The NLO property of a few designed inorganic-organic hybrid materials based on Nickel dithiolenes end-capped with donor-acceptor groups has been studied theoretically. All the designed molecules possess high first hyperpolarizability values indicating their potential use in optics, photonics, and as photosensitizers. Among the four designed systems, the BODIPY-containing systems significantly reduce the HOMO-LUMO energy gap resulting in a massive trek in the first hyperpolarizability (β) values. To judge their high NLO response, transition dipole moment (TDM) density has been plotted and it has been found that electron dissipation occurs through the molecular network with a large Δr index value. It is to be noted that high Δr index values are quantitative measurements to understand the type of transitions, and we noticed that a charge transfer transition occurs in all of our designed systems. Hence a nice correlation between the first hyperpolarizability, TDM density, and Δr index value has been observed. The global reactivity parameters are also studied and correlated nicely with polarizability and hole-electron transport ability.
Research Article
Design of a Few End-Capped Donor-Acceptor Inorganic-Organic Hybrid Nickel Dithiolene Derivatives for Efficient NLO Response
https://doi.org/10.21203/rs.3.rs-2431513/v1
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Journal Publication
published 22 Apr, 2023
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In recent times serious efforts have been devoted to the studies of nonlinear optical (NLO) properties of material because of their multifarious applications in the optoelectronic and photonic devices of telecommunication, optical data storage, optical transmission, optical switching, optical signal processing, optical computing and even so [1, 2]. These materials are also efficacious for biomedical applications such as photodynamic therapy, biological imaging, and optical sensors [3, 4]. For the above-cited applications, the essential requirement is to develop a molecule with large molecular first-order hyperpolarizability (\(\beta )\), which can be obtained from typical organic NLO chromophores constituted by a simple scheme of donor–(conjugate bridge)–acceptor (D–π–A) structure [5–7]. However, the bridge moieties can also selectively be replaced by organometallic groups to have an efficient NLO response as well [8]. In a number of reports, it is found that metal complexes with 1, 2-dithiolene are a very promising class of materials[9] for the above-cited application because of their extensive π-electron delocalization and the low-lying excited states[10]. Especially, Ni bis(dithiolene) complexes show unique properties [11] such as tunability, and high thermal and photochemical stability; consequently, they can be useful in organic electronics and are also suitable for Q-switching Nd: YAG lasers. It is also possible to use 1, 2-dithiolenes as an energy conversion material as well as a photodetector [12]. Moreover, they exhibit exceptionally long excited-state lifetimes as well as tunable absorption properties, making them suitable for photovoltaic applications [13].
Nickel bis(dithiolenes) complexes are also known as very strong chromophores that possess near-infrared (NIR) transitions. In the neutral state, this type of metal complex shows high absorption coefficients (≈ 30 000 M− 1 cm− 1) in a range of NIR wavelengths from 900 to 1600 nm, depending on the choice of the metal center and dithiolene substituent. In the presence of NIR, these complexes are stable in photothermal and photochemical conditions and do not produce singlet oxygen. Therefore, nickel bis(dithiolene) complexes are suitable for the photo-controlled release of drugs from organic nano-carriers and they can be used as a new photo-thermal therapeutic agent under NIR irradiation [14].
Nickel bis(dithiolenes) complexes of non-innocent ligands (e.g., 1,2-dithiolene) were synthesized for the first time in 1960 [15]. Afterward, a number of theoretical studies have been performed to understand the electronic structures and bonding schemes of these compounds. From these studies, it has been found that these complexes possess a high degree of electron delocalization [16]. Recently, INDO (intermediate neglect of differential overlap) computational studies by Herman et al. [17] on Ni(C2S2H2) complexes showed that the nickel atom is + 2 oxidation state with a d8 electronic configuration and this is very common for these types of complexes [18, 19]. Various theoretical and experimental studies on Ni complexes also confirmed the previous observation [20, 21].
It is a very common and effective strategy to fuse aromatic rings like benzene [22–24], functionalized benzene [25, 26], thiophene [27, 28], pyridine [29], quinoxaline [30, 31], and other heterocycles[32, 33] to the dithiolene core to increase electron delocalization and molecular planarity. In comparison to the others, thiophene fused metal dithiolenes are of particular interest because of their stable crystal packing, and more significant overlap of frontiers orbitals. In spite of that additional thiophene content would be expected to increase the weak intermolecular forces like hydrogen bonding, van der Waals forces, π-π interactions, and S— S/M—S interactions [34, 35] which consequently enhance their packing arrangement. All these facts motivate us to choose thiophene fused Ni dithiolene type systems for our present work. Moreover, a number of previous reports reveal that a coupler can also play a decisive role in designing efficient NLO-responsive molecules. Thus, by judicious choice of the inorganic-organic hybrid coupler, one can design a good, stable, biocompatible NLO active molecule for different applications.
