Molecular Recognition via Hydrogen Bonding in Supramolecular Complexes: A Fourier Transform Infrared Spectroscopy Study

We assess the assembly of supramolecular complexes by hydrogen bonding between azocompounds and a diacylaminopyridine monomer by temperature-dependent Fourier transform infrared spectroscopy (FT-IR) and density functional theory (DFT) calculations. The electronic delocalisation in the supramolecular rings formed by multiple hydrogen bonds stabilises the complexes, which coexist with dimeric species in temperature-dependent equilibria. We show how the application of readily available molecular modelling and spectroscopic techniques can predict the stability of new supramolecular entities coexisting in equilibria, ultimately assessing the success of molecular recognition.


Characterisation techniques
The temperature dependent FT-IR spectra were obtained using a Thermo Nicolet 470 FT-IR spectrometer over a frequency range 4000-400 cm -1 , with an accuracy of 4 cm -1 , as the average of 64 scans. The experiments were carried out in transmittance mode, on dispersions of the samples in potassium bromide, KBr, of 1% in weight. Homogeneous discs were obtained by grinding the corresponding amounts of sample and KBr into fine powder, and then pressing at around 7 atm for 5 min. A background measurement was taken at room temperature on a pristine KBr disc, and was subtracted to the sample spectra. Temperature was controlled by placing the discs into a Linkam TMS93 heating stage, with a temperature accuracy of ±0.1. Samples were heated to 150℃ and then cooled down to room temperature. The IR spectra were then measured on isothermal steps every 5 or 10 o C, allowing the sample to equilibrate at each temperature before measuring.
Additionally, we have also applied static 2D correlation spectroscopy to some of the experimental IR spectra [3][4][5] in order to detect cross correlation of spectral intensity variations with the temperature [6]. 2D correlation spectroscopy can discriminate potential overlapping peaks, and spreads these over the second dimension, in synchronous and asynchronous plots. The synchronous spectrum  ( ) signifies the simultaneous change in spectral intensities seen at the two different wavenumbers 1, and 2. If the spectral intensities at the two wavenumbers change in the same direction then the synchronous correlation sign will positive. If one is decreasing and the other increasing the sign will be negative. In case that  ( ) = 0, then the order of intensity variations is impossible to determine.
The asynchronous spectrum  ( ), alternatively, represents the out of phase changes of spectral intensities. When  ( ) =  then the variations of spectral intensities at the two wavenumbers are fully synchronized. If  ( )and  ( ) have the same sign then at wavenumber 1 , spectral intensity variations occur before they do at wavenumber 2 . If the signs are different, then the opposite occurs.
The theoretical conformations of the supramolecular systems were obtained by density functional theory, DFT, using simplified molecular models of our compounds. We have used the Gaussian 09 package [7], and more particularly the hybrid B3LYP functional. We have estimated the best minimum energy conformations by full geometrical optimisation, and obtained the theoretical infrared spectroscopy response of the conformers in the gas phase. The use of a diffusion function in a large basis set in this work, B3LYP/6-311++g** [8], allows the application of the original theoretical frequencies obtained by DFT, with only small deviations [9,10]. Moreover, considering that the scaling factor for our method and basis set is 0.960 in the high frequency range and 0.988 below 1800 cm -1 [11], we believe that the corrections would fall within the error of not considering the intermolecular environment of the complexes, which is inherent of condensed phases. Therefore, we consider our calculations to being valid to explain hydrogen bonding, but we note that the frequencies must be treated carefully, due to inherent deviations between gas and condensed phases.
In dimers and complexes, we have also calculated the dissociation energies, Edis, by sufficiently separating the individual molecules and obtaining the difference in energy. We justify the use of simplified DFT models of the compounds as a good approximation to study hydrogen bonding in the complexes, since the local environment of the groups involved are similar to those in the fully developed structures. Figure S2 shows an overview of the FT-IR spectra of the three pristine materials [12].            Tables S1 to S4 show comparisons between the experimentally obtained maxima, and the values estimated by DFT, used to assign bands to individual contributions. Table S5 summarises the lengths of the hydrogen bonds obtained in the models.   Table S5. Lengths (Å) of the hydrogen bonds estimated in the models by DFT.

Theoretical IR spectra calculated by DFT
Figures S16 to S28 show the theoretical spectra obtained in the gas phase, for all the species modelled by DFT. Figure S16. IR theoretical bands obtained by DFT for dAZOi-2HB-sym in the gas phase.