Article pubs.acs.org/JPCA
Cite This: J. Phys. Chem. A XXXX, XXX, XXX−XXX
Vacuum UV Polarization Spectroscopy of p‑Terphenyl Duy Duc Nguyen,†,# Nykola C. Jones,‡ Søren V. Hoffmann,‡ and Jens Spanget-Larsen*,† †
Department of Science and Environment, Roskilde University, Universitetsvej 1, DK-4000 Roskilde, Denmark ISA, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, Bldg. 1520, DK-8000 Aarhus C, Denmark
‡
S Supporting Information *
ABSTRACT: p-Terphenyl is used as a component in a variety of optical devices. In this investigation, the electronic transitions of pterphenyl are investigated by synchrotron radiation linear dichroism (SRLD) spectroscopy in the range 30000−58000 cm−1 (330−170 nm) on molecular samples aligned in stretched polyethylene, thereby extending the region investigated by polarization spectroscopy into the vacuum UV. The resulting partial absorbance curves reveal that the vacuum UV band system with a maximum at 55000 cm−1 (180 nm) is predominantly short axis-polarized. This result is of interest in the optical applications of p-terphenyl, for example as a wavelength shifter. The observed polarization spectra are compared with the results of quantum chemical model calculations. Convoluted versions of the transitions predicted with the semiempirical ZINDO method and with the long-range-corrected time-dependent density functional theory (TD−DFT) procedures TD−CAM-B3LYP, TD−LC-ωPBE, and TD−ωB97XD are in similar qualitative agreement with the observed partial absorbance curves throughout the investigated spectral regions, while TD−B3LYP fails to predict qualitatively the spectrum of p-terphenyl in the region above 40000 cm−1 (250 nm).
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region above 47000 cm−1 (210 nm).32−36 We were thus able to record SRLD spectra of pTP partially aligned in stretched PE in the range up to 58000 cm−1 (170 nm), leading to characterization of the polarization properties of this important chromophore in the vacuum UV region. The measured absorption bands are discussed with reference to electronic transitions predicted with a variety of quantum chemical computational procedures. Additional data is provided as Supporting Information, referred to in the ensuing text as parts S1−S17.
INTRODUCTION p-Terphenyl (pTP, Chart 1) is of considerable interest as a model compound for molecular wires (i.e., poly(p-phenylene)) Chart 1. D2 and C2h Symmetrical Rotamers of p-Terphenyl (pTP) with Definition of the Molecular Coordinate System
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EXPERIMENTAL SECTION Sample Preparation. A sample of pTP was obtained from Aldrich (purity 99%). The spectroscopic purity of the substance was checked by comparison of its UV absorption spectrum with literature data.37 Uniaxially stretched PE samples with pTP as a solute were prepared from pure low-density PE 100 μm sheet material using previously described procedures.32−36 Corresponding samples without solute were prepared for use as references. A number of sample replicas were prepared and measured in order to ensure reproducibility of the observed spectra. LD Spectroscopy. SRLD spectra were recorded at room temperature on the CD1 beamline30,31 on the storage ring
and as a backbone for molecular switches, laser dyes, detectors, scintillators, light emitters, wavelength shifters, and other optical devices.1−11 Its molecular structure and electronic transitions have been studied for decades.12−25 In this publication we report the results of an investigation of the absorbance spectrum of pTP in the near and vacuum UV regions (30000−58000 cm−1, 330−170 nm) by means of synchrotron radiation linear dichroism (SRLD) spectroscopy on molecular samples partially aligned in stretched polyethylene (PE). Linear dichroism (LD) spectroscopy provides information on the polarization directions of the observed transitions.26−29 The present study was performed with synchrotron radiation which provides increased photon flux and improved signal-tonoise ratio in the vacuum UV region, compared with a traditional source of radiation.30,31 This is advantageous with a solvent like PE which absorbs and scatters strongly in the © XXXX American Chemical Society
Received: October 22, 2017 Revised: December 5, 2017 Published: December 5, 2017 A
DOI: 10.1021/acs.jpca.7b10442 J. Phys. Chem. A XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry A
computed with ZINDO and TD−CAM-B3LYP/6-31++G(d,p) for the D2 and C2h rotamers of pTP are listed in Tables 1 and 2; full listings with MO energy diagrams are provided as Supporting Information, parts S6−S12. In the case of TD− CAM-B3LYP, 4000 cm−1 has been subtracted from the computed wavenumbers to facilitate comparison of observed and predicted trends.48 The ZINDO method considers a basis set of valence orbitals only, but to ease comparison with the TD-DFT results, the numbering of the ZINDO MOs in Table 1 includes core orbitals. Convolutions of the predicted transitions were performed by assigning a Gaussian function to each excitation wavenumber with an area proportional to the oscillator strength of that transition, using a constant width parameter, σ = 1500 cm−1 (Figures 2 and 3). Corresponding convoluted spectra obtained with TD−B3LYP, TD−LC-ωPBE, and TD−ωB97XD are provided as parts S14−S17.
