Enhancement of NLO Properties in OBO ... - ACS Publications

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C: Energy Conversion and Storage; Energy and Charge Transport

Enhancement of NLO Properties in OBO Fluorophores Derived From Carbazole-Coumarin Chalcones Containing Carboxylic Acid at the N-Alky Terminal End Manali Rajeshirke, Mavila C Sreenath, Subramaniyan Chitrambalam, I. Hubert Joe, and Nagaiyan Sekar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b02937 • Publication Date (Web): 01 Jun 2018 Downloaded from http://pubs.acs.org on June 1, 2018

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The Journal of Physical Chemistry

Enhancement of NLO Properties in OBO Fluorophores Derived From Carbazole-Coumarin Chalcones Containing Carboxylic Acid at the N-Alky Terminal End Manali Rajeshirke a, Mavila C. Sreenath b, Subramaniyan Chitrambalam b, Isaac H. Joe b, *, Nagaiyan Sekar a, ** a Department of Dyestuff Technology, Institute of Chemical Technology, Nathalal Parekh Marg, Matunga, Mumbai - 400 019. (India) b Centre for Molecular and Biophysics Research, Department of Physics, Mar Ivanios College, Thiruananthapuram, Kerala, 695015. (India) ** Corresponding author. E-mail: [email protected], [email protected] Tel.: +91 22 3361 1111/2222, 2707(direct), Fax. +91 22 3361 1020.

Abstract The NLO properties of carbazole-coumarin based chalcones and their OBO complexes of D-π-A and D-π-A-π-D type with carbazole as donor substituted with N-alky chain at the 9 position with carboxylic acid end group are studied in detail. These dyes exhibit good emission solvatochromism. NLO properties of dyes 5-8 were studied by two methods namely solvatochromic and computational method. The linear polarizability (α), hyperpolarizability (β) and second order hyperpolarizability (γ) of dyes 5-8 were studied by TD-DFT computational method using five different range separated functionals namely CAM-B3LYP, HISSbPBE, HSEH1PBE, wB97, and wB97X with triple zeta basis set 6-311++ G (d, p). The correlation between the NLO properties is well established with geometry of the dyes by understanding the 1 ACS Paragon Plus Environment

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enhancement of hyperpolarizability, second order hyperpolarizability and two photon absorption properties of complexed and uncomplexed carbazole-coumarin chalcones experimentally and theoretically. The dye 8 showed maximum enhancement in NLO properties like 167 GM two photon absorption cross section in toluene, which is due to the effective charge transfer in the molecule owing to presence of extra donor and rigidized planar conformation of the dye due to OBO complexation of the chalcone. z-Scan technique was used to study the third order nonlinearity (χ(3) ) of the dyes. The structure-property relationship has been well established in the studied dyes which can act as better organic NLOphores. 1. Introduction: The design and structure-property study of organic NLO materials have been widely researched in the past few decades for a variety of applications such as frequency mixing1–5, electro-optic modulation6,7, and second harmonic generation (SHG) use in optical devices8,9. The advantages of organic NLOphores over the inorganic NLO materials like lithium niobate are the ease of synthesis, large optical nonlinearity, low cut-off wavelengths, short response time, high laser damage thresholds, good solubility and compatibility with polymer matrix have brought the NLO research into forefront10–12. Organic compounds with high conjugated π electron system between electron donor and acceptor groups show good NLO properties with good thermal and photo stabilities. The chalcones are one of the promising NLO candidate by virtue of their pushpull type design consisting of D-π-A framework, with high absorption extinction coefficients in the ultraviolet (UV) region, good third-order nonlinearity and good optical power limiting property13–15. Chalcones have application in optoelectronic devices16,17, information storage, optical switching18,19, electrochemical sensing20 and Langmuir film21.

