Excited states of diphenylacetylene (tolan): Near and vacuum UV polarization spectroscopy European Journal of Chemistry 15 (2) (2024) 87-92 European Journal of Chemistry ISSN 2153-2249 (Print) / ISSN 2153-2257 (Online) – Copyright © 2024 The Authors – Atlanta Publishing House LLC – Printed in the USA. This work is published and licensed by Atlanta Publishing House LLC – CC BY NC – Some Rights Reserved. https://dx.doi.org/10.5155/eurjchem.15.2.87-92.2546 European Journal of Chemistry View Journal Online View Article Online Excited states of diphenylacetylene (tolan): Near and vacuum UV polarization spectroscopy Duy Duc Nguyen 1,#, Nykola C. Jones 2, Søren Vrønning Hoffmann 2, and Jens Spanget-Larsen 1,* 1 Department of Science and Environment, Roskilde University, Universitetsvej 1, DK-4000 Roskilde, Denmark 2 Centre for Storage Ring Facilities (ISA), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark * Corresponding author at: Department of Science and Environment, Roskilde University, Universitetsvej 1, DK-4000 Roskilde, Denmark. # Present affiliation: Duy Duc Nguyen, Intertek Vietnam Limited, Tan Binh District, Ho Chi Minh City, Vietnam. e-mail: spanget@ruc.dk (J. Spanget-Larsen). 10.5155/eurjchem.15.2.87-92.2546 Received: 9 March 2024 Received in revised form: 28 March 2024 Accepted: 4 April 2024 Published online: 30 June 2024 Printed: 30 June 2024 The UV absorbance spectrum of the important chromophore diphenylacetylene (tolan) is investigated by Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy using stretched polyethylene as an anisotropic solvent. The investigation covers the range of 58,000-28,000 cm–1 (172-360 nm). The observed linear dichroism provides information on the transition moment directions of the four main absorbance bands A, B, C, and D at 33,300, 44,400, 51,000, and 57,000 cm-1 (300, 225, 196, and 175 nm). The experimental wavenumbers, intensities, and polarization directions are compared with the results of quantum chemical calculations using the semiempirical all-valence-electrons method Linear Combination of Orthogonalized Atomic Orbitals (LCOAO) and Time-Dependent Density Functional Theory (TD-DFT) with the functional CAM-B3LYP. Magnetic Circular Dichroism (MCD) B-terms predicted with LCOAO suggest that a number of optically weak transitions may be observed by MCD spectroscopy. MCD B-terms Near and vacuum UV Synchrotron radiation Stretched polyethylene Polarization spectroscopy LCOAO and TD-DFT calculations Cite this: Eur. J. Chem. 2024, 15(2), 87-92 Journal website: www.eurjchem.com 1. Introduction Diphenylacetylene (tolan, DPA, Scheme 1) is a prototype of the oligo phenylene-ethynylene (OPE) systems which are of great interest in the fields of molecular wires and other molecular-based electronic devices [1-4]. The photophysical, photochemical, and spectroscopic properties of DPA have been studied for decades; for entries in the literature, see references [5-14]. We have previously studied the excited electronic states of the related compounds diphenyldiacetylene (DPDA) [15] and 1,4-bis(phenylethynyl)benzene (BPEB) [16] (Scheme 1). In the present publication, we report the results of a similar study of DPA, investigating the ground state absorbance spectrum by UV Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy on molecular samples