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J. Phys. Chem. A 2010, 114, 3190–3198
Spectroscopy and Photophysics of Structural Isomers of Naphthalene: Z-Phenylvinylacetylene† Josh J. Newby, Ching-Ping Liu,‡ Christian W. Mu¨ller, William H. James III, Evan G. Buchanan, Hsiupu D. Lee, and Timothy S. Zwier* Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907-2084 ReceiVed: September 25, 2009; ReVised Manuscript ReceiVed: NoVember 11, 2009
The fluorescence spectroscopy of Z-phenylvinylacetylene (Z-PVA) has been studied under jet-cooled conditions. The laser-induced fluorescence (LIF) spectrum shows vibronic activity up to 600 cm-1 above the ππ* electronic origin at 33 838 cm-1. In contrast, the single vibronic level fluorescence spectrum of the electronic origin shows strong intensity in transitions ending in ground state levels at least 1200 cm-1 above the ground state zero-point level. The double-resonance technique of ultraviolet depletion (UVD) spectroscopy was used to show that there are strong absorptions in Z-PVA that are not observed in the LIF spectrum due to the turn of a nonradiative process in this electronic state. The LIF and UVD spectra were compared quantitatively to calculate the relative single vibronic level fluorescence quantum yields. Upon inspection, there are some indications of state specific effects; however, the nature of these effects is unclear. Ab initio and density functional theory calculations of the ground and excited states were used to map the first two excited states of Z-PVA along the CtCH bending coordinate, determining them to be ππ* and πσ*, respectively, in character. The crossing of these two states is postulated to be the underlying reason for the observed loss in fluorescence intensity 600 cm-1 above the ππ* origin. The spectroscopy of Z-PVA has been compared to the previously characterized E isomer of phenylvinylacetylene [Liu, C. P.; Newby, J. J.; Muller, C. W.; Lee, H. D.; Zwier, T. S. J. Phys. Chem. A 2008, 112 (39), 9454.]. I. Introduction In studies of polyaromatic hydrocarbon (PAH) formation, m/z 128 is often assumed to be naphthalene; however, there are many isomers on the C10H8 potential energy surface that could be produced preferentially by radical or photochemical pathways. The unique chemical and photochemical properties of these C10H8 isomers may lead them either toward or away from naphthalene. Titan, one of the moons of Saturn, has an atmosphere rich in hydrocarbons formed by a series of photochemical reactions beginning with methane, its primary hydrocarbon constituent. Recent evidence from the Cassini mission and Huygens probe have particularly highlighted the existence and formation pathways leading toward complex hydrocarbons and nitriles, including benzene and larger PAH.1-3 Recent photochemical models4-6 have sought to include and understand the strong, altitude-dependent interplay between neutral and ionic pathways that lead toward PAH, since these pathways are likely to be major sources of the haze material that envelops Titan. Pathways still not included in these models are those involving photoisomerization. Reactions that produce other C10H8 isomers will undergo further chemical or photochemical processing. In molecules the size of C10H8, photoisomerization may be an efficient pathway for interconversion between isomers, since the phenyl derivatives initially formed will absorb at wavelengths longer than smaller atmospheric constituents, producing excited state molecules with energies well in excess †
Part of the “Benoît Soep Festschrift”. * Corresponding author. Email:
[email protected]. ‡ Current address: National Health Research Institutes Division of Medical Engineering Research, National Health Research Institutes, 35 Keyan Road, Zhunan 350, Taiwan.