In this work, we designed our systems (Scheme-1) by taking two such inorganic-organic hybrid materials coupled through aromatic spacer viz. furan, thiophene, pyrrole, and BODIPY, and the whole molecules are end-capped with donor-acceptor groups like amine and cyanide. As a consequence, the conjugation length is maximized along the molecular backbone [36]. Among the couplers, we consciously use boron dipyrromethene (BODIPY) in our investigation, as BODIPY-based dyes are known to have desirable photophysical properties, including strong absorption bands in the UV-vis region, and high fluorescence quantum yields [37]. There are a number of applications for BODIPY dyes in biolabeling, bioimaging, and photosensitizer in photodynamic therapy (PDT) [38–40]. Aza-BODPIY-based fluorescent dye [41] can also be potentially used in a Switchable Fluorescent Probe (SFP) in several formats, cell imaging, in vivo tissue imaging, temperature sensing, and tissue Ultrasound-Switchable Fluorescent probe (USF) imaging [42]. This finding prompted us to make a strategy for designing a new class of inorganic-organic hybrid NLO materials with high efficiency. In addition to this, the global reactivity parameters(GRP) such as ionization potential (IP), electron affinity (EA), softness (σ), hardness (ƞ), chemical potential (µc.p), electrophilicity index (ω), and ΔEback−donation along with light harvesting efficiency (LHE) parameters are calculated to have relevant information about the charge transport property of the studied molecules and for the necessary correlation with the NLO property.
In the presence of an oscillating electric field, the NLO properties can be defined as a power series of the molecular dipole moment according to Eq. 1[43].
$$E={E}^{0}-\sum _{i}{\mu }_{i}{F}_{i}-\frac{1}{2}\sum _{ij}{\alpha }_{ij}{F}_{i}{F}_{j}-\frac{1}{3}\sum _{ijk}{\beta }_{ijk}{F}_{i}{F}_{j}{F}_{k}-\frac{1}{4}\sum _{ijkl}{\gamma }_{ijkl}{F}_{i}{F}_{j}{F}_{k}{F}_{l}-$$
1
Here E and E0 are referred to as the energies of the perturbed and zero-field unperturbed states, the Fx’s are the field strength along a particular direction, the tensors µi, αij, βijk, and γijkl stand for the dipole moment, linear polarizability, first or static hyperpolarizability and second or cubic hyperpolarizability terms, respectively [44, 45]. In general, the field as well as polarizability and hyperpolarizability terms of the above equation are frequency dependent. The first static hyperpolarizability (βtotal) may be expressed by using x, y, z components as follows,
………………………(2)
and
β x = βxxx + βxyy + βxzz,
β y = βyyy + βxxy + βyzz,
β z = βzzz + βxxz + βyyz.
The total second-order hyperpolarizability, γtotal [46] is calculated by using Eq. 3.
$${\gamma }_{tot}=\frac{1}{5}\left[\left({\gamma }_{xxxx}+{\gamma }_{yyyy}+{\gamma }_{zzzz}\right)+2\left({\gamma }_{xxyy}+{\gamma }_{yyzz}+{\gamma }_{xxzz}\right)\right]$$
3
Ionization Potential (IP) and Electron Affinity (EA) signify the negative energy of the HOMO and LUMO energy states respectively of the corresponding molecules.
\(IP=-{E}_{HOMO}\) And \(EA=-{E}_{LUMO}\)
The chemical reactivity parameters such as chemical potential (µ), chemical hardness (ɳ) and softness (s), electrophilicity (ω), and back donation (ΔEback−donation) are calculated by the following equations:
$$\mu = -\frac{1}{2}({E}_{HOMO}+{E}_{LUMO})$$
$$ɳ= \frac{1}{2}({E}_{LUMO}-{E}_{HOMO})$$
$$s= \frac{1}{2} ɳ$$
$$\omega = \frac{{\mu }^{2}}{2ɳ}$$
$${\varDelta E}_{back-donation}= -\frac{ɳ}{4}$$
The structures of all the designed systems (C1, C2, C3, C4) were optimized using density functional theory (DFT) based methodology with B3LYP functional [47] in conjugation with 6-311 + + g (d,p) basis set incorporating effective core potentials (ECPs) basis, LANL2DZ for the transition metal atom. Indeed, employing a ECPs basis such as LANL2DZ for transition metals, while using all-electron basis sets for all other non-transition metal atoms, has become more and more popular in the computation of transition metals containing systems [48]. The frequency analysis at the same level of theory confirms that the molecules have been optimized at the minimum energy level without imaginary frequency, which implies the local minima of the optimized geometry. Therefore, all the obtained unrestricted B3LYP solutions are stable.