partially aligned in stretched poly- ethylene (PE). The measured LD provides information on the polarization directions of the observed transitions [17-22], and with synchrotron radiation [23,24] the investigated spectral range can be extended to about 58,000 cm-1 (172 nm). The observed energies, intensities, and polarizations are compared with the results of theoretical calculations using the Linear Combination of Orthogonalized Atomic Orbitals (LCOAO) model [25,26] and Time-Dependent Density Functional Theory (TD-DFT) [27-29] with the functional CAM- B3LYP [30]. The semiempirical all-valence-electrons LCOAO procedure was specifically developed for the prediction of the electronic absorption and Magnetic Circular Dichroism (MCD) spectra [31] of conjugated hydrocarbons [25,26] and has been applied to several π-systems with triple-bonded linkages [16, 26,32,33]. Additional data provided as Electronic supple- mentary information (ESI) is referred to in the ensuing text as Sup. S1-S5. Scheme 1. Diphenylacetylene (tolan, DPA), diphenyldiacetylene (DPDA), and 1,4-bis(phenylethynyl)benzene (BPEB). ABSTRACT RESEARCH ARTICLE KEYWORDS https://dx.doi.org/10.5155/eurjchem.15.2.87-92.2546 https://www.eurjchem.com/ https://dx.doi.org/10.5155/eurjchem.15.2.87-92.2546 mailto:spanget@ruc.dk http://www.eurjchem.com/ https://crossmark.crossref.org/dialog/?doi=10.5155/eurjchem.15.2.87-92.2546&domain=pdf&date_stamp=2024-06-30 88 Nguyen et al. / European Journal of Chemistry 15 (2) (2024) 87-92 2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.2.87-92.2546 Table 1. Observed features of the SRLD spectrum of diphenylacetylene (DPA) and vertical electronic transitions predicted with LCOAO. A graphical representation of the predicted transitions is shown in Figure 2 (see also Sup. S4). Observed LCOAO a 𝝂𝝂� b Abs c Pol d Term 𝝂𝝂� b f e B f Leading configurations g A 33.2 h 1.18 h z 1 1B1u 34.1 1.60 –0.63 98%[3b3u→3b2g] 1 1B2u 36.7 1∙10–3 +1.07 40%[1au→3b2g], 35%[3b3u→2b1g] 1 1B3g 36.8 0 0 40%[1b1g→3b2g], 35%[3b3u→2au] 2 1Ag 46.5 0 0 64%[3b3u→4b3u], 17%[2b2g→3b2g] 3 1Ag 46.9 0 0 69%[2b2g→3b2g], 24%[3b3u→4b3u] 2 1B3g 48.1 0 0 49%[1b1g→3b2g], 49%[3b3u→2au] B 44.4 h 0.42 h y 2 1B2u 48.7 0.87 –0.65 49%[1au→3b2g], 49%[3b3u→2b1g] C 51 0.50 z 2 1B1u 52.2 0.81 +2.11 43%[1au→2b1g], 42%[1b1g→2au] 4 1Ag 56.0 0 0 40%[1b1g→2b1g], 40%[1au→2au] 3 1B2u 56.1 0.04 –2.00 42%[1b1g→4b3u], 22%[2b2g→2au] 3 1B3g 56.1 0 0 43%[1au→4b3u], 20%[2b2g→2b1g] 3 1B1u 58.8 0.02 +0.58 53%[2b3u→3b2g], 42%[3b3u→4b2g] 4 1B3g 60.3 0 0 58%[2b2g→2b1g], 34%[1au→4b3u] D 57.1 0.51 y 4 1B2u 60.8 1.17 +8.72 57%[2b2g→2au], 36%[1b1g→4b3u] 4 1B1u 61.6 0.48 –8.58 79%[2b2g→4b3u], 6%[3b3u→4b2g] 5 1Ag 63.6 0 0 50%[1b1g→2b1g], 50%[1au→2au] a 16 lowest transitions, complete list provided as Sup. S4. b Peak wavenumber in 1000 cm–1. c Peak absorbance estimated from the partial absorbance curves in Figure 1c. d Polarization direction. e Oscillator strength. f MCD B-term in 10–3 βe D2/cm–1 (βe = Bohr magneton, D = Debye). g π-π* configurations, orbital energies and diagrams in Figure 3. h Onset. Figure 1. (a) Absorbance curves measured with polarized light for diphenylacetylene (DPA) in stretched polyethylene. 