It has been widely reported that the non-linear optical (NLO) response in dyes can be produced by addition of the benzothiazole group. Due to the electron withdrawing ability of dyes the benzothiazole group possess efficient optical and chemical properties. Benzothiazole cannot withdraw electrons efficiently and an additional acceptor molecule is required for this purpose. Benzothiazole is an unsymmetrical molecule because it possesses electron withdrawing and electron donating groups on different loci. Positions C-2 and C-6 have been found to be the most active charge–transfer (CT) sites. The NLO response strongly depends upon the added acceptor molecule. Moreover by incorporation of a robust acceptor to benzothiazole group at position C-2 produced robust effects. A model of compatible and incompatible positioning of 1,3-benzothiazole unit can help to easily understand the charge transfer performance of both types of substituent (electron-donating and electron-withdrawing) (Figure 1). The virtual dipole between C-2 (electron-deficient site) and benzene ring of benzothiazole (electron rich site) is oriented to facilitate the internal charge-transfer (ICT) within molecule, so enhances the dipole moment of excited state (cf., Figure 1).
The higher alteration between ground and excited state is shown by figure 1. Similarly the stability of lowest unoccupied molecular orbitals (LUMO) is improved with the compatible position of electron withdrawing group (EWG). So the excitation energy is decreased with increase in EWG strength. Both of these features play a very important role in determining high NLO responses. The synthesis of organic compounds is cheap, and gives high yield. That is why organic compounds are studied widely. The structural modifications affect the non-linear optical (NLO) response of substances which may allow the chemical structure to be fine tuned. Enhanced NLO properties have helped such complexes to gain worldwide attention. Intramolecular charge transfer (CT) is linked to the non-linear optical properties (second order). π-spacer or bridge helps the transfer of charge from donor to acceptor (D-A).
The NLO properties of simple molecules can be easily calculated. This tailored the path towards the development of models for molecules having high non-linear optical properties. The NLO characteristics of molecules can be enhanced by structural adjustments with the help of the addition of appropriate donor-pi-spacer-acceptor (D-π-A) structures. This kind of replacements is important in tuning of conjugation with NLO responses.
This computational design has been proposed for different molecules possessing different donor and pi-spacers. These molecules also possess dithiazole groups as additional acceptors at different unsymmetrical positions. Benzothiazole (BT) decreased the width of the band gap and this band gap acts to capture electrons. This facilitates the flow of electrons towards cyano acetic acid. Similarly two hetero-atoms containing aromatic compounds like phenothiazine and phenoxazine are used in DSSC devices as a hole transport material. These devices are helpful in the synthesis of intermolecular excimer by preventing molecular coalescence. D-π-A-A configuration is possessed by all the molecules. In this study the effect of polarity on the absorption properties and nonlinear optical characteristics are reported in detail. Here the absorption and optical properties of dyes have been calculated with help of quantum chemical calculations. This work paves the way for designing new and effective organic dyes. Hopefully this work will be helpful for other researchers in synthesis of NLO complexes with improved properties.
2 Computational details
Program package Gaussian 09 was used to perform calculations, while structures of dyes in vapor phase was optimized with the help of 6-31G (d, p) basis set and B3LYP functional. All simulations were done on 10-8 au SCF convergence criteria and 10-8 integration grid. The frequency calculations have been applied for optimization of computed geometries. All the values of wave numbers are not imaginary, this depicts the success of the geometry optimization.
Owing to the self-interaction error, the exchange correlation functional under estimates the charge transfer nature and excitation energy of excited states. This is why it is important to choose relevant functional properly describing the ultraviolet-visible absorption spectra of organic dyes possessing D–π–A configuration.
Computational methodology DFT-TDDFT thoroughly explains the optical properties of dyes.[9, 10] Initially λmax of dye (reference dye) was calculated in the influence of the basis and functional set. Therefore the CAM-B3LYP/6-31G (d,p) methodology is considered most appropriate for the study of optical properties of dyes. The absorption spectra of different dyes are also simulated by this methodology. CAM-B3LYP functional with Coulomb-attenuating methodology was used to calculate the optical properties of designed dyes. The transition energies of various dyes are mostly simulated by using CAM-B3LYP functional.Reported literature has indicated that consistent results are obtained from this functional for estimation of link of structure and property of organic chromophores TPA cross-sections.[8, 17] The absorption spectra of dyes was calculated by using TD-DFT method. Solvent effect (acetone) was explained with the help of Conductor-like polarizable continuum model (CPCM). The following equation is used for determination of average polarizability (α): (1)
βxxx, βyyy, βzzz, βxyy, βxxy, βxzz, βyzz, βxxy and βyyz; these components are present in Gaussian 09W output file. First hyperpolarizability (βtot) is calculated using following formula: (2)
3 Results and discussion
In this study, simulated designing of non-linear optical (NLO) dyes was performed. Various dyes along with D-π-A-A were designed. The structures of these dyes are provided in Figure 2. TD-DFT and DFT methodologies were completed to study the influence of two donor moieties on the NLO characteristics of molecules. Basic parameters: (i) hyperpolarizability (α), (ii) polarizability (β), (iii) light harvesting efficiency (LHE), and (iv) absorption wavelength were calculated.
