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SN Applied Sciences

, 1:259 | Cite as

Synthesis of red emitting triphenylamine derived NLOphoric D–π–A molecules: photophysical, and viscosity sensing studies

  • Dinesh S. Patil
  • Kiran C. Avhad
  • Mayuri M. Kadam
  • Nagayain SekarEmail author
Research Article
  • 151 Downloads
Part of the following topical collections:
  1. 1. Chemistry (general)

Abstract

Four small dyes with triphenylamine (TPA) as a donor and various acceptor groups connected with π-linker having chloro group are synthesized and studied for solvatochromism and DFT. Good solvatochromism is observed in compounds 2 and 4 having TPA attached to rhodamine-3-acetic acid and 4-aminosalicylic acid. The multilinear regression analysis supports the solvatochromic behaviour of the chromophores. The compound 4 shows viscosity sensing with 0.57 viscosity sensitivity (x) value as which is more as compared with the other compounds, and 14.14-fold increased emission intensity in viscous solution from 0 to 99% of PEG-400 in ethanol. These TPA based compounds 1, 2, 3, and 4 show two-photon absorption cross section in the range 702–1265 GM. The natural bond analysis shows charge transfer in compound having strong withdrawing groups, that is compound 1 and 2. The bond length alteration is correlated with the computationally obtained NLO properties and show linear correlation. The compound 1 is a better NLOphoric material than the other compound 2, 3, and 4 as revealed by solvatochromic and computational methods.

Keywords

Triphenylamine 4-aminosalicylic acid Viscosity sensor TD-DFT NLO Accepting group 

1 Introduction

Triphenylamine (TPA) is a planar aromatic compound and widely used as an electron donating unit in donor–π–acceptor (D–π–A) framework [1, 2, 3, 4]. TPA helps good charge transfer (CT) in several D–π–A dyes which are designed for various applications and it is widely used for the design of two photon absorption (2PA) chromophores [5, 6, 7, 8, 9, 10, 11]. TPA based D–π–A framework have attract much more attention as the best electron transporting organic materials in dye sensitized solar cell (DSSC) due to their in expensive synthesis, outstanding stabilities, and high molar extinction coefficient [12, 13, 14, 15, 16, 17, 18]. A large number of TPA based chromophores are used for their applications in red to deep red organic light emitting diodes (OLEDs) [19, 20, 21, 22, 23, 24, 25, 26]. The TPA has excellent intramolecular charge transfer (ICT) characteristics and it is attached to different electron withdrawing units for various applications [27, 28, 29, 30, 31, 32, 33, 34]. TPA based dyes are also widely investigated for their good AIE properties [35, 36, 37, 38, 39, 40, 41, 42, 43].

Recently fluorescent molecular rotors (FMRs), a class of fluorescent dyes which undergo twisted intramolecular charge transfer (TICT) upon photo excitation increased much attention [44, 45, 46]. 9-(Dicyanovinyl)-julolidine (DCVJ) is the best example exhibiting viscosity sensitivity. Different TPA derivatives exhibit FMR properties due to the freely rotating phenyl ring which is hindered in the viscous media causing fluorescence enhancement [47]. The TICT states are responsible for quenching of fluorescence in polar solvents and show more emission in a highly viscous solution. The electron withdrawing groups also play a major role in D–π–A systems used in DSSC and OLED, biological and FMR studies [48, 49, 50, 51, 52]. The functionalized carboxylic acid having electron withdrawing group in conjugation is used as a standard accepting group in DSSC [53, 54, 55, 56, 57, 58]. Recently fluorescent quantum dots have attracted much attention owing to their colorful fluorescence properties which can be widely used in the diagnosis of disease or cell detection (-COOH) [59]. Considerable effort is focused on fabricating fluorescent sensors for nucleic acid detection [59]. Herein, we developed a simple strategy having TPA as a donor and group containing -COOH as acceptor (except compound 3). Scheme 1 represents the structure of four chromophores studied.
Scheme 1

Chemical structure of the TPA based chromophores

For the first time 4-aminosalicylic acid as an acceptor group is employed in a Schiff base to explore superiority of Schiff base over conventional electron accepting groups. To understand the structural correlations and viscosity sensitivity of these dyes carrying different acceptor groups, we have synthesized four compounds having TPA based donor with chloro-group suitably placed on π-linker and cyanoacetic acid, rhodanine-3-acetic acid, dicyanovinylene or 4-amino salicylic acid as acceptor as shown in Scheme 1. The photo physical properties of the all compounds are well studied. The HOMO and LUMO energy level diagrams were obtained by theoretical DFT calculation. Density functional theory (DFT) computations were used to study the geometrical and electronic properties of the synthesized D–π–A dyes. The experimentally and theoretically calculated NLO properties of these chromophores showed that these chromophores are NLOphoric and have considerable two-photon absorption (2PA) cross section. The compound 4 behaves as good FMR with high viscosity sensitivity as compared to the known FMR, DCVJ.