𝐸𝐸𝑈𝑈 and 𝐸𝐸𝑉𝑉 indicate the absorbance measured with the stretching direction 𝑈𝑈 parallel and perpendicular to the electric vector of the radiation. (b) Family of reduced absorbance curves 𝑟𝑟𝐾𝐾 according to Equation 1 with 𝐾𝐾 varying from 0 to 1 in steps of 0.1. (c) Partial absorbance curves 𝐴𝐴𝑦𝑦 and 𝐴𝐴𝑧𝑧 as defined in Equations 2 and 3, indicating y- and z-polarized absorbance. 2. Experimental DPA [CAS 501-65-5] (98%) was purchased from Sigma- Aldrich. The spectroscopic purity of the substance was checked by comparison with the reference spectra available online [34]. Low-density polyethylene (PE) was obtained from Hinnum Plast, Denmark, as a pure 100 μm sheet material. DPA was introduced into the PE sample by submersion of a piece of the polymer sheet into a saturated solution of the compound in chloroform (Merck Uvasol) at room temperature for several days. Subsequently, the chloroform was allowed to evaporate, and the crystalline deposits on the surface were removed with methanol (Merck Uvasol). The PE sample was finally uniaxially stretched by ca. 500%. A sample without solute was prepared in the same manner for use as a reference. More details on stretched PE samples can be found in the literature [17-22]. The Synchrotron Radiation Linear Dichroism (SRLD) spectrum of DPA was measured at room temperature in the range 58,000-28,000 cm-1 (172-360 nm) on the CD1 beamline [23,24] at the storage ring ASTRID at the Centre for Storage Ring Facilities (ISA). Two absorbance curves were recorded as previously described [16] with the electric vector of the sample beam parallel (𝑈𝑈) and perpendicular (𝑉𝑉) to the stretching direction of the PE sample. The baseline-corrected absorbance curves 𝐸𝐸𝑈𝑈(𝜈𝜈�) and 𝐸𝐸𝑉𝑉(𝜈𝜈�) are shown in Figure 1a. The LD is defined as the difference between the two curves, LD = 𝐸𝐸𝑈𝑈(𝜈𝜈�) − 𝐸𝐸𝑉𝑉(𝜈𝜈�). A version of the spectrum with an indication of all peak wavenumbers and absorbance is provided in Sup. S1. 2.1. Theory/Calculation The electronic transitions of DPA were computed with the semiempirical all-valence-electrons method LCOAO [25,26] and with TD-DFT [27-29] using the functional CAM-B3LYP [30]. LCOAO calculation was performed with the computer program published in Reference [35]; a complete LCOAO bibliography is given in Sup. S5. CAM-B3LYP calculations were carried out with the Gaussian 16 software package [36]. The LCOAO calculation included the interaction between all singly excited singlet configurations generated by the promo- tion of an electron from the occupied π to unoccupied π* molecular orbitals (MOs), comprising 49 π-π* configurations. In addition to transition energies, intensities, and polarization directions, this calculation provided predictions of MCD B- terms [26,31] for the computed electronic transitions. The input geometry for the LCOAO calculation was taken as the one optimized with CAM-B3LYP and the basis set AUG-cc-pVTZ (see below). The main transitions obtained with LCOAO are listed in Table 1 and visualized in Figure 2, a complete listing of all LCOAO results is provided as Sup. S4. CAM-B3LYP and TD–CAM-B3LYP calculations were carried out with the basis sets AUG-cc-pVTZ and cc-pVTZ (with and without the inclusion of diffuse functions) [37,38]. The isotropic Nguyen et al. / European Journal of Chemistry 15 (2) (2024) 87-92 89 2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.2.87-92.2546 Table 2. Vertical electronic transitions for diphenylacetylene (DPA) predicted with TD-CAM-B3LYP/AUG-cc-pVTZ. A graphical representation is provided as Sup. S2. TD-CAM-B3LYP/AUG-cc-pVTZ a Term 𝝂𝝂� b f c Leading configurations d 1 1B1u 35.4 1.12 94%[3b3u(𝜋𝜋)→3b2g(𝜋𝜋*)] 1 1B2u 40.9 2∙10–4 49%[3b3u(𝜋𝜋)→2b1g(𝜋𝜋*)], 32%[1au(𝜋𝜋)→3b2g(𝜋𝜋*)] 1 1B3g 41.2 0 44%[3b3u(𝜋𝜋)→2au(𝜋𝜋*)], 35%[1b1g(𝜋𝜋)→3b2g(𝜋𝜋*)] 1 1Au 42.5 0 94%[8b2u(𝜋𝜋C≡C)→3b2g(𝜋𝜋*)] 1 1B3u 45.3 2∙10–3 76%[3b3u(𝜋𝜋)→14ag(𝜎𝜎*)], 11%[3b3u(𝜋𝜋)→15ag(𝜎𝜎*)] 2 1Ag 47.3 0 59%[3b3u(𝜋𝜋)→4b3u(𝜋𝜋*)], 18%[2b2g(𝜋𝜋)→3b2g(𝜋𝜋*)] 2 1B2u 48.7 0.52 48%[1au(𝜋𝜋)→3b2g(𝜋𝜋*)], 43%[3b3u(𝜋𝜋)→2b1g(𝜋𝜋*)] 2 1B3u 51.5 0.01 66%[3b3u(𝜋𝜋)→15ag(𝜎𝜎*)], 13%[3b3u(𝜋𝜋)→14ag(𝜎𝜎*)] 2 1B1u 52.1 0.73 46%[1au(𝜋𝜋)→2b1g(𝜋𝜋*)], 41%[1b1g(𝜋𝜋)→2au(𝜋𝜋*)] 3 1B3u 55.4 5∙10–3 54%[3b3u(𝜋𝜋)→17ag(𝜎𝜎*)], 33%[3b3u(𝜋𝜋)→16ag(𝜎𝜎*)] 3 1B1u 57.4 0.21 68%[3b3u(𝜋𝜋)→4b2g(𝜋𝜋*)], 14%[2b3u(𝜋𝜋)→3b2g(𝜋𝜋*)] 4 1B3u 57.7 8∙10–3 93%[8b2u(𝜋𝜋C≡C)→2b1g(𝜋𝜋*)] 3 1B2u 58.2 0.02 56%[8b2u(𝜋𝜋C≡C)→14ag(𝜎𝜎*)], 20%[8b2u(𝜋𝜋C≡C)→15ag(𝜎𝜎*)] 4 1B1u 58.3 0.65 30% 2b3u(𝜋𝜋)→3b2g(𝜋𝜋*)], 18%[3b3u(𝜋𝜋)→4b2g(𝜋𝜋*)] 5 1B3u 58.4 0.03 32%[1b1g(𝜋𝜋)→9b2u(𝜎𝜎*)], 17%[3b3u(𝜋𝜋)→16ag(𝜎𝜎*)] 6 1B3u 58.7 0.05 24%[1b1g(𝜋𝜋)→9b2u(𝜎𝜎*)], 19%[3b3u(𝜋𝜋)→17ag(𝜎𝜎*)] 4 1B2u 59.5 0.10 37%[2b2g(𝜋𝜋)→2au(𝜋𝜋*)], 20%[1b1g(𝜋𝜋)→4b3u(𝜋𝜋*)] 5 1B2u 61.0 0.50 43%[1b1g(𝜋𝜋)→4b3u(𝜋𝜋*)], 21%[2b2g(𝜋𝜋)→2au(𝜋𝜋*)] 6 1B2u 61.7 0.02 84%[3b3u(𝜋𝜋)→3b1g(𝜋𝜋*)] a Main transition only, complete list provided as Sup. S2. b Wavenumber in 1000 cm–1. c Oscillator strength. d πC≡C indicates the in-plane π component of the triple-bond. Figure 2. Gaussian convolutions of electronic transitions for diphenyl- acetylene (DPA) predicted with LCOAO. influence of the solvent was approximated by the Polarizable Continuum Model IEFPCM [39-42] using n-hexadecane as the solvent [36]. The ground state equilibrium geometry of DPA was optimized under the assumption of D2h symmetry with CAM-B3LYP using the respective basis sets and representing the dispersion effects by the model by Grimme [43] (keyword: empiricaldispersion=gd3bj [36]). The resulting nuclear coordinates are provided in Sup. S2 and Sup. S3 together with the results of frequency analyses. The TD-CAM-B3LYP calculations considered vertical transitions to the lowest 70 excited singlet states. The main transitions obtained with the basis set AUG-cc-pVTZ are listed in Table 2. Complete listings and graphical illustrations of all transitions computed with TD– CAM-B3LYP are provided as Sup. S2 and Sup. S3. The convolutions of the predicted transitions shown in Figure 2, Sup. S2, Sup. S3 and Sup. S4 were performed by assigning a Gaussian function to each excitation wavenumber with an area proportional to the oscillator strength or the MCD B-term for that transition, using a constant standard deviation, σ = 1,500 cm–1. 3. Results and discussion 3.1. Linear dichroism: Orientation factors and polarization directions The observed LD absorption curves 𝐸𝐸𝑈𝑈(𝜈𝜈�) and 𝐸𝐸𝑉𝑉(𝜈𝜈�) are shown in Figure 1a. Directional information that can be derived from the LD curves is given by the orientation factors 𝐾𝐾𝑖𝑖 = 〈cos2(𝑴𝑴𝑖𝑖 , 𝑈𝑈)〉, where (𝑴𝑴𝑖𝑖 , 𝑈𝑈) is the angle of the dipole moment vector 𝑴𝑴𝑖𝑖 of transition i with the polymer stretching direction 𝑈𝑈 and the pointed brackets indicate the average over all solute molecules in the light path [17-22]. The 𝐾𝐾𝑖𝑖 values may be estimated by considering the ‘reduced’ absorbance curves 𝑟𝑟𝐾𝐾(𝜈𝜈�) (Equation 1) [19]: 𝑟𝑟𝐾𝐾(𝜈𝜈�) = (1 − 𝐾𝐾)𝐸𝐸𝑈𝑈(𝜈𝜈�) − 2𝐾𝐾𝐸𝐸𝑉𝑉(𝜈𝜈�) (1) The contribution from transition i vanishes from the linear combination 𝑟𝑟𝐾𝐾(𝜈𝜈�) for 𝐾𝐾 = 𝐾𝐾𝑖𝑖 , and the 𝐾𝐾𝑖𝑖 value may thus be determined by visual inspection [19]. A family of curves 𝑟𝑟𝐾𝐾(𝜈𝜈�) for DPA with 𝐾𝐾 ranging between the limits 0 and 1 is shown in Figure 1b, leading to determination of 𝐾𝐾𝑖𝑖 values close to 0.64 and 0.20 for the main bands. According to the D2h molecular point group, dipole allowed transitions in DPA must be polarized along the molecular symmetry axes x, y, and z. We shall assume that the observed absorbance is primarily due to π-π* transitions and thus polarized along the in-plane y and z axes (Scheme 1); this assumption is supported by the theoretical results (Tables 1 and 2, Sup. S2, Sup. S3, Sup. S4). Aromatic hydrocarbons tend to align in stretched PE according to their molecular dimensions [17-22] and we thus assign the observed 𝐾𝐾𝑖𝑖 values 0.20 and 0.64 to the orientation factors of the short and long in-plane molecular axes y and z, �𝐾𝐾𝑦𝑦, 𝐾𝐾𝑧𝑧� = (0.20, 0.64). It is now possible to construct the partial absorbance curves 𝐴𝐴𝑦𝑦(𝜈𝜈�) and 𝐴𝐴𝑧𝑧(𝜈𝜈�) (Equations 2 and 3) corresponding to y and z polarized absorbance [19]: 𝐴𝐴𝑦𝑦(𝜈𝜈�) = �𝐾𝐾𝑦𝑦 − 𝐾𝐾𝑧𝑧�−1 𝑟𝑟𝐾𝐾𝑧𝑧(𝜈𝜈�) = −2.273 ∙ 𝑟𝑟0.64(𝜈𝜈�) (2) 𝐴𝐴𝑧𝑧(𝜈𝜈�) = �𝐾𝐾𝑧𝑧 − 𝐾𝐾𝑦𝑦�−1 𝑟𝑟𝐾𝐾𝑦𝑦(𝜈𝜈�) = +2.273 ∙ 𝑟𝑟0.20(𝜈𝜈�) (3) 90 Nguyen et al. / European Journal of Chemistry 15 (2) (2024) 87-92 2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.2.87-92.2546 Figure 3. Energies and symmetries of the five highest occupied and five lowest unoccupied π type MOs of diphenylacetylene (DPA) computed with LCOAO with indication of orbital amplitudes. The resulting partial absorbance curves are shown in Figure 1c. Inspection of the first strong-band system (band A) reveals that the sharp peaks in the vibrational progression in 𝐴𝐴𝑧𝑧(𝜈𝜈�) are associated with S-shaped “wiggles” in 𝐴𝐴𝑦𝑦(𝜈𝜈�). This phenomenon is often observed in reduction procedures with sharp peaks and can be explained by orientation-dependent inhomogeneous line-broadening (different solvent effects for differently oriented solute molecules) [17,18]. 3.2. Electronic transitions 3.2.1. Main bands The wavenumbers, relative absorbance, and polarization directions for the main band systems A, B, C, and D are listed in Table 1, where they are compared with the transitions computed with LCOAO (see also Figure 2 and Sup. S4). The spectrum starts with a strong band A with an onset at 33,200 cm–1 (301 nm). It is z-polarized and must be assigned to the 11B1u state predicted by LCOAO at 34,100 cm–1 (293 nm) (Table 1). It is well described by the HOMO-LUMO excitation, 3b3u(𝜋𝜋) → 3b2g(𝜋𝜋*) (Figure 3). Similar results are obtained with TD–CAM-B3LYP (Table 2). The next band B with an onset at 44,400 cm–1 (225 nm) has a partly resolved vibrational fine structure and a long and diffuse tail towards larger wavenumbers, overlapping the bands C and D. It is y-polarized and can be assigned to the 21B2u state computed by LCOAO at 48,700 cm–1 (205 nm). This transition is essentially due to the promotions 1au(𝜋𝜋) → 3b2g(𝜋𝜋*) and 3b3u(𝜋𝜋) → 2b1g(𝜋𝜋*) (SHOMO-LUMO and HOMO- SLUMO). Very similar results are predicted with TD-CAM- B3LYP (Table 2). The wavenumber of this transition is somewhat