Article pubs.acs.org/JPCA
Excitation Spectra of Large Jet-Cooled Polycyclic Aromatic Hydrocarbon Radicals: 9‑Anthracenylmethyl (C15H11) and 1‑Pyrenylmethyl (C17H11) Gerard D. O’Connor, George B. Bacskay, Gabrielle V. G. Woodhouse, Tyler P. Troy, Klaas Nauta, and Timothy W. Schmidt* School of Chemistry, The University of Sydney, NSW 2006, Australia S Supporting Information *
ABSTRACT: The 9-anthracenylmethyl (C15H11) and 1pyrenylmethyl (C 17 H 11 ) radicals were identified by a combination of mass-resolved laser spectroscopy of a jetcooled electrical discharge and quantum chemical methods. The 9-anthracenylmethyl radical was found to exhibit an origin band at 13757 cm−1, with vibrational structure observed in a1 modes, and even quanta of b1 and a2 modes. The 1pyrenylmethyl radical was found to exhibit an origin band at 13 417 cm−1, with a more complex vibrational structure as compared to 9-anthracenylmethyl, on account of its lower symmetry and larger size. The origin bands of these species were predicted to within 250 cm−1 by fitting a linear relationship between observed origin wavelengths of similar chromophores and the calculated TD-B3LYP transition energies. A refined fit including the title radicals provides estimated absorption energies for the larger 2-perylenylmethyl and 6-anthanthrenylmethyl species of 1.44 and 1.41 eV, respectively, with an estimated error of 30 meV.
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INTRODUCTION Polycyclic aromatic hydrocarbons (PAH) are networks of sp2 hybridized carbon atoms, whose “dangling bonds” are passivated with hydrogen. In the limit of large system size, one arrives at graphene,1 which has no band gap, whereas the smallest PAH is naphthalene, a white crystalline material that absorbs in the gas phase in the ultraviolet region.2−4 Though few would regard PAHs as quantum-confined graphenes, they indeed serve as theoretical benchmarks for larger carbon networks, and some PAHs have found technological applications in solar energy and molecular electronics.5,6 In their own right, PAHs are important pollutants on Earth, being produced in combustion,7,8 and are postulated to be important astronomical species.9−12 Most spectroscopic studies on the band gaps and vibrational frequencies of PAHs have focused on the closed-shell species, which are often commercially available,2,13 or readily synthesized.14 There are fewer studies on PAHs with unpaired electrons. Though not a PAH, the benzyl radical is a prototypical aromatic radical, where the unpaired electron is delocalized throughout the conjugated π-system. Radicals featuring this delocalization are known as resonance-stabilized radicals (RSRs). The benzyl radical has been the subject of extensive theoretical and experimental examination15−23 and is calculated to exhibit a radical stabilization energy (RSE) of 60 kJ/mol.24 That is to say, breaking a C−H bond in the methyl group of toluene requires 60 kJ/mol less energy than in methane (the methyl radical is defined © 2013 American Chemical Society
as having a RSE of 0). From this, one can predict that the C−C bond of 1,2-diphenylethane would be weakened by 2 × 60 kJ/ mol. As such, benzyl-type radicals will be preferentially formed through the breakup of large amorphous hydrogenated carbon networks, which are a postulated form of interstellar carbon.25−30 Of the larger benzylic radicals (Figure 1), only the naphthylmethyl radicals have been studied in the gas phase by laser spectroscopy.31 The larger radicals, based on the anthracene and pyrene frameworks, are the subjects of the present study. In the following, we use a combination of experiment and theory to predict the excitation wavelengths of the D1 ← D0 transitions of the 9-anthracenylmethyl (9-AnMe) and 1pyrenylmethyl (1-PyMe) radicals, pictured in Figure 1, and then identify them using the mass-resolved resonant two-color two-photon ionization technique, coupled to a pulsed discharge nozzle and supersonic expansion source. The spectra are discussed in the context of unidentified astronomical spectral features, and predictions are made for the absorption positions of still larger benzannulated benzyl radicals (BBRs).32 Special Issue: Terry A. Miller Festschrift Received: September 5, 2013 Revised: September 28, 2013 Published: September 30, 2013 13899
