Article pubs.acs.org/JPCA
Four-Component Fluorescence of trans-1,2-Di(1-methyl-2naphthyl)ethene at 77 K in Glassy Media. Conformational Subtleties Revealed Christopher Redwood,† V. K. Ratheesh Kumar,†,⊥ Stuart Hutchinson,† Frank B. Mallory,‡ Clelia W. Mallory,§ Olga Dmitrenko,∥ and Jack Saltiel*,† †
Department Department § Department ∥ Department ‡
of of of of
Chemistry and Biochemistry, Florida State University, Tallahassee, Florida 32306-4390, United States Chemistry, Bryn Mawr College, Bryn Mawr, Pennsylvania 19010, United States Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States, Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S Supporting Information *
ABSTRACT: The vibronic structure of the fluorescence spectrum of trans-1,2-di(1methyl-2-naphthyl)ethene (t-1,1) in methylcyclohexane (MCH) solution at room temperature was expected to become better defined upon cooling of the solution to 77 K. Instead, a broad, λexc-dependent fluorescence spectrum was observed in the glassy medium. Vibronically structured t-1,1 fluorescence spectra were obtained in the MCH glass only upon irradiation at the long-λ onset of the absorption spectrum. The application of singular value decomposition with self-modeling on the fluorescence spectral matrices of t-1,1 allowed their resolution into major and minor pairs of vibronically structured spectra that are assigned to two structural modifications of each of two relative orientations of the 1-methyl-2-naphthyl moieties. The difference between the two structures in each pair lies in the direction of rotation of each naphthyl group away from the plane of the olefinic bond. A complex but different conformer distribution is also responsible for the fluorescence spectra of t-1,1 in 5:5:2 (v/v/v) diethyl ether/isopentane/ethyl alcohol (EPA) glass at 77 K. The conformer distributions are also sensitive to the rate of cooling used in glass formation. Conformer distributions based on predicted small energy differences from gas-phase theoretical calculations are of little value when applied to volume-constraining media. The photophysical and photochemical properties of the analogues of the other two conformers of trans-1,2-di(2-naphthyl)ethene, trans-1-(1-methyl-2-naphthyl)-2-(3-methyl-2-naphthyl)ethene (t-1,3) and trans-1,2-di(3-methyl2-naphthyl)ethene (t-3,3), were determined in solution. However, it is the calculated geometries and energy differences of the t1,1 conformers [DFT using B3LYP/6-311+G(d,p)] that are essential guides to the interpretation of the experimental results.
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INTRODUCTION
conformers are regarded as s-trans,s-trans (s-t,s-t), s-trans,s-cis (s-t,s-c), and s-cis,s-cis (s-c,s-c), respectively.6 As a result of steric hindrance, methyl substitution at the 3 positions of the naphthyl groups is expected to simplify the conformational landscape in predictable ways. Consequently, the spectra of 3methylnaphthyl analogues were employed in the assignment of conformer structures to the resolved fluorescence spectra of tDNE4,5,8 and trans-1-(2-naphthyl)-2-phenylethene (t-NPE).6 Similarly, steric effects upon methyl substitution at the 1 positions of the naphthyl groups of DNE account for observations indicating that trans-1,2-di(1-methyl-2-naphthyl)ethene (t-1,1) and its cis isomer (c-1,1) behave as single conformers in solution.9
trans-1,2-Di(2-naphthyl)ethene (t-DNE) in solution exists as a mixture of conformers, designated A, B, and C in Scheme 1.1−6 The 1,2 bonds of the naphthyl moieties are shorter than the 2,3 bonds,7 and by analogy with trienes, the A, B, and C Scheme 1
Special Issue: Current Topics in Photochemistry Received: June 6, 2014 Revised: August 4, 2014 Published: August 18, 2014 © 2014 American Chemical Society
10575
dx.doi.org/10.1021/jp5056478 | J. Phys. Chem. A 2014, 118, 10575−10586
The Journal of Physical Chemistry A
Article
material containing a small amount of iodine was irradiated with visible light for 24 h to isomerize the mixture to exclusively the trans isomer. The solution was then poured through a short plug of alumina to remove the iodine. Rotary evaporation of the solvent gave a solid that after two recrystallizations from benzene afforded white crystals that were shown by GC/MS to be pure trans-1-(1-methyl-2-naphthyl)-2-(3-methyl-2naphthyl)ethene (mp 181.7−182.5 °C). 1H NMR (300 MHz, CDCl3): δ 8.12 (br d, J = 8 Hz, 1H), 8.10 (s, 1H), 7.88−7.73 (m, 5H), 7.67 (d, J = 15.8 Hz, 1H), 7.66 (br s, 1H), 7.55 (ddd, J = 8.4, 6.6, 1.5 Hz, 1H), 7.50−7.42 (m, 3H), 7.40 (d, J = 15.7 Hz, 1H), 2.82 (s, 3H), 2.61 (s, 3H). MS: m/z 308 (M+). Anal. Calcd for C24H20: C, 93.46; H, 6.54. Found: C, 93.51; H, 6.52. (Analysis was performed by M-H-W Laboratories, Phoenix, AZ.) Small, pure (as determined by GC and UV analyses) samples of the cis isomers of the dimethyl DNE derivatives were prepared from the trans isomers using fluorenone as the photosensitizer.13 Spectroscopic Measurements. Fluorescence spectra of dilute solutions of t-1,1 in MCH and 5:5:2 (v/v/v) diethyl ether/isopentane/ethyl alcohol (EPA) at 77 K were measured in cylindrical (2 mm inside diameter) quartz tubes immersed in liquid nitrogen in the Dewar of the phosphorescence accessory of a Hitachi F-4500 fluorometer. Condensation of atmospheric moisture on the Dewar in the course of fluorescence measurements was prevented by continuous purging of the sample chamber with argon. Controlled slow cooling of the fluorescence samples was achieved with the use of an Oxford OptistatDN cryostat. A FluoroMax 4P spectrofluorometer/ phosphorimeter equipped with a custom-made stage for the Oxford cryostat was employed to record the spectra using a cryogenic 1.00 cm2 quartz cuvette (NSG Precision Cells, model 65FL). Solute absorbances, measured on a Varian Cary 300B UV−vis spectrophotometer in 1.00 cm2 standard quartz cuvettes, were kept in the 0.1−0.3 range to ensure that the concentrations were sufficiently low to minimize distortion of the fluorescence spectra by self-absorption. Fluorescence spectra for quantum yield determinations in solution were recorded with the Hitachi F-4500 spectrophotometer. Suprasil fluorescence quartz cells (1.00 cm2) were mounted in a thermostated cell holder, and the emission was recorded at a right angle to the excitation beam. The excitation and emission slit widths were set at 2.5 nm. The emission spectra were baseline-corrected using the solvent as the reference and also corrected for nonlinearity of the instrumental response; t-1,3 and t-3,3 fluorescence quantum yields were determined using t1,1 as a fluorescence standard.9 Fluorescence lifetimes were determined as previously described14 using a Horiba Fluoromax 4 fluorometer equipped with a time-correlated single-photon counting accessory and an R928 PMT detector (Hamamatsu). The light source was a 295 nm nanoLED (Horiba) having a pulse duration of
