Optical properties of pyridine adsorbed polycyclic aromatic hydrocarbons using quantum chemical calculations

Abstract

Polycyclic aromatic hydrocarbons (PAHs), the molecular version of graphene, having edges saturated with hydrogen atoms, have recently emerged as a novel nanoplasmonic material. In this work, we investigate the optical properties of pristine and pyridine adsorbed circular and triangular PAHs. We base our calculations on computationally efficient first-principles time-dependent density-functional theory (TD-DFT) calculations. We find substantial changes in the optical absorption spectra induced by the presence of the pyridine molecule. In addition, with the help of electron difference density (EDD) maps, we demonstrate a strong optical interaction between PAHs and pyridine molecules. The main effect of pyridine adsorption is to split the plasmon band and to redistribute the optical absorption in a wider energy range. We believe that our findings can help in the design of novel plasmonic devices having PAHs as basic building blocks.

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Introduction

Plasmons, the collective oscillations of valence electrons in conducting materials, have the ability to confine light to nanometric spatial regions, where the optical field intensity is enhanced by several orders of magnitude.^1–3 This feature has been widely employed for applications in areas such as optical sensing^4–7 and photo-catalysis,^8–11 to list a few. Graphene has recently surfaced as a powerful plasmonic material^12–18 offering unique advantages of being electrically,^19–21 magnetically^22–24 and optically^25–28 tunable while displaying long excitation lifetimes and reduced losses,²⁹ making it amenable to display novel plasmonic properties. While graphene plasmons have been mainly investigated at lower frequencies (mid-infrared range) of the electromagnetic spectrum, extending to the higher frequencies (UV-vis spectral range) is highly demanding.²⁵ Results from recent TD-DFT calculations show that this can be possible via tailor-made graphene molecules known as PAHs.^30–36 They support molecular plasmons having outstanding structural and electrical tunability, demonstrated by a broad range of theoretical methods.^31,37,38 For example, Garcia de Abajo and coworkers showed that PAHs exhibit molecular plasmons³⁷ having remarkable structural and electrical tunability, much higher than graphene, allowing them to be switched on/off by addition or removal of a single electron.³¹ At the same time, Guidez and coworkers found the dominant band in the low-energy region of the PAH spectrum as the molecular plasmon, confirmed from the induced electron density profiles.³⁸

In a different context, many factors affect the interaction of light with molecules adjacent to a plasmonic structure.^39–42 The molecules that can be chemisorbed (such as pyridine) on the nanostructure surface show the largest local field enhancement in comparison to those that can be physisorbed (such as benzene).⁴³ Since the analyte molecules are much smaller than the optical wavelength, their interaction with light is very weak. Luckily, the huge local field enhancement generated by plasmons offers a solution to increase this interaction. By exposing the analyte molecules to the plasmons, their ability to absorb light is greatly improved.⁴⁴ This is the working principle of the techniques known as surface-enhanced infrared absorption and surface-enhanced Raman scattering. While the PAH molecular plasmons are well studied and understood theoretically, their interactions with molecules, to our knowledge, have not been addressed previously. Therefore, in the present work, using state-of-the-art TD-DFT calculations, we investigate the optical properties of different PAHs with respect to the changes in size and shape and due to the adsorption of pyridine molecules.

Computational aspects

All the calculations in this work were performed using the ORCA DFT/TD-DFT software (version 4.0).⁴⁵ We address both pristine and pyridine adsorbed PAHs having circular and triangular geometries. We consider the following circular PAHs: C1 (C₂₄H₁₂), C2 (C₅₄H₁₈), C3 (C₉₆H₂₄), C4 (C₁₅₀H₃₀) and C5 (C₂₁₆H₃₆) and the following triangular PAHs: T1 (C₂₂H₁₄), T2 (C₇₈H₂₄), T3 (C₁₄₁H₃₃), T4 (C₂₂₂H₄₂) and T5 (C₃₂₁H₅₁). Structural optimizations were performed using the Perdew–Burke–Ernzerhof (PBE) exchange–correlation functional and the def2-TZVP basis set (more details available in the ORCA online manual). The particular combination “PBE/def2-TZVP” was found optimal for structure optimization in terms of accuracy and performance. We employed a tight convergence criterion for the structural optimization by setting SCF convergence to 10⁻⁸, gradient convergence to 10⁻⁶ and energy convergence to 10⁻⁶. Optical absorption spectra are obtained by solving the time-dependent Kohn–Sham equations using the eigenvalue equation ΩF_I = ω_I²F_I, where Ω is a matrix consisting of products of occupied-virtual Kohn–Sham orbitals and the eigenvalues ω_I² correspond to squared excitation energies while the oscillator strengths are extracted from the eigenvectors F_I and employing the wB97X long range corrected hybrid exchange–correlation functional. Note that the TD-DFT technique has been widely used for computing the optical properties of a variety of nanostructures,^46–55 to list a few. All absorption spectra are broadened by a Gaussian smearing of width σ = 0.15 eV.

