Nonlinearity Enhancement by Solid-State Proton Transfer: A New Strategy for the Design of Nonlinear Optical Materials

Abstract

Theoretical considerations of the molecular hyperpolarizabilities (β) of donor-acceptor substituted aromatic molecules show that the transfer of a proton from the electron-donor group of an acid to the electron acceptor group of a base could increase β for both resulting ionic species relative to the corresponding neutral molecules. To take advantage of this, the cocrystallization of 4-nitrophenol with various organic bases was studied. Cocrystallization of 4-nitrophenol with 4-(dimethylamino)pyridine gives yellow crystals that display a strong second harmonic signal by the Kurtz and Perry powder test. The material crystallizes in the orthorhombic space group P2 1 2 1 2 1 and is composed of 4-(dimethylamino)pyridinium 4-nitrophenolate 4-nitrophenol (DMAPNP2). Theoretical calculations of the hyperpolarizabilities predict that the values of β are greater for the 4-(dimethylamino)pyridinium and 4-nitrophenolate ions than for the corresponding neutral molecules. A second noncentrosymmetric material has been grown from an equimolar melt of 4-hydroxypyridine and 4-nitrophenol. In this cocrystal, which forms in the noncentrosymmetric orthorhombic space group Pna2 1 , the pyridine derivative is present as the 4-pyridone tautomer. Though proton transfer occurs for only one of the two crystals presented here, the work with DMAPNP2 illustrates that new, highly efficient NLO materials can be designed by using acid/base chemistry.

Experimental and theoretical studies of the second- and third-order NLO properties of a semi-organic compound: 6-Aminoquinolinium iodide monohydrate

“…Evans et al [10] demonstrated the utilization of acid/base proton transfer to enhance the hyperpolarizabilities of the chromophores and our calculations show also an increase of the component β zzz upon protonation. If we consider the interaction between the cations and anions there is a further increase in the molecular NLO properties.…”

“…These systems exhibit high nonlinear optical efficiency in the visible region and good thermal and mechanical stability compared with those of purely organic crystals. Besides these advantages, it was demonstrated by Evans et al [10] that the proton transfer occurring in acid/base reactions, often used to synthesize these hybrid materials, can enhance the hyperpolarizabilities of the chromophores.…”

Growth and characterization of a nonlinear optical crystal—2,6-diaminopyridinium 4-nitrophenolate 4-nitrophenol (DAPNP)

“…In the past decades, many approaches have been developed to acquire noncentrosymmetric crystals [17][18][19][20] in which one way to include both ionic and hydrogen-bonding interactions in a system is to form ionic co-crystals from proton transfer between an organic acid and organic base [21][22][23]. 4-nitrophenolate (NP-) with a typical one-dimensional (1D) donor-acceptor p system and the proton transfer of phenolic OH with various organic and inorganic bases results in the increasing of hyperpolarizability of both species has been used to form a large number of noncentrosymmetric structures with the achiral counterions [24][25][26][27][28][29][30]. Pyridine is a good hydrogen-bond acceptor and a strong organic base.…”

Studies on the growth, optical, thermal and dielectric aspects of a proton transfer complex – Dimethyl amino pyridinium 4-nitrophenolate 4-nitrophenol (DMAPNP) crystals for non-linear optical applications

“…Moreover if a proton transfer between two separate NLO organic chromophores takes place and result in a non-centrosymmetric organic salt, then this could increase the hyperpolarisability of both species, provided one being an acid and second being a base. This first purposeful utilization of this strategy (proton transfer) to increase the hyperpolarizabilities of the chromophores was proposed by Evans and his co-workers [11] and they have reported the crystal structure and SHG signal test of the title compound dimethyl amino pyridinium-4 nitro phenolate 4-nitro phenol (DMAPNP). The title compound was synthesized and reported by Huang et al [9].…”