of barriers to isomerization. To that end, we have undertaken a series of studies of the spectroscopy and photoisomerization dynamics of C10H8 structural isomers. To date, this has included the three isomers of ethynylstyrene (o-, m-, and p-),7 the trans isomer of phenylvinylacetylene (E-PVA),8 and a cursory identification of transitions identified with the cis isomer, Z-phenylvinylacetylene (Z-PVA).8 The two PVA isomers were identified as primary products of the reaction of UV-excited diacetylene, C4H2* with styrene.9
Here, we expand our spectroscopic characterization of Z-PVA, exploring its vibronic-level spectroscopy via laser induced fluorescence (LIF), resonant two-photon ionization (R2PI), and single vibronic level fluorescence (SVLF) spectroscopy. We also combine the results of a double-resonance method, UV-depletion
10.1021/jp909243y 2010 American Chemical Society Published on Web 12/18/2009
Structural Isomers of Naphthalene: Z-Phenylvinylacetylene spectroscopy, with LIF spectra taken under identical conditions to extract relative single vibronic level fluorescence quantum yields. As we shall see, this data identifies the presence of a fast nonradiative process with a sharp threshold only 600 cm-1 above the 2A′ origin. Excited state calculations are used to associate this threshold with the presence of a close-lying πσ* state that is strongly bent in the CtCH subunit. II. Methods A. Experimental Details. The experimental techniques and apparatus used in this work have been described in detail elsewhere;10 therefore, only a brief description will be given here. Z-PVA was introduced into the fluorescence chamber via a Series 9 General Valve (800 µm) operating at 20 Hz, entrained in helium with a stagnation pressure of 3 bar. Laser-induced fluorescence (LIF) and single vibronic level fluorescence (SVLF) experiments are performed in a chamber designed with two, 4 in. spherical mirrors to increase collection efficiency of the fluorescence.10 LIF spectra are acquired using the doubled output of Nd:YAG pumped dye laser that crosses the supersonic expansion ∼1 cm downstream from the orifice. The fluorescence is imaged through a cutoff filter and onto a UV sensitive photomultiplier tube, while SVLF spectra are acquired by dispersing the fluorescence with a 0.75 m monochromator (50-100 µm slit width) and imaging on a 1024 × 256 pixel CCD (Andor DU440BU2). With this setup, a resolution of 6-8 cm-1 is achieved in the dispersed emission. Resonant ion-dip infrared (RIDIR) spectroscopy was used to probe the acetylenic C-H stretching frequency in both 1A′ and 2A′ electronics states. The time-of-flight mass spectrometer chamber used for the RIDIR spectroscopy has been described elsewhere.11 Tunable IR radiation from 3300 to 3400 cm-1 was produced using a Nd:YAG pumped parametric converter (3-6 mJ). In RIDIR spectroscopy, the IR laser (10 Hz) is counterpropagated with the molecular beam and fires ∼200 ns prior to the UV laser. IR transitions arising from the same ground state level as the probe laser resonance appear as depletions in the ion signal. Ground state RIDIR spectra (S0 RIDIRS) were recorded by monitoring, via active baseline subtraction, the difference between the ion signals due to the probe laser with the IR laser “on” or “off”. To record the IR spectrum of the 2A′ state, a variation of S0 RIDIRS is employed.12 To probe the excited state, the resonant ultraviolet pulse arrived first, followed immediately by the infrared pulse, with an ionizing ultraviolet pulse last. The timing of the IR laser is adjusted such that the IR laser fires slightly (1-5 ns) after the UV, while the ionization laser fires ∼2 ns later due to the short excited state lifetime of Z-PVA. When the IR is resonant with a 2A′ infrared transition, a portion of the excited state population is excited by over 3000 cm-1, which increases their nonradiative rate by forming highly vibrationally excited states that are difficult to ionize, thus, IR absorption is detected as a depletion in the ion signal. To further probe the excited state, a second double-resonant spectroscopy was employed. We use the term “UV depletion (UVD) spectroscopy” to describe a variant of UV-UV holeburning that is especially useful in detecting absorptions involving short-lived upper states that are not easily detected via fluorescence or R2PI.13,14 Usually, ultraviolet hole-burning (UVHB) spectroscopy is used to record conformer/isomer specific UV spectra. In UVHB, a 10 Hz pump laser is parked on a transition observed in the excitation spectrum (usually the 2A′-1A′ origin transition) while a probe laser operating at 20 Hz is scanned through the region of interest while the difference
J. Phys. Chem. A, Vol. 114, No. 9, 2010 3191 in signal between successive probe laser pulses is recorded using active baseline subtraction through a gated integrator. UVD is different in that the hole-burn laser is tuned while the probe laser is fixed. Since the probe provides a constant signal, transitions to the excited state can be observed even if the states to which the pump laser is resonant have low-fluorescence quantum yields. E- and Z-PVA were synthesized according to the procedure of Michel, Gennet, and Rassat.15 Each isomer of PVA was stored as a dilute solution (
