ARTICLE

Solvent effects on nonlinear optical response of certaintetrammineruthenium(II) complexes of modified1,10-phenanthrolinesMuhammad Ramzan Saeed Ashraf Janjua, Asif Mahmood, and Farooq Ahmad

Abstract: In this research paper, we have reported solvent effects on nonlinear optical properties of tetrammineruthenium(II)complexes of modified 1,10-phenanthrolines. Polarizability and hyperpolarizability were calculated in the gas phase, benzene(� = 2.3), THF (� = 7.52), dichloromethane (� = 8.93), acetone (� = 21.01), methanol (� = 33.00), acetonitrile (� = 36.64), and water(� = 80.10) using density functional theory. These solvents cover a wide range of polarities. The results of theoretical investigationhave shown that nonlinear optical properties significantly increased with the increase of solvent polarity. Solvent stronglyaffected hyperpolarizability as compared with polarizability. Nonlinear optical properties were also changed by the change offunctional. Hyperpolarizability significantly changed with the change of functional as compared with polarizability. The resultsof this study indicate that like structural modification, polarity of the medium can significantly change the nonlinear opticalproperties.

Key words: solvent effects, nonlinear optics, hyperpolarizability, polarizability, DFT.

Résumé : Nous décrivons dans le présent article les effets du solvant sur les propriétés optiques non linéaires de complexestétraamine ruthénium(II) de 1,10-phénanthrolines modifiées. Nous calculons la polarisabilité et l’hyperpolarisabilité en phasegazeuse, ainsi que dans le benzène (� = 2,3), le THF (� = 7,52), le dichlorométhane (� = 8,93), l’acétone (� = 21,01), le méthanol(� = 33,00), l’acétonitrile (� = 36,64) et l’eau (� = 80,10) par la méthode de la théorie de la fonctionnelle de la densité. Ces solvantscouvrent une grande gamme de polarités. Les résultats des investigations théoriques montrent que les propriétés optiques nonlinéaires augmentent considérablement lorsque la polarité du solvant s’accroît. Le solvant affecte plus fortement l’hyperpolarisabilitéque la polarisabilité. Le changement de fonctionnelle modifie aussi les propriétés optiques non linéaires. Comparativement a lapolarisabilité, le changement de fonctionnelle modifie considérablement l’hyperpolarisabilité. Les résultats de l’étude indiquentque, comme la modification structurale, la polarité du milieu peut modifier appréciablement les propriétés optiques nonlinéaires. [Traduit par la Rédaction]

Mots-clés : effets du solvant, optique non linéaire, hyperpolarisabilité, polarisabilité, DFT.

IntroductionNonlinear optics (NLO) is the study of interaction of electromag-

netic radiations with matter, resulting in new electromagneticradiations with change in frequency, phase, or other physicalproperties.1 NLO materials are very important due to their poten-tial applications in optical and electrooptical devices.2

Recently, organometallic complexes have attracted significantattention due to good NLO properties.3 A large number of theo-retical studies have been carried out to design high-response NLOmaterials by the modification of donor, �-spacer, and accep-tor.4–14 The effect of metal variation has also been studied.15,16

Solvent effect on NLO properties was theoretically explored in afew studies.17–22

Theoretically, NLO properties have been mostly calculated inthe gas phase but experimentally, NLO properties are studied incondensed phases. The impact of solvent on NLO properties hasbeen proven experimentally. The nature of the solute−solventinteraction determines the extent of the influences of solvent. Inthe presence of solvents, the electronic and geometrical struc-tures of the solute molecule change. Different solvents form dif-

ferent configurations with solute molecules. Solvent changes theNLO properties of NLO material. The dramatic effect of condensedphases on optical properties of molecules has been well demon-strated.23–30 Quantum chemistry is an efficient and easy way tounderstand the solvent effect on NLO properties.

