VOLUME 1, NÚMERO 3, DEZEMBRO​​ DE 2018

ISSN: 2595-8402

DOI:​​ 10.5281/zenodo.2529378

 

 

COMPUTATIONAL STUDY OF NAPHTHYL BASED SCHIFF BASE MOLECULAR RECEPTOR: COMPARISON OF THEORETICAL WITH EXPERIMENTAL

 

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

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

makandchem@gmail.com

 

ABSTRACT

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

 

 

 

1INTRODUCTION

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-.

 

2COMPUTATIONAL METHOD

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.

 

3RESULTS AND DISCUSSION

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).

 

4CONCLUSIONS

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.

 

5 ACKNOWLEDGEMENTS

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].

 

6 REFERENCES

[1] H.N. Kim, W.X. Ren, J.S. Kim and J. Yoon. Fluorescent and colorimetric sensors for ​​​​ detection of lead, cadmium and mercury ions (2012) Chem Soc Rev 41: 3210.

[2] B. E. Kim, T. Nevitt and D. J. Thiele. Mechanisms for copper acquisition, distribution and ​​ ​​​​ regulation. (2008) Nat Chem Biol 4: 176.

[3] R. Uauy, M. Olivares and M. Gonzalez. Essentiality of copper in humans (1998) Am J Clin ​​​​ Nutr 67: 952.

[4] D Karak, S Lohar, A Banerjee, A Sahana, I Hauli,et al. Interaction of soft donor sites with a ​​ ​​​​ hard metal ion: crystallographically characterized blue emitting fluorescent probe for Al (III) ​​​​ with cell staining studies (2012) RSC Adv 2: 12447.

[5] P. de Silva, D. B. Fox, A. J. M. Huxley and T. S. Moody, Coord. Progress in Heterocyclic Chemistry: A Critical Review of the 2000 literature (2000) Chem Rev 205: 41.

[6] R. Martınez, A. Espinosa, A. Ta rraga and P. Molina. Bis (indolyl) methane derivatives as highly selective colorimetric and ratiometric fluorescent molecular chemosensors for Cu2+ cations (2008) Tetrahedron 64: 2184.

[7] AP de Silva, AJM Huxley and TS Moody. Combining luminescence, coordination and electron transfer for signalling purposes (2000). Coord Chem Rev 205: 41.

[8] V. Amendola, L. Fabbrizzi. Anion receptors that contain metals as structural units (2009) ​​​​ 

Chem Commun 513.

[9] A. Ajayaghosh, P Carol, S Sreejith. A Ratiometric Fluorescence Probe for Selective Visual Sensing of Zn2+ (2005) J Am Chem Soc 127: 14962.

[10] D Wang, Y Ke, H Guo, J Chen, W Weng. A novel highly selective colorimetric sensor for aluminum (III) ion using Schiff base derivative (2014) Spectrochim. Acta, Part A 122: 268.

[11] V Amendola, L Fabbrizzi, M Licchelli, C Mangano, P Pallavicini, Et al. Molecular events switched by transition metals (2000) Coord Chem Rev 192: 649.

[12] AP Silva, HNQ Gunaratne, T Gunnlaugsson, AJM Huxley, CP McCoy, et al. Signaling ​​​​ 

Recognition Events with Fluorescent Sensors and Switches (1997) Chem Rev 97: 1515.

[13] P. D. Beer and P. A. Gale. Anion Recognition and Sensing: The State of the Art and Future Perspectives (2001) Angew Chem 40: 486.

[14] C. Perez-Casas and A. K. Yatsimirsky, J. Detailing Hydrogen Bonding and Deprotonation Equilibria between Anions and Urea/Thiourea Derivatives (2008) Org Chem 73: 2275.

[15] FY Wu, ZC Wen, N Zhou, YF Zhao, YB Jiang. A novel thiourea based dual fluorescent anion receptor with a rigid hydrazine spacer (2002) Org Lett 4: 3203.

[16] J. M. Linares, D. Powell and J. K. Bowman, Coord. Chem. Rev 2003, 240, 57.

[17] X. J. Peng, Y. K. Wu, J. L. Fan, M. Z. Tian and K. L. Han, J. Org. Chem., 2005, 70, 10524.

[18] Chellappan, N. J. Singh, I. C. Hwang, J. W. Lee and K. S. Kim. A ​​ ​​​​ calyx [4]imidazolium[2]pyridine as an anion receptor (2005) Angew Chem 44: 2899.

[19] L. Yang, X. Li, J. Wang, Y. Qu and J. Hua. Colorimetric and Ratiometric Near-Infrared ​​​​ 

Fluorescent Cyanide Chemodosimeter Based on Phenazine Derivatives (2013) ACS Appl ​​​​ Mater Interface 5: 1317.

[20] J Ren, W Zhu and H Tian. A highly sensitive and selective chemosensor for cyanide (2008) ​​ ​​​​ Talanta 75: 760-764

[21] M. A. Kaloo and J. Sankar. A molecular Boolean mimic with OR, NOR, YES and INH functions: dual-ion recognition driven fluorescence ‘‘turn on’’ (2014) New J. Chem 38: 923.

 

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