2 Experimental

2.1 Chemicals and instruments

All the commercial reagents and the solvents were procured from S. D. Fine Chemicals Pvt Ltd. Mumbai and were used without purification. Triphenylamine was procured from Alfa Aeser. The reaction was monitored by TLC using 0.25 mm silica gel 60 F254 percolated plates, which were visualized with UV light. 1H NMR and 13C NMR spectra were recorded on Agilent 500 MHz instrument (USA using TMS as an internal standard). The UV visible absorption spectra of the compounds 1, 2, 3, and 4 were recorded on a Perkins-Elmer Lambda 25 spectrometer; fluorescence emission spectra were recorded on Varian Cary Eclipse fluorescence spectrophotometer.

2.2 Computational details

Ground state geometries of the molecules were optimized using the DFT hybrid functional, B3LYP [62]. Pople’s basis set with both diffuse and polarization functions, 6-311++G(d,p) was used for all the atoms. The B3LYP is a combination of Becke’s three parameter exchange functional (B3) [63] with the nonlocal correlation functional by Lee, Yang, and Parr (LYP) [64]. The first excited singlet states were optimized using TD-DFT B3LYP and CAM-B3LYP/6-311++G(d,p). Frequency computations were carried out on the optimized geometry of the low-lying vibronically relaxed first excited state of molecules [65]. The TD-DFT calculations with B3LYP functional (the global hybrid) were performed and compared with the experimental data. The polarizable continuum model (PCM) [66, 67] as implemented in Gaussian 09 was used for optimization in solvents. The Gaussian 09 [68] program was used for all the DFT and TD-DFT computation.

3 Results and discussion

3.1 Synthesis and characterization

Synthesis of intermediates and final dyes with purity was reported in our earlier paper [60] and also given in supporting information.

3.2 Photophysical properties

Absorption and emission properties of 1, 2, 3, and 4 are studied in various solvents with different polarities. Table 1 shows absorption and emission maxima, molar extinction coefficient, and Stokes shift of all the four chromophores in various solvents. Figure 1 represents normalized absorption and emission spectra for all the four chromophores in seven different solvents. The first absorption smaller peak at around 372 nm is due to the π–π* transition of compound 2 [61] and the second peak near 500 nm is due to the intramolecular charge transfer transition (ICT). The red shifted absorption spectra were observed when the rhodamine-3-acetic acid group is attached to triphenylamine moiety. Also, with the presence of strong electron accepting group cyanoacrylic acid in compound 1, are markable change in the molar extinction coefficient was observed (Table 1). The chromophore having strong electron withdrawing group cyanoacetic acid (1) shows the highest molar extinction coefficient, while the chromophore having electron withdrawing group p-amino-salicylic acid (4) shows the least molar extinction coefficient (Table 1).
Table 1

Photophysical properties of the compounds in seven solvents

Solvent

λ abs a

ε max b  × 104

fwhmc

λ ems d

Stokes shifte

Φ F f

fg

µeg(Abs)h

(nm)

(M−1 cm−1)

(nm)

(nm)

(nm)

(cm−1)

(debye)

1

         

 CHCl3

482

23.2

149

631

149

4899

0.039

4.68

22.17

 EAi

462

23.6

91

620

158

5515

0.033

4.95

22.09

 Acetone

479

21.4

102

588

109

3870

0.029

4.91

21.83

 ACN

474

14.0

177

567

93

3460

0.015

3.68

18.77

 MeOH

462

23.5

91

621

159

5541

0.009

4.95

22.08

 DMF

415

21.3

76

562

147

6302

0.019

4.64

20.40

 DMSO

473

22.4

89

628

155

5218

0.023

4.29

20.80

2

         

 CHCl3

497

12.0

96

638

141

4446

0.042

3.03

17.91

 EAi

483

12.9

92

626

143

4729

0.056

3.25

18.29

 Acetone

482

10.5

96

630

148

4873

0.039

2.50

16.04

 ACN

486

13.2

94

662

176

5470

0.024

3.24

18.32

 MeOH

488

11.8

99

673

185

5632

0.035

3.10

17.95

 DMF

485

13.4

97

653

168

5304

0.015

3.54

19.13

 DMSO

501

13.7

102

668

167

4990

0.038

3.61

19.63

3

         

 CHCl3

509

11.9

86

643

134

4094

0.034

2.07

14.99

 EAi

485

12.1

88

645

160

5114

0.031

2.51

16.11

 Acetone

488

11.4

95

617

129

4284

0.040

2.52

16.19

 ACN

486

10.2

93

595

109

3769

0.015

2.19

15.08

 MeOH

485

10.8

94

563

78

2856

0.029

2.46

15.95

 DMF

494

13.2

105

591

97

3322

0.032

3.86

20.16

 DMSO

497

12.3

99

633

136

4322

0.028

2.79

17.18

4

         