A low HOMO−LUMO energy gap is required for better NLO prop-erties.31 The energy gap between HOMO and LUMO can be reducedby increasing the conjugation. Structural modification is one wayto decrease the HOMO–LUMO gap. Use of polar solvents is anotherway to decrease the HOMO–LUMO gap.

In this study, we have used solvent polarity to decrease theHOMO–LUMO gap for the enhancement of NLO properties. Wehave studied the dependence of the polarizability and hyperpolariz-ability on medium polarity. This study was performed to understandthe change of NLO properties of certain tetrammineruthenium(II)complexes of modified 1,10-phenanthrolines with an increase of me-dium polarity. Density functional theory calculations were per-formed in the framework of the integral equation formalismversion of the polarizable continuum model (PCM).32 We selectedsolvents with a wide range of dielectric constants (0.00–80.10).Calculations were performed in the gas phase, benzene (� = 2.3),

Received 15 August 2013. Accepted 16 September 2013.

M.R.S.A. Janjua and A. Mahmood. Department of Chemistry, University of Sargodha, Sargodha, Pakistan.F. Ahmad. College of Chemical Engineering and Material Sciences, Zhejiang University of Technology, Zhejiang, China.Corresponding authors: Muhammad Ramzan Saeed Ashraf Janjua (e-mail: Dr_Janjua2010@yahoo.com, Janjua@uos.edu.pk) and Asif Mahmood(e-mail: asifmahmood023@gmail.com).

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Can. J. Chem. 91: 1303–1309 (2013) dx.doi.org/10.1139/cjc-2013-0377 Published at www.nrcresearchpress.com/cjc on 20 September 2013.

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THF (� = 7.52), dichloromethane (� = 8.93), acetone (� = 21.01),methanol (� = 33.00), acetonitrile (� = 36.64), and water (� = 80.10).These solvents cover a wide range of polarities.

Computational procedureAll computations were performed using the Gaussian 09 pro-

gram package.33 All systems were optimized using density func-tional theory. The three functionals B3LYP, CAM-B3LYP, and LC-BLYPwere used. Polarizability and hyperpolarizability were also calcu-lated using these functionals. The 6-311+g(d) basis set for sulfurand 6-31g(d) basis set for other nonmetal atoms were adopted. TheStuttgart/Dresden double-� (SDD34,35 basis set was used for theruthenium atom. It is a relativistic effective core potential thatreplaces the inner core 28 electrons, leaving the outer core 14 elec-trons (4s2 4p6 4d6) as the valence electrons for Ru(II).36 The B3LYPfunctional is a combination of Becke’s three-parameter hybrid ex-change functional37 and the Lee–Yang–Parr37 correlation functional.B3LYP is a widely used functional.36 Coulomb-attenuated CAM-B3LYP is quite successful among the class of new functionals.38 Itis successfully used to calculate NLO properties.20 Due to thesereasons, CAM-B3LYP has been used along with B3LYP. The long-range-corrected BLYP functional (LC-BLYP) is an efficient func-tional that can be used in small and large systems.39

To study the solute−solvent interactions, the choice of theoret-ical solvent model is critical. The continuum and discrete ap-proaches are two basic ways to study the solvent effects. The PCMbecomes a popular solvent model and has been applied to manyNLO systems.40–42 The PCM model treats the solvent environmentas a homogenous dielectric continuum medium characterized byits dielectric constant �, while the solute molecule inside a cavityis immersed in the medium.43

Average polarizability (�) is determined by considering only thediagonal elements:16

(1) � � 1/3(�xx � �yy � �zz)

Ten components (�xxx, �xxy, �xyy, �yyy, �xxz, �xyz, �yyz, �xzz, �yzz,and �zzz) are present in the Gaussian 09W output file. First hyper-polarizability (�tot) is calculated using following formula:16

(2) �tot � �(�xxx � �xyy � �xzz)2 � (�yyy � �xxy � �yzz)

2

� (�zzz � �xxz � �yyz)2�1/2

Results and discussionUnderstanding of NLO properties is very important in the de-

sign of materials for signal processing, communication technol-ogy, optical memory devices, and optical switches.