Results and discussion

Before discussing the optical properties of pristine and pyridine adsorbed PAHs in detail, it is instructive first to analyze the optical absorption spectrum of a pyridine molecule. To this aim, we plot in Fig. 1 the optical absorption spectrum of a pyridine molecule in an energy range of 0 to 5.0 eV. One finds two different well-localized bands in the spectrum: one band (focused and dominant) located at 4.75 eV and one low-intensity band located at 4.20 eV.

Fig. 1 TDDFT calculated optical absorption spectrum of a pyridine molecule.

Let us now discuss the optical absorption spectra of pristine circular PAHs, see Fig. 2. With the help of the EDD map corresponding to the dominant band, we gain further insight into the nature of optical excitation. It should be stressed that the dominant band is composed of many single particle transitions. Let us begin our analysis starting from the smallest circular PAH considered in this work, i.e., the C1 structure (C₂₄H₁₂). Fig. 2 shows that the absorption spectrum of this structure consists of a band located at 4.90 eV and the corresponding EDD map exhibits plasmon-like resonance having similar characteristics to the molecular plasmons.³¹ When moving to the C2 structure (C₅₄H₁₈), one finds that the dominant band is red-shifted to 3 eV and three new low-intensity bands emerge right to the dominant band. The EDD map corresponding to the dominant band exhibits typical plasmon-like resonance. Interesting optical response modifications emerge starting from the C3 structure (C₉₆H₂₄). One finds two dominant bands in the absorption spectrum. The first band is located at 2.75 eV and the second band is located at 4.60 eV. As expected, the EDD map corresponding to the first dominant band shows typical plasmon-like resonance. Interesting changes appear in the case of the C4 structure (C₁₅₀H₃₀). One sees that the second dominant band is now more focused and both the bands are slightly red-shifted. In the case of the largest circular PAH considered in this work [C5 (C₂₁₆H₃₆)], we find two dominant bands having comparable absorption intensities. It is worth noting that with increasing size, both the dominant bands red-shift systematically. Now let us shed some light on the optical response modifications of circular PAHs upon pyridine adsorption, see Fig. 3. We placed the pyridine molecule of size 0.5 nm on top of PAHs. The prima facie analysis provides some interesting findings. In the case of pyridine@C1, pyridine@C2, pyridine@C3 and pyridine@C4, the key effect of pyridine adsorption is to transform the dominant band into multiple low-intensity bands distributed over a wider energy range. This could be due to the mixing of the PAH plasmon with the pyridine states. Interestingly, pyridine adsorption has only a marginal effect on the optical properties of the pyridine@C5 system since both the absorption spectrum and the EDD map look similar to those of the corresponding pristine C5 structure. More precisely, in the case of the large-sized PAHs, the plasmonic and pyridine excitation can be clearly identified and separated. It is useful to see how the optical gap (the energy difference between the ground state and the first excited state) of the circular PAHs change as a function of the PAH size. One easily concludes that the optical gap increases more or less monotonically with the reduction in size, see Table 1. This is understandable since by decreasing the PAH size, the density of states transforms from a continuum to a discrete set of levels, exhibiting the molecular character of the PAHs.

Fig. 2 TDDFT calculated optical absorption spectra of circular PAHs as a function of their size. EDD map (with two different orientations for an easy visual inspection) and the root number corresponding to the dominant band are also shown in the figure. Red and green colors represent electron density accumulation (depletion) regions during the excitation, respectively. Both densities are plotted using the same iso-surface contour value. The values of the density of states are shown in the ESI,† see Fig. S1.

Fig. 3 TDDFT calculated optical absorption spectra of pyridine adsorbed circular PAHs as a function of their size. EDD map (with two different orientations for an easy visual inspection) and the root number corresponding to the dominant band are also shown in the figure. Red and green colors represent electron density accumulation (depletion) regions during the excitation, respectively. Both densities are plotted using the same iso-surface contour value. The values of the density of states are shown in the ESI,† see Fig. S2.