TD-DFT computations were performed for the determination of electronic structures, and absorption spectra, and using this data a contribution analysis has been carried out in order to highlight the NLO active segments in the molecules. For calculating 𝛌max (nm) values we have employed CAM-B3LYP/6-311 + + G (d,p) methodology with the same ECPs. The Columb-attenuated hybrid exchange-correlation functional (CAM-B3LYP) allows sufficient spatial overlap between the donor and acceptor parts of the molecule, which is necessary for the accurate description of the excited states, especially for the charge transfer state [49]. In one of our previous works[50]we optimized the computational methodology with the help of experimental data provided for synthesized systems and hence used the same methodology for calculating excitation energies for our designed systems (C1, C2, C3, C4).
The NLO behavior and first hyperpolarizability (β), have been quantitatively evaluated for all the studied species by using CAM-B3LYP/6-311 + + g(d,p) methodology with ECPs. We have already benchmarked our computational model in one of our previous works [50] with a very common push-pull system and could reproduce the β-value to a reasonable extent with the same computational model and hence rely on that. The transition dipole moment (TDM) has been calculated based on the dipole moment integral between occupied and virtual MOs. The contribution of each MO pair to the transition dipole moment can be straightforwardly evaluated and visualized with the help of the Multiwfn wave function analyzer [51]. ∆r index which proposed to measure the charge transfer length during electron excitation has also been calculated in Multiwfn. All the computations were performed using Gaussian 16 program package [52].
4.1 The electronic structure of Nickel dithiolene (NiBDT)
Bisdithioglyoxalnickel, Ni(S2C2H2)2, or simply Nickel dithiolene (NiBDT) is an example of a square planar complex having a “non-innocent” conjugated sulfur-based ligand. So far, several bonding structures have been proposed for NiBDT [53], where the central atom possesses 0, + 2, and + 4 oxidation state. However, in most of the interesting studies, it is concluded that these complexes are of Ni (II) with each ligand bearing a radical [54, 55]. We also noticed that the ligand-based part mainly constitutes the frontier orbital rather than the metal-based part [56]. So, the change in oxidation state takes place at the ligand center than at the metal center. Upon addition of an electron to NiBDT (i.e formation of [NiBDT]−), the extra electron occupies an antibonding orbital of ligand (in case of a “non-innocent” ligand) or metal d-orbital of MII ion (in case of “innocent” ligands). Thus, two types of different spin states may exist: open-shell doublet and quartet. The magnetic susceptibility data of some synthesized systems based on NiBDT core i.e., Tetrabutylammonium Bis(5-(2-thienyl)-2,3-thiophenedithiolato)-nickelate(II), [Bu4N]NiTDT and various salts of NiTDT were measured[57] and the obtained susceptibility values are found to be greater than 1.73 µB, indicating S = 1/2. Hence, all the subsequent computations on the studied complexes are performed assuming the doublet state as the ground state.
4.2. Frontier Molecular Orbitals
The frontier molecular orbital (FMO) theory is remarkable in predicting the chemical stability of the molecules under investigation [58]. It also gives information about these molecules' electronic circulation and molecular geometry. The study of the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) theory is one of the most significant studies that remarkably explain the UV/Vis spectra of the molecules. Generally, HOMO expresses the electron donation facility while LUMO denotes the capability of accepting an electron [59]. Besides, the HOMO-LUMO energy gap (HLG) is an important parameter for the prediction of the dynamic stability, chemical reactivity, chemical hardness, and softness of a molecule [60]. A high HLG value indicates chemically hard molecules with high kinetic stability and less chemical reactivity. In contrast, a low HLG value reveals less kinetic stability i.e., chemically soft molecules. It is well-known that polarizability increases with the increasing softness of a molecule which surely facilitates the NLO response as well [61].
