Infrared Band Assignments and Structure of Even-Numbered 2-Alkyl-7,7,8,8-Tetracyanoquinodimethane in Cast Films-Two Components of the CH2 Scissoring Vibrations not Related to Crystal Field Splitting

ArticleinGuang pu xue yu guang pu fen xi = Guang pu 30(4):892-6 · April 2010with6 Reads
DOI: 10.3964/j.issn.1000-0593(2010)04-0892-05 · Source: PubMed
Infrared (IR) spectra were measured for 2-octyl-, 2-dodecyl-, and 2-octadecyl-7, 7, 8, 8-tetracyanoquinodimethane (C8TCNQ, C12 TCNQ and C18 TCNQ) in cast films, and it was found that each spectrum shows two components for the CH2 scissoring band at 1 471 and 1 462 cm(-1). Polarized IR measurements showed that the micro-crystallites in the cast films take a random orientation in the plane of the plate. The intensity ratio of the two bands at 1 471 and 1 462 cm(-1) (I1 471/I1 462) decreases observably with the increase in the length of the alkyl chain. Moreover, the relative intensity of the 1 471 cm(-1) CH2 band to a band at 1 529 cm(-1) (C=C stretching mode of the TCNQ chromophore ring) does not change significantly for the three kinds of C(n) TCNQ while the relative intensity of the 1 462 cm(-1) CH2 band to the band at 1 529 cm(-1) increases markedly with the length of the alkyl chain. The above variations of the CH2 scissoring doublet of C(n) TCNQ are quite different from those of long-chain fatty acids (stearic acid and lignoceric acid) where the splitting of the CH2 scissoring vibration occurs due to a crystal field splitting. Considering the crystal structure of C12 TCNQ and the above spectral variations, the authors assign the two components of the CH2 scissoring bands at 1 462 and 1 471 cm(-1) of the C(n) TCNQ cast films to the interdigitated and non-interdigitated parts of the alkyl chains, respectively. Furthermore, the conclusion that the length of the non-interdigitated part of the alkyl chain is almost unchanged in the three kinds of even-numbered C(n) TCNQ could also be reached.
  • [Show abstract] [Hide abstract] ABSTRACT: The combination of various experimental techniques with theoretical simulations has allowed elucidation of the mode of incorporation of fluorene based derivatives into phospholipid bilayers. Molecular dynamics (MD) simulations on a fully hydrated 1,2-dimyristoyl-sn-glycero-3-phosphatidylcholine (DMPC) bilayer, with benzene (B), biphenyl (BP), fluorene (F) and tri-(9,9-di-n-octylfluorenyl-2,7-diyl), TF, have provided insights into the topography of these molecules when they are present in the phospholipid bilayer, and suggest marked differences between the behavior of the small molecules and the oligomer. Further information on the interaction of neutral fluorenes within the phospholipid bilayer was obtained by an infrared (IR) spectroscopic study of films of DMPC and of the phospholipid with PFO deuterated specifically on its alkyl chains (DMPC-PFO-d34). This was complemented by measurements of the effect of F, TF and two neutral polymers: polyfluorene poly(9,9-di-n-octylfluorenyl-2,7-diyl), PFO, and poly(9,9-di-n-dodecylfluorenyl-2,7-diyl), PFD, on the phospholipid phase transition temperature using differential scanning calorimetry (DSC). Changes in liposome size upon addition of F and PFO were followed by dynamic light scattering. In addition, the spectroscopic properties of F, TF, PFO and PFD solubilised in DMPC liposomes (absorption, steady-state and time-resolved fluorescence) were compared with those of the same probes in typical organic solvents (chloroform, cyclohexane and ethanol). Combining the insight from MD simulations with the results at the molecular level from the various experimental techniques suggests that while the small molecules have a tendency to be located in the phospholipid head group region, the polymers are incorporated within the lipid bilayers, with the backbone predominantly orthogonal to the phospholipid alkyl chains and with interdigitation of them and the PFO alkyl chains.
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