Electronic and Vibrational Transition Moment Directions in 7

ARTICLE pubs.acs.org/JPCA

Electronic and Vibrational Transition Moment Directions in 7-Dimethylamino-3-methyl-N-methyl-d3-4-phenylethynylcarbostyril Deborah L. Casher,† Lukas Kobr,† and Josef Michl*,†,‡ † ‡

Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309-0215, United States Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic, Flemingovo nam. 2, Prague 16610, Czech Republic ABSTRACT: We report the synthesis and photophysical characterization of 7-dimethylamino-3methyl-N-methyl-d3-4-phenylethynylcarbostyril, a chromophore of interest as a rotator in surfacemounted molecular rotors. Measurement of UVvis absorption and fluorescence spectra, steady state fluorescence and excitation anisotropy, and linear dichroism in the IR and UVvis permitted a determination of absolute vibrational and electronic transition moment directions in this previously unreported chromophore. The first singletsinglet absorption and fluorescence are polarized perpendicular to the axle of the rotator. Density functional theory calculations of electronic excitation and vibrational frequencies gave results in very good agreement with those observed. Calculated IR transition moment directions showed rather poor agreement with experiment.

’ INTRODUCTION Chromophores for Use as a Rotator in Surface-Mounted Azimuthal Molecular Rotors. Surface-mounted molecular ro-

tors are of interest for studying thermal and driven rotation of nanosized objects and promise a wide range of applications in nanoelectronics.113 The molecular rotors of interest to us consist of (i) an anchor group or groups that attach to a surface and carry (ii) an axle mounted perpendicular7,8 (azimuthal rotor) or parallel10,12,14 (altitudinal rotor) to the surface and (iii) a rotator that is free to turn about the axle. We have worked on the synthesis of a Tinkertoy15 kit1618 of rods, connectors, and rotators for this purpose and have reported the attachment chemistry and orientation of altitudinal rotors on Au.10,12,14 The present study focuses on the characterization of a chromophore that appears promising as a rotator for time-resolved fluorescence depolarization studies of dipolar azimuthal rotors. These measurements will yield the most information if the rotator is a highly efficient and photochemically robust UVvis fluorophore with a rigidly mounted axle, and the permanent ground state dipole moment and the transition dipole moments of emission and of one or more of the accessible absorptions all are oriented approximately perpendicular to the axle. These requirements are met relatively rarely, and few such fluorophores have been adequately characterized. The 7-dimethylamino-3-methyl-N-methyl-d3-4-phenylethynylcarbostyril rotator 1 (Figure 1) represents a potential candidate. It is derived from the commercially available sturdy laser dye Carbostyril 165 (2, Figure 1)19 by incorporation of phenylethynyl and trideuteriomethyl groups. The former provides a rigid axle for the mounting of the rotator, and both represent localized IR chromophores with predictable transition moment directions that can be expected to help us establish absolute polarizations in the molecular framework. r 2011 American Chemical Society

Figure 1. Rotator axle (thin dashed line), principal orientation axis z (bold solid line), and structures of 1 and 2.

According to time-dependent density functional theory (TDDFT) calculations, 2 is a charge-transfer chromophore whose electron transfer direction in the first transition, located near 360 nm, is from the dimethylamino group to the lactam carbonyl group, with a transition moment approximately parallel with the line connecting the two groups (perpendicular to the axle shown in Figure 1). The TD-DFT theory with standard functionals is of questionable value for charge-transfer transitions, and an Special Issue: Pavel Hobza Festschrift Received: April 4, 2011 Published: June 03, 2011 11167

dx.doi.org/10.1021/jp2031253 | J. Phys. Chem. A 2011, 115, 11167–11178

The Journal of Physical Chemistry A experimental determination of transition moment directions was deemed important. Given that diphenylacetylene absorbs at ∼300 nm,20 much higher energy than the lowest energy absorption in 2, it appeared unlikely that introduction of the phenylethynyl substituent to form 1 would significantly perturb the lowest energy transition moment direction. Presently, we report the synthesis of 1 from the known21 7-dimethylamino-4-hydroxy-3-methylcarbostyril (3) and the results of an experimental determination of its electronic transition moment directions. While the determination of relative transition moment directions in the UVvis region from steady-state fluorescence and excitation anisotropy is straightforward, the determination of the absolute polarization directions requires an oriented sample with known second moments of the orientation distribution function (orientation factors). We used stretched polyethylene22 as a uniaxial anisotropic solvent for 1 and obtained the orientation factors from the linear dichroism of its infrared transitions (IR LD) whose moment directions were known from the nature of the normal mode or from density functional theory (DFT) calculations.

’ EXPERIMENTAL PART AND CALCULATIONS Materials. Starting materials for the synthesis of 1 as well as pure 2 (g99%) were purchased from Sigma-Aldrich and used as received. Absorption and fluorescence measurements were performed in EPA, a 5:5:2 by volume mixture of diethyl ether/ 2-methylbutane/ethanol. Measurement of fluorescence quantum yield was performed in EPA and in cyclohexane with cross-calibration standards 9,10-diphenylanthracene (DPA, Aldrich Gold Label 99%) in cyclohexane and anthracene (Aldrich Gold Label 99.9%) in ethanol. All solvents used for photophysical characterization were of spectrophotometric grade and were used without further purification. Solvents used for fluorescence measurements were checked for background emission. Synthesis Methods. Anhydrous tetrahydrofuran was freshly distilled from sodium and benzophenone. Anhydrous diisopropylamine was distilled from calcium hydride. Anhydrous dimethylformamide was distilled from calcium hydride under vacuum. All reactions were conducted under a dry argon atmosphere using Schlenk techniques unless otherwise noted. IR spectra were recorded with a Nicolet Avatar 360 FT-IR spectrophotometer. UVvis spectra were recorded with a HewlettPackard 8452A spectrophotometer. NMR spectra were recorded with an Inova 400 spectrometer or Bruker Avance-III 300 Spectrometer. A Hewlett-Packard 5989B mass spectrophotometer was used to record mass spectra. Elemental analyses were performed by Columbia Analytical Services, Tucson, AZ. 7-Dimethylamino-3-methyl-4-trifluoromethanesulfonyloxycarbostyril (4). A dried three-necked round-bottom flask, equipped with septa, a magnetic stir bar, and a connection to a Schlenk line, was charged with neat sodium hydride (732 mg, 1.2 equiv, 30.5 mmol) and with 3 (5.554 g, 1.0 equiv, 25.4 mmol) under argon. The reaction flask was immersed in an ice bath, and anhydrous dimethylformamide (110 mL) was added rapidly to the stirred mixture. After the evolution of hydrogen slowed down, the ice bath was removed, and the reaction mixture was stirred at room temperature for an additional 45 min. The reaction flask was then again immersed in an ice bath, and Nphenylbis(trifluoromethanesulfonimide) (10.896 g, 1.2 equiv, 30.5 mmol) was added. The reaction mixture was left in the ice bath, and it was stirred overnight (12 h) at room temperature as

