Electronic structure and VUV photoabsorption measurements of thiophene

Thiophene (C₄H₄S) is a heterocyclic aromatic compound of significant importance in materials science, as it is a building block in the synthesis of conjugated π-systems. In particular, the electronic properties of thiophene polymers can be adjusted through additional side-chains and by controlling the polymer lengths, giving them high potential as components of organic electronic devices.¹ Thiophene derivatives are also biologically active compounds and form the basis of many pharmaceutical drugs.² However, the metabolism of some thiophene compounds can induce toxicity.³ The use of thiophene-derived materials in medicine and technology makes the understanding of the structure and electronic and vibrational spectroscopic character of the base thiophene unit important. The desire to control and understand thiophene-based material optoelectronic properties such as photo-stability and conductivity has led to significant investigations of thiophene photochemistry. In particular, the photochemical behaviour of thiophene and α-oligothiophenes has attracted significant experimental and theoretical attention. These investigations address complex issues regarding the vertical and adiabatic ordering of its lowest-lying singlet and triplet states and the competition between possible decay routes, i.e., inter-system crossing, internal conversion, and fluorescence.^4–10 

The relatively simple structure of thiophene, along with its strong structural similarities to furan and pyrrole, has made it an important target for experimental and theoretical investigations. This has led to numerous investigations on the structure of thiophene in its neutral and cationic states to determine the ionization energies and the ion vibrational structure,^11–13 ionized orbital characters,¹⁴ and interaction potentials.¹¹ These ionization investigations have been supported by numerous theoretical calculations.^9,15–17 Rennie et al.¹⁵ also reported absolute cross sections for photoabsorption (8.9–35.0 eV) through synchrotron photoionization experiments. The synchrotron photoabsorption spectrum for thiophene was observed using a transmission method reported by Palmer et al.¹⁸ and was recently remeasured for thiophene using a Fourier transform spectrometer at the Soleil DESIRS beamline.¹⁹ In the more recent work by Holland et al., a relative photoabsorption spectrum was obtained (5.0–12.5 eV) and normalized to the absolute photo-ion spectrum.¹⁵ 

The technological importance of thiophene, and thiophene-based materials, has also stimulated interest in its interactions with electrons. This interest has attracted recent attention, as electron scattering phenomena play important roles in efficiently breaking chemical bonds (through dissociative electron attachment) and driving reactions in plasma-like environments.^20,21 This is of particular relevance to materials science where non-thermal plasmas are gaining increasing use for functionalizing surfaces and synthesizing novel nanomaterials.^22,23 To this end, total electron scattering cross sections for thiophene have recently been reported.²⁴ Differential cross sections for elastic scattering have been reported as a function of incident electron energy for fixed scattering angles to investigate resonance behaviour in thiophene,²⁵ building on early electron transmission spectroscopic investigations.²⁶ These investigations have been supported by theoretical calculations at the R-matrix^25,27 and Schwinger Multichannel (SMC)²⁸ level, with cross sections having also been calculated using an additivity rule.²⁹ The electronic structure of thiophene has also been investigated by electron energy loss spectroscopy.^18,30,31

Recent investigations have demonstrated the important role that the ground and excited electronic states play in electron scattering calculations.^32–39 In this respect, adequately describing electronic excited states is essential in Schwinger Multichannel (SMC) calculations, where multichannel coupling allows flux to flow out of the elastic scattering channel and into discrete electronic state excitation processes. In ring-compounds, in particular, it has been observed that the channel space must be built from states that well-describe all energetically available electronic states to enable sufficient flux to couple into inelastic scattering channels to produce a reliable elastic scattering cross section.³⁴ The ability to combine experimental and theoretical photon- and electron-impact collisional investigations has been important in building complete datasets that are suitable for modelling collisional environments, such as radiation damage, plasma processing, and charged-particle transport,^40–44 and in part has formed the motivation for this work.

In this manuscript, we therefore report a new absolute photoabsorption cross section measurement and supplement this work with new time dependent density functional theory (TDDFT) calculations. In Sec. II, we describe our experimental and theoretical methods. Our results are presented and discussed in Sec. III. Finally, our conclusions from this work are summarized in Sec. IV.

