initio Study of the Reactions between a Series of Substituted Singlet Nitrenium Ions and Water

“…Unlike the isoelectronic carbenes, examples in the literature of nitrenium ions characterized by direct spectroscopic methods are scarce. Aryl-substituted nitrenium ions have been found to be relatively stable to allow their detection by UV−Vis spectrophotometry and NMR spectroscopy . In addition, the aminylium ion ( + NH 2 ) has been characterized by IR, EPR and laser spectroscopies .…”

Synergy between the Assembly of Individual PEDOT Chains and Their Interaction with Light

“…Liquid-state 1 H hr-NMR spectroscopy was used to investigate the reaction kinetics in the range from 30 min to 48 h for EDOT solution in d 6 -ethanol and d 2 -formic acid (3:7 vol %), see Supporting Information, Figure S1. The representative 1 H NMR spectra are depicted in Supporting Information, Figure S1, and the signal assignment is based on literature data. , We must emphasize that the 1 H NMR peaks are very narrow, which proves that acid-assisted polymerization proceeds without the formation of cation radicals …”

“…The 13 C NMR spectrum also reveals the formation of an ester from the solvents (signals marked as “10”, “11”, and “12”); however, pure d 6 -ethanol (signals “7” and “8”) and d 2 -formic acid (signal “9”) are present in the mixture. Additionally, the narrowness of the peaks in the 13 C NMR spectra indicates the absence of cation radical formation during acid-assisted polymerization, which forms vortex field and makes it impossible to carry out measurements …”