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the ice melted and the bath warmed up to ambient temperature. The resulting mixture was quenched with a few drops of water, and it was then slowly poured into diethylether (1.5 L) while stirring. The resulting suspension was washed with water (200 mL), 5% aqueous sodium hydroxide (200 mL), and brine (200 mL). The solvents were removed under reduced pressure, and the crude product was recrystallized from ethyl acetate to yield 6.420 g (72%) of white crystals, mp (CH2Cl2) 253 C. 1H NMR (300 MHz, CDCl3) δ 11.15 (s, 1H), 7.55 (d, J = 9.1 Hz, 1H), 6.73 (dd, J = 9.1, 2.4 Hz, 1H), 6.44 (d, J = 2.4 Hz, 1H), 3.09 (s, 6H), 2.26 (s, 3H). 19F NMR (282 MHz, CDCl3) δ 72.99 (s). 13C{1H} NMR (75 MHz, DMSO) δ 162.43, 151.84, 150.79, 138.93, 121.94, 117.99 (q, J = 320.2 Hz), 115.68, 109.31, 103.70, 95.05, 39.60, 10.67. IR (KBr, cm1): 499, 588, 596, 669, 692, 726, 747, 756, 789, 821, 880, 922, 980, 1044, 1134, 1151, 1210, 1220, 1243, 1279, 1292, 1347, 1364, 1377, 1391, 1409, 1445, 1488, 1527, 1565, 1629, 1659, 2829, 2867, 2926, 2994, 3093, 3157. UVvis (CH2Cl2, nm) λmax (εmax): 369 (23140), 360 (22010), 305 (4570), 286 (5050), 254 (10470), 229 (38810). Elemental analysis: Calcd C, 44.57%; H, 3.74%; N, 8.00%. Found: C, 44.52%; H, 3.65%; N, 7.97%. 7-Dimethylamino-3-methyl-N-methyl-d3-4-trifluoromethanesulfonyloxycarbostyril (5). A flame-dried, two-necked, round-bottom flask equipped with a septum was charged with 4 (400 mg, 1.0 equiv, 1.142 mmol) and sodium hydride (32.8 mg, 1.2 equiv, 1.370 mmol). The flask was immersed in an acetoneice cooling bath (20 C), and anhydrous dimethylformamide (10 mL) was added at once. The mixture was stirred under argon, at 20 C for 30 min, and then for 1 h at room temperature. Afterward, the mixture was recooled to 20 C, and methyl-d3 iodide (198.5 mg, 1.2 equiv, 1.37 mmol) was added. The reaction mixture was allowed to warm to room temperature, stirred for an additional 4 h, quenched with a few drops of water, diluted with diethyl ether (150 mL), and washed with water (50 mL). The organic layer was dried over anhydrous sodium sulfate and filtered. The solvent was removed under reduced pressure, and the crude product was purified by flash chromatography in ethyl acetate to yield 313 mg (75%) of yellowish crystals, mp (CH2Cl2) 170 C. 1H NMR (300 MHz, CDCl3) δ 7.60 (d, J = 9.1 Hz, 1H), 6.73 (dd, J = 9.1, 2.4 Hz, 1H), 6.37 (d, J = 2.4 Hz, 1H), 3.11 (s, 6H), 2.25 (s, 3H). 19F NMR (282 MHz, CDCl3) δ 72.98 (s). 13C{1H} NMR (75 MHz, CDCl3) δ 163.56, 152.27, 150.61, 140.34, 123.91, 118.70 (q, J = 118.7 Hz), 116.48, 108.80, 105.85, 94.97, 40.43, 11.89. 2H NMR (61 MHz, CDCl3) δ 3.69 (s). 13C CP MAS NMR (101 MHz, neat solid) δ 160.88, 151.48, 148.15, 138.90, 121.42, 120.05, 113.82, 109.99, 103.04, 91.82, 39.03, 27.65, 11.35. IR (KBr, cm1): 505, 594, 620, 667, 694, 747, 815, 868, 946, 965, 1000, 1043, 1140, 1167, 1207, 1228, 1247, 1315, 1347, 1379, 1403, 1489, 1532, 1554, 1605, 1645, 1893, 2081, 2812, 2930, 3108. UVvis (CH2Cl2, nm) λmax (εmax): 370 (24 400), 360 (22 670), 305 (5250), 284 (7160), 231 (42 920). HRMS (ESIþ): 368.0969 (calculated for MHþ: 368.0965). Elemental analysis: Calcd C, 45.77%; H, 4.12%; N, 7.63%. Found: C, 46.02%; H, 3.99%; N, 7.50%. 7-Dimethylamino-3-methyl-N-methyl-d3-4-trimethylsilylethynylcarbostyril (6). The triflate 5 (300 mg, 1.0 equiv, 0.817 mmol), tetrakis(triphenylphosphino)palladium (94.3 mg, 0.1 equiv, 0.082 mmol), cuprous iodide (31.1 mg, 0.2 equiv, 0.163 mmol), trimethylsilylacetylene (802.0 mg, 10.0 equiv, 8.170 mmol), anhydrous diisopropylamine (0.5 mL), and anhydrous dimethylformamide (6 mL) were combined in a flame-dried Schlenk flask (25 mL) under argon. The reaction mixture was 11168