3.1 HOMO-LUMO and band gap
To understand the photovoltaic performance of the molecules, electron density distribution and molecular geometry of organic dyes should be taken into account.
These statistics helped to identify the spectroscopic as well as electronic properties of the pi-con]jugated bridges or spacers. The geometry optimizations were accomplished with DFT and the B3LYP 6-31G (d,p) method. Potential energy surface possesses a local minima indicating the absence of imaginary frequencies. This is used for testing of optimization of ground state geometries.
Figure 2 shows two things: (i) iso-density graphs of frontier molecular orbitals, and (ii) energy levels of the isolated dyes.Analysis illustrates that phenoxazine and phenathiazine are key contributors for HOMO, while the cyano acetic acid is key contributor for LUMO. This is completely in agreement to the estimation that an electron travels from phenoxazine or phenathiazine to cyanoacetic acid (within molecule) upon excitation. The comparison of band gap, HOMO and LUMO are given in Table 1 and frontier molecular orbitals are shown in Figure 3. The decreasing order of band gap of the designed systems is: 4, 2, 3, 1, 8, 6, 7 and 5. Band gap of the systems comprises of polarity is smaller than systems with reverse polarity.
3.2 Non-linear optical properties
Non linear optical properties are significantly important in signaling, communication and optical switches. The electrical characteristics of these substances determine the extent of optical response. The power of optical reply is a function of polarizability or linear response (α) and hyperpolarizabilities or nonlinear response (β, γ). Therefore the non-linear optical potential of substances can be calculated from their non-linear optical properties.
Data for four dyes i.e. values of average polarizability and major contributing tensors, are given in Table 2. Decreasing trend of average polarizability of designed systems is from System 7 to System 1. Polarizability of systems having common polarity was greater than the reverse polarity containing systems. Usually, a large linear polarizability is essential to get large hyperpolarizabilities.  x and y direction transitions are mostly used for determination of polarizability. The dipole polarizability along the x direction is explained with the help of the following equation as an example: (3) Where Mxgm and Egm are transition moment and transition energy between the ground and mth excited state respectively. From eq 3, we can observe that the value of α is directly proportional to the transition moment raised to power two and the α and transition energy are inversely related to each other. Generally dye with a large value of Mxgm will have a large value of α.
The second order polarizability (β) can be estimated from the non-linear optical response. Beta and charge-transfer in a molecule is correlated. This correlation moves from donor towards the acceptor side and travels via pi-conjugation. This pi-conjugation acts as a link between acceptor and donor. This is helpful in flow of charge transfer (ICT) within molecule. This interaction affects the dipole moment and non-linear optical response (second order) of molecules.
CAM-B3LYP functional was used for measuring the hyperpolarizabilities and the computed βtot values and major contributing tensor of systems are given in Table 3. Decreasing order βtot of four dyes (systems) is as follows: 7, 5, 8, 6, 3, 4, 1 and 2. Polarizability of systems based on polarity was greater than reverse polarity containing systems. The charge transfer through pi-spacer is responsible for efficient NLO responses.
3.3 UV-Vis spectra of dyes
The CAM-B3LYP/6-31+G* level was used to perform TD-DFT calculations to simulate the C-PCM in acetone and the information about excited states are obtained for absorption spectra analysis in Figure 4. Ten lowest singlet-singlet transitions were calculated during TD-DFT calculations.
In Table 4, the computed oscillator strengths (f), maximum absorption wavelengths (λmax) and nature of the transitions are summarized. The systems based on polarity are Systems 1-4 and on reverse polarity are Systems 5-8. The absorption spectra of Systems 5-8 are red shifted compared to those of Systems 1-4. HOMO to LUMO transitions are the major observed transitions for all systems having polarity and reverse polarity as shown in Figure 3. Furan ring containing systems i.e. 1, 2, 5 and 6 show red-shift compared with the thiophene ring containing systems i.e. 3, 4, 7 and 8. Red shifted absorption wavelengths were also observed in case of phenoxazine. Moreover the optical efficiency of dyes is linked to their light harvesting efficiency (LHE). The LHE values of dyes should be high so that the photocurrent response is maximized. The LHE of a dye can be expressed as: (4) Where f is the frequency of oscillator.