 CHCl3

375

6.0

52

456

81

4736

0.087

1.01

8.94

 EAi

370

6.6

46

441

71

4351

0.064

0.95

8.64

 Acetone

371

7.6

53

500

129

6954

0.045

1.34

10.29

 ACN

370

8.1

49

464

94

5475

0.041

1.20

9.72

 MeOH

369

7.0

51

484

115

6439

0.056

1.13

9.45

 DMF

374

6.6

48

474

100

5640

0.024

0.96

8.77

 DMSO

376

6.0

50

482

106

5848

0.028

0.89

8.45

aabsorption maxima, bmolar extension coefficient, cfull width half maxima, demission maxima, eStokes shift, fquantum yield, goscillator strength, hdipole moment and iethyl acetate

Fig. 1

Normalized absorption and emission spectra of all four compounds in seven different solvents

The molar extinction coefficient increases in the order: 4 < 3 < 2 < 1 showing that the carboxycyano vinylene accepting group increases ICT in compound 1. A shoulder peak appearing at around 370 nm in the absorption spectra of compound 2 is due to the presence of rhodamine-3-acetic acid acceptor group. The absorption λmax increases in the order: 4 < 3 = 1 < 2 in acetonitrile. The compound 2 is the reddest shifted which is due to the acceptor, rhodamine-3-acetic acid which has more π-conjugation as compared to the other chromophores. A similar trend was observed in the Stokes shifts (Table 1). In the emission spectra, the same trend was observed, a red shift in emission λmax was observed from compound 4 to 2 (500 to 673 nm). So, it can be predicted that there is a better intramolecular charge transfer (ICT) after varying withdrawing groups from moderate to strong electron withdrawing groups.

The photophysical properties of the four compound 1, 2, 3, and 4 in different solvents are evaluated and tabulated in Table 1 and Table S1. They show quenching of fluorescence in polar solvents causing a lowering of quantum yield and red shifted emissions. The compound 1 shows the highest values of molar extinction coefficient, FWHM, Stokes shift, oscillator strength and transition dipole moment compared the other three compounds, while compound 4 shows the highest values of quantum yield, fluorescence lifetime and minimum non-radiative decay constant. The compound 1, and 4 show a lower value of non-radiative decay constant (Knr) than the other compound 2, and 3. All the compounds 1, 2, 3, and 4 show lower values of the non-radiative decay constant (Knr) than their corresponding radiative decay constant (Kr) in all the solvents (Table S1). In polar solvents, the value of Knr increases for compounds 1, 2, and 4 (except compound 3).

3.3 Solvatochromism

The D–π–A compounds show good solvatochromic behaviour. In D–π–A chromophores the solvent polarity affects the excited state by stabilizing it through dipole–dipole interactions, and hydrogen bonding. Hence, we have studied the solvent dependent absorption and emission spectrum of these compounds in different solvents. We observed that the compound showed a red shift in absorption (67 nm) and a large red shift in emission (69 nm). These results are supported with the positive difference in dipole moment of the S0 and S1 state and increased Stokes shift in polar solvents as compared to the non-polar solvents (Table 1). The normalized emission spectra of the compounds 1, 2, 3, and 4 are shown in Fig. 1 which indicates the highest red shift in emission spectra (69 nm) of compound 1. The increase in Stokes shift with an increase in the solvent polarity in these compounds can be correlated by using Lippert-Mataga (LM) equation [62].

Figure 2 shows a linear relationship suggesting that solvent parameters such as dielectric constant and refractive index are responsible for the red shifted emission in compound 2. These compound 1, 2, 3, and 4 show different types of interactions with polar and non-polar solvents. Mac Rae (McR) model is the upgraded version of LM model where solute polarizability in addition to the solvent polarizability was taken into account [63]. We plotted the McR function (ΔfMR) vs Stokes shift for the compound 1, 2, 3, and 4 as shown in Figure S1 and there exists a linear relationship suggesting that the polarity originating from the dipole created by this compound 1, 2, 3, and 4 and it also responsible for the solvatochromic shift in polar solvents. A similar molecular microscopic solvent polarity function \((E_{T}^{N} )\) by Reichardt [64, 65] which correlates much better with the solvatochromic shift of dipolar molecules rather than the traditionally used LM polarity functions is also plotted with the Stokes shift and linear relationship is observed as shown in Figure S2.
Fig. 2

Lippert-Mataga plots for compounds (a) 1, (b) 2, (c) 3, (d) 4 in different solvents

3.4 Intramolecular charge transfer (ICT) and dipole moment

ICT occurs upon excitation in D–π–A chromophores, and this process becomes solvent dependent. Large Stokes shift (71 to 185 nm) observed for this compound 1, 2, 3, and 4 suggests enough structural reorganization or a polar S1 state as compared to S0 state. The difference in dipole moment (μ) between the S0 and S1 state was calculated to understand the polarity of the excited state using LM, and McR functions. Table 2 gives the transition dipole moment (μeg), difference in dipole moments (Δμeg) by experimental and DFT method and the ratio μeg using Bilot-Kawaski (BK), Bakhshiev and Liptay correlations. Transition dipole moment (μeg) is a measure of the probability of radiative transitions from S0 to S1 state and corelates with the oscillator strength. The positive values obtained for the difference in dipole moment (µe > µg), and ratio of dipole moment (μe/μg) by experimental as well as DFT, confirmed that the S1 state is polar than S0. The compound 4 shows the highest values of Δμeg obtained by three different correlations (i.e. LM, McR, and BK), while compound 1 shows the higher values of μeg obtained by the other three different correlations (i.e. Bilot-Kawaski, Bakhshiev and Liptay).
Table 2

Difference and ratio of dipole moment between ground and excited state of compound 1, 2, 3, and 4 by experimental as well as DFT method

Dye

µeg

(Debye)

Δμeg (Debye)

µ (DFT)

(Debye)

μeg (Debye)

LM

McR

BK

BK

Bakhshiev

Liptay

1

22.17

9.41

5.36

14.98

1.41

14.58

14.30

15.81

2

17.91

13.10

7.52

11.50

1.52

10.97

15.21

18.01

3

14.99

12.72

7.50

4.37

2.47

9.88

14.26

18.75

4

8.94

15.71

9.44

16.49

1.39

8.65

10.67

11.24

3.5 Viscosity study

We studied the viscosity sensitivity (x) of the TPA based chromophores in ethanol: PEG-400 mixture. The presence of extra phenyl ring in TPA rotates around a single bond. TPA based compounds are known to show enhancement of fluorescence after hindering the rotation in viscous media [47]. A decrease in fluorescence intensity was observed in compound 1, 2, 3, and 4 as the polarity of solvent increases and these emissions are due to the TICT state formed in polar solvents. We used PEG-400 as a viscous solvent for viscosity studies. In 5 µM solutions of the chromophores in ethanol, when the percentage of PEG was increased from 0 to 99% enhancement of fluorescence was observed with the increased percentage of PEG-400. Figure 3 represents viscosity induced emission of the chromophores with increasing percentage of PEG 400 in ethanol.
Fig. 3

Viscosity study of all four compounds in 0% PEG to 99% PEG

As seen from Fig. 3, a minimum 2.67 times and a maximum 14.14 times enhancement of fluorescence intensity respectively for compound 3 and 4 is observed. It is almost three to four times for compound 2 and 1 (8.64 and 9.81 respectively). The compound 4 shows 14.14 times fluorescence enhancement. The very high fluorescence enhancement observed for compound 4 is due to the formation of TICT state in the excited state which gets blocked in viscous media and show increased fluorescence with slightly blue shifted emission spectra. Between compound 1 and 2, comparatively better enhancement in fluorescence was observed for compound 1, which is coplanar with the π-bridge. Due to the good sensitivity of compound 4 to viscosity, it can be considered as a fluorescent molecular rotor (FMR) as it has emission wavelength, 465 nm. The compound 1 and 2 are also good candidates for the FMRs as they show longer emission wavelengths (631 and 673 nm) as compared to well-known FMR DCVJ. All the compound 1, 2, and 3 except compound 4 emitted above 600 nm which is due to the presence of strong withdrawing group, and they can be considered as good FMRs in the red to the deep red region for biological applications. Similarly, compound 4, 2 and 3 show high Stokes shifts, 129, 159 and 185 nm respectively compared to DCVJ. The compound 4 show high value of viscosity sensitivity (x) 0.58 as compared to another compound 1, 2, and 3 (Figure S3).

3.6 Multiple linear regression analysis

Multiple linear regression analysis provides the information on different types of interactions between solute and solvent. It is also used to explain the mechanism of electronic transition. From the existing literature, it is well known that the intensity, position and shape of the excitation spectrum can explain the spectral shifts with the nature of solvent and solute. The spectral shifts induced by solvents were calculated using Kamlet–Taft parameters α, β, and π*. In order to get more information on the solvatochromic properties of dyes, the spectral properties are correlated with Kamlet–Taft parameters. The wave numbers of absorption (νabs), fluorescence (νem) and Stokes shift (ΔνSt) are correlated with as α, β, and π* using multiple linear regression analysis. This analysis is used to explain the mechanism of transitions between ground and excited states based on the information of solute-solvent interactions. The solvatochromic param-eters α, β, and π* are summarized in Table 3.
Table 3

Estimated coefficients (y0, a, b, c, d), their standard errors and correlation coefficients (r) for the multi-linear analysis of (ῡemi) of the compound 1, 2, 3 and 4 as a function of Kamlet–Taft solvent scales

Kamlet

emi

cπ*

r

n

1

18.76 ± 0.74

(− 2.41) ± 0.67

(− 0.64) ± 1.19

(− 2.74) ± 0.51

0.75

10

2

16.29 ± 1.13

(− 0.18) ± 0.12

(− 0.32) ± 1.83

(− 1.28) ± 0.31

0.62

10

3

15.78 ± 1.07

(− 0.16) ± 0.97

(− 0.66) ± 1.74

(− 1.36) ± 0.19

0.66

10

4

17.30 ± 0.89

(− 0.11) ± 0.79

(− 0.75) ± 1.43

(− 1.92) ± 0.81

0.71

10

$$\nu = \, \nu_{0} + \, a\alpha \, + \, b\beta \, + \, c\pi^{ *}$$
(1)
where ν represents the property to be correlated, ν0 stands for the property related to a standard process, α is the hydrogen bond donor acidity, β is the hydrogen bond acceptor basicity, π* is the dipolarity-polarizability, while a, b and c are coefficients characteristic of a given compound.
Moreover, the new four-parameter Catalan solvent scale, which considers polarizability, dipolarity, acidity and basicity of a solvent, is also applied. It is shown that this new four-parameter Catalan solvent scale gives a better fit than the three-parameter scales.
$$\nu = \, \nu_{0} + \, a_{SA} SA + \, b_{SB} SB \, + \, c_{SPP} SPP \, + \, d_{SP} SP$$
(2)

SPP, SP, SA and SB characterize the dipolarity, polarizability, acidity, and basicity respectively.

The values of Kamlet–Taft parameters (α, β, and π*) are tabulated in Table 3.

The analysis of emission data according to Kamlet–Taft Eq. 1 using a solvent scale in which solvent (di)polarity and polarizability effects are combined in the single parameter shows a poor-fit, as assessed by the value of r (0.66) and the large standard errors on the estimated parameters aα, bβ, and cπ* as a quality-of-fit criterion. Conversely, use of Catalan solvent parameters, where (di) polarity and polarizability effects split, gives a perfect multi-linear fit to νemi (r = 0.82). Because this multiple linear regression gives relatively small estimates for aSA and bSB with comparatively large associated standard errors. We decided to perform the multi-linear analysis according to (Eq. 2) with solvent dipolarity (SPP) and polarizability (SP) as only solvent scales. This analysis gives better results (r = 0.82).

Multi-linear analysis is carried out in different solvents using emission data represented in Table 3. In the case of emission spectra again Catalan parameters show better correlation coefficient (r = 0.82) than Kamlet–Taft parameters (0.75). The negative value observed for all three solvent parameters (acidity, dipolarity and polarizability) both by Kamlet–Taft and Catalan analysis supports the red shifted emissions observed in polar solvents. Estimated coefficients for dyes by Kamlet–Taft and Catalan method are represented in Tables 3 and 4. By seeing the sign of each estimated coefficient as well as the extent of standard error and correlation coefficient obtained by both the methods, solvent dipolarity emerges out as the main factor responsible for the shift in absorption as well as in emission. Hence solvent dipolarity can be considered as the main factor responsible for the slightly red shifted absorption as well as highly red shifted emission spectra of all the dyes.
Table 4

The correlation coefficients (r) for the multi-linear analysis of (ῡemi) of compounds 1, 2, 3 and 4 as a function of Catalan solvent scales

Catalan

emi

aSA

bSB

cSPP

dSP

r

n

1

19.39 ± 2.54

(− 2.47) ± 1.34

(− 0.45) ± 1.15

(− 3.27) ± 1.02

(− 2.32) ± 1.27

0.82

10

2

17.08 ± 3.68

(− 1.49) ± 1.14

(− 0.26) ± 1.60

(− 2.64) ± 1.47

(− 2.45) ± 1.75

0.72

10

3

18.61 ± 2.68

(− 1.55) ± 1.41

(− 0.02) ± 1.17

(− 2.85) ± 1.07

(− 2.32) ± 1.46

0.78

10

4

16.52 ± 2.29

(− 1.25) ± 1.21

(− 0.58) ± 1.99

(− 2.45) ± 0.92

(− 2.71) ± 1.95

0.75

10

3.7 Theoretical investigation

3.7.1 Optimized geometries

Geometry optimizations of the compound 1, 2, 3, and 4 in gas phase as well as in solvents are carried out using time-dependent density functional theory (TD-DFT) calculations with the Gaussian 09 program using the popular global hybrid functional, B3LYP [66, 67, 68]. The optimized geometries of all chromophore are displayed in Figure S4. One phenyl ring in TPA is always twisted. The frontier molecular orbital (FMO) of all chromophore are displayed in Fig. 4. The electron density is uniformly distributed over the dye, D unit, π-linker and A unit. In HOMO, the electron density is located on the donor group (TPA) and π-linker, and in the LUMO electron density is located on the withdrawing unit and π-linker. The minimum overlap of electron density between HOMO and LUMO is observed in the case of compound 1 and 3 as compared with compound 2 and 4 showing that the more effective charge transfer is possible in the presence of strong withdrawing groups. MEP diagram supports the FMO diagram of all the chromophores (Figure S5).
Fig. 4

Frontier molecular orbital (FMO) of compounds 1, 2, 3, and 4 from left to right

The energy gap between HOMO and LUMO for the compound 1, 2, 3, and 4 are calculated in chloroform and are represented in Figure S6. As expected, compound 3 shows the lowest energy difference between HOMO and LUMO (2.51 eV) which is due to the strong withdrawing unit present in this compound. The compound 4 shows the highest difference between HOMO and LUMO (2.91 eV) due to the presence of weak withdrawing unit, 4-aminosalyclic acid. The computationally obtained lowest energy transitions i.e. vertical excitations, oscillator strengths and dipole moments in chloroform are compared with the experimentally observed absorptions, oscillator strength and dipole moment derived from absorption λmax in chloroform solvent (Table 5). In general, computationally obtained vertical excitation values are well in agreement with the experimental value. A similar trend for the experimentally and computationally observed oscillator strengths is found, and the only exception is of compound 1 which shows almost double the value of experimental oscillator strength compared to the computational value. Though the experimentally calculated dipole moment increases in the order 4 < 3 < 2 < 1, computational dipole moment increases in the order 2 < 1 < 4 < 3.
Table 5

Selected properties of the compounds (experimental and computational) in chloroform

Chromophores

λabs (nm)

f

µeg (Debye)

a

b

a

b

a

b

1

482

546

4.68

2.27

22.17

12.64

2

497

583

3.03

2.12

17.91

11.55

3

509

542

2.07

2.28

14.99

14.28

4

375

491

1.01

2.52

8.94

13.46

aexperimental value, bcomputational value

3.7.2 NBO study

The charge distribution and electron transfer in D–π–A compound 1, 2, 3, and 4 are understood by the natural bond orbital (NBO) analysis. The charges on the atoms in the ground state give some idea about the electron delocalization inside the chromophore. The natural charges on selected atoms inside the chromophores in chloroform are tabulated in Table 6. It was observed that the negative charge on nitrogen atom (N12) increased in the order: 1 < 3 < 2 < 4 indicating that the lone pair of nitrogen atom migrates from D to A. The C41 carbon acts as acceptor which has increased positive charge on carbonyl carbon. In case of compound 1 and 2 C41 carbon shows more positive charge than the other two compound 3, and 4 which is due to the presence of the strong electron withdrawing group. In case of compound 2 C50 carbon atom shows more positive charge than C41 because C50 was the carbonyl carbon atom. In case of 1, 2, and 3 negative charge increased over N46, N45 and N44 respectively showing that the electrons are delocalized towards the positive charge density.
Table 6

Natural charges of theselected atom at ground state (GS) at B3LYP with 6-311++G(d,p) in CHCl3

1

2

3

4

Atom no.

N C

Atom no.

N C

Atom no.

N C

Atom no.

N C

N12

− 0.4773

N12

− 0.4891

N12

− 0.4795

N12

− 0.4950

C1

0.2235

C1

0.2080

C1

0.2226

C1

0.2027

C2

− 0.2650

C2

− 0.2615

C2

− 0.2639

C2

− 0.2336

C3

− 0.0779

C3

− 0.0891

C3

− 0.0780

C3

− 0.1526

C4

− 0.2075

C4

− 0.1758

C4

− 0.1867

C4

− 0.1749

C11

0.0882

C11

0.0595

C11

0.0840

C11

0.0486

C13

− 0.3082

C13

− 0.2903

C13

− 0.3062

C13

− 0.2814

Cl37

0.0194

Cl37

− 0.0029

Cl37

0.0187

Cl37

− 0.0035

C38

− 0.0422

C38

− 0.1203

C38

− 0.0453

C38

0.1208

C40

− 0.3339

C40

− 0.2926

C40

− 0.3232

C40

− 0.4709

C41

0.7982

C41

0.6649

C41

0.3093

C41

0.1769

O42

− 0.6810

C43

− 0.1027

N42

− 0.3530

C42

− 0.2763

O44

− 0.6310

S44

− 0.1067

C43

0.3147

C44

0.3513

C45

0.2828

N45

− 0.4985

N44

− 0.3579

C48

− 0.2379

N46

− 0.3770

C49

0.8131

  

C50

0.7944

  

O53

− 0.6096

  

O53

− 0.6966

3.7.3 Electronic transitions

The calculated electronic vertical transition energy, oscillator strengths (f), maximum absorption wavelength (λmax) and nature of the transitions are collected in Table 7. The electronic transition could be influenced by the polarity of solvents. From Table 7, the strongest absorption value for all sensitizers are mainly related to the transition from the HOMO to the LUMO. Major contributions of electronic transitions were observed 98–99% from HOMO to LUMO. Moreover, one can find that the TD-DFT calculated maximum vertical excitation of compound 2 was remarkably red-shifted in all solvents as compared to other TPA chromophores and the increasing order is: 4 < 3 < 1 < 2 (as obtained from both DFT functional). Similar trends were observed in the experimental and computational methods using the functional CAM-B3LYP and B3LYP. Computationally compound 2 shows vertical excitation at a longer wavelength than the other compound 1, 3, and 4. The trend in experimental absorption is similar.
Table 7

Electronic transition data obtained by TD-B3LYP and CAM-B3LYP/6-311++G(d,p) level of each molecule in CHCl3 solution

Dyes

functional

λmax (nm)

States

f

Excited energy (eV)

Transition character

1

B3LYP

546

S1

1.156

2.27

104 → 105 (99.82%)

CAM-B3LYP

457

S1

1.425

2.71

104 → 105 (89.95%)

2

B3LYP

583

S1

1.208

2.12

131 → 132 (99.71%)

CAM-B3LYP

475

S1

1.575

2.61

131 → 132 (85.76%)

3

B3LYP

542

S1

1.135

2.28

90 → 100 (99.72%)