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The Journal of Physical Chemistry A stirred at 120 C for 24 h under argon. The solvent was removed under reduced pressure, and the remaining solid was dissolved in diethyl ether (100 mL). The organic layer was washed with water (2 40 mL), dried over anhydrous sodium sulfate, and filtered. The solvent was removed under reduced pressure, and the crude product was purified by gradient flash chromatography in a hexanes/ethyl acetate mixture (2:1 to 100% ethyl acetate) to yield 196 mg (76%) of yellow crystalline solid, mp (CH2Cl2) 166 C. 1H NMR (400 MHz, CDCl3) δ 7.79 (d, J = 8.9 Hz, 1H), 6.64 (dd, J = 8.9, 2.2 Hz, 1H), 6.25 (d, J = 2.2 Hz, 1H), 3.04 (s, 6H), 2.37 (s, 3H), 0.32 (s, 9H). 2H NMR (46 MHz, CDCl3) δ 3.67 (s). 13C{1H} NMR (75 MHz, CDCl3) δ 162.77, 151.39, 140.12, 128.35, 127.95, 127.65, 110.89, 108.23, 107.23, 99.90, 95.10, 40.42, 31.0026.96 (m), 16.03, 0.04. IR (KBr, cm1): 479, 628, 651, 700, 723, 754, 795, 803, 816, 843, 864, 948, 963, 1015, 1030, 1087, 1113, 1165, 1235, 1246, 1308, 1345, 1376, 1399, 1528, 1547, 1580, 1609, 1632, 2140, 2801, 2863, 2911, 2954. UVvis (CH2Cl2, nm) λmax (εmax): 380 (15 560), 262 (27 970), 232 (42 470). HRMS (ESIþ): 316.1920 (calculated for MHþ: 316.1919). Elemental analysis: Calcd C, 68.52%; H, 7.66%; N, 8.88%. Found: C, 68.81%; H, 7.64%; N, 8.60%. 7-Dimethylamino-4-ethynyl-3-methyl-N-methyl-d3-carbostyril (7). Compound 6 (160 mg) was dissolved in a mixture of dichloromethane (15 mL) and methanol (15 mL) in a roundbottom flask (100 mL). A solution of potassium carbonate (345 mg) in water (30 mL) was added. The reaction mixture was stirred for 4 h at room temperature. Afterward, it was diluted with diethyl ether (60 mL), and the aqueous layer was separated. The organic layer was dried over anhydrous sodium sulfate and filtered. The solvent was removed under reduced pressure to yield 123 mg (100%) of yellow crystalline product, mp (CH2Cl2) 171 C. 1H NMR (300 MHz, CDCl3) δ 7.86 (d, J = 9.0 Hz, 1H), 6.70 (dd, J = 9.0, 2.4 Hz, 1H), 6.35 (d, J = 2.4 Hz, 1H), 3.73 (s, 1H), 3.09 (s, 6H), 2.40 (s, 3H). 2H NMR (46 MHz, CDCl3) δ 3.69 (s). 13C{1H} NMR (101 MHz, CDCl3) δ 162.45, 151.23, 139.93, 128.01, 127.62, 127.36, 110.68, 108.07, 94.84, 88.83, 78.57, 40.29, 30.0627.83 (m), 15.92. IR (KBr, cm1): 435, 492, 505, 578, 624, 637, 675, 720, 753, 801, 839, 944, 957, 1027, 1059, 1105, 1153, 1169, 1210, 1237, 1266, 1309, 1341, 1370, 1395, 1411, 1432, 1452, 1487, 1522, 1580, 1602, 1629, 1715, 1786, 1882, 2073, 2094, 2124, 2231, 2581, 2798, 2858, 2911, 2989, 3204. UVvis (CH2Cl2, nm) λmax (εmax): 379 (17 470), 294 (4240), 257 (23 850), 232 (45 940). HRMS (ESIþ): 244.1515 (calculated for MHþ: 244.1523). Elemental analysis: Calcd C, 74.04%; H, 6.63%; N, 11.51%. Found: C, 74.23%; H, 6.65%; N, 11.66%. 7-Dimethylamino-3-methyl-N-methyl-d3-4-phenylethynylcarbostyril (1). A flame-dried Schlenk flask (25 mL) was charged with 6 (140 mg, 1.0 equiv, 0.575 mmol), iodobenzene (141 mg, 1.2 equiv, 0.690 mmol), tetrakis(triphenylphosphino)palladium (66.4 mg, 0.1 equiv, 0.058 mmol), cuprous iodide (21.9 mg, 0.2 equiv, 0.115 mmol), anhydrous tetrahydrofuran (5 mL), and anhydrous diisopropylamine (0.5 mL) under argon. The mixture was stirred at 40 C under argon for 24 h. The solvent was removed under reduced pressure, and the remaining solid was dissolved in diethylether (100 mL). The organic layer was washed with water (2 40 mL), dried over anhydrous sodium sulfate, and filtered. The solvent was removed under reduced pressure, and the crude product was purified by gradient flash chromatography in a hexane/ethyl acetate mixture (2:1 to 100% ethyl acetate) to yield 149 mg (81%) of yellow crystals, mp (CH2Cl2) 186 C. 1H NMR (300 MHz, CDCl3) δ