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bar of argon at 368 K before being supersonically expanded into a vacuum and passed through the two electrodes of the PDN. A potential of −800 V, in series with an 18 kΩ resistor and pulsed for a duration of 80 μs, was applied to the inner electrode at a time coincident with the 230 μs gas pulse. For 1-PyMe radical production, 1-methylpyrene was seeded into 9 bar of argon at 343 K before being supersonically expanded into a vacuum and passed through the two electrodes of the PDN. A potential of −1100 V, in series with an 18 kΩ resistor, pulsed for a duration of 70 μs, was applied to the inner electrode at a time coincident with the 170 μs gas pulse. The radical-containing plasma was then cooled by supersonic expansion. The coolest part of the jet passed through a 2 mm skimmer into the differentially pumped interrogation region, between the electrostatic grids of a time-offlight mass spectrometer. In the interrogation region, the radicals were first excited by a Nd:YAG-pumped dye laser and subsequently ionized by a second laser pulse. For 9-AnMe, ionization photons were produced by the 193 nm (6.4 eV) output of a small ArF excimer laser, whereas 1-PyMe ionization photons were produced by a frequency doubled Nd:YAG-pumped dye laser containing Exalite 417, producing 206 nm (6.0 eV) photons. Resulting cations were accelerated by a +2.4 kV potential along a time-offlight tube before being detected by a tandem multichannel plate. Ion signal, corresponding to the appropriate mass/charge ratio (m/z 191 for 9-AnMe, m/z 215 for 1-PyMe), was plotted against excitation wavelength to obtain the final spectrum. Hole-Burning. Hole-burning depletion was undertaken through the addition of a Nd:YAG-pumped dye laser pulse, 100 ns prior to the excitation and ionization laser pulses. Due to low signal-to-noise, hole-burning depletions were obtained by subsequent one minute acquisitions at fixed frequencies, alternating between activation and deactivation of the depletion laser.
Figure 1. Structures of the benzyl radical (top left) and benzannulated analogs: 2-naphthylmethyl (top right); 9-anthracenylmethyl (bottom left); 1-pyrenylmethyl (bottom right).
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THEORETICAL METHODS The quantum chemical calculations of ground- and excited-state geometries and frequencies of 9-AnMe and 1-PyMe were carried out using density functional theory33 (DFT) and timedependent DFT34 (TD-DFT), utilizing the B3LYP functional35,36 and the 6-311++G(d,p) basis set, as well as by complete active space self-consistent field (CASSCF) theory37 with seven active electrons in seven active orbitals. The 6-31G(d) basis38 was used in the CASSCF calculations, but for technical reasons the H basis in the 1-PyMe radical was reduced to the minimal STO-3G set.39 (Analogous calculations on 9-AnMe have demonstrated that such simplification in the basis results in negligibly small changes in the geometries, frequencies, and relative energies.) The excitation energies were obtained at both the TD-B3LYP/ 6-311++G(d,p) and complete active space second-order perturbation (CASPT240−42) level of theory, with an active space of 11 electrons in 11 orbitals, in conjunction with the 6311G(d) and 6-31G bases on the C and H atoms respectively. In addition to harmonic frequency calculations at the TDB3LYP/6-311++G(d,p) level, excited-state potential energy surfaces of 9-AnMe were constructed by computing single point TD-B3LYP excited-state energies at geometries distorted by application of the calculated excited-state vibrational coordinates. The calculated points were then fit with a cubic spline, allowing the variational calculation of (anharmonic) vibrational states. The B3LYP and TD-B3LYP calculations were carried out using the Gaussian0343 and Gaussian0944 suites of software. The Dalton programs45 were used to obtain CASSCF geometries and frequencies, whereas the CASPT2 calculations were carried out using MOLPRO.46
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RESULTS AND DISCUSSION Excitation Energies (TD-DFT and CASPT2). The TDB3LYP/6-311++G(d,p) excitation energies for 9-AnMe and 1PyMe are reported in Table 1. As odd-alternant RSRs, the D1 ← D0 transitions of BBRs are expected to be dominated by the combination of two single-electron transitions: from the highestoccupied molecular orbital (HOMO) to the singly occupied molecular orbital (SOMO); and from the SOMO to the lowestunoccupied molecular orbital (LUMO).54 In the D1 ← D0 Table 1. Dn ← D0 Vertical Electronic Transition Energies, En, and Oscillator Strengths, f, of the 9-AnMe and 1-PyMe Radicals, Calculated at the TD-B3LYP/6-311++G(d,p) Level of Theorya
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9-AnMe
EXPERIMENTAL METHODS Excitation Spectra. Mass-resolved resonant two-color twophoton ionization spectroscopy (R2C2PI) was used to record the D1 ← D0 excitation spectra reported in this paper. The apparatus used has been described previously31,47−52 and is summarized here. Argon, containing traces of 9-methylanthracene or 1methylpyrene, was expanded into a vacuum chamber through a pulsed discharge nozzle (PDN).53 Discharge conditions were optimized separately for 9-AnMe and 1-PyMe radical production. For 9-AnMe radicals, 9-methylanthracene was seeded into 4
a
13900
1-PyMe
n
Γμn0
En (eV)
f
Γμn0
En (eV)
f
1 2 3 4 5 6 7 8
A1 A1 B2 A1 B2 B2 B2 B2
1.95 2.76 2.8 3.13 3.2 3.39 3.63 3.87
0.0003 0.0889 0.0006 0.0032 0.0013 0.1319 0.0696 0.0009
A′ A′ A′ A′ A′ A′ A′ A′
1.96 2.61 2.86 3.06 3.15 3.2 3.5 3.7
0.0011 0.0015 0.2499 0.0164 0.0499 0.0174 0.0982 0.0022