CAM-B3LYP

456

S1

1.434

2.71

90 → 100 (90.25%)

4

B3LYP

491

S1

1.182

2.52

122 → 123 (98.75%)

CAM-B3LYP

394

S1

1.673

3.14

122 → 123 (80.41%)

(104, 131, 90 and 122 are HOMOs of compounds 1, 2, 3, and 4 respectively)

3.7.4 BLA and BOA study

Bond length alteration (BLA) and bond order alteration (BOA) are the geometrical and electronical parameters that depend on the π-conjugation length and the strength of the D and A units. The BLA and BOA values of ground state optimized geometries were calculated using the B3LYP optimized geometries. BLA and BOA values are shown in Table 8. From Table 8 it is seen that the BLA values increase in the order 3 < 4 < 2 < 1 whereas BOA value shows the reverse trend. The magnitudes of µ, α, and β increases with increasing donor to acceptor strength and increasing π-conjugation length, especially in case of compound 1.
Table 8

BLA and BOA of all compounds obtained by B3LYP/6-311++G(d,p)

Sensitizer

1

2

3

4

BLA

0.09734

0.07172

0.06561

0.06922

BOA

− 0.54078

− 0.32598

− 0.29824

− 0.10201

3.8 NLO study

3.8.1 Solvatochromic method for determination of NLO properties

The values of the linear polarizability αCT of TPA based D–π–A chromophores are obtained by using absorption and emission spectroscopy. The solvatochromic method is utilized for determination of dipole moment of the lowest lying CT excited state. The chromophores show good CT character in one direction is essentially coincident with the molecular long axis (X-axis) [69, 70]. The linear polarizability is sensitive to solvents and is evident in both the experimental and computational studies. The compound 2 and 3 show the higher αCT value in polar solvents. There is a substantial increase from18 to 29 × 10−24 esu to 122–202 × 10−24 esu. The values forthe first hyperpolarizability obtained using the solvatochromic method (Table 9) are based on several assumptions and thus allow only an approximate estimate of the dominant tensor of total hyperpolarizability along the direction of CT which is the major contributor to the total hyperpolarizability. The solvatochromic βCT values are given in Table 9. The solvatochromic values of β for all compound 1, 2, 3 and 4 though differ in their values, are in order of (× 10−30) esu. The βCT values are higher in polar solvents such as methanol, DMF, and DMSO.
Table 9

Effect of solvent on values of polarizabilities (α) (all values in × 10−24 esu), first hyperpolarizabilities (β) (all values in × 10−30 esu) and static second hyperpolarizability (γ) (all values in × 10−36 esu) for compounds 1, 2, 3, and 4

Solvents

1

2

3

4

α

β CT

γ

α

β CT

γ

α

β CT

γ

α

β CT

γ

CHCl3

164

344

− 1010

111

249

− 271

127

185

− 1067

20.8

39.4

45.6

EA

165

345

− 946

118

261

− 339

163

242

− 1841

20.2

37.6

43.1

Acetone

124

262

− 417

120

236

− 380

122

173

− 908

25.9

45.9

41.1

ACN

168

354

− 991

117

260

− 340

124

163

− 906

24.7

46.1

48.2

MeOH

173

371

− 1091

93.9

195

− 173

154

221

− 1581

29.7

55.2

49.6

DMF

121

220

− 415

124

278

− 400

202

271

− 2716

20.2

38.3

45.2

DMSO

139

292

− 607

131

289

− 520

162

234

− 1789

18.3

32.9

36.9

A comparison between B3LYP computed and experimental values shows that compound 2 has the computed values of 76 (× 10−30) esu to 88 (× 10−30) esu and experimental values from 163(× 10−30) esu to 271(× 10−30) esu across the solvents. The value of β0 or total first order hyperpolarizability of the molecule gives an estimate of the NLO properties of the organic molecules. This value considers all the directional tensor components of hyperpolarizability. The solvent effect is consistently shown higher in polar solvents (methanol, DMSO, and DMF) by the solvatochromic method.

The second-order hyperpolarizability at molecular level originating from the electronic polarization in the non-resonant region can be treated by a three-level model [71, 72, 73, 74, 75].

The second order hyperpolarizability can be expressed as the ‘‘solvatochromic descriptor” and the values calculated are given in Table 10. The solvent effect is also evident in this parameter, higher the polarity of the solvent (methanol), higher the second order hyperpolarizability. The compound 4 has solvatochromic values of 37(× 10−36) esu to 50(× 10−30) esu. The individual components of the second order static hyperpolarizability γ are obtained computationally. The values are proportional to the solvatochromic descriptor γ of the second order hyperpolarizability. The values have a trend where higher polarity solvents show higher values of γ, and a similar trend is followed by the solvatochromic obtained values.
Table 10

Comparison between solvatochromic and theoretical NLO value of compounds 1, 2, 3, and 4 in chloroform

Dyes

Experimental

Computational

 

μ

α

β

γ

μ

0

β

γ

  

10−24

10−30

10−36

 

