Optics Communications 293 (2013) 119–124

Contents lists available at SciVerse ScienceDirect

Optics Communications

0030-40

http://d

n Corr

E-m

journal homepage: www.elsevier.com/locate/optcom

Nonlinear optical characterizations of dibenzoylmethane in solution

Yashashchandra Dwivedi, Gabriel Tamashiro, Leonardo De Boni n, Sergio C. Zilio

Instituto de Fısica de S ~ao Carlos, Universidade de S ~ao Paulo, Caixa Postal 369, 13560-970 S ~ao Carlos, SP, Brazil

a r t i c l e i n f o

Article history:

Received 9 October 2012

Received in revised form

22 November 2012

Accepted 23 November 2012Available online 10 December 2012

Keywords:

Femtosecond Z-scan

Two-photon absorption cross-section

First hyperpolarizability

Hyper-Rayleigh scattering

18/$ - see front matter & 2012 Elsevier B.V. A

x.doi.org/10.1016/j.optcom.2012.11.065

esponding author.

ail address: deboni@ifsc.usp.br (L.D. Boni).

a b s t r a c t

This work reports on the two-photon absorption (2PA) cross-section and first hyperpolarizability of

dibenzoylmethane solutions using femtosecond Z-scan and hyper-Rayleigh scattering techniques. The

2PA spectrum, spanning the wavelength range from 460 to 740 nm, presents a band centered at

510 nm, with a cross-section value estimated as 37 GM at this wavelength. Owing to the molecular

symmetry, this band is not observed in the linear absorption spectrum. The sum-over-state approach

was adopted to evaluate various spectroscopic parameters. Experimental and theoretical values of the

first hyperpolarizability values were estimated in ethanol and DMSO solutions.

& 2012 Elsevier B.V. All rights reserved.

1. Introduction

The dibenzoylmethane (DBM) molecule has attracted consider-able attention because of its application in a variety of fields,especially in biomedicine and display devices [1]. DBM is a memberof naturally occurring compounds that belong to the b-diketonefamily, exhibiting enol and kito tautomeric forms. Quantum che-mical calculation of electronic spectra of both forms were alreadystudied in the past [2,3]. It was shown to inhibit the growth ofvarious types of cancer cells in vitro and to prevent the carcinogen-esis in various animal models. In particular, it inhibits the carcino-genesis of mammary glands induced by the well-known carcinogen7,12-dimethylbenz[a]anthrance, both in vivo and in vitro [4–6].When compared to curcumin, which also belongs to the b-diketonefamily, DBM lacks the phenolic hydroxyl groups and the reducibleunsaturated alkyl groups. As a result of these structural modifica-tions, dibenzoylmethane shows very small antioxidant abilityin vitro and is well absorbed and distributed in tissues in vivo

[7,8]. Marin et al. [9] already discussed the photochemical andphotophysical properties of DBM derivatives within proteins. How-ever, the application of this molecule in photodynamics therapy islimited by its absorption in the ultraviolet (UV) region only, whichis far from the ‘‘therapeutic window’’ region (between 600 and800 nm), where biological tissues are relatively transparent. Hence,light cannot penetrate deeply into the tissues, thus limiting itsapplication to superficial lesions. Additionally, the distribution ofDBM inside the cell cannot be tracked by linear microscopy. Onesolution to this problem is to use a 2PA process, which is a well

ll rights reserved.

established technique used in applications such as: two-photonpolymerization [10], three-dimensional optical data storage [11],fluorescence excitation microscopy [12], optical limiting [13]and two-photon photodynamic therapy [14].

Considering the abovementioned properties of the DBM mole-cule, it seems worth to investigate its nonlinear optical propertiesin detail. It possesses considerable planarity in the ground-state,suggesting good p-electron delocalization due to the ability ofperforming intramolecular charge transfer [15,16], thus acting as apush–pull compound. However, the nonlinear optical characteriza-tion of dibenzoylmethane in solutions was not reported, except forthe strong second harmonic generation observed by Li et al. inlanthanide complexes containing DBM [17] and the calculation ofthe first hyperpolarizability performed by Wostyn et al. in lantha-nate complexes with DBM [18].

