Elsevier

Inorganica Chimica Acta

Volume 494, 1 August 2019, Pages 13-20
Inorganica Chimica Acta

Research paper
Tuning the electronic and optical properties of Pt(diimine)(dithiolate) complexes through different anchoring groups; A DFT/TD-DFT study

https://doi.org/10.1016/j.ica.2019.05.006Get rights and content

Highlights

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    DFT and TDDFT computation supports the effect of changing anchors for PtN2S2 sensitizers.

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    The electronic and photophysical properties can be efficiently modulated by tuning anchors on the diimine ligand.

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    Low-energy bands in the UV–Vis spectra of the complexes are MMLL′ CT transitions.

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    The UV absorption can be enhanced by solvents such as benzene and toluene.

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    Anchors 1 and 7 are expected as the promising candidates for DDSCs.

Abstract

The development of novel anchoring groups is important for the effective utilization in dye-sensitized solar cells (DSSC). Among metal complex sensitizer, Pt(II) diimine dithiolate (PtN2S2) complexes have been proposed as alternative sensitizers owing to their ultimate photophysical and photochemical properties. Herein, a series of [Pt(bpy)(bdt)] sensitizers with different anchoring groups on the bpy (diimine) ligand have been designed and their geometrical, electronic, and photophysical properties for DSSC applications have been systematically examined both in the presence and absence of a solvent using density functional theory (DFT) and time dependent density functional theory (TD-DFT) methods. A precise electronic structure is also presented on the basis of NBO analysis. The seven anchoring groups are considered including aldehyde (1), hydroxamate (2), cyanoacrylic acid (3), carboxylic acids (4a−4c), boronic acid (5), phosphonic acids (6a−6c), and sulfonic acid (7) substituents. The results indicate that the gap energy (HLG) and electrophilicity (ω) values obtained for these structures alter by anchor addition. Notably, the anchors 1 and 7 have remarkable effect on the electronic features of [Pt(bpy)(bdt)] sensitizer. From the TD-DFT calculations, it is found that the substitution of the considered anchoring groups enhances the intensity of the absorption and the overall absorption spectrum can be red shifted significantly by lowering the LUMO energy. Besides, the values of light-harvesting efficiency (LHE) have calculated and compared with each other. These findings highlight the importance of using anchors 1 and 7 to boost the LHE of DSSCs. Thus, our results may give a useful guidance for the molecular design of the metal complex sensitizers used in DSSCs.

Introduction

Solar energy is one of the important sources of renewable energy at the moment and remarkable growth was found since two decades to meet the global energy demand. The light to energy conversion followed photovoltaic effect where the sun light illuminated on photosensitizers, absorbed energy which is then converted into electricity [1], [2]. Dye sensitized solar cells (DSSC), hetero junction cells, quantum dot cells, polymer solar cells and hot carrier cells belong to the 3rd generation solar cells [3], [4], [5], [6]. As one of the most flexible photovoltaic devices, dye sensitized solar cells (DSSCs) are promising alternatives to the conventional silicon solar cells due to their low cost and high efficiency. Compared to traditional power cells, dye-sensitized solar cells (DSSCs) represent an effective alternative to provide clean energy from the sun [7], [8], [9].

DSSCs have received considerable attention as a promising low-cost candidate for transforming solar energy into electricity from the beginning; high-efficiency DSSC was first reported by Grätzel in 1991 [10]. The efficiency of DSSC is affected by the sensitizing molecules that chemically adsorbed on the porous nanocrystalline-TiO2 surface as well as by the ability of these molecules to harvesting solar light ranging from the visible to the near-IR regions [11], [12], [13], [14]. Many metal-organic compounds have been reported having good photoelectrical properties with potential application as light harvester, storage and in optoelectronic devices [2]. Up to now more and more attentions have been paid to luminescent transition metal complexes, especially heavy metal complexes, such as Ir(III), Fe(II), Pt(II), Os(II), Ru(II) and Re(I) complexes [15], [16], [17], [18], [19], [20]. Among metal complex sensitizers, Pt(II) diimine dithiolate (PtN2S2) complexes are notable in having substantially lower-energy charge-transfer absorptions (610–650 nm), making them more interesting as potential photosensitizers for proton reduction [21], [22]. In 1997, Connick and Gray reported the complete structural and spectroscopic characterization of the photooxidation products of [Pt(bpy)(bdt)] (bpy = 2,2ʹ-bipyridine, bdt = 1,2-benzenedithiolate); they identified both a monosulfinate and a disulfinate complex depending on the oxidation conditions [23].

The anchoring group determines the binding energy of the dye on TiO2 (largely affecting its long-term stability) and the injection rate (by mediating the electron transfer from the chromophore to the semiconductor) and it can also modulate the injection energy by altering the energy of the dye’s excited state. Historically, the most frequently used anchors in DSSCs are carboxylic acid, cyanoacrylic acid groups and, sometimes, phosphonic acid groups [24], [25]. Owing to the recent exponential growth of DSSC research, many new anchors have emerged and been tested; this trend has significantly increased the choice of materials available, and facilitates the understanding of DSSCs. On the other hand, computational chemistry has been used to disentangle some of the phenomena that are relevant to the functioning of DSSC and to introduce an element of design in the systematic exploration. The excellent agreement of calculated results with available experimental data indicates that the approach could be used as an efficient predictive tool to research the new photosensitizer [26], [27].

To the best of our knowledge, there has not been any comprehensive theoretical explanation to the common and uncommon anchoring groups on the efficiency of these Pt(NN)(SS) complexes as sensitizers for DSSCs. In this work, a series of these [Pt(byp)(bdt)] complexes with seven anchoring groups were designed, illustrated in Fig. 1 and labeled from (1) to (7). Note that the choice of some anchoring groups is based on the available literature and several considerations have been used for similar metal complex sensitizers [28], [29], [30], [31], [32], [33], [34], [35]. Meanwhile, we analyzed and compared these considered dyes through molecular structures, optoelectronic properties, and other parameters in detail and hope that our project could support which designed dyes are the most useful to enhance performance of DSSC

Section snippets

Computational details

In the present work, the geometric structures of the [Pt(bpy)(bdt)] complex with seven anchors in their ground state (S0) were optimized using density functional theory (DFT) method using the functional B3LYP [36]. The 6–31G(d) basis set [37] and the Los Alamos effective core potential (LANL2DZ) [38] were employed for the lighter (H, C, N, B, S, O and P) and heavier (Pt) elements, respectively. For this task, the program Gaussian09 was used [39]. It should be noted that the CAM‐B3LYP, B3PW91

Results and discussion

The optimized geometry structures of the sensitizers (1–7) are presented in Fig. 2. The important geometrical parameters around the platinum center are collected in Table S4 and are compared with the available experimental data in Table S5 (Supplementary Information). The calculated geometrical parameters of complexes are in excellent agreement with experimentally reported values [28], [35], indicating that the computational results are reliable. The bond angles around platinum indicate a

Conclusions

DFT and TD-DFT calculation were employed to investigate the effect of seven anchoring groups on the electro-optical features of [Pt(bpy)(bdt)] sensitizer. The results revealed that the nature of Ptsingle bondN and Ptsingle bondS bonds remained nearly unaltered upon anchor substitutions. Additionally, the HLG energy values decreased in the range of 1.08%−16.75%; whereas the electrophilicity values significantly increased (4.21%−62.96%) in comparison to the isolated sensitizer. In this regard, the electronic

Acknowledgement

We are grateful to the Shahid Chamran University of Ahvaz for financial support (Grant No. 1397).

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