Pyridinium complexes supported on a polymeric biomaterial : Synthesis of NLO-phores at molecular level

It is known that pyridinium compounds such as betaines have interesting electronic characteristics including NLO properties. The main strands of this work include electronic modifications on pyridinium cation. We carried out a novel easy synthesis and characterization of new anchored pyridinium moieties into a polymeric backbone. Moreover, DFT calculations have been performed on suitable model to explore molecular parameters involved in lineal and non-lineal optical properties.


Introduction
Pyridinium salts are very useful synthetic building blocks to obtain substituted pyridines, dihydropyridines or piperidines.Furthermore, pyridinium dyes present multiple application fields including biological 1 and optical responses due to their photophysical features, in particular, high fluorescence, charge transfer character and solvatochromic properties. 2 Different methods are available for the synthesis of pyridinium salts depending on the symmetry around the cation.One of them involves the use of pyrylium salts as precursor. 3These salts are cationic organic molecules with trivalent oxygen in a six member aromatic rings. 4The factors that govern the reactivity of these cations have been previously reported. 5 increased number of compounds with NLO properties have been reported based on inorganic salts, organic zwitterions or push-pull molecules. 6Linear and non-linear optical behaviours are controlled by different parameters at both macro-and microscopically level.At molecular level is necessary to know parameters such as dipole moment, polarizatility, first-order hyperpolarizability, among others.
On the other hand, chitosan as polymeric support has a wide working window in numerous areas of everyday life.This polymer was obtained from agroalimentary wastes of chitin crab shells and its transformation leave high added value products.
In this communication, we have focused on synthesis and characterization of a new anchored pyridinium moiety into a polymeric backbone.Moreover, DFT calculations have been performed on suitable model to explore molecular parameters involved in lineal and non-lineal optical properties.

Experimental Methods
All chemicals were purchased and used without further purification.Evaporations were conducted under reduced pressure.TLC was performed on silica gel plates (DC-Alufolien F254, E. Merck).The synthesis of pyrylium tetrafluorobate from aldehydes and ketones was previously carried out 3 and detection of compounds was accomplished with UV light (254 nm) and by charring with H 2 SO 4 and characterization with NMR spectroscopy and Mass spectrometry.
For the synthesis of pyridinium salts, reaction between different pyrylium salts and amines were carried out by using acetic acid as catalyst.

Results and discussion
For generating the NLO-phore units on the polymer backbone is previously necessary to obtain the corresponding pyrylium complexes.These pyrylium salts are employed for reacting with the biopolymer.The substituents were chosen for modifying the electronic properties of the Chitosan derived system.
The synthesis of compounds 1-3 was carried out directly from aromatic aldehydes and ketones mediated with BF 3 .Et 2 O (Scheme 1).After 24 h, fluorescent pyrylium salts were obtained with moderate yields.

Scheme 1. Synthesis of pyrylium salts from aldehydes and ketones
As previously described, 7 subsequent nucleophilic attack of the Chitosan amino groups to these pyrylium compounds gave the pyridinium complexes (4-6) supported on Chitosan as polycationic biomaterial (Scheme 2).Chitosan used for the synthesis presents a degree of deacetylation (DD) of 84 % and a molecular weight (M w ) of 87875 gmol -1 determined by HPLC-SEC.In the case of compound 6, the counter-anion acetate was confirmed by 1 H NMR.
The obtained compounds were characterized by both 1 H and 13 C NMR. 1 H NMR spectra show new protons in the range 7.89-6.84ppm corresponding to the aromatic moiety introduced into Chitosan backbone.On the other hand, characteristic protons at 4.93 and 3.24 ppm are assigned to H-1 and H-2 of the carbohydrate moiety, respectively, were also observed.
Scheme 2. Synthesis of NLO-phore (4-6) from pyrylium salts and chitosan For studying the NLO properties at molecular level, we performed DFT calculations at B3LYP level of theory (Table 1).Ideally, a good NLO-phore must have moderate dipole moment (µ) and highly first-order hyperpolarizability (β).All results obtained were compared with p-nitroaniline (PNA) as a simple model with non lineal optical properties.

Furthermore, a
decrease of β value (7.08 × 10 -30 esu) with fluorine atoms at 2,6 positions is observed in compound 5. Conclusions Synthesis of Chitosan pyridinium complexes (4-6) were easily carried out from pyrylium salts previously synthesized.Chitosan provides an environment noncentrosymmetric and thermal and chemical stability.By means of quantum calculation NLO parameters for model compounds have been explored.Taking into account the results obtained, compounds 4 and 6 proved to have better NLO properties than pnitroanilene.The best NLO-phore studied turned to be compound 6 (R 1 = R 3 = F; R 2 = OMe) with a high polarizability and hyperpolarizability values.