10−24

10−24

10−30

10−36

1

22

164

133

− 1010

10

109

26481

845

4108

2

17

111

249

− 271

11

107

24361

772

3918

3

15

127

185

− 1067

15

84

14292

605

1827

4

9

20.8

39.4

45.6

14

91

15727

517

2995

Static dipole moment [µ (Debye)], mean polarizability [α0 (10−24 esu)], c polarizability anisotropy [Δα (10−24 esu)], first hyperpolarizability [β0 (10−30 esu)], second hyperpolarizability [γ (10−36esu)]

3.8.2 NLO properties by using DFT and TD-DFT method

The first and second order hyperpolarizability (β and γ), static polarizability (α0), and related properties dipole moment(μ), ∆α, αvδ, β, for all the chromophores are calculated using the B3LYP and CAM-B3LYP functionals with 6-311+G (d,p) basis set [76]. The values calculated of the TPA chromophores in different solvent environments using Eqs. (13–17) in supporting information are enlisted in Table 11 and Table S3. A comparison of the calculated hyperpolarizabilities suggests that the results obtained by using the B3LYP functional are better than those obtained CAM-B3LYP functional. B3LYP functional predicts higher β values as compared to the CAM-B3LYP functional.
Table 11

Theoretical NLO value of compounds 1, 2, 3, and 4 in chloroform

 Dyes

μ

α 0

∆α

β

γ

  

10−24

10−24

10−30

10−36

1

     

 B3LYP

10

109

26481

845

4108

 CAM- B3LYP

13

76

8504

416

1253

2

     

 B3LYP

11

107

24361

772

3918

 CAM-B3LYP

10

94

14586

414

2008

3

     

 B3LYP

15

84

14292

605

1827

 CAM-B3LYP

14

76

9122

417

1313

4

     

 B3LYP

14

91

15727

517

2995

 CAM-B3LYP

13

82

9507

250

1209

Static dipole moment [µ (Debye)], mean polarizability [α0 (10−24 esu)], c polarizability anisotropy [Δα (10−24 esu)], first hyperpolarizability [β0 (10−30 esu)], second hyperpolarizability [γ (10−36 esu)]

3.8.3 Vibrational contribution to the linear and NLO response

The connection between an electronic polarization and the vibrational motions leads to a considerable vibrational input to the non-linear optical (NLO) properties. DFT is a current technique to calculate the vibrational input towards polarizability and hyperpolarizability of NLO materials. The vibrational contributions to electronic hyperpolarizabilities and polarizabilities were calculated by using B3LYP functionals and 6-311++G(d,p) basis set in ethanol. The vibrational input to an element of α and β tensor has been calculated and compared with the corresponding electronic counterpart and listed in Table S4. The results show that the power of an individual component indicates a significant delocalization of charges.

3.8.4 Limits for the hyperpolarizability values

The limits for NLO values are obtained by the limiting theory proposed by Kuzyk [77].The γ values have two fundamental limits are negative limit a centrosymmetric and positive limit for an asymmetric NLO molecule. The limiting values for β and γ are compared with the corresponding obtained values by solvatochromic and theoretical calculations for compound 1, 2, 3, and 4 in Table S3. Table S3 shows that βvalues below the βmax limit and γ values are within the predictable limiting values for all chromophores in all solvents.

4 Conclusion

A triphenylamine based donor with chloro-group suitably placed on π-linker with different electron withdrawing groups. Rhodamine-3-acetic acid as an electron withdrawing group offers an increased length of π-conjugation in compound 2, and it showed red shifted absorptions as well as emissions as compared to other three chromophores. The compound 2 and 4 show good linearity with respective the LM and McR polarity functions. The observed trends in LM and McR for compounds 1 to 4 are: 1 < 3 < 4 < 2. The chromophores can be separated into two groups as ICT (2) and TICT (1, 2, and 4) depending viscosity sensitivity which is moderate (2.12) to very high (14.14) respectively. The compound 4 shows high viscosity sensitivity value (x = 0.57) as compared to the very famous FMR DCVJ have x value 0.40 in same viscous media. The multilinear analysis of absorption and emission spectrais carried out by using Kamlet–Taft and Catalan parameters which shows that solvent dipolarityand polarizability are main factors responsible for the slightly red shifted absorption and emission. BLA values show a linear correlation with the computationally obtained NLO parameters. The NLO properties of all the chromophores are investigated experimentally and computationally. The compound 1 is a better NLO material than the other three compound 2, 3, and 4. The compound 4 is a good FMR for viscosity sensitivity study.

Notes

Acknowledgements

The authors, Dinesh Patil, Kiran Avhad and Mayuri Kadam are thankful to UGC-SAP, India for Junior and Senior Research Fellowships.

Compliance with ethical standards

Conflicts of interest

The authors declare that they have no conflict of interest.

Supplementary material

42452_2019_268_MOESM1_ESM.docx (2.4 mb)
Supplementary material 1 (DOCX 2482 kb)

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© Springer Nature Switzerland AG 2019

Authors and Affiliations

  1. 1.Department of Dyestuff TechnologyInstitute of Chemical Technology (ICT)MumbaiIndia

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