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In the current work, we focused on the study of the chalcones with coumarin as an acceptor and carbazole as a donor linked via π- conjugated framework. Among the organic NLOphores coumarin derivatives have good molecular nonlinearity which is suitable for variety of applications such as electroluminescence (EL) materials22, fluorescent labels23, frequency doubling properties as studied by Yankelevich et a124 and in various optics phenomena. Coumarin derivatives are known for good charge mobility in some coumarin-doped polymers25, excellent biological and medical activities26. D-A-D coumarin chalcones possess promising NLO properties27,28. Carbazole containing NLOphores are widely studied in recent years29–31. The nature of N-substituent influences the emission. The alkyl chain linked via sp3 carbon to the 9 position of carbazole usually suppresses the association of dye which lead to fluorescence quenching. Carbazole with easy derivatizable alky carboxyl group show good photoexposure delay (PED)32, act as a derivatization reagents in HPLC analysis, and also forms metal complexes with Zn, Cd metal ions. 33. The bulk nonlinear properties of organic chromophores is attributed to effective tuning and alteration of intermolecular and intramolecular interactions which is correlated to the dipole moment and the molecular hyperpolarizability of the molecule17. The intramolecular charge transfer (ICT) help in increasing the hyperpolarizabilities of the molecule. Similarly, the intermolecular interaction like solvent-solute interaction direct the alignment of molecular dipole which affects the second harmonic generation of the molecule. Thus architecture for a betterperforming NLOphores could be achieved by understanding the roots of the NLO at molecular level17. Oudar’s two level model says that the hyperpolarisability and second order hyperpolarizability increases with an increase in transition dipole moment and the decrease in the energy gap amongst the ground and the first excited singlet state. Thus the two level model helps 3 ACS Paragon Plus Environment


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to design and understand the trend of hyperpolarizability which can be further supported by the DFT and TD-DFT computations. The theoretical descriptions of two level model is given in the supporting information. In our research, we have studied the NLO properties of newly designed coumarin-carbazole chalcones and their BF2 complexes with acid anchoring group at the alkyl end on the carbazole donor as seen iin Scheme 1. The effect of enforced planarity by geometry restriction by BF2 complexation of the chalcones 5 and 6 on the NLO properties is studied in detail. The ClaisenSchmidt reaction was employed for preparation of chalcones and their BF2-complexes. The aldehyde of 3-(9H-carbazol-9-yl) propionic acid (2) on reaction with 3-acetyl, 4-hydroxycoumarin (3a) and 3-acetyl, 4-hydroxy, 7-N, N-diethyl coumarin (3b) gave the respective chalcone 5 and 7. The BF2 complexes 4a and 4b reacted with the aldehyde of 3-(9H-carbazol-9yl) propionic acid (2) to form 6 and 8 BF2 complexes of chalcones respectively. The complete synthesis method and characterization of 5-8 are given in the supplementary material. The effect electron donor group on the coumarin ring, the BF2 complexation of chalcones, and the anchoring acid group on the alky end of carbazole donor are investigated for shift in the spectral behavior and consecutively on NLO properties of the studied compounds. The dipole moment ratio and generalized Mulliken-Hush (GMH) analysis are employed to study the charge transfer (CT) characteristics of 5-8. The polarisability parameter αCT, first hyperpolarisability parameter βCT, solvatochromic descriptor of γSD, and two photon properties (σ2PA) of couamrin-carbazole chalcones and their BF2 complexes with acid anchoring group at the alkyl end on the carbazole donor were obtained from solvatochromic two-level microscopic model. The static first hyperpolarisability (βο) and its related properties (µ, α0, ∆α, β0, ￣γ) were determined by the


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computational methods. The Z-scan technique was used to study the nonlinear absorption coefficient (β) and nonlinear refractive index (n2) for the studied dyes.