overestimated by the present calculations. A corresponding situation was observed for (E)-1,2-diphenyl- ethene (trans-stilbene), which is π iso-electronic with DPA [44]. Band C has a broad z-polarized maximum around 51,000 cm–1 (196 nm). It is easily assigned to the 21B1u state predicted by LCOAO at 52,200 cm–1 (192 nm) (Table 1), primarily involving the orbitals next to the frontier region: 1au(𝜋𝜋) → 2b1g(𝜋𝜋*) and 1b1g(𝜋𝜋) → 2au(𝜋𝜋*) (Figure 3). Again, the results are consistent with those obtained with TD–CAM-B3LYP (Table 2). Nguyen et al. / European Journal of Chemistry 15 (2) (2024) 87-92 91 2024 – European Journal of Chemistry – CC BY NC – DOI: 10.5155/eurjchem.15.2.87-92.2546 The band D peaking at 57,100 cm–1 (175 nm) in the vacuum UV region is predominantly y-polarized. It can possibly be assigned to the 41B2u state predicted by LCOAO at 60,800 cm–1 (164 nm) (Table 1). With the AUG-cc-pVTZ basis set, TD–CAM- B3LYP introduces additional states in this region; the 41B2u state predicted by LCOAO therefore corresponds to the 51B2u state computed with TD-CAM-B3LYP at 61,000 cm–1 (164 nm) (Table 2). But TD–CAM-B3LYP predicts a strong z-polarized transition at 58,300 cm–1 (172 nm), which seems in poor agreement with the observed spectrum (Sup. S2). In any case, the theoretical prediction of electronic states in the vacuum UV is difficult, and the suggested assignment of the absorbance observed in this region must be considered as tentative. 3.2.2. Additional transitions Much attention has been devoted to the 11Au state of DPA which is of prime importance in the photochemistry of the compound [6-13]. TD–CAM-B3LYP predicts this state at 42,500 cm–1 (235 nm) (Table 2) primarily due to the promotion 8b2u(𝜋𝜋C≡C) → 3b2g(𝜋𝜋*), where 𝜋𝜋C≡C indicates the in-plane π component of the triple-bond. In ground state absorption spectroscopy, the state is optically forbidden in the D2h point group, but it gains intensity in distorted conformations. Comparison with the optical properties of the “molecular rotor” diphenyldiacetylene (DPDA, Scheme 1) seems relevant [15]. Apart from the 11B1u state responsible for band A, the lowest optically allowed state is predicted to be 11B2u. LCOAO and TD-CAM-B3LYP predict this state at 36,700 cm–1 (272 nm) and 40,900 cm–1 (244 nm), respectively (Tables 1 and 2). The leading configurations are 1au(𝜋𝜋) → 3b2g(𝜋𝜋*) and 3b3u(𝜋𝜋) → 2b1g(𝜋𝜋*), similar to the 21B2u state giving rise to band B. The 11B2u and 21B2u states are essentially minus and plus combi- nations of the two configurations, a consequence of the approximate pairing symmetry [25,26,45] of the DPA π system. Because of the minus character, the transition to the 11B2u state is predicted to be weak (“parity forbidden”) and it is likely to be buried under the strong absorbance due to the 11B1u state (band A). The transition may possibly be observed directly in the MCD spectrum, since positive and negative B-terms are predicted for 11B1u and 11B2u (Table 1, Figure 2, Sup. S4). It should be mentioned that a variety of CAS-based procedures predict 11B2u as the lowest excited singlet state [7,12]. The present calculations clearly predict 11B1u as the lowest excited singlet state of DPA, in agreement with the results of semiempirical [5,6], TD-DFT [8,12], and ADC(2) [10] calculations. See the detailed