The symmetry of the transition moment is given as Γμn0. dx.doi.org/10.1021/jp4088833 | J. Phys. Chem. A 2013, 117, 13899−13907
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Table 2. 9-AnMe and 1-PyMe D1 ← D0 Excitation Energies (cm−1)
transition, the transition moments of these single electron transitions interfere destructively, resulting in a relatively weak transition. A stronger, higher-energy, transition occurs when the transition moments of the same two single-electron transitions interfere constructively (Table 1). Interestingly, the lowest energy transition of 9-AnMe is calculated to be parallel to the C2 axis. The lowest transition in the benzyl radical itself is perpendicular to the symmetry axis in the plane of the molecule. However, in benzyl, the D2 state is just 400 cm−1 higher in energy than D1 and is accessed by a transition parallel to the symmetry axis,16 the near degeneracy arising from the degenerate LUMOs of benzene. In 9-AnMe the LUMO is of b1 symmetry, and thus the lowest transition is parallel to the symmetry axis. The TD-B3LYP/6-311++G(d,p) method has been shown to overestimate D1 ← D0 transition energies of RSRs, and a comparison between calculated and experimental values for benzylic PAH radicals suggests that this error may occur in a systematic way. The experimental D1 ← D0 excitation energies of the BBRs previously analyzed under jet-cooled conditions, benzyl,19 1naphthylmethyl and 2-naphthylmethyl radicals,31 were plotted against the presently calculated TD-B3LYP values (Figure 2). A
9-AnMe
a
1-PyMe
method
Te
T0
Te
T0
CASPT2 TD-B3LYP scaled DFTa experiment
13375 15232
12698 14945 13590 13757
14638 15204
14227 14854 13660 13417
See Figure 2.
experiment. In contrast, the ab initio CASPT2 method underand overestimates these energies by ∼1000 cm−1. As found in previous work on related smaller molecules,31,51 both theoretical approaches provide acceptable estimates of the excitation energies, especially when an empirical scaling is applied. 9-Anthracenylmethyl (9-AnMe) Radical Spectrum. Figure 3 shows the R2C2PI spectrum obtained in the region 12 850−14 500 cm−1 by monitoring the m/z 191 mass channel, using 9-methylanthracene as a precursor. Hydrogen abstraction in a discharge has been observed previously to generate many RSRs,31,52 and thus it is here proposed that the spectrum obtained is due to 9-AnMe. As shown in Table 2, the CASPT2 calculated value for T0 is 1059 cm−1 smaller than the observed transition, whereas the empirically scaled TD-B3LYP value is just 167 cm−1 less than the observed transition. The agreement between the calculated excitation energies, the structure of the precursor, and the observed m/z of the spectral carrier all support an assignment of the observed spectrum to the 9-AnMe radical. 9-AnMe Spectral Assignments. The 9-AnMe radical is of C2v symmetry, and assignments of vibrational modes have been made in this point group. The D1 ← D0 transition of 9-AnMe is calculated to be of A1 symmetry (B1 ← B1), and thus vibrational levels with total a1 symmetry are Franck−Condon allowed, and levels of b2 and b1 symmetry are vibronically allowed. The electronic origin band is therefore Franck−Condon allowed and the strong band observed at 13757 cm−1 is thus assigned (000). The energies of the additional vibronic bands are reported relative to this band. The TD-B3LYP D1 excited-state frequencies for 9-AnMe are tabulated in the Supporting Information. The 72 vibrational modes are labeled in the Mulliken convention in the C2v point group, showing 25 a1, 10 a2, 13 b1, and 24 b2 vibrational modes. It is expected that several modes, especially out-of-plane bending, may exhibit anharmonicity, and thus anharmonic frequencies for these modes were calculated. For completeness, other modes assigned in the spectrum were also calculated. Calculated anharmonic frequencies for several modes are listed in Table 3, with assigned frequencies in Table 4. The TD-B3LYP/6-311++G(d,p) excited-state harmonic frequencies of a1 symmetry, ν21−25, correspond well to the observed relative frequencies of strong transitions, with a maximum disagreement of just 12 cm−1 (Table 4). Similar agreement was observed by Zwier and co-workers for the 1hydronaphthyl radical.55 However, the strong band, observed at +108 cm−1, is at a significantly lower frequency than the lowest frequency a1 mode (ν25). It is possible that multiple isomers could be generated by a discharge, as seen previously.31 To explore this possibility, spectral hole-burning was used to confirm that the +108 cm−1 band is due to the same molecule as the putative origin and the strongest low-frequency bands. Indeed, the strong band in
Figure 2. Correlation of experimental and TD-B3LYP/6-311++G(d,p) D1 ← D0 electronic excitation energies. Experimental values for benzyl,19 1-NpMe, and 2-NpMe radicals31 are from the literature. The experimental excitation energy for the 9-AnMe and 1-PyMe radicals are from data reported in this paper (open symbols). The line is a fit through the solid data points, and the origin.