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7.86 (d, J = 8.9 Hz, 1H), 7.667.57 (m, 2H), 7.447.34 (m, 3H), 6.64 (dd, J = 8.9, 2.4 Hz, 1H), 6.23 (d, J = 2.4 Hz, 1H), 3.03 (s, 6H), 2.44 (s, 3H). 2H NMR (46 MHz, CDCl3) δ 3.63 (s). 13 C{1H} NMR (75 MHz, CDCl3) δ 162.65, 151.28, 140.07, 131.77, 129.06, 128.54, 128.46, 127.79, 126.74, 122.71, 110.84, 108.07, 101.03, 95.03, 84.55, 40.32, 32.2626.61 (m), 16.08. IR (KBr, cm1): 435, 492, 505, 578, 624, 637, 675, 720, 753, 801, 839, 944, 957, 1027, 1059, 1105, 1153, 1169, 1210, 1237, 1266, 1309, 1341, 1370, 1395, 1411, 1432, 1452, 1487, 1522, 1580, 1602, 1629, 1715, 1786, 1882, 2073, 2094, 2124, 2231, 2581, 2798, 2858, 2911, 2989, 3204. UVvis (CH2Cl2, nm) λmax (εmax): 387 (13 000), 296 (25 320), 283 (25 590), 232 (43 840). HRMS (ESIþ): 320.1839 (calculated for MHþ: 320.1836). Elemental analysis: Calcd. C, 78.96%; H, 6.31%; N, 8.77%. Found: C, 78.69%; H, 6.33%; N, 8.62. UVvis Absorption and Fluorescence. Room-temperature (RT) spectra were measured in a 1 cm path length Suprasil quartz cell. All low-temperature (77 K) spectra were measured in a 1 cm path length Suprasil quartz cell equipped with a stopcock, immersed in liquid nitrogen in a quartz Dewar flask with Suprasil windows, and equilibrated for 5 min before measurement. To correct for the contraction of the EPA at 77 K, absorbance was multiplied by a factor of 0.771.23 RT absorption spectra of 1 and 2 in EPA (∼1 105 M) were measured with a Hewlett-Packard 8452A diode array spectrophotometer. The 77 K absorption spectrum of 1 in EPA was measured with an OLIS RSM 1000 spectrometer that was calibrated with a holmium oxide filter. Prior to measurement, samples of 1 in EPA were sonicated for ∼10 s until all solid material appeared to be dissolved. Room-temperature fluorescence spectra of 1 (2 scans with excitations at 324 nm) and 2 (1 scan with excitation at 358 nm) in EPA were measured with a SPEX spectrofluorimeter with a 0.5 m double monochromator for excitation and a 0.75 m monochromator for emission. The low-temperature (77 K) fluorescence spectrum of 1 (1 scan with excitation at 382 nm) was measured in EPA. All emission spectra were corrected for the wavelengthdependent lamp output and detection efficiency and by a factor of λ2 for conversion to wavenumbers.24a Fluorescence Quantum Yield. The fluorescence quantum yields of 1 in EPA and in cyclohexane were computed from a series of fluorescence (two scans from 350 to 650 nm in steps of 2 nm with a 1 s collection time per step) and absorption (single scan integrated for 0.1 s) spectra on three to five samples and reference standards of varying concentrations (A324 nm = ∼0.01 to ∼0.1) measured with the SPEX spectrofluorimeter and the Hewlett-Packard 8452A diode array spectrophotometer described above. The accuracy of the spectrofluorimeter measurements was checked by cross-calibration, and samples were referenced against a single standard. To calculate fluorescence quantum yield Φ, integrated fluorescence counts were plotted as a function of absorbance intensities at 324 nm and fitted with a line function constrained to pass through the origin. Φ is related to the slope or gradient G of the fitted line by Φx ¼ ΦR ðGx nx 2 Þ=ðGR nR 2 Þ where the subscripts R and x refer to the reference and sample compounds, respectively, and n is the refractive index of the solvent. Values of n for all solvents were taken at the sodium D line (589 nm) at 20 C,25 and nEPA was estimated to be 1.35 based on the refractive indices of its component solvents. 11169


The Journal of Physical Chemistry A

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Polarized Emission and Excitation. Steady state polarized fluorescence spectra of 1 in EPA at 77 K were measured using a SPEX spectrofluorimeter with a Glan-Taylor polarizer positioned between the excitation source and the sample, a Polacoat26 polarizer placed between the sample and the detector, and a 29 angle between the excitation light and the collection optics. Polarized spectra were measured with the polarizer transmission directions in four configurations: both horizontal (HH), both vertical (VV), the first (excitation) horizontal and the second (emission) vertical (HV), and vice versa (VH). Polarized fluorescence spectra of 1 were measured with excitation at 396, 319, and 302 nm in 2 nm steps. Polarized excitation spectra of 1 were measured while monitoring the emission at 466 nm, near the center of the 77 K fluorescence band. The anisotropy of both steady state emission and excitation was determined from the polarized spectra by solving the quadratic eq 1 for N27

N 2 cos2 ψ þ N sin2 ψ ¼ ðVV HHÞ=ðHV VHÞ

ð1Þ

where ψ is the angle between the excitation light and the collection optics. The positive root of N is related to the fluorescence polarization degree P and anisotropy r0 by P ¼ ðN 1Þ=ðN þ 1Þ r0 ¼ 2P=ð3 PÞ

ð2Þ

For purely polarized transitions, the angle β between the emitting and absorbing transition moments is given by24b cos2 β ¼ ð5r0 þ 1Þ=3

ð3Þ

The uncertainty in r0 and β was determined by standard error propagation methods assuming a (3 error in the angle ψ and a 2.5% uncertainty in each of the intensities HH, VV, HV, and VH due to the limited degree of polarization of the Polacoat polarizer over the wavelengths examined (90% at worst28). IR and UVvis Linear Dichroism. IR linear dichroism (LD) measurements on 1 in stretched polyethylene were made in a dry air-purged sample chamber of a Nicolet Nexus 670 FT-IR spectrometer equipped with a DTGS detector and a Molectron IGP225 polarizer positioned alternately parallel (EZ) and perpendicular (EY) to the polyethylene sheet stretching direction Z. Error due to the limited degree of polarization of the polarizer was determined to fall well within other experimental uncertainties and was not considered further. IR LD spectra were all corrected with blank polyethylene (the same sheet later used for the sample spectrum) and are the accumulated average of 1000 scans at a resolution of 2 cm1. Fringing in the IR spectrum was reduced by subtracting the spectrum of a dilute sample, in which no absorption bands were apparent above the fringe pattern, from a more concentrated sample, using various scaling factors as needed. LD and absorbance measurements in the UVvis region were made using a JASCO J-720 spectropolarimeter operated in the LD mode. The spectra were corrected with a polyethylene blank and are the accumulated average of three scans. The absorbance spectrum is derived from a second channel measurement of the high tension (HT) voltage during measurement of the LD spectrum. Software provided with the JASCO instrument was used to convert the HT voltage to absorbance units. Optical densities obtained by this method were determined to be identical to those obtained with the Hewlett-Packard 8452A diode array spectrophotometer described above.