An explanation linking the structure and property is required before explaining the reason for second order NLO properties. Two state model (sum-overstates (SOS) approach) explains the association between low lying charge transfer transition and hyperpolarizability. In the static case, the following model equation is used to find βCT: (5)
Where Egm is transition energy, Δμgm is the change of dipole moment among the ground and mth excited state and fgm is the oscillation value of the transition from the ground state (g) to the mth excited state (m). According to this model, the hyperpolarizability and product of transition dipole moment and oscillating strength are directly related with each other and hyperpolarizability and transition energy raised to power three are inversely related with each other.
An NLO material possessing low energy charge transfer excited states and high value of oscillation is optimum design. This suggests a very high value of hyperpolarizability. The dipole moment values of all the simulated systems are almost same which is why the value of Δμgm is taken as constant. That is why the NLO response (second order) and oscillation of designed dyes are directly related with each other. Moreover NLO response is inversely related to the cube of transition energy. The small value of transition energy gives the high value of hyperpolarizability.
4 Experimental Section
After describing systems (1-8) theoretically, the author was inspired to probe into the reliability of these theoretical results. In order to confirm these theoretical predications, system 1 has been synthesized as a check on all other studied systems. It was noticed that the synthesis of system 1 verified the hypothesis. The experimentally measured spectrum of system 1 matches with that of simulated dye (system 1) which indicates that the theoretical methods used in this paper are reliable. The experimental value of system 1 measured to be 435 nm which is in good agreement with the theoretically value calculated to be 431 nm (shown in Table 4 and Figure 5).
Phenoxazine, bromothiophene, methyl nitrile and cyanoacetic acid were purchased from Sigma-Aldrich. All the solvents were distilled before use.
4.2 Synthesis of System-1
A mixture of phenoxazine (S-1) (10g), chlorodecane (7mL) was mixed in dry acetone (10 ml) and anhydrous K2CO3(1 g). This mixture was refluxed for 24 hours. 2 g Ferric bromide (FeBr3), bromothiophene (3mL) and S-2 molecule (7g) of compound were mixed in ethanol and refluxed for 4 hours. In this way product S-3 is formed. Then compound S-3 (5 g) and N-bromosuccinimide (2g), carbon tetrachloride and refluxed for 3 hours. Then compound S-4 (4 g) was dissolved in tetrahydrofuran and n-butyl lithium (2 g) and refluxed for 4 hours at -78 °C. Then compound S-5 (3 g), 2-bromo-2,3-dihydro-1,3-benzothiazole (3 g) and trimethoxyborone (2 g) was mixed and refluxed at -78 °C for 1 hour. Following this compound S-7 (2 g) and palladium tetra(triphenyl phosphonium), sodium carbonate (2 M) and toluene were refluxed for 24 hours. Then compound S-8 (1 g) was dissolved in pipperidine and refluxed for 3 hours in the presence of 2 mL methyl nitrile and cyanoacetic acid (2 g) . All the products were filtered, washed and cooled. All the products were also recrystallized from ethanol.
4.3 Characterization: UV-Visible Spectra of System-1
System-1 (0.2 mg) was dissolved in acetone and its spectrum was recorded on a UVD3500 UV-Visible spectrophotometer, in the 300-550 nm wavelength range.
The UV-Visible spectrum of dye is given in Figure 5. The spectrum possess peaks at 342 and 435 nm wavelength. The absorbance of peak (435 nm) is maximum among all peaks. These peaks are present at 338 and 431 nm wavelength in simulated pattern of dye (Figure 4 system 1). The experimentally measured spectra of the system-1 shown in Figure 5 matches with that of the simulated dye. It shows that peak of experimental pattern is slightly red-shifted as compared to that of simulated pattern. This occurs due to strains present in structure, solvent effect or instrumental error.
The effect of polarity of the benzothiazole group on the non-linear optical properties is studied using DFT and TD-DFT methods. The systems with polarity possess greater polarizability and hyperpolarizability than the systems with reverse polarity. The systems with commonly used polarity show higher polarizability and hyperpolarizability compared to the systems with reverse polarity. The absorption spectra of systems with commonly used polarity are observed red shifted compared to that of systems with reverse polarity. These simulations will be helpful in designing dyes with desired properties and performance in optics and electronics. The results obtained in this work may be useful in the design of advanced materials for special applications. Lastly, it is concluded that the synthesis of system 1 verified the hypothesis which indicates that the theoretical methods used in this paper are reliable. The experimentally measured spectrum of system-1 matches with that of simulated dye (system 1). The experimental value of system 1 measured to be 435 nm which is in good agreement with the theoretically value calculated to be 431 nm.
The author would like to acknowledge the support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. SR161009.
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About the article
Published Online: 2017-06-14
Citation Information: Open Chemistry, Volume 15, Issue 1, Pages 139–146, ISSN (Online) 2391-5420, DOI: https://doi.org/10.1515/chem-2017-0017.
© 2017 Muhammad Ramzan Saeed Ashraf Janjua. This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 3.0 License. BY-NC-ND 3.0