linear fit of these data, including the point (0,0) results in an empirical correction to the calculated excitation energies for 9AnMe and 1-PyMe. This line, with formula Eexp = 0.864ETD−B3LYP + 0.0004 was used to estimate the D1 ← D0 transition energies of 9-AnMe and 1-PyMe. From the respective ETD‑B3LYP energies of 1.95 and 1.96 eV, the expected origin band positions for these radicals are calculated to lie near 1.69 eV, or 730 nm. This formula makes predictions for the known radical excitation energies with errors of 0.02 eV, and so we could not necessarily expect the predictions for 9-AnMe and 1-PyMe to be better than this. The computed D1 ← D0 excitation energies (adiabatic and zero-point corrected) are listed in Table 2 along with the experimental values. Although TD-B3LYP overestimates the excitation energies of 9-AnMe and 1-PyMe, respectively, by ∼1200 cm−1, the scaled TD-DFT values are within ∼250 cm−1 of 13901
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Figure 3. R2C2PI spectrum of a m/z 191 product of an electrical discharge containing 9-methylanthracene, assigned to the 9-anthracenylmethyl radical (structure shown). Top: lower power spectrum, revealing more accurate relative intensities.
calculated to appear at 188 cm−1. But, given that the energies of both 2ν47 and 2ν48 are overestimated by 16 and 18 cm−1, respectively, 10 cm−1 is a tolerable error. Due to the appreciable intensity of the bands assigned to modes 2ν48 and ν25, it is expected that combination bands featuring these modes may have observable intensities. Further assignments are made to combination bands containing either 2ν48 or ν25, with the relative frequencies estimated from alreadyassigned bands. Small differences between the expected and observed frequencies are observed but are generally within a few cm−1 (Table 4). The bands observed at +364 and +411 cm−1 do not correspond to the calculated anharmonic frequency of any mode, or combination of modes, with total symmetry a1. However, vibronic coupling with higher B2 electronic transitions could allow single quanta of b2 modes to be observed. Good agreement between calculated anharmonic excited-state frequencies for b2 modes ν72 and ν71 with bands observed at relative frequencies +364 and +411 cm−1 support this conjecture. As seen in Table 1, TD-B3LYP predicts several strong transitions of B2 symmetry. The D6 ← D0 transition, which TD-B3LYP calculates as 1.44 eV above the reported D1 ← D0 transition, is calculated to be more than 400 times as intense. With good correspondence between calculated frequencies and the observed spectrum, as seen in Table 4, the assignment of this spectrum to 9-AnMe is secure. 1-PyMe Spectral Assignments. Figure 4 shows the R2C2PI excitation spectrum obtained in the region 12 800− 15 800 cm−1 by monitoring the m/z 215 mass channel for a discharge containing 1-methylpyrene. It is expected that the spectrum is due to 1-PyMe, formed from 1-methylpyrene by hydrogen abstraction, in a manner similar to that for 9-AnMe. The D1 ← D0 transition of 1-PyMe is calculated to have in-plane A′ symmetry (A″ ← A″). Vibrational modes of a′ symmetry are
Table 3. Anharmonic Frequencies of 9-AnMe in the D1 Excited State Computed at the TD-B3LYP/6-311++G(d,p) Level of Theory ν
Γν
v=1
v=2
21 22 23 24 25 35 46 47 48 71 72
a1 a1 a1 a1 a1 a2 b1 b1 b1 b2 b2
702 642 510 387 248 106 297 130 58 419 374
1404 1283 1020 774 496 220 595 265 126 841 753
question, as well as those labeled 2510 and 4720 in Figure 3, were all verified as belonging to the same molecule as the origin. Combinations of even-numbered quanta of a2, b1, or b2 modes will also have a total symmetry of a1, and these possible assignments were considered. Due to potentially significant anharmonicity of the out-of-plane a2 and b1 modes, calculated anharmonic frequencies were used for the remainder of the assignment. The band observed at +108 cm−1 is assigned as two quanta of the lowest frequency b1 mode, 2ν48, with a calculated anharmonic excited-state frequency of 126 cm−1 . Similar two-quanta b1 assignments can be made for the bands at +249 cm−1 (2ν47) and +600 cm−1 (2ν46). The band with relative frequency +221 cm−1 can be assigned to two quanta of the lowest-frequency a2 mode, 2ν35. The band observed with a relative frequency of +178 cm−1 may be assigned to the combination band 47104810. From anharmonic calculations of single quanta of these modes, the band is 13902
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Table 4. Assignment of Observed Vibrational Frequencies of the 9-AnMe Radical in the D1 Excited State and Comparison with Computed Harmonic and Anharmonic Frequencies Obtained at the TD-B3LYP/6-311++G(d,p) Level of Theorya a1 modes
b1 modes
a2 modes b2 modes combination bands
a
assignment
observed
harmonic
anharmonic
Δanh
expected
Δexp
ν25 ν24 ν23 ν22 ν21 2ν48 2ν47 2ν46 4ν48 2ν35 ν72 ν71 ν47ν48 2ν352ν48 2ν472ν48 ν252ν48 ν252ν35 ν252ν47 ν25ν24 ν254ν48 ν242ν48 ν232ν48
244 385 519 637 690 108 249 600 290 221 364 411 178 338 360 353 467 *488 629 531 *493 *628
246 387 527 641 702 88 246 590 176 196 374 421 167 284 334 334 442 492 633 422 475 615
248 387 510 642 702 126 265 595 281 220 374 419 188 346 391 374 468 513 635 529 513 636
+4 +2 −9 +5 +12 +18 +16 −5 −9 −1 +10 +8 +10 +8 +31 +21 +1 +25 +6 −2 +20 +8
329 357 352 465 493 629 534 493 627
−9 −3 −1 −2 +5 0 +3 0 −1
The starred frequencies are tentative observed values based on partially resolved bands.
Figure 4. R2C2PI spectrum of 1-methylpyrene discharge recorded for m/z 215. Spectrum assigned to 1-pyrenylmethyl radical (structure shown). † tentative assignment.
417 cm−1 is thus assigned. This assignment is in good agreement with the D1 ← D0 transition energy calculated by both CASPT2
therefore Franck−Condon allowed and the electronic origin band is allowed and expected. The lowest frequency band at 13 13903
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(14 227 cm−1) and scaled TD-B3LYP/6-311++G(d,p) (13660 cm−1). Although it might at first be surprising that the 1-PyMe radical, with a larger chromophore, absorbs at an energy so similar to 9AnMe, this is easily rationalized in terms of the electronic structure arising from a radical site interacting with the peripheral π system of the PAH. Both anthracene and pyrene possess a periphery with 14 C−C bonds, rendering them aromatic according to Hückel’s rule.56 As such, if the PAHs are considered to be perturbed cyclic systems, then, in the spirit of Hückel’s rule, they would be expected to absorb at similar energies, as they do. As such, it is unsurprising that the radicals built on these chromophores also absorb at similar energies. The TD-B3LYP D1 excited-state frequencies for 1-PyMe are also tabulated in the Supporting Information. The 78 vibrational modes are labeled in Mulliken convention in Cs symmetry, showing 53 a′ and 25 a″ vibrational modes. The assignment of the 1-PyMe vibronic structure is focused on low frequency modes with vibrational energies below 900 cm−1, assignment of higher-frequency bands being complicated by multiple valid assignments fitting observed bands. All peaks in this region show good agreement with the calculated TD-B3LYP harmonic excited-state frequencies of a′ vibrational modes (Table 5). These assignments are displayed in Figure 4 and
two lasers; and exponential decay caused by the decaying excited state. Analyzing the ionization-lifetime transient of 9-AnMe, shown in Figure 5, reveals a laser cross-correlation of 5.1 ns and a D1
Figure 5. Ionization lifetime scan of 9-AnMe showing a D1 excited-state lifetime of