Polyethylene sheets used in these experiments were cut from polyethylene bags (US Plastic Corp.) into ∼100 squares which were purified by soaking in chloroform for several hours, rinsed with fresh chloroform, and air-dried for 12 h. The cleaned sheets were stretched to approximately five times their original length and lightly scratched with lens tissue before measurements of the blank and subsequent addition of the compound. To absorb 1 into the polyethylene, several drops of a saturated solution in CHCl3 were placed on the stretched sheet, and the sheet was left sitting above a pool of CHCl3 in a covered dish overnight. After removal from the covered dish, the sheet was left to air-dry for ∼5 min. Residual crystals on the surface were subsequently removed by rinsing with MeOH. To help confirm the band positions in the polarized spectra, the FT-IR spectrum of 1 dropcast from a solution in EtOH onto a Ge crystal was measured on a Nicolet Nexus 670 with a dry air-purged chamber and corrected with a Ge blank. The resolution was 2 cm1, and 100 scans were collected. Computational Methods. Starting structures for 1 and 2 were built in ChemDraw and Cartesian coordinates for the atoms were obtained from Chem3D 7.0 Ultra (CambridgeSoft Corporation, Cambridge, MA). Geometries were constrained to Cs symmetry and optimized using the semiempirical AM1 method29 and subsequently density functional theory (DFT) at the B3LYP/6-31G(d,p) level using Gaussian 98.30 Electronic spectra of the two compounds were calculated by time-dependent (TD) DFT, and the vibrational spectrum of 1 was calculated by DFT/ B3LYP/6-31G(d,p) using Gaussian 98.

’ RESULTS Synthesis. Compound 1 was synthesized by the sequence of reactions shown in Scheme 1. The hydroxy derivative 321 was converted into the triflate 4 which was then methylated using sodium hydride and methyl-d3 iodide. Direct Sonogashira coupling of 5 and phenylacetylene yielded a complex mixture. However, when the more reactive trimethylsilylacetylene was used, the reaction proceeded in good yield to the product 6 which was then deprotected with potassium carbonate. Subsequent Sonogashira coupling of 7 with iodobenzene afforded the desired product 1. UVvis Absorption and Fluorescence. UVvis absorption and fluorescence spectra of 1 and 2 in EPA are shown in Figure 2 and are summarized in Table 1. Spectra of 2 in EPA with an absorption maximum at 27 900 cm1 (358 nm) and fluorescence maximum at 24 600 cm1 (406 nm) compare well with published spectra in various solvents.19,31 The shoulder around 28 900 cm1 (346 nm) in the absorption spectrum of 2 is believed to be due to vibrational structure and is not typically observed in more polar solvents.31 Addition of the phenylethynyl group to the parent chromophore 2 to produce 1 broadens the lowest energy absorption and the fluorescence bands and red shifts the absorption to 26 200 cm1 (382 nm). In the region between 30 000 and 40 000 cm1 the band structure of 1 and 2 differs, with one or two very weak bands at 33 800 cm1 (296 nm) and 36 000 cm1 (278 nm) in the spectrum of 2 and a broad band with significant vibrational structure, or possibly several overlapping bands, around 32 500 cm1 (308 nm), 34 20034 500 cm1 (292290 nm), and 36 000 cm1 (278 nm) in the spectrum of 1. The spectra of the two compounds are similar in the high 11170



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Scheme 1

Figure 2. UVvis absorption and normalized fluorescence spectra in EPA and TD-DFT angles, R, and transitions of: (A) 2 at RT and (B) 1 at RT (black line) and 77 K (purple line). (Inset: Anisotropy of fluorescence, λexc (nm) = 396 (upper curve) and 319 (lower curve), and of fluorescence excitation (monitored at 466 nm) of 1 in EPA at 77 K (orange line).) (C) UVvis absorption (thick black line), LD (gray line), EZ cEY (thin black line), and EZ dfEY (thick lines, various colors) of 1 in stretched polyethylene for 1.08 e c e 1.56, increments of 0.04, df = 1.12 (uppermost), 1.48, and 1.52 (lowest). Inset: 1.1 e c e 1.5, increments of 0.08, df = 1.18 (upper), 1.26.

lowest energy band at 25 100 cm1 (399 nm). The 77 K fluorescence maximum at 22 400 cm1 (446 nm) is blue-shifted compared to that at RT, which has its maximum at 20 700 cm1 (482 nm). Fluorescence Quantum Yield. Table 1 summarizes the fluorescence quantum yield measurements of 1 with excitation at 30 900 cm1 (324 nm) and literature values31 for 2. Crosscalibration of our experimental setup reproduced the previously reported values Φ = 0.933 for DPA in cyclohexane and Φ = 0.2734 for anthracene in ethanol. In EPA, Φ1 = 1.09 ( 0.1, and in cyclohexane, Φ1 = 0.13 ( 0.01. Fluorescence Polarization. Steady-state fluorescence anisotropy curves of 1 with excitation at 25 250 and 31 350 cm1 are shown in Figure 2. Excitation at 33 100 cm1 (302 nm) gave identical results to excitation at 31 350 cm1 (319 nm) and is not shown. Excitation at all wavelengths examined gave a nearly constant anisotropy across the fluorescence band, demonstrating that the fluorescence band is purely polarized. Excitation near the origin of the first absorption band, at 25 250 cm1, gave a fluorescence anisotropy r0 of 0.38 ( 0.01. The first absorption also seems to be quite purely polarized, with an average r0 value of 0.36 ( 0.01. Upon excitation into the second band at 31 350 cm1, the anisotropy drops sharply to r0 = 0.16 ( 0.01 and stays close to this value up to ∼33 600 cm1. At least in this region, the second absorption band appears to be purely polarized as well. At higher energies, the anisotropy rises moderately to values close to zero. Linear Dichroism in Stretched Polyethylene.32 The information obtained from polarized absorption measurement on a uniaxially aligned sample is contained in absorbances EZ and EY measured with the electric vector parallel and perpendicular, respectively, to the unique sample axis Z. These absorbances are related to the squares of the cosines of the angles between the observed transition moments and the Z axis, averaged over the molecules observed EZ ¼

energy region, with relatively intense absorption bands observed at 43 50043 900 cm1 (228232 nm). At 77 K, the absorption bands of 1 are slightly red-shifted with respect to those at RT, with the largest shift observed for the