Scheme 1 Design strategy for dyes 5-8 2. Experimental 2.1. Materials and equipment All the chemical reagents used were purchased from S.D. fine chemicals Ltd., Mumbai, India. Pre-coated silica gel aluminum based plates Kiesel gel 60 F254 Merck, India were used to moniter the reactions. Recrystallization and column chromatography were employede for purification of all the compounds wherever necessary. An instrument from Sunder Industrial Product Mumbai was used to measure the melting points. 1H NMR and 13C NMR spectra were recorded on a 500 MHz and 125 MHz respectively on Agilent Technology instrument using TMS as an internal standard. The Perkin-Elmer Lambda 25 UV-visible spectrophotometer and Varian Cary Eclipse 5 ACS Paragon Plus Environment


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fluorescence spectrometer were used to record the absorption and emission spectra of the dyes respectively. FT-IR spectra were recorded on a Jasco 4100 Fourier Transform IR instrument (ATR accessories). Rhodamine 6G was used as the reference standard to calcutae the relative quantum yield of dyes 5-8. 2.2. Z-Scan experimental The nonlinear responses of 5, 6, 7, and 8 namely the absorption coefficient (β) and nonlinear refractive index (n2) were determined by z-scan technique. In this technique, a laser beam was focused to a minimum waist at the focal point where Z=0 (Z beam propagation direction). The DMSO and acetone liquid solutions of the samples were moved along the beam path, by varying the intensity of incident beam. The the nonlinear refractive index was determined from thee transmittance through a small circular aperture placed at the far field position (closed aperture) whereas the open-aperture (OA) Z-scan arrangement was used to the measure the nonlinear absorption coefficient. 0.5mM solutions of all dyes were subjected to Z-scan measurement wherein 532 nm Nd: YAG laser having 5ns pulses at a repetition rate of 10 Hz was used. The microscopic parameters β, n2 and Χ(3) of 5, 6, 7, and 8 were evaluated by Z-scan. 2.3. Computational method Gaussian 09 revision D.01 package was used to perform all the computations. CAM-B3LYP functional with 6-31G (d) basis set was used to obtain the ground state optimized geometry for 58 in studied solvents. The hyperpolarizabilities analysis of th dyes was done using different range hybrid functionals, namely CAM-B3LYP, HISSbPBE, HSEH1PBE, wB97, and wB97X with triple zeta basis set 6-311++ G (d, p). The optimization in solvents of different polarity were obtained using the self-consistent reaction field (SCRF) incorporated in the Polarizable 6 ACS Paragon Plus Environment

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Continuum Model (PCM) as implemented in Gaussian 09. The volume computations using Onsager model in Gaussian 09 was performed to obtain the values of Onsager cavity radii 5, 6, 7, and 8 in toluene, DCM, methanol, and DMF solvent using the DFT-optimized ground state geometries in the respective solvents. 3. Results and discussion The dyes 5-8 showed better emission solvatochromism than absorption solvatochromism as presented in S.Table 1. Bathochromic shift was observed in the order 5 < 7 < 6 < 8 for both absorption and emission spectra (S.Table 1).Thus, the main interest of our research here was to study the change in NLO properties as response of CT characteristics of these dyes. This CT characteristics of dyes is studied from electronic vertical excitation spectra, GMH analysis and the dipole moment ratio of ground to excited state of the dyes using two different approaches. 3.1. Vertical excitation Spectra The energies, oscillator strength and their orbital contributions of the electronic transition were obtained from the optimized vertical excitations of the ground state geometries of 5-8 using TDDFT which are listed in Table 1. The experimental and computed absorption data have been compared which are in good agreement with each other. The orbitals involved in the electronic absorption are mainly the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) orbitals for all the dyes. The absorption band implies the intramolecular charge transfer (ICT) characteristic of the dyes which is supported by the oscillator strength values of that transition. The energy gap decreases from nonpolar toluene to polar DMF solvent for 5-8 like for dye 8 energy gap in toluene is 2.5359eV and in DMF the value is 2.4358eV, this decrease is credited to the solvent relaxation of the S1 first excited state of 7 ACS Paragon Plus Environment


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the dye molecule in polar medium (Table 1). The order of energy gap among the studied dyes follow the observed experimental trend as 5 (3.3739eV) < 7(2.7444eV) < 6(2.5409eV) < 8(2.5359eV) in toluene. Thus the experimental values are well clarified by the TD-DFT computed energies of all the dyes. Table 1 Experimental absorption and computed photophysical properties of 5-8 TD-DFT computational data

µeg[a]

Solvent

Expt. λabs (nm)

[b]

Vertical excitation (nm)

f[d]

[c]

HOMO to LUMO, Major contribution

Energy gap (eV)

5 Toluene

5.24

421

368.11

0.2459

118 ->119, 45.52%

3.3681

DCM

7.17

430

367.62

0.4986

118 ->119, 58.39%

3.3726

MeOH

6.83

430

367.95

0.5515

118 ->119, 61.75%

3.3696

DMF

3.91

440

369.00

0.6089

118 ->119, 63.51%

3.3600

gas

-

-

357.76

0.1592

118 ->119, 65.28%

3.4655

Toluene

7.74

521

487.95

0.9598

129 ->130, 69.93%

2.5409

DCM

7.42

525

497.91

0.9328

129 ->130, 69.91%

2.4901

MeOH

5.61

536

498.22

0.8969

129 ->130, 69.82%

2.4885

DMF

6.76

547

501.32

0.934

129 ->130, 69.89%

2.4732

gas

-

-

455.54

0.7716

129 ->130, 69.64%

2.7217

Toluene

7.23

442

451.78

0.9254

138 ->139, 69.17%

2.7444

MeOH

7.76

447

467.13

1.0229

138 ->139, 70.15%

2.6542

DMF

5.24

454

465.33

0.8981

138 ->139, 69.22%

2.6644

gas

-

-

430.06

0.7355

138 ->139, 70.09%

2.8829

Toluene

9.09

562

503.50

1.2433

149 ->150, 70.27%

2.4625

DCM

8.74

559

505.75

1.2039

149 ->150, 70.35%

2.4515

MeOH

8.82

576

509.01

1.2287

149 ->150, 70.32%

2.4358

DMF

7.74

-

454.01

1.12

149->150, 69.76%

2.7309

gas

-

559

488.91

1.2642

149 ->150, 70.06%

2.5359

6

7

8


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[a] transition dipole moment (in x 10-18 esu), [b] experimental absorption maxima in nm, [c] computed vertical excitation by TD-DFT in nm, and [d] TD-DFT computed oscillator strength of transition.

3.2. Dipole Moment The transition dipole moment (µeg) of a chromophore help to understand better the charge transfer characteristics of chromophore. Two factors: strength of donor-acceptor group and the microenvironment of dye molecule which alters the electronic spectral behavior of the push-pull chromophore governs the dipole moment of the dyes. The transition dipole moment which gives the extent of the probability of π-π* a transition from ground to excited state is given by equation (1);

μ = . ×

×ῡ

(1)

Thus, the transition dipole moment (µeg) calculated using equation (1) and results obtained are presented in Table-1. The µeg value increases from 5 to 7 owing to increase in one donor group in 7, while the BF2 complexes exhibit higher µeg than the corresponding chalcones. The electronic spectral performance of the chromophore also depict the ratio of dipole moment of excited to ground state µe/ µg of the dye. This ratio could be obtained using the spectroscopic data of the studied dyes by two ways: 1) by using the Stokes shift 2) by using the solvent induced shift in the absorption maxima. 3.2.1 Dipole Moment ratio using Stokes Shift



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Bilot-Kawski34, Bakhshiev35and Liptay36,37 polarity functions plots using the Stokes shift data were used to obtain the ratio of excited state dipole moment to the ground state dipole µe/ µg for all the studied dyes. The detailed equations for computing the dipole moment ratio by BilotKawski, Bakhshiev, and Liptay functions plots is provided in the supporting information. The dipole moment ratios of all the dyes are presented in Table 2, wherein 5-8 have ratio greater than unity indicative of more polar excited state than the ground state of the dyes resulting in the charge transfer in the excited state supported by the positive solvatochromism observed for the studied dyes. Though the methods used for calculation of dipole moment ratio are based on several assumptions and approximations the trend in dipole moment rato remains the same. Thus dipole moment ratio states CT characteristics of the dyes.