Table 1 TD-DFT calculated optical gap of the circular and triangular PAHs

PAH	Optical gap
C1 (C24H12)	3.97
C2 (C54H18)	3.00
C3 (C96H24)	2.40
C4 (C150H30)	1.96
C5 (C216H36)	1.62
T1 (C22H14)	3.30
T2 (C78H24)	2.78
T3 (C141H33)	2.63
T4 (C222H42)	2.49
T5 (C321H51)	2.36

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Having analyzed the optical properties of pristine and pyridine adsorbed circular PAHs, we next analyze the optical properties of triangular PAHs. This analysis is important to understand the influence of shape effects on the optical properties. Let us analyze the pristine triangular PAHs, see Fig. 4. We begin our discussion starting from the smallest triangular PAH, i.e., the T1 structure (C₂₂H₁₄). Fig. 4 shows that the absorption spectrum consists of a dominant band located at 3.50 eV and a couple of low-intensity bands located between 2 and 3 eV. Note that the EDD map corresponding to the dominant bands does not exhibit plasmon-like resonance. Interesting modifications in the absorption spectrum emerge in the case of the T2 structure (C₇₈H₂₄). Note that a new low-intensity band appears at 0.50 eV and the dominant band is red-shifted by 0.50 eV. Starting from the T3 structure (C₁₄₁H₃₃), one observes that the low-energy band splits into several bands and a few new bands emerge to the left of the dominant band. Further splitting of the low-energy band can be observed in the case of the T4 structure (C₂₂₂H₄₂). Interestingly, one finds that the dominant band starts to split further. Finally, in the case of the T5 structure (C₃₂₁H₅₁), one finds several new peaks emerging in the low-energy spectral region and the dominant band is now clearly split into two well-focused bands. EDD maps shown in Fig. 4 suggest that the electron excitation corresponding to the dominant band is not plasmonic. This is a vital result since it shows the importance of shape in exhibiting plasmons. In addition, the optical gap of triangular PAHs increases more or less monotonically with the reduction in PAH size, see Table 1. Now let us provide some insight into the triangular PAHs having pyridine molecules adsorbed on top, see Fig. 5. One readily notices that the effect of pyridine adsorption is to transform the dominant band into multiple low-intensity bands distributed in a wider energy range, similar to the circular PAHs shown in Fig. 3. Finally, it is also interesting to see how the molecular electrostatic potential energy (MEP) map changes due to pyridine adsorption. Some representative results are shown in Fig. 6.

Fig. 4 TDDFT calculated optical absorption spectra of triangular PAHs as a function of their size. EDD map (with two different orientations for an easy visual inspection) and the root number corresponding to the dominant band are also shown in the figure. Red and green colors represent electron density accumulation (depletion) regions during the excitation, respectively. Both densities are plotted using the same iso-surface contour value. The values of the density of states are shown in the ESI,† see Fig. S3.

Fig. 5 TDDFT calculated optical absorption spectra of pyridine adsorbed triangular PAHs as a function of their size. EDD map (with two different orientations for an easy visual inspection) and the root number corresponding to the dominant band are also shown in the figure. Red and green colors represent electron density accumulation (depletion) regions during the excitation, respectively. Both densities are plotted using the same iso-surface contour value. The values of the density of states are shown in the ESI,† see Fig. S4.

Fig. 6 Molecular electrostatic potential map (MEP) of C2, pyridine@C2, T2, and pyridine@T2 structures.

Conclusion

Using time-dependent density-functional-theory calculations, we have demonstrated the optical properties of pristine and pyridine adsorbed circular and triangular PAHs. Our results reveal the complex nature of the optical properties with the help of optical absorption spectra and electron difference density maps. Our simulations allow us to draw the following conclusions: (1) pristine circular PAHs exhibit plasmon-like resonances; (2) pyridine adsorbed circular PAHs [C1, C2, C3 and C4] damp the plasmon resonance, whereas in the case of C5, the plasmonic and pyridine excitation can be separately identified; (3) pristine triangular PAHs do not exhibit plasmon-like resonances and the dominant band in the optical absorption spectra was found to be damped as a result of pyridine adsorption. We hope that our theoretical findings are pivotal for the emerging field of PAH based molecular sensing applications.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The research reported in this publication was supported by funding from Kuwait College of Science and Technology (KCST).

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Footnote

† Electronic supplementary information (ESI) available: Density of states of PAHs and pyridine adsorbed PAHs. See DOI: 10.1039/c8cp06744a

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