discussion by Robertson and Worth [12]. LCOAO predicts the 31B2u state in the vacuum UV at 56,100 cm–1 (178 nm) (Table 1). The corresponding state computed with TD-CAM-B3LYP is 41B2u at 59,500 cm–1 (168 nm) (Table 2). This state is not clearly observed in the present spectrum, but it may contribute to the y-polarized absorbance in the region between the bands B and D, possibly gaining intensity by vibronic coupling with these strong bands. It may possibly be observed in the MCD spectrum, since a large negative B-term is predicted for this state (Table 1, Figure 2). However, as indicated above, the results in the high-wavenumber region should be treated with caution. 4. Conclusions The absorbance spectrum of diphenylacetylene (DPA) is characterized by four characteristic bands A, B, C, and D in the region 58,000–28,000 cm–1 (172–360 nm), similar to the spectrum of trans-stilbene [44]. According to the present results, bands A and C are z-polarized, while bands B and D are y-polarized. The LCOAO model provides an adequate theoretical description of the four main bands. The TD-CAM- B3LYP results are consistent with those obtained with LCOAO, except in the region of band D in the vacuum UV, where the two methods differ in the prediction of individual electronic transitions. The optically allowed states 11B2u and 31B2u are predicted with too low intensity to be observed in the present experimental spectra, but the MCD B-terms predicted by LCOAO suggest that they may be observed by MCD spectroscopy. Acknowledgements This investigation was supported by grants of beam time on the CD1 beamline at the Centre for Storage Ring Facilities (ISA), Aarhus University. The stay of Nguyen Duc Duy at Roskilde University was enabled by a Ph.D. scholarship granted by the Vietnamese Ministry of Education and Training. The Danish International Development Agency (DANIDA) provided additional support via the Enhancement of Research Capacity (ENRECA) program. The authors are grateful to Signe Høgsberg Andersen for preliminary investigations of the title compound and to Eva M. Karlsen for technical assistance in the spectroscopy laboratory. Supporting information Electronic supplementary material available: Spectroscopic peak data. Nuclear equilibrium coordinates calculated with CAM-B3LYP/AUG-cc-pVTZ and CAM-B3LYP/cc-pVTZ. Complete list and graphical representations of electronic transitions predicted with TD-CAM-B3LYP/AUG-cc-pVTZ, TD- CAM-B3LYP/cc-pVTZ and LCOAO. LCOAO bibliography 1980-2023. Disclosure statement Conflict of interest: The authors declare that they have no conflict of interest. Ethical approval: All ethical guidelines have been adhered to. Data availability: Spectroscopic data are available from the UV/Vis+ Photochemistry Data Base (https://science-softcon.de/spectra/). CRediT authorship contribution statement Conceptualization: Duy Duc Nguyen, Jens Spanget-Larsen; Methodology: Duy Duc Nguyen, Jens Spanget-Larsen; Formal Analysis: Jens Spanget-Larsen; Investigation: Duy Duc Nguyen, Nykola C. Jones, Søren Vrønning Hoffmann, Jens Spanget-Larsen; Resources: Nykola C. Jones, Søren Vrønning Hoffmann, Jens Spanget-Larsen; Data Curation: Jens Spanget-Larsen; Writing - Original Draft: Jens Spanget-Larsen; Writing - Review and Editing: Nykola C. Jones, Søren Vrønning Hoffmann, Jens