ASTRID at the Centre for Storage Ring Facilities (ISA). As previously described,32−36 two dichroic absorbance curves EU(ν̃) and EV(ν̃) were recorded with the electric vector of the sample beam parallel (U) and perpendicular (V) to the stretching direction of the PE sample. Examples of the observed baseline corrected absorbance curves are shown in Figure 1.
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RESULTS AND DISCUSSION Linear Dichroism: Orientation Factors and Partial Absorbance Curves. The observed SRLD absorption curves EU(ν̃) and EV(ν̃) for pTP partially aligned in stretched polyethylene are shown in Figure 1a. The orientational properties that can be determined from the LD spectra are represented by the orientation factors Ki for the transition moments of the observed transitions i:26−29 K i = ⟨cos2(M⃗ i , U )⟩
(1)
where (M⃗ i ,U) is the angle between the moment vector M⃗ i of transition i and the stretching direction U of the polymer; the pointed brackets indicate an average over all solute molecules in the light path. Two molecular conformations of pTP are expected to contribute to the observed spectra, namely the two rotamers of D2 and C2h symmetry (Chart 1). In the case of D2 symmetry, optically allowed transitions have moment vectors M⃗ i along the three molecular symmetry axes x, y, and z, corresponding to transitions to excited states of B3, B2, and B1 symmetry, respectively. In the case of C2h symmetry, allowed transitions are either polarized along the molecular symmetry axis z, or in the x, y plane perpendicular to z, corresponding to transitions to states of Au or Bu symmetry, respectively. The orientation factors Ki for the observed transitions may be determined by the graphical TEM procedure.26,27 In the present application, we consider the reduced absorbance curves rK(ν̃):49
Figure 1. (a) Synchrotron radiation linear dichroism (SRLD) absorbance curves EU(ν̃) and EV(ν̃) for p-terphenyl (pTP) in stretched polyethylene recorded with the electric vector of the linearly polarized radiation parallel and perpendicular, respectively, to the uniaxial stretching direction U. (b) Family of reduced absorbance curves rK(ν̃) = (1 − K)EU(ν̃) − 2KEV(ν̃) according to the TEM procedure26,27,49 with K varying from 0 to 1 in steps of 0.1.
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THEORETICAL SECTION Quantum chemical calculations were performed by using the Gaussian09 software package38 (full reference available as Supporting Information, part S1). The gas phase equilibrium geometries of pTP were computed with B3LYP39−41 density functional theory (DFT) and the 6-31+G(d,p)38 basis set. Two rotamers of D2 and C2h molecular symmetries (Chart 1) were predicted with torsional angles equal to 40.2° and 39.5°, respectively, and with essentially similar energies; the predicted difference in electronic energy was less than 10−6 atomic units, or less than 10−3 kcal/mol. The energy of the two rotamers was ca. 4 kcal/mol below those of the planar and nonplanar D2h conformations (torsional angles equal to 0° and 90°, respectively). Nuclear coordinates and energies of the D2 and C2h equilibrium geometries and of the stationary points corresponding to planar and nonplanar D2h configurations are provided as Supporting Information S2−S5. Electronic transitions were computed with the ZINDO semiempirical all-valence-electrons method42 and with timedependent DFT (TD-DFT)43 procedures using the functional B3LYP39−41 and the long-range-corrected functionals CAMB3LYP,44 LC-ωPBE,45,46 and ωB97XD47 with the basis sets 321+G, 6-31+G(d,p), or 6-31++G(d,p).38 The main transitions
rK (ν)̃ = (1 − K )EU (ν)̃ − 2KEV (ν)̃
(2)