Table 2 Dipole moment ratio ( ) of 5-8 by various methods. 5 0.51 0.48 2.18

Bilot-Kawski Bakhshiev Liptay

6 1.56 1.66 1.71

7 0.35 3.23 3.51

8 2.68 0.32 0.31

3.2.2 Dipole Moment ratio obtained from solvent induced shift in absorption maxima Paul Suppan

38

established a relation between the solvatochromic shift induced by solvents of

different dielectric constants with similar refractive index to get an insight of the dipole moment nature in the excited state of the solute. The Suppan equation (2) for solvents 1 and 2 correlates the change in permanent dipole moment to absorption energy shift as 39–41; −∆ʋ =

∆

())

∆!"(#$ ) − "(%$ )& +

"(#) = ()*) and "(%) =

( (

∆"(%$ )

(2)

(+ ( ) (+ ( )


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where dielectric constant of solvent is represented as ε, refractive index of solvent as η, h is the Planck’s constant, velocity of light as c, onsager radius as a0, the ground and excited state dipole moment of solute as µg and µe respectively. Ayachit et al. further simplified equation (2) to linear equation (3) as; ,

-.

/

+ - = 1 (3) (

1=

∆!()2 )(+2 )&. ( ∆ʋ. (

with intercept 4 =

and 3 =

∆(+2 ). (

∆

and 4 =

∆ʋ. (

( (

The least square fit method is used to obtain the values of the intercepts C1 and C2 which are used to get the dipole moment ratio as;

-

= -. − 1 (4) (

Thus the dipole moment ratio for dyes 5-8 are calculated using equation (4) using absorption maxima in different solvents (S. Table 1) and values listed in Table (3). The ratio for all the dyes is greater than unity suggestive of higher excited state dipole moment than the ground state dipole moment. The results obtained by both the methods show the same trend in dipole moment ratio. Though the values of dipole moment ratio not accurate rather overestimating still these spectroscopic derived methods help to understand the trend in dipole moment ratio and the excited state of the molecule. Further, this property is also studied using GMH analysis. Table 3. The dipole moment ratio of dyes 5-8 calculated from absorption spectra. 5 6 7 8



C1/C2

5.02

10.78

4.47

9.91

µe/µg

4.02

9.78

3.47

8.91

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3.3. Generalized Mulliken-Hush Charge Transfer Analysis The CT characteristics of dyes 5-8 were studied using GMH analysis. The NLO property of the material can be depicted from the strength of intramolecular charge transfer (ICT), greater CT in excited state larger is the dipole change better the NLO properties. GHM analysis is derived from two-state model for weakly interacting localized diabatic states with zero off-diagonal dipole moment matrix elements wherein the strength of electronic coupling (HDA) based on spectroscopic data for charge transfer is obtained by simple two-state GMH treatment. The GMH equation (5) relates the strength of electronic coupling HDA between the ground (S0) and CT excited states with the vertical excitation energy (∆Eeg), the difference between the adiabatic 6 dipole moments of the ground and excited states (∆µeg), ∆5 is the difference in diabatic state

dipole moments, and the transition dipole moment µge as follow42: 768 =

∆9 : ∆

=

∆9 ( )./( ( (∆ *∆

(5)

The adiabatic states are assumed to be made up of these three diabatic states – a donor ground state (GS), a donor locally excited state (LE), and a CT state with the ‘‘transferring electron” localized on the acceptor. Few CT systems have carefully used this approach 43,44. The following expression, Equation (6)45 is used to get the centre-to-centre separation distance (RDA) between the donor and acceptor through π-bridge,

Enhancement of NLO Properties in OBO ... - ACS Publications

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