The present work reports on the nonlinear optical characteriza-tion of the DBM molecule in solutions. The 2PA cross-section andthe first hyperpolarizability were determined through Z-scan andhyper Rayleigh scattering measurements. Theoretical calculationsof the hyperpolarizability were also carried out in the gas phaseand with solvents for comparison with the experimental data.

2. Experimental

Dibenzoylmethane of analytical purity was purchased fromSigma-Aldrich. Spectroscopic grade ethanol and dimethyl sulf-oxide (DMSO) were used as solvents. Solutions with differentconcentrations were prepared and ultrasonicated for 10 minbefore performing the experiments in order that the propermixing and thermodynamic equilibrium could be established

240 270 300 330 360 390 4200.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

2PA wavelength (nm)

1PA wavelength (nm)

Abs

orba

nce

2PA

Cro

ss-s

ectio

n (G

M)

480 540 600 660 720 780 840

0

10

20

30

Fig. 1. Absorbance (A¼� log10(I/I0), solid line) and 2PA cross-section (open

circles) spectra of DBM in ethanol solution. The dotted line shows the 2PA

spectrum calculated with the sum-over-state approach. The linear absorbance

spectrum of DBM in DMSO solution is given by the dashed line.

Y. Dwivedi et al. / Optics Communications 293 (2013) 119–124120

between the two existing tautomeric forms [19]. The solutionsobtained in this way were colorless and transparent.

Absorption spectra in the UV/vis region were measured with aShimadzu UV-1800 spectrometer, with a sample concentration of4.3�1014 molecules/cm3 (7.2�10�7 mol L�1), while nonlinearabsorption measurements were performed in solutions contain-ing 4.3�1019 molecules/cm3 (7.2�10�2 mol L�1). Both linearand nonlinear measurements were carried out with the solutionsplaced in 2 mm-thick silica cuvettes.

The 2PA cross-section spectra were obtained with the open-aperture (OA) Z-scan technique [20] with 120 fs pulses from anoptical parametric amplifier pumped by a Ti:Sapphire chirpedpulse amplified laser system (775 nm and 1 kHz repetition rate),allowing wavelength tuning from 450 to 800 nm. The OA Z-scanmethod basically consists in translating the sample through thefocal plane of a Gaussian beam and monitoring changes in thesample’s transmission. To ensure a Gaussian profile of the laserbeam, spatial filtering was performed before the Z-scan setup. Thebeam waist radius at the focus was measured to be in the rangefrom 15 to 1771 mm, and the laser pulse energies varied from 50to 100 nJ, depending on the excitation wavelength. If an absorp-tive nonlinearity is present, the light field induces an intensitydependent absorption, a¼a0þbI, where I is the laser beamirradiance, a0 and b are the linear and nonlinear absorptioncoefficients, respectively. For a positive b value, the inducednonlinear absorption increases as the sample approaches thefocal plane and the measured transmittance presents a diparound this position. If the sample is transparent (a0¼0) thisdip is related just to the 2PA process and any population effect,such as reverse saturable absorption, can be neglected. Thisprocedure generates an absorption Z-scan signature, and themagnitude of the nonlinear process can be extracted from it. Z-scan curves were obtained for each excitation wavelength.Assuming a Gaussian temporal profile, the normalized energytransmittance T (z) can be expressed as [20]:

T zð Þ ¼1ffiffiffiffi

pp

q0 z,0ð Þ

Z 1�1

ln½1þq0 z,0ð Þe�t2

�dt ð1Þ

where q0 z,0ð Þ ¼ bI0Lef f ð1þZ2=Z20Þ�1, Leff¼[1�e�a0

L]/a0, L is thesample thickness, z0¼kw0

2/2 is the Rayleigh length, w0 is the beamwaist, k¼2p/l is the wave vector, l is the laser wavelength, z is thesample position, and I0 is the on-axis irradiance at the focus (z¼0).This expression must be used in the thin-sample approximation.However, it can also be employed when the sample thickness isclose to the Rayleigh range because in this case the results differonly by a few percent from those obtained using the thick-sampletheory [21] (the differences are generally smaller than the experi-mental error). That is the case in the present experiments. The 2PAcoefficient b can be determined by fitting the Z-scan curves with Eq.(1). We have used the CGS unit system for all calculations. The 2PAcross-sections can be determined using d¼hnb/N, where hn is theexcitation energy and N is the number of molecules per cm3.Usually, d is expressed in Goppert–Mayer (GM) units, where1 g¼1�10�50 cm4 s molecule�1 photon�1 [22].