A family of curves rK(ν̃) for pTP is shown in Figure 1b. A peak or a shoulder due to transition i will vanish from the linear combination rK(ν̃) for K = Ki, and the Ki value can thus be determined by visual inspection. However, in the present case application of the procedure is complicated by the presence of broad and generally featureless bands, apart from the sharp peak close to 47000 cm−1, prominent in EV(ν̃). In ethanol solution this feature of pTP is seen as a diffuse shoulder at 47600 cm−1 (210 nm).37 The Ki value for this feature is estimated to be close to 0.05. Previous experiments20,26 have shown that the alignment of pTP in stretched PE is very efficient and rodlike.26,27 This means that Kx = Ky ≪ Kz, where Kz is the orientation factor for the long molecular axis z and Kx = Ky = Kxy is the orientation factor for any molecular axis perpendicular to z. Since Kx + Ky + Kz = 1, we have 2Kxy + Kz = 1. Hence, the observed Ki value 0.05 must correspond to Kxy, B
DOI: 10.1021/acs.jpca.7b10442 J. Phys. Chem. A XXXX, XXX, XXX−XXX
44 44 46.7
48.1 54.7
55.6
Cg Dg E
F G
H
0.91
0.86 0.46
0.3 0.2 0.96
1.01 0.16
absc
x, y
z z
x, y z x, y
z x, y
pold 1 B1 1 1B2 1 1B3 2 1B2 3 1B2 2 1B1 2 1B3 4 1B2 3 1B1 5 1B1 6 1B1 6 1B2 5 1B3 8 1B3 7 1B2 10 1B2
1
term 33.9 34.7 36.1 36.1 44.9 42.6 46.0 46.4 45.8 51.4 54.5 52.3 52.3 54.1 54.1 56.5
ν̃b 0.99 0.01 4 × 10−3 3 × 10−3 0.12 3 × 10−3 0.39 0.83 1.52 0.18 0.21 0.13 0.08 0.13 0.19 0.24
fe 88% 39% 34% 27% 46% 34% 39% 36% 37% 29% 33% 26% 27% 32% 25% 25%
(14b2 → 14b3) (14b2 → 18a), 18% (14b2 → 19a) (14b2 → 18b1), 18% (17a → 14b3) (14b2 → 19a), 22% (13b3 → 18b1) (16b1 → 14b3), 26% (14b2 → 19a) (14b2 → 15b3), 14% (13b3 → 15b2) (14b2 → 18b1), 32% (17a → 14b3) (17b1 → 14b3), 32% (14b2 → 18a) (17b1 → 18a), 29% (16b1 → 19a) (13b2 → 14b3), 18% (13b3 → 15b2) (13b2 → 15b3), 22% (13b3 → 15b2) (13b3 → 18b1), 15% (13b2 → 18a) (13b2 → 18b1), 20% (13b3 → 19a) (17a → 15b3), 17% (16b1 → 15b2) (17b1 → 15b3), 19% (17a → 15b2) (16b1 → 15b3), 20% (13b2 → 19a)
leading configurationsf
ZINDO: D2 rotamera
44.8 42.6 46.4 45.8 51.4 54.2 52.2 53.4 53.9 55.8 57.0
3 1Au 5 1Au 6 1Au 6 1Bu 7 1Bu 8 1Bu 9 1Bu 10 1Bu
33.9 34.7 36.1
ν̃b
3 1Bu 2 1Au 4 1Bu
1 Au 1 1Bu 2 1Bu
1
term
1.52 0.20 0.14 0.20 0.04 0.33 0.03 0.22
0.20 3 × 10−3 1.12
0.99 0.01 7 × 10−3
fe
35% 25% 21% 28% 48% 24% 28% 41%
(18ag (12bg (12bg (14bu (13bg (18ag (12bg (17ag
→ → → → → → → →
17au), 15bu), 16bu), 19ag), 20au), 16bu), 17au), 16bu),
29% 22% 17% 13% 24% 19% 24% 14%
(17ag (14bu (14bu (18ag (14bu (16au (12bg (18ag
→ → → → → → → →
18au) 14bg) 14bg) 15bu) 20ag) 14bg) 18au) 16bu)
47% (17ag → 15bu), 26% (13bg → 18au) 32% (13bg → 16bu), 16% (14bu → 14bg) 37% (18ag → 15bu), 33% (13bg → 17au)
88% (13bg → 15bu) 39% (13bg → 17au), 19% (13bg → 18au) 26% (13bg → 18au), 22% (14bu → 19ag)
leading configurationsf
ZINDO: C2h rotamera
a
Main transitions only. Complete list of calculated transitions provided as parts S9 and S10. bPeak wavenumber in 1000 cm−1. cPeak absorbance estimated from the partial absorbance curves in Figures 2 and 3. dPolarization direction. eOscillator strength. fMO numbering includes core orbitals, diagrams in Figures 4 and 5 (see also parts S7 and S8). gDiffuse shoulder.
34.4 35.7
A B
ν̃b
observed
Table 1. Assignment of Observed Features of the SRLD Spectrum of p-Terphenyl (pTP) to Electronic Transitions Predicted with the ZINDO Method
The Journal of Physical Chemistry A Article
C
DOI: 10.1021/acs.jpca.7b10442 J. Phys. Chem. A XXXX, XXX, XXX−XXX
D
46.7
48.1
54.7
E
F
G
0.91
0.46
0.86
0.96
0.2
0.3
1.01 0.16
absc
x, y
z
z
x, y
z
x, y
z x, y
pold 34.1 36.2 37.6 37.7 42.2 44.2 44.7 46.2 46.2 46.4 47.2 47.6 48.3 54.6 55.0 55.3 56.6 57.7 52.7 55.2 55.4 56.1 58.5
3 1B3 5 1B2 6 1B2 3 1B1 4 1B1 10 1B1 12 1B1 13 1B1 14 1B1 15 1B1 8 1B2 12 1B3 12 1B2 13 1B2 16 1B2
ν̃b,e
B1 1 B2 1 B2 1 B3 1 B2 1 B2 1 B3 1 B1
1 1 2 1 3 4 2 2
1
term
0.14 0.21 0.11 0.38 1.05 0.04 0.04 0.04 0.11 0.08 0.10 0.18 0.15 0.10 0.05
0.96