∑f Æcos2 ðZ, Mf ÞæAf

ð4Þ

∑f ½1 Æcos2ðY , Mf ÞæAf

ð5Þ

EY ¼ ð1=2Þ 11171



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Table 1. UVvis Absorption and Fluorescence and Its Anisotropy in EPA, Fluorescence Quantum Yield in Various Solvents, and TD-DFT Calculated Spectra of 1 and 2 flsc

abs

1 (RT)

~νmaxb

εmaxc

fd

ν~maxb

26.2

11.1

0.21

20.7

r0e

TD-DFT/B3LYP/6-31G(d,p)a Φ

~νb

1.09i

25.6

0.16

1 f 1 (3 f 1)

86

30.2 32.2

0.11 0.66

2 f 1 (3 f 1) 3 f 1

13 5

32.4

1 104

4 f 1

oop

34.3

0.15

1 f 2

-86

36.2

3 104

1 f 3

80

37.4

0

7 f 1

oop

37.6

0.19

1 f 4 (2 f 2)

33

38.2

0.0018

5 f 1 (3 f 3)

88

39.8

0.0077

6 f 1 (2 f 2, 3 f 2)

78

f

f

dominant contributionsg

Rh

0.13j 32.5k 34.2

13.9 19.9

36.0

20.5

43.5

1 (77 K)l

25.1 31.3

2 (RT)

36.8

14.1

0.73

0.25

22.4

33.1

31.5 25.1

43.1

45.3

20.8

0.38 0.16

24.5

36.0

27.9

0.66

0.16

0.83

0.82

24.6

0.9-

41.4

0.088

3 f 2 (2 f 2)

9

41.8

0.038

6 f 1 (1 f 5, 3 f 2, 2 f 2)

6

42.8

0.018

2 f 3

89

43.5

0.015

3 f 3

87

43.9

0.068

1 f 5 (3 f 4)

48

44.8

0.024

3 f 4, 8 f 1, 3 f 4

40

45.0 45.6

0 2 104

4 f 2 1 f 6

oop oop

46.3

0.63

2 f 4, 3 f 4

87

47.3

0.068

8 f 1 (2 f 4)

78

31.5

0.33

1 f 1 (2 f 2)

82

0.35m 33.8 36.0 43.9

5.5 6.1 43.4

35.1

0.0044

2 f 1 (1 f 2)

82

35.3

0

3 f 1

oop

38.8

0.077

1 f 2

30

43.5

0.0065

1 f 3, 4 f 1

42

45.8

0

3 f 2

oop

46.4

0.42

1 f 3 (4 f 1, 2 f 3, 2 f 2)

51

46.7

0.41

2 f 2

-82

Showing up to 20 states for Renergies up to 47 600 cm1. In bold, transitions assigned to those observed. b In 103 cm1. c In 103 M1 cm1. d Oscillator ~max. f Oscillator strength, f = 4.319 109 ε(ν~)dν~ (ref 32). e Fluorescence anisotropy, (0.01, with excitation at the corresponding absorption ν strength. g Dominant molecular orbital (MO) contributions to the excitation amplitude MO (occ.) f MO (unocc.), 1 (HOMO), 2, 3, ... from least to most bonding, and 1 (LUMO), 2, 3, ... from most to least stable, including all significant contributions to the first state and all contributions greater than half the dominant one for additional states, in the order of decreasing contributions (small contributions in parentheses). h Angle, in degrees, with respect to the CtC triple bond axis (Figure 1) or oop (polarized perpendicular to the molecular plane of symmetry). i In EPA, (0.1. j In cyclohexane, (0.01. k Shoulder. l εmax and f at 77 K, corrected for EPA volume contraction of 0.771 (ref 23). m In various solvents, in order of decreasing polarity (ref 31). a

where the sums are over all transitions f, Mf is the transition moment of the fth transition, (Z, Mf) is the angle between Mf and the sample axis Z, and Af is the absorbance contribution from transition f (three times its contribution to the absorbance of an isotropic sample of the same concentration and optical path). The pointed brackets are used to indicate averaging over all observed molecules. To determine the direction of each transition moment M with respect to the molecular framework, we define the molecular orientation axes, x, y, and z, as those that diagonalize the orientation tensor with elements Kuv = Æcos u cos væ, u,v = x, y, z. The average orientations of these axes along the Z axis

are described by the orientation factors Kx = Kxx, Ky = Kyy, and Kz = Kzz, respectively, where Ku ¼ Æcos2 ðZ, M u Þæ

ð6Þ

From the properties of direction cosines, Kx þ Ky þ Kz = 1, such that only two of these factors need to be treated as unknown. By convention, we choose Kx e Ky e Kz. The molecular z axis is called the principal orientation axis (the axis that makes the smallest angle, on average, with Z). In the molecular orientation axes system, the experimentally determined Kf values are related to the orientation factors Ku and the unknown angles between 11172



ARTICLE

Table 2. Observed (in Stretched Polyethylene) and TD-DFT Calculated UVvis Spectrum of 1 experimental ~maxa ν

a

K

calculated

|φfz|b

Rexptl

c

ν~

a

Rcalcdc

26.2

0.371 ( 0.004

48 ( 2

86 ( 5

25.6

86

31.8

0.432 ( 0.003

29 ( 6

9(7

30.2

13

33.8

0.387 ( 0.004

43 ( 2

5 ( 5

32.2 34.3

5 86

36.1

0.425 ( 0.003

32 ( 5

70 ( 6 or 6 ( 6

37.6

33

42.9

0.359 ( 0.004

51 ( 3

89 ( 5 or 13 ( 5

46.3

87

3

1 b

In 10 cm . Angle, in degrees, between the fth transition moment and the principal orientation axis z. c Angle, in degrees, counterclockwise from the CtC triple bond axis (Figure 1).

the fth transition moments and molecular x, y, and z axes by32a

∑u ðKu Kf Þcos2 jfu ¼ 0, u ¼ x, y, z

ð7Þ

for each transition f. For planar aromatic molecules such as 1 (Cs symmetry), the molecular orientation axis x is perpendicular to the symmetry plane, and transitions are polarized either along x or perpendicular to it, in the yz plane of the molecule. For transitions polarized in the yz plane, |jfx| = 90, and the sum in eq 7 reduces to two terms, giving tan2 jfz ¼ cot2 jfy ¼ ðKz Kf Þ=ðKf Ky Þ