For characterization of the first hyperpolarizability, hyper-Rayleigh scattering (HRS) experiments were conducted by usinginfrared (1064 nm) pulse trains containing approximately 20pulses of 100 ps separated by 13 ns, delivered by a Q-switchedand mode-locked Nd:YAG [23]. As the complete Q-switch pulsetrain was used to pump the samples, each measurement involvedseveral different intensities of the mode-locked pulses present inthe Q-switch envelope. To avoid noise and other than hyper-Rayleigh contributions (e.g. solvent ionization) the maximumintensity was kept below the ionization threshold, by meansof two crossed polarizers. To improve the detection efficiency,the scattering point is located between a spherical mirror and a

telescope, and the signal is collected by a Hamamatsu H5783Pphotomultiplier with a 3 ns rise time. Furthermore, a referencesignal for the laser intensity is collected by a PIN detector with aresponse time of 1 ns. The first hyperpolarizability was calculatedby using the external reference method, where p-NA (solute) inDMSO (solvent) was employed as reference. Detailed descriptionof the experimental arrangement can be found in Ref. [23].

To confirm the magnitude of the HRS signal, quantum chemicalcalculations were performed. Initial geometry of the dibenzoyl-methane molecule generated from standard geometrical para-meters was minimized without any constraint in the potentialenergy surface using DFT with Becke-3-Lee-Yang-Parr (B3LYP) [24]level, adopting the 6�31þG atomic basis set. This geometry wasthen re-optimized at B3LYP level, using basis set 6�311þG*, forbetter description of geometry. These calculations were carried outusing the GAUSSIAN 03 [25] program package. From these struc-tures, the energy of the excited states and the dipole moment werecalculated in a TD-DFT approach from the CIS optimized structure.The optimized geometry obtained from ab initio/DFT geometry wasused as an input for DFT/PCM calculation to estimate the compo-nents of the hyperpolarizability tensor [26].

3. Results and discussion

3.1. Absorption

The absorbance spectrum of the DBM in ethanol and DBMsolutions (7.2�10�7 mol L�1), shown in Fig. 1, presents twobands; a strong one at 347 nm, which is ascribed due to p-p*transition in the CO conjugated ethylene system (enol form), andanother at 240 nm, assigned to the benzene ring (keto form). Theone-photon absorption (1PA) cross-section of the DBM moleculeat 388 nm was estimated to be 1.06 and 5.3�10�16 cm2 inethanol and DMSO solutions, respectively. It is obvious from theabsorption spectrum that for wavelength longer than 440 nm, thesample is transparent.

For 2PA measurements, the highly concentrated (7.2�10�2 molL�1) ethanol solution was used but no aggregation was observed.

0

10

20

30

40

2PA

S3S2

S1

S0

Ener

gy( x

103 c

m-1

)

1PA

Fig. 3. Energy-level-diagram for DBM used to fit the SOS model. The solid lines

correspond to allowed transitions.

Y. Dwivedi et al. / Optics Communications 293 (2013) 119–124 121

The 2PA cross-section spectrum, shown as open circles in Fig. 1, wasdetermined with the aid of Eq. (1) to fit the Z-scan measurements ofFig. 2 at several wavelengths. These Z-scan signatures present adecrease in the normalized transmittance at the focal volume,corresponding to a 2PA process, once the sample is transparent(non-resonant regime). Fig. 2 also displays curves of normalizedtransmittance change versus irradiance for excitation wavelengthsof 500, 510 and 470 nm. The linear dependence between normalizedtransmittance and input irradiance reveals the 2PA nature of theprocess.