All of our designed systems (C1 to C4), possess a very low HOMO-LUMO energy gap (Table 1) indicating the chemical softness of the molecules. Upon incorporation of different spacers in the NiDT-based framework, the HOMO-LUMO energy gap (HLG) reduced significantly for all four systems (around 0.90 eV), but in the case of BODIPY spacer (for the system C4), HOMO-LUMO gap is extremely small (0.53 eV). It is to be noted that the HOMO for all the systems except C4 is delocalized through the molecular framework which is responsible for the very low HLG. In this connection, it is worth mentioning that as all the designed systems possess a very low HLG, hence would exhibit excellent NLO response. Another important point to note is that in the case of the BODIPY spacer system, the HOMO is more stabilized by the NiBDT part capped with the acceptor group whereas the donor-capped NiBDT part stabilizes the LUMO. This difference in the generation of MOs can be correlated with their first hyperpolarizability value, which is higher in C4 than the others.
| Molecule | Energies in gas-phase (eV) | β × 10− 30 esu | ||
|---|---|---|---|---|
| HOMO | LUMO | HLG | ||
| C1 | −3.076 | −2.176 | 0.900 | 7168.133 |
| C2 | −3.059 | −2.146 | 0.913 | 8000.719 |
| C3 | −3.126 | −2.231 | 0.895 | 9084.767 |
| C4 | −3.123 | −2.591 | 0.530 | 14464.200 |
4.3. Absorption Properties
The absorption maxima for all of our designed systems are studied in the gas phase by using CAM-B3LYP/6-311 + + G(d,p)/LANL2DZ level of theory and the values are in the range of near-infrared (NIR) region (900 to 1600 nm)(Table 2) which indicates their photothermal and photochemical stability. Therefore, like other synthesized nickel bis(dithiolene) complexes, our designed hybrid molecules may also be used for the photo-controlled release of drugs from organic nano-carriers and can be used as photothermal therapeutic agents under NIR irradiation as mentioned earlier. Light harvesting efficiency (LHE) which is related to the oscillator strength (f) of vertical excitation has also been calculated for all the systems under study and values are reported in Table 2. LHE involves the capacity of transporting excitation energy to enhance the optical capture cross-section of photochemical reaction. For every molecule that participates in charge separation, a large number of other molecules capture the energy via light harvesting and this is the key requirement for developing solar cells. To understand the type of transition we have calculated the Δr index value (Table 2). It has been reported in the literature that the type of excitation can be judged with the help of the Δr index. Δr ≤ 1.5 Å indicates local excitations, whereas Δr ≥ 2.0 Å indicates long-range excitations [62]. Hence charge transfer type of transitions is occurring here.
| Compound | State | Excited energy (eV) | 𝛌max (nm) | f | LHE | MO transition | Δr (A0) |
|---|---|---|---|---|---|---|---|
| C1 | S6 | 0.8975 | 1381.45 | 0.7628 | 0.8273 | 184B→186B | 6.085 |
| C2 | S7 | 0.8935 | 1387.69 | 0.8433 | 0.8565 | 184B→186B | 6.397 |
| C3 | S6 | 0.9474 | 1308.71 | 0.7133 | 0.8065 | 188B→190B | 3.842 |
| C4 | S9 | 1.0131 | 1223.76 | 0.6850 | 0.79346 | 215B→218B | 6.238 |
4.4. Nonlinear optical properties
Finally, for the assessment of NLO properties of investigated systems, linear response, i.e., polarizability (α), and non-linear response, i.e., hyperpolarizability (β and γ) need to be evaluated. The average polarizability (Δα) of the investigated molecules increases from C1 to C4 and in all the cases it is showing regular variation with the dipole moment. The first and second-order hyperpolarizability (β and γ) values are following the same trend i.e., increases from C1 to C4. It is known that the first hyperpolarizability (β) value depends on several factors like HLG, the nature of charge transfer, electron delocalization, etc. However, HLG has a dominant effect on the β value and is in an inversely proportional relationship. The first hyperpolarizability is also related to the intramolecular charge transfer (ICT) through the molecular network from one donor site to the acceptor one and we noticed high charge transfer in all the systems. All the designed systems especially BODIPY-based systems possess extremely large first and second hyperpolarizability values (β = 14464.200 × 10− 30 esu and γ = 457056.500× 10− 36 esu) which surely make the molecules imperative for diverse applications. To explain the origin and variation of the NLO response, the transition dipole moment (TDM) density is also calculated for the systems under investigation. Table 4 shows the oscillator strength and pictorial representation of TDM for the corresponding transition. These data are also in agreement with the fact that the oscillator strength (f) is directly related to the square of the TDM (in atomic unit),
, where ΔE denotes the transition energy between the two states. From the reported TDM plot in Table 4, we observed that for the energy state with high oscillator strength electron dissipation occurs mainly through the organometallic network. All our designed systems are showing the nearly same type of electron-hole separation and it can be evident from their Δr index values as well. Hence, a nice correlation between Δr index, TDM, and hyperpolarizability has been observed.