Spanget-Larsen; Visualization: Jens Spanget- Larsen. ORCID and Email Duy Duc Nguyen duy.nguyen@intertek.com https://orcid.org/0009-0002-4241-8256 Nykola C. Jones nykj@phys.au.dk https://orcid.org/0000-0002-4081-6405 Søren Vrønning Hoffmann vronning@phys.au.dk https://orcid.org/0000-0002-8018-5433 Jens Spanget-Larsen spanget@ruc.dk https://orcid.org/0000-0002-4212-7603 References [1]. Bunz, U. H. F. Poly(aryleneethynylene)s: Syntheses, properties, structures, and applications. Chem. Rev. 2000, 100, 1605–1644. [2]. Li, Y.; Zhao, J.; Yin, G. Theoretical investigations of oligo(phenylene ethylene) molecular wire: Effects from substituents and external electric field. Comput. Mater. Sci. 2007, 39, 775–781. [3]. Whitten, D. G.; Tang, Y.; Zhou, Z.; Yang, J.; Wang, Y.; Hill, E. H.; Pappas, H. C.; Donabedian, P. L.; Chi, E. Y. A retrospective: 10 years of oligo(phenylene-ethynylene) electrolytes: Demystifying nanomaterials. Langmuir 2019, 35, 307–325. [4]. Chen, H.; Sangtarash, S.; Li, G.; Gantenbein, M.; Cao, W.; Alqorashi, A.; Liu, J.; Zhang, C.; Zhang, Y.; Chen, L.; Chen, Y.; Olsen, G.; Sadeghi, H.; Bryce, M. R.; Lambert, C. J.; Hong, W. 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Excited states of trans-stilbene and 1,4-diphenylbutadiene. Near and vacuum UV polarization spectroscopy. J. Mol. Struct. 2023, 1293, 136206. [45]. Pariser, R. Theory of the electronic spectra and structure of the polyacenes and of alternant hydrocarbons. J. Chem. Phys. 1956, 24, 250–268. Copyright © 2024 by Authors. This work is published and licensed by Atlanta Publishing House LLC, Atlanta, GA, USA. The full terms of this license are available at https://www.eurjchem.com/index.php/eurjchem/terms and incorporate the Creative Commons Attribution-Non Commercial (CC BY NC) (International, v4.0) License (http://creativecommons.org/licenses/by-nc/4.0). By accessing the work, you hereby accept the Terms. This is an open access article distributed under the terms and conditions of the CC BY NC License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited without any further permission from Atlanta Publishing House LLC (European Journal of Chemistry). No use, distribution, or reproduction is permitted which does not comply with these terms. Permissions for commercial use of this work beyond the scope of the License (https://www.eurjchem.com/index.php/eurjchem/terms) are administered by Atlanta Publishing House LLC (European Journal of Chemistry). https://hrcak.srce.hr/file/261320 https://omlc.org/spectra/%20PhotochemCAD/html/114.html https://omlc.org/spectra/%20PhotochemCAD/html/114.html http://dx.doi.org/10.13140/2.1.3455.6482 https://www.eurjchem.com/index.php/eurjchem/terms http://creativecommons.org/licenses/by-nc/4.0 https://www.eurjchem.com/index.php/eurjchem/terms 1. Introduction 2. Experimental 2.1. Theory/Calculation 3. Results and discussion 3.1. Linear dichroism: Orientation factors and polarization directions 3.2. Electronic transitions 3.2.1. Main bands 3.2.2. Additional transitions 4. Conclusions Acknowledgements Supporting information Disclosure statement CRediT authorship contribution statement ORCID and Email References PrintField10: PrintField11: PrintField12: PrintField13: PrintField14: PrintField15: PrintField20: PrintField21: PrintField22: PrintField23: PrintField24: PrintField25: