Electronic Spectroscopy of 1-(Phenylethynyl)naphthalene

Article pubs.acs.org/JPCA

Electronic Spectroscopy of 1‑(Phenylethynyl)naphthalene Philipp Constantinidis,† Melanie Lang,† Jörg Herterich,† Ingo Fischer,*,† Johannes Auerswald,‡ and Anke Krueger‡ †

Institute of Physical and Theoretical Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany

‡

S Supporting Information *

ABSTRACT: Recently 1-(phenylethynyl)naphthalene (1PEN) was suggested to be the primary dimerization product of phenylpropargyl radicals and therefore an important polycyclic hydrocarbon in combustion processes. Here we describe a spectroscopic investigation of a genuine 1-PEN sample by several complementary techniques, infrared spectroscopy, multiphoton ionization (MPI), and threshold photoelectron spectroscopy. The infrared spectrum recorded in a gas cell confirms that 1-PEN is indeed the previously observed dimerization product of phenylpropargyl. The origin of the transition into the electronically excited S1 state lies at 30823 cm−1, as found by MPI. Considerable vibrational activity is observed, and a number of low-wavenumber bands are assigned to a progression in the torsional motion. Values of 6 cm−1 (S0) and 17 cm−1 (S1) were derived for the fundamental of the torsion. In the investigated energy range the excited state lifetimes are in the nanosecond range. Spectra of the 1-PEN/Ar cluster exhibit a red shift of the electronic origin of 22 cm−1, in good agreement with other aromatic molecules. A threshold photoelectron spectrum recorded using synchrotron radiation yields an ionization energy of 7.58 eV for 1-PEN. An excited electronic state of the cation is found at 7.76 eV, and dissociative photoionization does not set in below 15 eV.

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INTRODUCTION Recently we studied the dimerization of phenylpropargyl radicals, C9H7, in a pyrolysis flow reactor by IR/UV ion dip spectroscopy.1 The main product of this reaction turned out to be 1-(phenylethynyl)naphthalene, from here on abbreviated as 1-PEN and depicted in Scheme 1, which is most likely formed

work on combustion products found a substantial amount of H-rich polycyclic aromatic hydrocarbons (PAHs) in soot,6 an observation that might be explained by the presence of PAH molecules with aliphatic chains. In astrochemistry on the other hand polycyclic molecules are generally assumed to be the carriers of the unidentified infrared bands (UIBs), but recent work suggested that some of the IR bands are probably due to aliphatic side chains.7,8 Molecules that are formed from resonance-stabilized radicals in a reaction with low activation barriers might subsequently be interesting candidates for the UIBs. 1-PEN has been used also in synthetic organic chemistry for the preparation of larger polycyclic aromatic hydrocarbons.9 Our earlier results and the possible importance of molecules such as 1-PEN motivated us to investigate the structure and properties of this molecule by (1) infrared spectroscopy, (2) resonance-enhanced multiphoton ionization (REMPI) to gain insight into the excited electronic states, and (3) threshold photoelectron spectroscopy using synchrotron radiation to determine the ionization energy (IE). Previously we chose this approach of combining different spectroscopic techniques in our group to study pyracene,10 a molecule consisting of fiveand six-membered rings and various substituted [2.2]paracyclophanes.11

Scheme 1. Chemical Structure of 1(Phenylethynyl)naphthalene

in a kinetically controlled reaction. It is generally assumed that the resonance-stabilized phenylpropargyl, first studied in an electric discharge2,3 and later investigated by photoionization,4 will accumulate in combustion processes and contribute to the growth of polycyclic aromatic hydrocarbons and soot. Therefore, its dimerization product 1-PEN might be relevant for combustion chemistry as well. Interestingly, a photoionization study of the reactions of the phenyl radical in a chemical reactor identified an intense peak at m/z = 228, the molecular mass of 1-PEN, but the authors could not conclusively assign it.5 Earlier © 2014 American Chemical Society

Received: December 20, 2013 Revised: February 25, 2014 Published: March 31, 2014 2915

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excitation energies in the neutral and cation of 1-PEN were computed by density functional theory (DFT) and timedependent DFT respectively, using the B3LYP (cation) or ωB97xD (neutral) functionals and either a 6-311G** or a 6311++G** (TD-DFT computations of neutral) basis set. The ionization energy was in addition calculated using the complete basis set CBS-QB3 approach. In all cases the Gaussian 09 package was employed.23 All computed vibrational wavenumbers reported below are given unscaled.

No detailed spectroscopic information is yet available on 1PEN, but related molecules have been investigated, because they are possible building blocks for molecular wires.12,13 Tolane, 1,2-diphenylacetylene, was studied by laser-induced fluorescence14 and high-resolution photoelectron spectroscopy,15 while 1,4-bis(phenylethynyl)benzene was explored by cavity ringdown spectroscopy.16 The spectra of both molecules showed extensive torsional structure, which is therefore expected for 1-PEN as well. Below, we will refer to these related experiments several times for comparison.

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RESULTS AND DISCUSSION a. Gas-Phase Infrared Spectrum of 1-PEN. As discussed in the Introduction, we investigated the dimerization of phenylpropargyl radicals by IR/UV ion dip spectroscopy using free electron laser (FEL) radiation.1,24 The fingerprint region IR spectrum recorded in the m/z = 228 mass channel in experiments on 3-phenylpropargyl is depicted in the upper trace of Figure 1. In ref 1 it was assigned to 1-PEN by

EXPERIMENTAL SECTION 1-PEN was synthesized from 1-bromonaphthalene and phenylacetylene by a modified Sonogashira coupling according to literature.17 Details of the synthetic steps and the characterization of the intermediate products are given as Supporting Information. For the REMPI spectra and lifetime measurements the substance was placed in the sample compartment of a modified pulsed solenoid valve (General Valve). The source was heated to approximately 120 °C to evaporate 1-PEN, which was diluted in 1.4−2 bar of either argon or nitrogen. The gas mixture was expanded through a 0.5 mm diameter nozzle into the vacuum. For the REMPI experiments a nanosecond laser system was employed, consisting of a 10 Hz Nd:YAG laser pumping a dye laser operating with DCM in dimethyl sulfoxide (DMSO). The dye laser output was frequency-doubled in a potassium dihydrogen phosphate (KDP) crystal. Its wavelength was calibrated in a hollow cathode lamp filled with neon. Around 1 mJ/pulse unfocused laser light were employed for the [1 + 1]-REMPI process. The spectra were not corrected for laser power. The ions were detected in a time-of-flight (TOF) mass spectrometer with a dual stage microchannel plate detector. The ion signals were recorded using a digital storage oscilloscope and typically averaged over 50 shots. The setup was synchronized using a digital delay generator. The detailed setup has been described previously.18 For lifetime measurements a picosecond laser system with 10 Hz repetition rate was employed. The output of a tunable optical parametric generator, pumped by the third harmonic of a Nd:YLF laser, was used to excite 1-PEN. For ionization the output of the nanosecond laser system was utilized, and the two laser systems were synchronized electronically by another digital delay generator. Again details of the setup have been reported elsewhere.10 The infrared spectrum was recorded in a Fourier transform (FT) spectrometer (Bruker IFS 120HR), using a high-temperature gas cell designed to investigate gasphase vibrational spectra of molecules with low vapor pressure.19,20 Threshold photoelectron (TPE) spectra were recorded at the vacuum-UV (VUV) beamline of the Swiss Light Source (SLS).21 Experiments were performed in a differentially pumped vacuum chamber employing the iPEPICO (imaging photoelectron/photoion coincidence) technique, which enables the mass selective detection of threshold photoelectrons by detecting them in coincidence with ions.22 The sample was heated to 100 °C and then effused into the chamber. The spectrometer combines a Wiley−McLaren TOF mass spectrometer and a velocity map imaging photoelectron spectrometer. The photon energy was scanned in steps of 20 meV, and the data were recorded with an acquisition time of 5 min per data point. Computations were carried out to aid in the interpretation of the experimental data. Vibrational frequencies and vertical

Figure 1. Infrared spectrum of 1-PEN, recorded in a gas cell (lower trace), in comparison to the IR/UV ion dip spectrum of m/z = 228, recorded with FEL radiation in experiments on the 3-phenylpropargyl radical (upper trace).

comparison with a computed spectrum. Here we compare the earlier FEL spectrum (upper trace) with a gas-phase spectrum of a synthesized 1-PEN sample (lower trace). Although the resolution of the FT-IR spectrum is better and a splitting in the major band around 760 cm−1 is visible, the two spectra agree in both band position and intensity. Even small features that are hardly distinguishable from the noise in the FEL spectrum are discernible in the FT-IR spectrum. This similarity confirms that 1-PEN is indeed the carrier of the m/z = 228 IR/UV spectrum and thus the dimerization product of both 1- and 3phenylpropargyl. b. Photoionization. We investigated the photoionization of 1-PEN by synchrotron radiation for two reasons: First, in many aromatic molecules the first excited singlet state is located roughly halfway to the ionization limit, so the accurate IE has to be known in advance before REMPI experiments are conducted. Second, combustion-relevant molecules are often detected in flames online by photoionization using synchrotron radiation.25−27 Since assignment to a given structure is based on the IE, prior information on the photoionization properties is advantageous in this type of experiment. The threshold photoelectron spectrum of 1-PEN is depicted in Figure 2 together with a Franck−Condon (FC) simulation28 of the ground and the first excited cationic states. The onset of the signal is found at 7.50 eV, and the maximum is reached around 7.6 eV. A second broad band maximizes around 7.8 eV. 2916

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Figure 2. Threshold photoelectron spectrum of 1-PEN. Figure 3. Part of the REMPI spectrum of 1-PEN, composed of three shorter scans. Numerous vibrations were identified by the presence of combination bands with the torsion T02 (indicated by dashed lines) and the 56a′ (solid lines).

An IEad of 7.67 eV was computed by CBS-QB3, while excited states of the cation are calculated to appear at +0.3 eV and +0.72 eV (vertical excitation energies). Note that the computed energies are zero point energy corrected. The cationic ground state was found to be planar. To extract a more accurate IE, the spectrum was simulated using the optimized geometries and vibrational frequencies of the neutral and the cationic ground state for computing FC factors of the photoionization. The stick spectrum (red lines) was then convoluted with a Gauss function of 70 meV full width at half-maximum (fwhm). As visible in Figure 2 the best representation to the first band is obtained with an IE of 7.58. Several modes are active in the cation, but no dominant progression is visible. Since an effusive beam was employed, the molecules contain significant thermal energy, which contributes to the broadening of the bands. The simulation was repeated for the excited state of the cation. Based on the computed geometries, it is justified to assign the second broad band around 7.8 eV to the first excited electronic state of the cation and not to an intense fundamental vibration. From the results we conclude that the origin of the first excited state lies 0.18 eV above the IE; thus experiment and theory are in good agreement. Beyond 8 eV no further assignments are possible because of the broad and unstructured spectrum. The TPE signal increases to the blue due to further excited states of the cation. We also explored the dissociative photoionization of 1-PEN and found this process to set in only above 15 eV. Two channels were identified, loss of H2 and loss of acetylene; however no full breakdown diagram was recorded. c. REMPI Spectra. One-color [1 + 1] resonance-enhanced multiphoton ionization spectra were recorded to gain insight into the excited electronic state of 1-PEN. Figure 3 shows an overview spectrum over a large energy range that is composed of three shorter scans. Note that the spectrum has not been corrected for laser power. The dominant band at low wavenumbers appears at 30823 cm−1 (3.822 eV) and is assigned to the origin of the S1 ← S0 transition. A scan 400 cm−1 further to the red did not yield any additional bands. Note that half of the ionization energy, i.e., the minimum energy that is accessible in a [1 + 1] process, corresponds to 30570 cm−1. Considerable vibrational activity is visible in the spectrum (Figure 3), and in particular a rich structure of low wavenumber bands is apparent. The TD-DFT computations yield a vertical excitation energy of 3.63 eV for a transition from the X 1A′ ground state into the A 1A′ state. Both states are found to be planar. Since the transition into the B 1A′ state is computed to be at 4.23 eV, only the A state is relevant in the energy region

investigated here. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of 1-PEN can be viewed as linear combinations of the HOMOs and LUMOs of benzene, naphthalene, and acetylene respectively and thus extend over the whole molecule. A graphic representation is given in the Supporting Information. The orbitals resemble closely the ones reported for tolane, PhCCPh.29 The HOMO−LUMO transition has π → π* character and is associated with a reduced bond order at the bridge. Thus, the lengths of the CC triple bond, r(C2C3), increases upon electronic excitation (see Scheme 1 for numbering of carbon atoms). The distances between the acetylenic carbons and the ring carbon atoms, i.e., r(C1−C2) and r(C3−C4), decrease and partial double bond character develops. In the case of tolane the laser-induced fluorescence spectrum was rationalized assuming two close-lying electronically excited states.14 However, in 1-PEN the two chromophores coupled via the bridge are inequivalent, which leads to a downward shift in energy of the first excited state. It is interesting to note that the order of excited states is less clear away from the ground state equilibrium geometry. In tolane it was found that a dark state of πσ* character is shifted downward in energy upon a reduction of the CCC twist angle.30 Such a state, which might be present in 1-PEN as well, can have a considerable impact on the molecular photophysics. A close-up of the origin region is depicted in the lower trace of Figure 4. To the red of the origin band two bands are present that can reasonably be assigned to hot or sequence bands. To the blue a series of bands appear that are 11−12 cm−1 apart and show an intensity alternation. It might be explained by a progression in the torsion around the CC triple bond, which also dominates the spectrum of tolane.14 Note that the torsion is of a″ symmetry and can only appear with its even overtones, so a very low wavenumber has to be associated with this mode. The second possible explanation for the pattern is the presence of two low-frequency modes that appear with several overtones and combination bands. Further information on the assignment is obtained from experiments using N2 as the carrier gas. One expects less efficient cooling in the jet with nitrogen and thus a higher contribution from hot and sequence bands. Figure 4 compares the low-wavenumber region of the spectra recorded with the two gases. It is evident that some bands are significantly more 2917

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supports our assignment for the torsional mode in 1-PEN. Another issue of interest is the height of the barrier for internal rotation. When a sufficient number of transitions is identified, barriers can be obtained from a fit to the experimental data.31 In this work the number of transitions and the accuracy of the transition energies turned out to be insufficient for a reliable fit. For the rotational barrier V0 in the electronic ground state we calculated a value of 280 cm−1 by DFT, using the B3LYP functional. In the computations the torsional angle was varied and all other geometry parameters were optimized at each value of the torsional angle. The value is similar to the one of 202 cm−1 derived both experimentally14 and computationally32 for tolane. The increase in the torsional wavenumber upon electronic excitation indicates an increase in the force constant and a higher barrier. As discussed previously an analysis of the computations shows that upon excitation the bond lengths r(C1−C2) and r(C3−C4) decrease and the bonds become stronger, which overcompensates for the reduced bonding in the acetylenic bridge. Again the same reasoning can be applied to the torsional wavenumbers of tolane. Here a torsional barrier of 1600 cm−1 was estimated for both excited states, i.e., an increase by almost an order of magnitude. Neither in 1-PEN nor in the previous tolane experiments a tunneling splitting was experimentally observed. Note that barrier heights are not easily computed. In 1,4bis(phenylethynyl)benzene a torsional barrier of 230 cm−1 was determined experimentally, whereas 731 cm−1 was obtained computationally by DFT.16 Therefore, computed barrier heights have to be considered with care. One has to also keep in mind that barriers to internal rotation can vary significantly. In dimethylacetylene for example V0 is smaller than 6 cm−1.33 Several other vibrational modes can be identified in the REMPI spectrum in Figure 3 due to the presence of combination bands, in particular with T02. Since the S2 state is also of A′ symmetry and will not induce A″ modes by vibronic coupling, we will only consider totally symmetric vibronic states. As visible in Figures 3 and 4 a prominent peak shows up at +120 cm−1 and is assigned to the fundamental of the 56a′ deformation mode, which can be described as a motion of the phenyl ring relative to the CC triple bond. It is computed at 131 cm−1 in the S0 and 124 cm−1 in the S1 state and appears also in combination with the sequence bands of the torsion, in particular visible with nitrogen as the carrier gas, confirming the preceding conclusions. Several other bands also appear in combination with the 56a′, as marked in Figure 3. The band at +227 cm−1 is assigned to the 55a′ fundamental, computed at 244 cm−1 (S0) and 242 cm−1 (S1). Alternatively, it might be due to the (56a′)2 overtone, but judged by the band intensity higher members of the progression would be expected, which cannot be identified. We therefore assign (56a′)2 to a small band at +238 cm−1. The band at 177 cm−1 also shows combinations with the torsion at low intensity but does not match any of the computed a′ fundamentals. We therefore assign it to (25a″)2, the first overtone of an out-ofplane deformation mode computed at 87 cm−1. For the bands at +303, +313, +370, and +467 cm−1 combination bands with the torsion and 56a′ can be identified as indicated in Figure 3 by dashed and solid lines, but spectral congestion starts to set in and assignments become ambiguous. A comparison with related molecules might aid: The electronic spectrum of naphthalene features two prominent ring deformation modes, the most intense band being the 8(b1g) at +436 cm−1. The

Figure 4. REMPI spectrum of 1-PEN using Ar (lower trace) and N2 (upper trace) as carrier gas.

intense relative to the origin band, in particular the bands at +12 and +22 cm−1, indicating that they are not due to a S1 fundamental but rather originate from vibrationally excited states that get more populated under the warmer conditions in a nitrogen jet. Thus, we rule out the second explanation and assign the two bands to sequence bands of the torsional motion, T11 and T22. The band at +33 cm−1 on the other hand shows up with comparable relative intensity with both carrier gases and constitutes the first overtone of the torsion, T02, in S1. Assuming that the bands at −12 and −22 cm−1 relative to the origin are the T20 and T40 hot bands of the torsion, one derives a fundamental of 6 cm−1 for the torsion in the electronic ground state and 17 cm−1 in the S1 state. This permits a consistent assignment of all major bands in the low-energy region. The fairly regular spacings indicate that the torsion can be reasonably well treated as a vibrational mode rather than an internal rotation. To support the experiment, vibrational wavenumbers were also computed. For the torsional vibration T, which corresponds to 27a″, a value of 5 cm−1 was obtained for the S0 state, in excellent agreement with the experimental one. In the S1 state a wavenumber of 43 cm−1 was computed, a factor of 2 higher than the experimental value. Two other lowfrequency modes were found in the computations, the 57a′ inplane deformation at 33 cm−1 and the 26a″ out-of-plane deformation at 44 cm−1 (values for S0). The only unexplained feature is the enhanced intensity of T40 compared to T20. In the experiments with Ar this might be explained by the dissociation of a 1-PEN/Ar cluster (see below). Since this enhancement is also evident with nitrogen, we suggest that another sequence band coincidentally appears at almost the same wavenumber. In the N2 spectrum all bands are broader compared to Ar. One reason is the higher rotational excitation in the ground state; the second reason might be the near-coincidence of various hot bands. The T42 transition will for example appear very close to T11 and thus lead to a broadening of this band. The torsional wavenumbers are considerably smaller than those found for tolane. For the latter molecule values of 17 cm−1 (S0),14 42.5 cm−1 (B2u),14 48 cm−1 (B1u),14 and 54 cm−1 (X+ 2B3u)15 were reported, roughly a factor of 2−3 larger. To verify our theoretical results, we carried out DFT computations on tolane and found a value of 17 cm−1 for the S0 torsion using the ωB97xD functional. The excellent agreement for tolane suggests that such frequency calculations are rather reliable for the electronic ground state of substituted acetylenes and thus 2918

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second band is the 8(b2u)2 at 708 cm−1, which is symmetryforbidden in the fundamental.34,35 In benzene the 6(e2g) appears particularly prominent with 521 cm−1.36 All three vibrations are thus expected to appear in 1-PEN as well. The calculations reveal a′ ring deformation modes in the S1 state at 357, 478 (naphthyl part), and 498 cm−1 (phenyl part). We therefore assign the three intense bands at +313, +370, and +467 cm−1 to these modes. The assignment of the 303 cm−1 band is less certain, because no further band is computed in this wavenumber region. An overtone of the 23 or 24a″ band or, alternatively a combination band of either a″ fundamental with the 56a′ mode, might be responsible for this band. In the fluorescence excitation spectrum of tolane14 two prominent vibronic bands appear at +341 cm−1 in the B1u state and at +229 cm−1 in the state tentatively assigned as B2u, but less vibronic structure is visible, due to the higher symmetry of tolane as compared to 1-PEN (D2h vs Cs). With increasing wavenumber, the vibrational density of states becomes rather large and individual bands cannot be unambiguously assigned anymore. The vibrational assignments are summarized in Table 1.

origin.37 Only at excess energies above 3000 cm−1 it falls below 100 ns, although intersystem crossing (ISC) is clearly evident. The much shorter lifetime of 1-PEN might indicate a more efficient ISC in comparison or alternatively an interaction with a πσ* state.30 Nevertheless the S1 state is expected to fluoresce, like the low-lying states of tolane.14 When Ar is used as a carrier gas, 1-PEN/Ar clusters are observed. A typical REMPI spectrum is shown in Figure 6. A

Figure 6. REMPI spectrum of the 1-PEN/Ar cluster.

Table 1. List of Vibrational Fundamentals Determined in the Present Work

first band is observed at 30801 cm−1. It is assigned to the origin of the S1 ← S0 transition of the cluster, which is therefore redshifted by 22 cm−1 relative to the free molecule. The transition energy coincides with the T40 transition of 1-PEN, so cluster fragmentation might contribute to the signal on the low-energy side of the origin in the spectrum of the free molecules. The red shift is comparable to the one observed in benzene/Ar (21 cm−1),38 naphthalene/Ar (15 cm−1),39 and 1-naphthol/Ar (15 cm−1).40 Most likely the Ar atom is located above the naphthalene unit of 1-PEN, because a stronger dispersion interaction can be assumed due to the larger polarizability of naphthalene.41 The low-wavenumber vibrational structure cannot be resolved, because the two intermolecular bending modes contribute additional low-energy bands that, in combination with the torsion, result in a broadened band. In dimethylnaphthalene/Ar for example42 S1 wavenumbers of 8.4 and 14.4 cm−1 were found for the bending modes, i.e. values on the same order of magnitude as the molecular torsion, and 42.5 cm−1 for the intermolecular stretch. However, the intramolecular vibrations 56a′ as well as the 55a′ and 54a′ are also visible in the spectrum of the cluster. The wavenumber of 54a′ is reduced from 313 to 300 cm−1, a small peak on the red side might be due to the band that appears at +303 cm−1 in the bare molecule. The slightly larger wavenumber reduction of the naphthyl-centered deformation mode 54a′ compared to the bare molecule might confirm that the Ar is attached to the naphthyl unit.

S1/cm−1

a

mode

expt

computed

27a″ (torsion T) 25a″ 56a′ 55a′ not assigned 54a′ 53a′ 52a′

17 89a 120 227 303 313 370 467

43 87 124 242 357 478 498

First overtone divided by 2.

We also measured the excited state lifetimes of 1-PEN. A picosecond laser was employed to pump several bands. As visible in Figure 5 a lifetime of 32 ns was determined for the

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SUMMARY AND CONCLUSION We have investigated 1-(phenylethynyl)naphthalene (1-PEN) by a number of spectroscopic techniques, because it has been suggested to be an important aromatic hydrocarbon in combustion processes. Infrared spectra of 1-PEN recorded in a gas cell show the same vibrational structure in the fingerprint region as the IR/UV spectra of the dimerization products of phenylpropargyl radicals that have been recently recorded with a free electron laser in experiments aiming at 3-phenylpropargyl. Thus, they confirm the previous conclusion that was solely based on a computed IR spectrum. The ionization energy was determined to be 7.58 eV by threshold photoelectron

Figure 5. Lifetime of the origin band of 1-PEN.

origin band from a fit to the data by a monoexponential function. Due to the laser bandwidths of 20−30 cm−1 the T11 and T20 bands are also excited. Scans at further selected wavelengths up to +1600 cm−1 above the origin also yielded lifetimes around 30 ns. Thus, the S1 state of 1-PEN is long-lived in the investigated energy region. However, in naphthalene a fluorescence lifetime τfl of 299 ns was measured for the 0° 2919

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spectroscopy, and an excited electronic state of the cation was found to set in at 7.76 eV. The spectrum was simulated using computed Franck−Condon factors. Such photoionization data can aid in the in situ identification of the molecule in flames that is often conducted by photoionization. The A 1A′ (S1) ← X 1A′ (S0) transition was studied by resonance-enhanced multiphoton ionization, and the origin of the transition was found at 30823 cm−1. The low-frequency vibrational structure is assigned to the torsional motion. From the spectrum, a value of 6 cm−1 was derived for the torsion in S0, in very good agreement with the 5 cm−1 computed by density functional theory. For S1 we found a torsional wavenumber of 17 cm−1. Interestingly, the values are significantly lower than the ones found in the related molecule tolane. A barrier of 280 cm−1 was computed for the internal rotation. The excited state lifetimes are in the nanosecond regime; a value of 32 ns was found for the origin band. We also recorded spectra of the 1-PEN/Ar cluster. The origin of the S1 ← S0 transition is red-shifted by 22 cm−1, in agreement with similar clusters. The low-frequency vibrational structure could not be resolved due to the presence of the intermolecular bending vibrations in addition to the torsion. Details of the synthesis of 1-PEN are given as Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

Text describing details of the synthesis of 1-(phenylethynyl)naphthalene including the characterization of all intermediates and full ref 23, figure showing a graphic representation of the HOMO and LUMO of 1-PEN, and tables listing computed vibrational wavenumbers of S0 and S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ingo.fi[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the “Deutsche Forschungsgemeinschaft” through Grant FI575/9-1 and the graduate research school GRK 1221. The iPEPICO experiments were carried out at the VUV beamline of the Swiss Light Source, Paul Scherrer Institut. We would like to thank Matthias Kastner and Patrick Hemberger for their assistance in the photoionization experiments and Thiemo Gerbich for his help in the lifetime measurements.

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REFERENCES

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