Comparing the two spectra of Fig. 1, one concludes that theyoriginate from different electronic transitions. The absorbancespectrum presents two bands at 240 and 347 nm, while the 2PAspectrum exhibits just one band at 510 nm, corresponding to anenergy level at 255 nm, with 2PA cross-section of about 37 GM.These different spectra are expected owing to the centrosym-metric nature of the DBM molecule. The selection rules forcentrosymmetric molecules allow 1PA between states with dis-tinct parities, while a 2PA process is allowed only between stateswith the same parity. This enables the visualization of thosestates that are invisible to 1PA, like in the present case. On thebasis of the 1PA and 2PA spectra, an energy-level-diagram can beconstructed, as depicted in Fig. 3. The knowledge of the two-photon allowed states enables to model the experimental 2PAcross-section spectrum using the simplified sum-over-states(SOS) approach.

In the SOS approach, the 2PA cross-section can be expressedas [27]:

d nð Þ ¼ 4ð2pÞ4

5ðnhcÞ2L4 n2

n01�nð Þ2þG2

01

�9m019

29m1292G02

n02�2nð Þ2þG2

02

þ9m019

29m1392G03

n03�2nð Þ2þG2

03

!ð2Þ

where c is the speed of light in cm s�1, L is the local field correctionfactor, h is the Planck constant, n0x and G0x represent, respectively, thetransition frequency and the damping constant of the | 04-|x4transition. The transition dipole moment between the ground and thefirst-excited states can be estimated using the relation [28]:

m01 ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3

8p2

cffiffiffiffipp

N

ln 10A� �

L

o2

ffiffiffiffiffiffiffiffiffiffiffilnð2Þ

p h

nof

vuutð3Þ

where c is the speed of light in cm s�1, A is the absorbance, o is thefull width at half maximum of the band (in Hz), h is the Planckconstant, nof is the frequency in Hz of the respective transition, N is

-0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

0.80

0.85

0.90

0.95

1.00

500 nm

Z (cm)

520 nm 510 nm

Nor

mal

ized

Tra

nsm

ittan

ce

Fig. 2. (a) Experimental open aperture Z-scan curves for DBM in ethanol solution ex

theoretical fit obtained in Eq. (1). (b) Normalized transmittance as a function of the in

the number of molecules per cm3 and L is the optical path length incm. The value of m01 is found to be 3.7 Debye. Using the simplifiedSOS model, the values of transition dipole moments between the firstand higher states, m12 and m13, are 0.9 and 0.6 Debye; the dampingconstants G01, G02 and G03 were found to be 6.8, 6.5, and 6.1(�1013 Hz); and transition frequencies n01, n02 and n03 are 0.87,1.2 and 1.3 (�1015 Hz) respectively.

3.2. First hyperpolarizability

Many organic molecules containing conjugated electrons arecharacterized by large values of molecular first hyperpolarizabil-ity. We have measured the first hyperpolarizability using the HRS,where the intensity of the scattered light is proportional to thesquare of the incident light intensity, according to [29,30]:

Ið2oÞ ¼ G rsvb2svþrstb

2st

� �I2ðoÞ ð4Þ

where I(o) and I(2o) are, respectively, the irradiance of theincident light and the light scattered at the double of thefrequency of the incident light and G is a geometrical factor. Themolecular density (r) and first hyperpolarizability (b) of thesolvent and solute are represented as rsn, rst and bsv, bst,

Nor

mal

ized

Tra

nsm

ittan

ce

0 20 40 60 80 100 120 140 160

0.80

0.85

0.90

0.95

1.00

510 nm 500 nm 470 nm

Irradiance (GW/cm2)

citing with 500, 510 and 520 nm at a power of 0.08 mW. The solid lines are the

put irradiance for different excitation wavelengths.

Y. Dwivedi et al. / Optics Communications 293 (2013) 119–124122

respectively. By keeping the solvent density constant and varyingthe solute density, a linear behavior of I(2o)/I2(o) versus rst isobserved, with an angular coefficient of Gb2

st. Fig. 4 shows the dataacquired for DBM in ethanol and DMSO, and the respective lineardependences on the solute concentrations.

Since DBM molecules do not absorb at 532 nm, the scatteredlight does not require any intensity correction. We observed thatthe noise is somewhat higher in the case of ethanol, probablybecause it is more volatile and ionizes easier than DMSO, for agiven laser irradiance.

The first hyperpolarizability can be calculated by taking thesquare root of the ratio between the angular coefficients (a) of theDBM sample and that of the reference, and multiplying by thetabulated value of the first hyperpolarizability of the reference(26.2�10–30 cm5/esu for pNA in DMSO) [31]:

bsample ¼

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiasample

aref erence

sbref erence ð5Þ

The calculated experimental values are presented in Table 1 asbexp.

In order to compare the experimental results with the theore-tical values obtained from quantum chemistry calculation, wemade the following procedure. The static first hyperpolarizabilityof the DBM molecular system was calculated by using the semi-empirical quantum chemical method, based on the finite-field

0.0

0.5

1.0

1.5

2.0

0.0 3.5 7.0

14 21 28 35 42

0.8

1.2

1.6

2.0

2.4

I(2ω

) (ar

b. u

nits

)

I(ω) (arb. units)

Ethanol

Fig. 4. Hyper-Rayleigh scattering signals for DBM in ethanol and in DMSO solutions

quadratic coefficient and the concentrations of DBM in ethanol (left) and DMSO (right

Table 1

Theoretically calculated tensor components and first hyperpolarizability (bth(2o))

in vacuum and in solutions (ethanol and DMSO). Experimental first hyperpolariz-

ability (bexp(2o)) measured in these solutions are also given. The unit of b is

10�30 esu.

Vacuum Ethanol DMSO

bxxx 1.224 652.26 683.46

bxxy 0 0.002 0.048

bxyy �0.588 �118.65 �119.67

byyy 0 0 0

bxxz 1.827 567.4 583.69

bxyz 0 0.013 0.03

byyz 0.343 68.97 70.38

bxzz 0.372 205.58 215.5

byzz 0 �0.04 �0.06

bzzz �2.758 �1068.2 �1108.5

bthe 1.2 7.2 7.3

bexp 7.070.9 8.870.8

approach at the DFT level. The first hyperpolarizability is a third-rank tensor that can be described by a 3�3�3 matrix. The 27components of this matrix can be reduced to only 10 componentsdue to the Kleinman symmetry [32]. The components of b aredefined as the coefficients in the Taylor series expansion of theenergy in the external electric field. When the electric field isweak and homogeneous, this expansion becomes:

E¼ E0�X

i

miFi�

1

2

Xij

aijFiFj�

1

6

Xijk

bijkFiFjFk�

1

24

Xijkl

nijklFiFjFkFl

þ

ð6Þ

where E0 is the energy of the unperturbed molecule; F is the fieldat the origin; and mi, aij, bijk, and gijkl are the components of dipolemoment, polarizability, the first and second hyperpolarizabilitiesrespectively. The cartesian components of the first-order hyper-polarizability can be written as:

bi ¼ biiiþ1

3

Xia j

bijjþbjijþbjji

� �ð7Þ

and are used to estimate the static hyperpolarizability:

b¼ b2xþb

2yþb

2z

� �1=2ð8Þ

According to the two-state model [33], the first hyperpolariz-ability can be expressed as:

b 2oð Þ ¼3m2

egDmeg

E2eg

�o2

eg

1�4o2=o2eg

� �o2

eg�o2� � ð9Þ

where meg, Eeg, and oeg, are the dipole moment, energy andfrequency of the transition between the ground-state 9g4 andthe excited-state 9e4 . The first term in the right-hand side is thestatic hyperpolarizability, b(0), which can be theoretically calcu-lated, while the second factor is a correction that takes intoaccount resonance enhancement effects due to the optical dis-persion. In order to obtain the second factor correction for thefirst hyperpolarizability of DBM molecule, we have taken intoaccount the solvent effect to perform the quantum chemicalcalculation, in presence of ethanol and DMSO solvents. Beforeperforming the calculations, energy optimizations was carried outwith the self-consistent reaction field (SCRF) methodology, basedon the IEFPCM (integral equation formalism of polarized con-tinuum solvation method) model [33,34], defining ethanol andDMSO as solvents. The calculated values of b for isolated DBMmolecules (vacuum) and those corrected including dispersion

0.0 0.3 0.6 0.9

5 10 15 20 25

0.6

1.2

1.8

I(ω) (arb. units)

DMSO

at different concentrations. The insets show the linear dependence between the

).

Y. Dwivedi et al. / Optics Communications 293 (2013) 119–124 123

effect (solution) are given in Table 1, where one can see that thevalues experimentally measured and theoretically calculated arein good agreement.

The value of the hyperpolarizability indicates the extent of theoptical nonlinearity of the molecular system. It is usually associatedto intramolecular charge transfer, resulting from the movementof the electron cloud through p conjugated framework, fromelectron–donor to electron–acceptor groups. As a general rule, thefirst hyperpolarizability (b) vanishes in centrosymmetric molecules,but there are a few symmetry breaking cases as investigated bothexperimentally and theoretically [35,36]. The existence of weakasymmetric interactions such as solvent field fluctuations orsymmetry breaking substituents would destroy the initial inversionsymmetry of the electronic states, thus leading to nonvanishing bvalues in symmetric systems [37]. We expect solvent field fluctua-tions to be one of the possible causes for a nonvanishing b value inpolar solvents like DMSO. In addition, DBM has a non-planargeometry in which benzene rings are tilted �3.4 and 16.71 relativeto the molecular plane [38]. Our quantum calculation shows thatdue to the slightly tilted molecular structure, the molecule has anonzero dipole moment in the ground-state and hence only a smallfirst hyperpolarizability appears in calculation, which is enhancedin presence of polar solvent molecules like EtOH and DMSO. Thedominance of particular components indicates a substantial delo-calization of charges in those directions. It is obvious from Table 1that the leading component of hyperpolarizability is in the directionof the principal dipole moment axis (bxxx) and parallel to the chargetransfer axis, which shows the delocalization of electron cloud ismore in that direction.

To investigate the origin of the above mentioned nonlinearcharacteristics of DBM, a molecular orbital description is impor-tant. The key transitions of the DBM molecule consist of manycomponents, in which the main constituent is the HOMO-LUMOtransition. The HOMO and LUMO plots of DBM molecule aredepicted in Fig. 5. The LUMO is delocalized over the whole C–Cbond, while the HOMO is located mainly on oxygen atoms.

Fig. 5. Molecular orbital distribution of dibenzoylmethane molecule in highest

occupied and lowest unoccupied states.

Consequently, the HOMO-LUMO transition implies an electrondensity transfer from the benzene ring to oxygen atoms. Theseresults clearly indicate a relation between the molecular chargedelocalization and the third-order nonlinear optical response.

The HOMO–LUMO energy gap for DBM molecule, calculatedusing B3LYP/6�311þG* level, reveals that the energy gap reflectsthe chemical activity of the molecule. LUMO, as an electronacceptor, represents the ability of obtaining an electron, whileHOMO represents the ability of donating an electron. The value ofHOMO energy and LUMO energy was found to be �10.12 eV and�6.39 eV, respectively. A lower HOMO–LUMO energy gap(�3.73 eV), explains the fact that eventual charge transfer inter-action is taking place within the molecule.

4. Conclusions

In summary, two-photon absorption and the first hyperpolar-izability of dibenzoylmethane molecule were investigated insolutions using femtosecond Z-scan and hyper Rayleigh scatteringtechniques, respectively. The 2PA spectrum reveals a band at255 nm, not present in the 1PA spectrum. The appearance of thisband is expected due to the centrosymmetric nature of themolecule. The value of maximum 2PA cross-section was esti-mated to be 37 GM in the ethanol solution. First hyperpolariz-ability values were determined with HRS experiments in ethanoland DMSO solvents, while theoretical calculations were carriedout in DFT framework. The nonlinear nature of DBM molecule wasexpected due to efficient charge delocalization induced by oxygenatoms, as appear in molecular orbital distribution calculation.

Acknowledgment

The authors are grateful to Fundac- ~ao de Amparo �a Pesquisado Estado de S~ao Paulo (FAPESP) and Conselho Nacional deDesenvolvimento Cientıfico e Tecnologico (CNPq) for financialsupport.

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