The electronic and vibrational spectra of N-R-N'-R-symmetrical dialkyl-3,4 : 9,10-peryIenebis(dicarboximide)s (R = CSHII, C,H,, and C,H,,) are reported. Lmgmuir monolayers were prepared and surface pressurearea isotherms were measured for three non-amphiphilic molecules. Langmuir-Blodgett (LB) films of mixed layers only were fabricated on solid substrates. Fundamental vibrational frequencies were observed using surface-enhanced resonance Raman scattering, transmission and reflection-absorption (RAIRS) Fourier transform infrared spectroscopy. The molecular orientation in the LB films was established from the transmission and RAIRS spectra. Molecular aggregation is reduced by the presence of bulky alkyl substitwnts. The adsorption of electron acceptor molecules and its effect on the fluorescence yield are also reported.This paper reports part of our studies of the series of alkyl derivatives of 3,4 : 9,10-perylenebis(dicarboximide)s (PTCD). These electroactive and photoactive organic materials may be applied as thin solid films, e.g. to form organic p n heterojunctions.' The objectives of this work were to characterize these three new materials, to form floating monolayers and to fabricate of LangmuirBlodgett (LB) films on solid substrates. I t was of interest to examine the effect of the size of the alkyl substituent in the formation of molecular aggregates and LB fabrication. Visible absorption and emission spectra were recorded to identify the monomer species and monitor dimer and excimer formation. The excimer emission is a characteristic spectral feature of PTCD derivatives and is well documented.'-The vibrational characterization includes the study of transmission Fourier transform infrared (FTIR) spectra of KBr pellets and transmission FTIR of LB films formed on ZnS or CaF, substrates. Reflection-absorption FTIR spectroscopy (RAIRS) was used to obtain the infrared spectra of LB films transferred to smooth Ag films evaporated on to glass. Raman spectra were recorded using the surfaceenhanced resonance Raman scattering (SERRS) technique for a single LB monolayer deposited on a silver island film on glass. The vibrational assignment of characteristic frequencies was carried out by a comparison with the parent perylene molecule and the tetracarboxylic derivatives. The effect of increased alkyl group size from CS-H,, (PPTCDP) to C,H,, (HPTCDH) and C8H,, (OPTCDO) on the properties of the floating layers and the spectroscopy of these molecules is discussed.
EXPERIMENTALThe synthesis and purification of the thrce materials have been described elsewhere.6 LB monolayers of the non-amphiphilic molecules were prepared at 13 "C in a Lauda Langmuir film balance equipped with an electronically controlled dipping device (Lauda Filmlift FL-1 ). The spreading solvent was H PLC-grade chloroform. Water purified using a Milli-Q system (Millipore) with a measured resistivity of 18.2 MR cm was used as the subphase. n-A isotherms, shown in Fig. 1, were recorded at a compression rate of 10-3 nm' molecule s -' . In order to improve t...
The molecular organization in Langmuir-Blodgett (LB) monolayers for five organic dyes with a common chromophore moiety was studied. With the use of transmission and reflection-absorption (FT)IR spectroscopy (RAIRS), the orientation of the chromophore units in the LB monolayer was determined. The results obtained with carbonyl and C=C stretching vibrations exclude a flat-on orientation of the chromophore moiety and may be rationalized in terms of a tilted head-on organization of the chromophore on the Ag, ZnS, and Ge surfaces. The tilt angle, φ, formed between the direction of the dynamic dipole moment and the normal to the surface of the film was determined from the combined infrared data obtained in a transmission geometry and in RAIRS. The aromatic-aromatic interaction plays a significant role in determining the molecular alignment in the Langmuir and LB monolayers.
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