Most experimental measurements of NLO properties are carriedout in the condensed phase. The solvent environment changes theproperties of solvated molecules and sometime significantly in-fluences the dynamics of the processes. In the case of NLO re-sponse, the major influences due to the solvent environment are(i) change of wave function due to change of structure in theperturbing environment, (ii) change of geometrical structure ofthe molecule, (iii) change of response properties, (iv) change of theperturbing local field acting on the molecule, and (v) change of thedynamics of the excitation processes.44

This study was performed to understand solvent effect on NLOproperties of certain tetrammineruthenium(II) complexes ofmodified 1,10-phenanthrolines. All of the calculations were per-formed in the gas phase with different solvents. Structures ofsystems studied in this research work are given in Fig. 1.

Solvent effects on the HOMO−LUMO energy gapReactivity of molecules depends upon the frontier orbitals

HOMO and LUMO. The frontier orbital gap determines the chem-ical reactivity and kinetic stability of the molecule. A moleculewith a smaller frontier orbital gap is more polarizable and isgenerally associated with high chemical reactivity and low kineticstability and it is also termed a soft molecule.45 Moving fromapolar to polar solvent, the HOMO−LUMO energy gap decreased inall systems. Polar solvents stabilized both the HOMO and LUMOlevel, but stabilization of LUMO was greater. This means that the

Fig. 1. Structures of systems studied in the present research work.

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decrease in the energy of LUMO was greater than that in theHOMO energy. As a result, the energy gap between HOMO andLUMO became low.

HOMO–LUMO energy gaps of all systems calculated by B3LYP,CAM-B3LYP, and LC-BLYP functionals are given in supplementalTables 1, 2, and 3 (see Supplementary material section). From thetables, it is clear that HOMO−LUMO energy gaps decreased withincreasing medium polarity.

HOMO and LUMO compositions of all systems are given in Fig. 2.For systems 1, 2, and 3, HOMO and LUMO composition is the same.HOMO and LUMO composition of systems 4, 5, 6, and 7 is differ-

ent. This indicates the significant charge transfer in systems 4, 5,6, and 7.

Solvent effects on polarizabilityPolarizability is the ability of a molecule to be polarized. Polar-

izabilities determine the dynamical response of a bound system toexternal fields and provide insight into a molecule’s internalstructure. Polarizability increases with increasing solvent polar-ity. Solvent effect was evaluated for solvents of various polarities.Polarizability was calculated in the gas phase in different solventswith a wide range of polarities.

Fig. 2. HOMO and LUMO orbitals of the studied systems calculated using CAM-B3LYP.

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From Fig. 3, it is clear that the change of polarizabilityis directlyproportional to the polarity of the solvent. The trend of changeof polarizabilities is shown in Fig. 3. Polarizability of all systemsincreased with increasing medium polarity. Solvent polarity sig-nificantly changed the polarizability of system 6. Polarizability ofsystem 6 calculated by B3LYP, CAM-B3LYP, and LC-BLYP changedfrom 78.39 × 10−24 esu (gas phase) to 139.59 × 10−24 esu (water), from74.87 × 10−24 esu (gas phase) to 165.78 × 10−24 esu (water), and from54.21 × 10−24 esu (gas phase) to 138.80 × 10−24 esu (water), respec-tively. Increase in the polarizability of system 6 was 78%, 121%, and156% for B3LYP, CAM-B3LYP, and LC-BLYP, respectively. In thesame way, polarizabilities of other systems also significantlychanged with changing medium polarity. In an ab initio studyof solvent effect on the polarizability and hyperpolarizability ofp-nitroaniline, Mikkelsen et al.46 reported that values of polariz-ability increase about 18% in water solution compared with thegas phase. Bartkowiak and Lipinski reported a 9% increase for thepolarizability value in water compared with the gas phase.47

