ISSN: 2595-8402

DOI:​​ 10.5281/zenodo.2529378





Masood Ayoub Kaloo1*, Bilal A Bhat1, Sheikh Abdul Majid1​​ and Murtaza Gani1​​ 

1Department of Chemistry, Govt. Degree College Shopian, 192303-Kmr




In this work,​​ molecular​​ receptor​​ (R)​​ which is a​​ Schiff​​ base​​ of naphthalene moiety tethered with diaminomalenonitrile (DAMN),​​ reported by Sankar et al.​​ (New J. Chem. 2014, 38, 923-926)​​ was studied​​ through density functional theory (DFT) and time-dependent density functional theory​​ (TD-DFT).​​ Absorption​​ spectra of neutral receptor (R) as well as​​ its deprotonated​​ form​​ (R)​​ were calculated using 6-311++G​​ (d,​​ p) basis set​​ in DFT/TD-DFT calculations.​​ Mullikan​​ charge distribution analysis was carried out in order to understand the​​ polarized nature of N-H bonds,​​ and​​ to locate the chemically significant regions​​ on the molecule.​​ Spectral modulations during anion recognition were attributed to the intramolecular charge transfer (ICT) enhancement and photoinduced electron transfer (PET) process.​​ The calculated​​ results were​​ further compared with the experimental data.


Key words:​​ Receptor, Fluoride, DFT, TD-DFT ICT, PET





Owing to crucial​​ role of cations and anions in​​ innumerable​​ process of biological, chemical and environmental significance,​​ momentous​​ attention has been paid towards​​ exploration and​​ development of effective​​ and​​ molecule based detection methods​​ “receptors”​​ [1-2]. Molecular receptors are valuable tools to perform ion recognition,​​ amid prompt selectivity and sensitivity. Most importantly, these are easy to prepare, operate and realize high sample through output. In this course, library of ion sensing receptors has been proposed [3-8]. While developing these receptors,​​ number of interactions, such as hydrogen bonding, electrostatic force, metal-ligand​​ coordination, hydrophobic and vander Waals forces have been taken onto consideration​​ [9-12].​​ In this course, tethering​​ of​​ N-H polarization based molecular functionalities in the form of amides, ureas,​​ thioureas;​​ ammonium, imidazole​​ and Imidazolium​​ on diverse molecules have been used and proposed, in addition to chemical reaction based methods​​ [13-20].

In the present study, one such newly explored receptor molecule (R) possessing​​ free and polarized ̶​​ NH2​​ motif,​​ was​​ bought under study. The​​ absorption spectroscopic analysis of receptor​​ R​​ and​​ its deprotonated form,​​ R​​ (Fig.​​ 1)​​ has been carried out theoretically and compared with the experimental. In addition, the mechanism of anion interaction has also been​​ worked out.​​ The combined use of DFT/TD-DFT (B3LYP) functional and standard basis sets 6-311++G(d,p) provides an excellent balance between accuracy and computational efficiency of absorption spectra. The literature survey reveals that, to the best of our knowledge, no intensive observation of theoretical [DFT/TD-DFT] investigation has been reported so far with this type of receptor​​ design possessing free –NH2​​ based recognition site. Therefore, the present investigation was undertaken to study the absorption spectra and inter and intra molecular interaction between HOMO and LUMO energy levels of the​​ newly explored molecular receptor.

Fig 1. Molecular structure of receptor R and​​ R-.



In the present work, one of the hybrid methods; Lee-Yang-Parr correlation function (B3LYP)​​ was carried out​​ by​​ basis sets 6-311++G(d,p) using GAUSSIAN 09W program. In DFT methods; Becke’s hybrid function combined with the B3LYP function​​ predict the best results of Mullikan charge density on N and H of amine (NH2) group,​​ and absorption spectra of​​ receptor (R) and its deprotonated form (R). The theoretical result of electronic absorption transitions was compared with the experimental.