ð8Þ

Once the orientation factors Kz and Ky are known, this equation permits the evaluation of transition moment directions for all transitions whose Kf values can be obtained from the observed dichroism. For nonoverlapping transitions, these values are given by32b Kf ¼ df =ð2 þ df Þ

ð9Þ

where df = EZ(f)/EY(f) is the dichroic ratio of the fth transition. For overlapping bands, df is the value of the coefficient c in the linear combination EZ cEY for which the fth spectral feature just disappears from the spectral curve and is determined in a stepwise reduction by trial and error.32b While the exact orientation of the principal orientation z axis in the molecular frame in a particular stretched polymer sample is usually unknown a priori for compounds of low symmetry, it is typically close to the longest axis of the molecule. It is often possible to deduce it if Kf can be measured for one or more transitions of known polarization direction. There still remains an ambiguity in the sign of jfz, which gives the absolute transition moment direction in the molecular framework, and this can sometimes be resolved by consideration of fluorescence polarization or of calculated transition moment directions. (i). Electronic Spectra. UVvis absorption (A) and LD spectra of 1 in stretched polyethylene are shown in Figure 2 along with the stepwise reduction curves EZ cEY and are summarized in Table 2. EZ and EY were found from the measured curves A = (EZ þ EY)/2 and LD = EZ EY. The orientation factors Kf (see Table 2) range from 0.359 ( 0.004 to 0.432 ( 0.003, with the uncertainty based on the values of the next highest and lowest reduction curves shown and our ability to discern the best curve between them. While the lowest and highest energy observed bands each reduce with a single df value, the central band shows three features with different reduction factors.

Figure 3. Infrared spectra of 1 dropcast on Ge (black line) and in stretched polyethylene (EZ red line, EY green line) with DFT/B3LYP/ 6-31G(d,p) transitions.

TD-DFT calculated electronic spectra for 1 and 2 are also shown in Figure 2 and are summarized in Table 1. In the spectrum of 2, four intense transitions are calculated between 25 000 and 50 000 cm1. The calculated bands appear at slightly higher frequencies than the corresponding bands in the experimental spectrum, but the relative intensities agree well. In the spectrum of 1, there are six intense calculated transitions between 25 000 and 50 000 cm1. The lowest energy band calculated at 25 600 cm1 has its transition moment calculated at 86 from the direction of the CC triple bond axle. Four intense bands are calculated to lie in the region of the observed broad central band between 30 000 and 46 000 cm1. The two lower energy bands in this region have transition moments calculated to lie nearly parallel to the triple bond axle, at 13 and 5, while the higher energy bands in this region lie at 86 and 33 from this direction. Above 40 000 cm1, the spectrum of 1 shows a single intense transition with a calculated polarization at 87 from the triple bond axle. TD-DFT calculations suggest that for each compound the first excited state is dominated by HOMOLUMO excitation. Examination of the orbital coefficients of the HOMO and LUMO (not shown) in each case indicated that the first excitation involves a displacement of charge from the N,N-dimethylamino-substituted isocyclic ring to the lactam ring as expected for a charge transfer transition. The ground state dipole is calculated to be 5.7 D at 84 from the triple bond axle. (ii). Vibrational Spectra. Polarized IR spectra (EZ and EY) of 1 in polyethylene stretched along Z are shown in Figure 3 along with the spectrum of 1 dropcast on Ge. Regions of the spectrum that are obscured by intense CH stretching (3000 2800 cm1), scissoring (15001450 cm1), and rocking (740710 cm1) bands of polyethylene are excluded from further analysis. Many sharp, but extremely weak, overtones between 2000 and 1700 cm1 (not shown) were apparent in the spectrum of 1 on Ge. In the polarized EZ and EY spectra from 2000 to 1700 cm1 (also not shown), only some of these weak bands were observed, and the rest were obscured by the tail of the intense band at 1637 cm1 and/or the intense fringe pattern which we were unable to eliminate sufficiently from the background in this region. We do not consider the bands in this region further. Accessible band frequencies and intensities are listed in Table 3. Selected linear combinations of the EZ and EY curves (EZ cEY) are shown for various regions of the IR spectrum in Figure 4. The reduction factor df for each transition is indicated above each band. Orientation factors Kf determined from the reduction factors by eq 9 are listed in Table 3 and range from 0.24 ( 0.01 to 11173



ARTICLE

Table 3. Experimental and calculated vibrational frequencies, intensities and transition moment directions for 1 cast film (on Ge) ~d ν