Discriminant analysis is performed to understand the solventeffects on polarizability. Discriminant analysis is a technique forclassifying a set of observations into predefined classes. It oper-ates on raw data and the technique constructs a discriminantfunction for each group.48 Discriminant function is a set of linearcombinations of the variables of the variables whose values are asclose as possible within groups and as far apart as possible be-tween groups. A simple linear discriminant function transformsan original set of measurements on a sample into a single discrim-inant score.49 A discriminant analysis plot is shown in Fig. 4 andfactor weightings are given in supplemental Table 7. From theFig. 4, it is clear that the circles of the solvents that have closedielectric constant values are closer. The figure clearly demon-

strates the difference in the polarizabilities calculated in differentmedia. System 6 showed high polarizability in the gas phase andall solvents for three functionals. The higher polarizability of sys-tem 6 can be explained on the basis of significant charge transferand better delocalization of �-electrons. HOMOs are present onthe donor side, while LUMOs are present on the acceptor side.Different HOMO and LUMO compositions indicate a significantcharge transfer.

Polarizabilities of systems calculated by B3LYP are found infollowing order: system 6 > system 4 > system 3 > system 2 > sys-tem 7 > system 5 > system 1, while the order of polarizabilities ofsystems calculated by CAM-B3LYP is system 6 > system 7 > system4 > system 5 > system 2 > system 3 > system 1, but the order ofpolarizabilities of systems calculated by LC-BLYP is system 6 > sys-tem 7 > system 4 > system 5 > system 3 > system 2 > system 1.These orders are given according to the trend shown in Fig. 3. Theorder of polarizabilities of systems calculated by CAM-B3LYPandLC-BLYP is quantitatively similar, but the order of polarizabilitiesof systems calculated by B3LYP is different from that calculated byCAM-B3LYP and LC-BLYP. This means that CAM-B3LYP and LC-BLYPhave produced qualitatively the same results. Discriminant anal-ysis was also performed to determine the difference between thepolarizabilities of systems calculated by the three different func-tionals. A discriminant analysis plot is shown in Fig. 5 and factorweightings are given in supplemental Table 7. From the Fig. 5, it isclear that polarizabilities of systems calculated by CAM-B3LYP andLC-BLYP are quite similar, while polarizabilities calculated bythese functionals are different from those calculated by B3LYP.

Solvent effects on hyperpolarizability�tot (� total), �vec (� vector), and �� (� parallel) are the types of

hyperpolarizabilities that have been discussed in the literature. In

Fig. 2 (concluded).

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this investigation, we have reported hyperpolarizabilities as �tot(reported in 10−30 esu). The �tot is calculated by three functionals,B3LYP, CAM-B3LYP, and LC-BLYP. Results are given in supplemen-tal Tables 9, 10, and 11.

Solvent polarity can affect the � value by modifying the electrondistribution within the conjugated bridge between the electrondonor and acceptor groups in the molecules, thereby affecting thedegree of bond length alternation.50

The change of hyperpolarizability with solvent polarity is de-picted in Fig. 6. From the figure, it is clear that hyperpolarizabilitysignificantly changed with solvent polarity. For the hyperpolariz-abilities calculated by B3LYP, system 3 showed maximum changemoving from the gas phase to water (368.98 × 10−30 to 1134.40 ×10−30 esu) (280% increase), while in the case of hyperpolarizabili-ties calculated by CAM-B3LYP, system 2 showed maximum change

moving from the gas phase to water (265.99 × 10−30 to 1273.40 ×10−30 esu) (380% increase), but in the case of hyperpolarizabilitiescalculated by LC-BLYP, system 7 showed maximum change mov-ing from the gas phase to water (206.32 × 10−30 to 1304.45 ×10−30 esu) (533% increase). From the results, it is clear that solventis a key factor like structural modification. Solvent can be used asan efficient tool to increase the NLO response. Change of solvent isvery easy as compared with the structural modification. The effectof solvent polarity on hyperpolarizability was more prominent ascompared with the polarizability. These results also indicate thathyperpolarizability is significantly affected by functional. Bart-kowiak and Lipinski have also reported a significant increase inhyperpolarizability of p-nitroaniline in water compared with thegas phase.47 Strong electrostatic (dipole–dipole) interactionscompared with hydrogen bond interactions in water solution areresponsible for the large change in hyperpolarizability.51