| Molecule | µ (Debye) | Δα × 10−24 (esu) | βtotal × 10−30 (esu) | γ × 10− 36 (esu) |
|---|---|---|---|---|
| C1 | 28.107 | 191.860 | 7168.133 | 76289.200 |
| C2 | 28.530 | 202.043 | 8000.719 | 80597.860 |
| C3 | 30.236 | 211.318 | 9084.767 | 104823.900 |
| C4 | 40.914 | 216.578 | 14464.200 | 457056.500 |
Table 4. Dominant energy state, Oscillator strengths (f), and transition dipole moment densities for four designed systems. The purple color implies charge increase upon excitation and the turquoise color implies charge depletion. The isovalue is 2×10−3 in a.u.
4.5. Global reactivity parameters
The global reactivity parameters (GRP) such as ionization potential (IP), electron affinity (EA), softness (σ), hardness (ƞ), chemical potential (µc.p), electrophilicity index (ω), and
ΔEback−donation is calculated to have relevant information about the charge transport property of a molecule [63–65] and results are tabulated in Table 5.
| σωΔ | |||||||
|---|---|---|---|---|---|---|---|
| Molecule | IP (eV) | EA (eV) | µc.p (eV) | ƞ(eV) | σ (eV) | ω (eV) | ΔEback−donation (eV) |
| C1 | 3.076 | 2.176 | 2.626 | 0.450 | 0.225 | 7.662 | 0.112 |
| C2 | 3.059 | 2.146 | 2.602 | 0.456 | 0.228 | 7.423 | 0.114 |
| C3 | 3.126 | 2.231 | 2.678 | 0.447 | 0.223 | 8.022 | 0.112 |
| C4 | 3.123 | 2.591 | 2.857 | 0.266 | 0.133 | 12.619 | 0.067 |
The IP and EA of studied molecules are in the range of 3.059 eV to 3.126 eV and 2.146 eV to 2.591 eV respectively. It is well known that molecules with a higher EA value have a higher electron transport ability while molecules with a lower IP value have a higher hole transport ability. Here molecule C2 has the lowest IP value of 3.059 eV and C4 has the highest EA value of 2.591 eV. Therefore, molecules C2 and C4 will transport holes and electrons better than other molecules. It is interesting to note that with decreasing the HLG, IP and EA values both increases. The global hardness and softness of all the designed molecules are very less (less than 1eV), which in turn makes the molecule more polarizable. Moreover, the electrophilicity index and ΔEback−donation values are very high and low respectively which also suggest a facile charge transfer ability of the compounds amongst their HOMO and LUMO orbitals, which results in more polarizability and desirable NLO properties.
In the present computational study, the non-linear optical properties of four designed inorganic-organic hybrid systems based on NiBDT have been studied. All the systems possess very low HLG and consequently high hyperpolarizability values. This observation has also been supported by excitation energy calculations with the TDDFT approach. By systematic analysis, we also observe a nice correlation between Δr index, TDM, and hyperpolarizability. Further, we calculate the global index parameter in order to have an idea of the charge transport property and NLO activity of the designed inorganic-organic hybrid molecules. In a nutshell, we can nicely justify the higher order NLO response of all our designed systems with the help of different global parameters and hence anticipated that if synthesized, these molecules may prove to be useful.
Acknowledgment
Financial support from SERB-DST (India) (CRG/2019/000262) is thankfully acknowledged. This work is dedicated to Prof. P. K. Chattaraj on the occasion of his 65th birthday.
Author contributions PG has carried out the work. All four authors analyzed the results and wrote the main manuscript text. AM has supervised the work.
Funding Source of the funding for this work comes from SERB-DST, India through the Grant No. CRG/2019/000262.
Conflict of interest The authors declare that they have no conflict of interest.
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Scheme 1 is available in the Supplementary Files section.
No competing interests reported.
- SupportingInformation.docx
- Graphicalabstract.png
- Scheme1.png
Scheme 1. Structures of the Ni bis(dithiolene) based: (C1) furan coupled, (C2) pyrrole coupled, (C3) thiophene coupled, and (C4) BODIPY coupled designed systems.