Mullikan charge distribution analysis

The Mullikan​​ charge on the atoms is used to understand the charge distribution on the chemical bonding.​​ The analysis​​ facilitates positive and negative regions in the molecular space, at which the protons and electrons concentrate. Thus​​ chemically significant regions and bonds can be identified.​​ This also gives the description​​ of the​​ possible mechanism in the form of​​ electrophilic, nucleophilic substitutions, hydrogen bonding​​ or​​ proton​​ abstraction/transfer. Normally​​ the charges are distributed evenly over the molecule which​​ imparts​​ neutral​​ character to​​ the molecule. Whenever the substitutions are added to the molecule, the charge distribution is completely altered with respect to the substitution or the removal of any atom/moiety. Here in the case of​​ receptor​​ R, the negative Mulliken charges populated over the highly electronegative atoms (N-atoms) which splits the positive charges among hydrogen atoms. Hence the N and H of the molecule have negative and positive space respectively. The Mulliken charges of N and H atoms of amine (-NH2) of R​​ and​​ R​​ are presented in table 1.


UV–Vis spectra and molecular orbital analysis

​​ The UV–vis spectra and the electronic transitions of the receptor molecule and its deprotonated form​​ were​​ calculated using the TD-DFT/6-311++G​​ (d,​​ p) method. As it is clear from the figure 2, F​​ induced deprotonation​​ resulted in increase of electron density on the nitrile group of ICT channel​​ (D-π-A) in LUMO,​​ while as there is a decrease in electron density in HOMO. The​​ alteration​​ clearly indicate Intramolecular charge transfer (ICT) enhancement.​​ The ICT enhancement further results in the decrease of electron density over​​ naphthyl​​ moiety​​ ring in the LUMO of​​ R-​​ compared to its HOMO. This clearly indicates blockade of PET channel in the molecule after deprotonation. Hence fluorescence “turn on” could be understood in the molecule after fluoride addition due to ICT enahncement. The electron delocalization overall leads to​​ the decrease in the energy gap between HOMO and LUMO of molecule from 2.74​​ eV​​ to 1.39 eV​​ (Fig 2).​​ Hence the​​ presence of fluoride anion realizes spectral shift (red-shift),​​ thus​​ offering naked-eye optical detection of anion.

Fig 2. Excitation energies and HOMO and LUMO transitions of R and R.


The mechanism of recognition​​ can be explained in terms of the strong basicity of​​ F​​ in​​ the non-aqueous environments​​ (DMSO, THF, CH3CN, etc).​​ Under these conditions, F​​ interacts with polarized –N–H moiety of amine group in the receptor molecule. The recognition is accompanied by the transfer of proton from –NH2​​ to the​​ F.​​ The recognition​​ is further driven by the formation of the thermodynamically stable HF2​​ dimer​​ which was reported​​ by author during the course of the work [21].

Due to this deprotonation, receptor​​ R​​ in the presence of F​​ displayed​​ a red shift​​ both experimentally​​ as well as​​ theoretically.​​ In this study, the trends in spectral shift are as per experimental observations, but not empirically close with experimental​​ (Fig 3 and Table 2). This​​ may be because of solvent effect and the nature of interaction.​​ 





Fig.​​ 3​​ Theoretically​​ calculated electronic absorption spectra of R​​ (a)​​ and R-​​ (b).



In summary, for the first time​​ we have theoretically​​ studied​​ the structural and​​ spectral​​ aspects of​​ a naphthyl​​ moiety​​ based molecular receptor tethered with DAMN​​ fragment. For understanding​​ fluoride recognition mechanism and the nature of bonds in the receptor, negative and positive electronic regions were revealed on N and H atoms of the molecule with the help of Mullikan charge distribution analysis. The theoretical calculations (DFT/TD-DFT) of the receptor displayed red shift of absorption spectra after interaction with the F. The reduction in the energy gap (ΔE) between highest occupied and lowest unoccupied energy levels were also revealed, hence strong the red shift​​ of its​​ spectral​​ characteristics.​​ Most​​ importantly,​​ during the course of proton transfer of –NH2 to F, a decrease in the pi-electron density over napthyl moiety in the LUMO contrary to its​​ HOMO​​ revealed blockade of PET process and hence fluorescent turn on response.



We highly acknowledge department and lab staff and other faculty of chemistry at GDC Shopian for their discussions and support during various stages of the work. M. A Kaloo gratefully acknowledges Department of Science and Technology, New Delhi for INSPIRE FACULTY award [DST/INSPIRE/04/2016/000098].



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