Discriminant analysis is used to study solvent effects on hyper-polarizability. Factor weightings are given in supplemental Ta-ble 12. A discriminant analysis plot is shown in Fig. 7. From thefigure, it is clear that hyperpolarizability significantly changedwith solvent polarity. The circles for different solvents are atgreater distance as compared with that in the case of polarizabil-

Fig. 3. Trend of polarizability (�) of seven systems in differentsolvents calculated by (a) B3LYP, (b) CAM-B3LYP, and (c) LC-BLYP.

Fig. 4. Discriminant analysis plot of polarizability calculated indifferent media.

Fig. 5. Discriminant analysis plot of polarizability calculated usingB3LYP, CAM-B3LYP, and LC-BLYP.

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ity. This indicates that hyperpolarizability is more sensitive tosolvent as compared with polarizability.

For all systems (except system 6), hyperpolarizability increasedwith increasing solvent polarity. Change of hyperpolarizabilitywith the change of solvent polarity is depicted in Fig. 6. The rrderof hyperpolarizabilities calculated using B3LYP is system 3 > sys-tem 2 > system 4 > system 6 > system 5 > system 7 > system 1,while the order of hyperpolarizabilities calculated using CAM-B3LYP is system 2 > system 3 > system 4 > system 5 > system7 > system 1. Hyperpolarizabilities calculated using CAM-B3LYPare found in the following order: system 7 > system 5 > system4 > system 2 > system 3 > system 1. In the order of hyperpolariz-abilities calculated using CAM-B3LYPand LC-BLYP, system 6 is notgiven because hyperpolarizability of system 6 increased up to THFand then started to decrease up to water. It did not show a smoothtrend.

From Fig. 6, it is clear that hyperpolarizability is also sensitive tofunctional. Different functionals produced different results. Thetrend of change of hyperpolarizability is different for the threefunctionals, but in the case of polarizability, the effect of func-tional was minimum. To understand the effect of functional onhyperpolarizability, discriminant analysis is performed. A dis-

criminant analysis plot is shown in Fig. 8. The circles of the func-tionals are at a longer distance from each other. This indicatesthat hyperpolarizability is significantly changed with functional.A plot of discriminant analysis is shown in Fig. 8. From the figure,it is clear that the three functionals produced different results.From the figure, it is clear that hyperpolarizability is more sensi-tive to functional as compared with polarizability. Hyperpolariz-abilities of all systems calculated by B3LYP and CAM-B3LYPshowed almost the same trend for all systems (except system 6).For system 6, hyperpolarizabilities calculated by B3LYP are low ascompared with those calculated by CAM-B3LYP and LC-BLYP.

ConclusionThe results of the theoretical investigation showed that NLO

properties strongly increased with increasing solvent polarities.Like the structural modification, change of solvent polarity isanother way to enhance the NLO properties. Solvent stronglyaffected the hyperpolarizability as compared with polarizability.NLO properties are also sensitive to the choice of functional. Hy-perpolarizability significantly changed with the change of func-tional as compared with polarizability.

Supplemental materialSupplementary material for this article is available on the journal

Web site at http://nrcresearchpress.com/doi/suppl/10.1139/cjc-2013-0377.

Fig. 7. Discriminant analysis plot of hyperpolarizability calculatedin different media.

Fig. 8. Discriminant analysis plot of hyperpolarizability calculatedusing B3LYP, CAM-B3LYP, and LC-BLYP.

Fig. 6. Trend of hyperpolarizability (�) of seven systems in differentsolvents calculated by (a) B3LYP, (b) CAM-B3LYP, and (c) LC-BLYP.

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