The high resolution vacuum ultraviolet (VUV) photoabsorption spectrum of thiophene was measured using the AU-UV beam line at the ASTRID2 storage ring, Aarhus University, Denmark. The experimental configuration has been described in detail elsewhere.^45,46 Briefly, the monochromatized light with a resolution of 0.08 nm passes through a gas cell that is filled with thiophene vapour. Light transmitted through the cell passes through a MgF₂ window and is detected using a photomultiplier tube. The MgF₂ window sets the lower limit of detection at 115 nm. The pressure of the thiophene vapour in the gas cell is monitored using a 1 Torr full scale capacitance manometer (Chell model CDG100D). Absolute photoabsorption cross sections (σ in units of Mb ≡ 10⁻¹⁸ cm²) are obtained using the Beer-Lambert attenuation law

Here I₀ is the intensity of the incident light (measured as that transported through an evacuated gas cell), I_t is the measured intensity of the light transmitted through a gas cell of l absorption path length, l = 15.5 cm, that is filled with thiophene vapour at a molecular number density, N. The light intensity at each wavelength is kept quasi-constant through ASTRID2 operating in a “top-up” mode that compensates for the constant beam decay. To compensate for the slight intensity variations (∼3%), the incident flux is normalized to the accurately determined beam current in the storage ring. Using this procedure, the absolute photoabsorption cross sections are determined to within ±5%.

Our photoabsorption cross section measurements were supported by theoretical calculations performed at the time-dependent density functional theory (TDDFT) level of approximation.⁴⁷ The thiophene geometry was optimized, with a frequency calculation performed using a B3LYP functional⁴⁸ and an augmented correlation consistent polarized valence triple-zeta basis (aug-cc-pVTZ).^49,50 The TDDFT calculation was then performed at the optimized geometry, using the same model chemistry. We have implemented this methodology for assisting in the interpretation of the thiophene photoabsorption spectrum, as similar model chemistries have enabled us to assign the photoabsorption spectra of other large ring-based compounds.^32,37,51 All of these calculations were performed in the Gaussian 09 package.⁵² For discussion of our theoretical assignments, we orient the molecule in the yz-plane (see Fig. 1 inset).

The VUV photoabsorption spectrum of thiophene measured at room temperature is shown in Fig. 1. The absolute cross sections are similar to those observed by Rennie et al.¹⁵ in the overlapping regions (8.8–10.7 eV). Here Rennie et al. determined the absolute photoabsorption cross section using synchrotron radiation and a double ion chamber to collect all photo-ions produced. This absolute scale was used to normalise relative photoabsorption data from Holland et al.¹⁹ over the 5.0–12.5 eV range. We see good agreement between our photoabsorption cross section and that produced by Holland et al. over the entire overlapping regions. This demonstrates the reliability in the reported high-resolution photoabsorption cross sections for thiophene. To assist in the interpretation of this spectrum, our TDDFT calculations are summarized in Table I. We now discuss the photoabsorption behaviour and the excited state electronic structure in energetic bands identified in Table I and indicated in Fig. 1. The spectroscopic assignments for the various bands are detailed in Tables II–V. To assist in understanding the vibrational assignments, our calculated vibrational frequencies, along with the experimental and theoretical vibrational frequencies available for thiophene in both the neutral and cationic states, are summarized in Table VI.

In Band I, we see a broad spectral feature (see Fig. 2), overlayed with a large number of vibrational progressions that are detailed in Table II. The photoabsorption cross section in this region has significant intensity, peaking at 19.8 Mb. Analysis of this region indicates that 8 progressions of the ν₆ vibrational mode are prominent in this band, with the average ν₆ vibrational spacing being 120 meV. Our calculations suggest that there are two prominent ππ^* electronic transitions, lying close in energy and with comparable oscillator strengths (f), that may contribute to this feature. These transitions are dominated by HOMO-LUMO and (HOMO-1)-LUMO promotions in the ¹B₂ [1a₂ → 4b₁] (5.67 eV) and ¹A₁ [3b₁ → 4b₁] (5.75 eV) states, respectively. The existence of two-closely lying and comparable intensity electronic features may explain the high density of ν₆-progressions observed in this region. This observation is consistent with previous high level calculations, also placing these two electronic transitions within 0.3 eV of each other and with comparable optical oscillator strengths.¹⁹ The vertical S₀-S₁ band origin has been previously identified at 5.157 eV, see Palmer et al.¹⁸, while the lower energy bands are associated with hot bands, sequence bands, and S₀-S₂ transitions (possible origin at 5.051 eV) accessible through a Franck-Condon overlap.⁵ The complexity in the vibronic structure makes further refinement of the spectroscopic features in this region difficult such that we label each ν₆-progression identified.