EZ (in PE) ν~d

rel. int.e

Ka

EY (in PE) ν~d

103A

103A

|φfz|b

Rexptlc

deg

deg

~d,f ν

int.g 5.73 11.3 11.3 20.2 3.20 24.0 5.47 2.18 14.2 19.3

νCHh νCHh νCHi νCHi νCHh νCHi νCHi νCHi νas CD νCtC

24 72 12 84 6 6 83 3 oop 15

DFT/B3LYP/6-31G(d,p) assignment

Rcalcd deg

3099sh 3083

0.0129 0.0203

3100sh 3082

1.21 2.91

3100sh 3082

1.05 2.25

0.44 0.44

27 ( 12 27 ( 12

65 ( 13 or 11 ( 13 65 ( 13 or 11 ( 13

3057 3034 3019 2233sh 2208 2141 2127 2079 1632 1606

0.0244 0.0210 0.0206 0.0238 0.0313 0.0140 0.0162 0.0196 0.393 1

3055 3034 3020

3.80 1.82 0.656

3055 3034 3019

3.11 1.31 0.479

6.86 1.05 1.60 3.61 130 301

2210 2141 2127 2078 1637 1609sh

5.92 1.24 1.86 3.14 103 246

42 ( 4 42 ( 4 33 ( 8 90 39 ( 5

80 ( 6 or 4 ( 6 80 ( 6 or 4 ( 6 71 ( 9 or 5 ( 9

2210 2141 2127 2078 1637 1609sh

0.39 0.39 0.42 0.25 0.40

77 ( 6 or 1 ( 6

3110 3073 3053 3049 3047 3042 3031 3023 2227 2230

0.40

39 ( 5

77 ( 6 or 1 ( 6

2115

30.0

νsCD

11

0.38 0.38

45 ( 4 45 ( 4

83 ( 6 or 7 ( 6 83 ( 6 or 7 ( 6

1587 1551 1528

0.333 0.0937 0.135

1586sh 1550 1526

96.8 34.6 49.6

1586sh 1550 1526

72.1 28.3 36.4

0.40 0.39 0.39

39 ( 5 42 ( 4 42 ( 4

77 ( 6 or 1 ( 6 80 ( 6 or 4 ( 6 80 ( 6 or 4 ( 6

1436 1418 1401 1377

0.0576 0.0683 0.304 0.167

1434 1414sh 1398 1377

18.2 26.4 92.5 55.4

1434 1413sh 1398 1377

12.8 22.0 67.1 44.8

0.34 0.40 0.40 0.36

57 ( 8 39 ( 5 39 ( 5 51 ( 5

95 ( 9 or 19 ( 9 77 ( 6 or 1 ( 6 77 ( 6 or 1 ( 6 89 ( 6 or 13 ( 6

1347 1311

0.123 0.167

1341 1308

39.8 58.2

1341 1308

31.0 44.6

0.39 0.38

42 ( 4 45 ( 4

80 ( 6 or 4 ( 6 83 ( 6 or 7 ( 6

1239 1213 1171 1151 1107

0.0631 0.0179 0.0713 0.166 0.0276

80 ( 6 or 4 ( 6

474 685 17.4 47.1 90.3 70.6 4.20 3.44 18.2 16.2 157 153 7.03 122 112 84.3 29.9

n n

1667 1612 1602 1580 1534 1518 1437 1434 1431 1408 1386 1376 1356 1339 1305 1295 1243 1210 1176 1157 1114 1085 1078 1069 1065 1064 1033 1032 1007 966 948 851 805 793 759 690

νCdO νCCj νCCi νCCj νCCj νCCh δCHk νCCi δCHk,l δCHl δCHk,l δCHk δCH,h,k,l νCNm δCC,j δCHj,k δCH,h,k νCNm δCHh δCHi,j δCHh δCHh,l δCHh δCD δCHi δCD δCH,k δCD δCHl δCD δCHh,i δCD, δCHh,i δCH,k δCD δN(CH3)2l δCD, δCC, δCHj,k δCC, δCHj,k δCHh δCHh δCHi δCHi

47 82 10 8 44 77 oop 77 36 19 31 47 45 52 76 71 23 43 71 65 31 86 oop 50 28 45 26 4 28 50 54 66 oop oop oop oop

1235

23.1

1235

16.0

0.39

42 ( 4

n

n

n

n

n

n

1168 1151 1107

27.3 51.9 9.74

1168 1151 1107

20.2 37.5 9.03

0.39 0.39 0.41

42 ( 4 42 ( 4 36 ( 6

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

n

1066

0.0633

1066

20.6

1066

13.7

1031 1006 965 950 851 820 792 755 690

0.0322 0.0168 0.0770 0.0299 ∼4 104 0.137 0.0875 0.274 0.119

n

0.39

42 ( 4

n

n

80 ( 6 or 4 ( 6 80 ( 6 or 4 ( 6 74 ( 7 or 2 ( 7

80 ( 6 or 4 ( 6 n

1031

7.71

1031

7.25

0.39

42 ( 4

n

n

n

n

n

n

964 950 851 806 793sh 755 689

20.0 10.6 4.01 16.8 ∼1.7 50.3 26.0

964 950 852 806

14.7 9.68 4.02 0.0292

0.39 0.41

42 ( 4 36 ( 6

n

n

0.24

90

n

n

755 689

72.7 40.3

0.24 0.24

90 90

80 ( 6 or 4 ( 6 n

80 ( 6 or 4 ( 6 74 ( 7 or 2 ( 7 n

n

26.5 63.7 5.99 4.14 3.61 5.77 24.9 35.0 6.14 8.60 8.14 28.7 14.6 5.10 40.8 8.55 35.5 19.1

a Experimental orientation factor, (0.01. b Angle between the fth transition dipole moment and the principal orientation axis z. c Two possible absolute directions for the experimental transition moments. Preferred choice in boldface. d In cm1. e Relative to the most intense band at 1606 cm1. f Calculated frequencies scaled by 0.954 (above 2300 cm1), 0.967 (between 1261, after scaling, and 2300 cm1), 0.98 below 1261 cm1 (after scaling). g In km/mol. Bands with intensities below 2.4 km/mol are not included in the table except in the aromatic CH stretching region where all bands are listed. h N,N-Dimethylamino-substituted ring of the carbostyril chromophore. i Phenyl ring of 1. j Carbostyril chromophore. k Lactam ring methyl group. l Dimethylamino group. m Lactam ring of the carbostyril chromophore. n Too weak to be determined reliably. sh = shoulder

0.44 ( 0.01, with the uncertainty derived as described above for the electronic spectra (for clarity, not all curves examined are shown in Figure 4). Calculated vibrational bands, transition moment directions (with respect to the CtC bond direction), and band assignments for 1 are included in Table 3. Calculated bands are shown along with the experimental spectra in Figures 3 and 4. Three

frequency scaling factors were used to obtain reasonable agreement with the experimental bands: 0.95 for bands above 2300 cm1, 0.967 from 2000 to 1261 cm1 (scaled), and 0.98 for bands below 1261 cm1 (scaled). Assignments are based on the calculated normal modes and the degree of contribution of the internal coordinates to the modes and are supported with references to the literature where possible. 11174



ARTICLE

specific bands with bands previously reported in the literature is difficult for such a complex ring system. Orientation factors in this region vary from 0.44 to 0.39 ((0.01). There are three calculated bands in the region between 2230 and 2115 cm1. The CtC stretch calculated at 2230 cm1 is assigned to the band observed at 22102208 cm1, in the region where disubstituted CtC stretches generally appear (22602190 cm1).35b The asymmetric stretch of the deuterated methyl group calculated at 2227 cm1 is assigned to the shoulder on the CtC band that appears around 2233 cm1 in the dropcast sample and is evident in the linear combinations of EZ and EY shown in Figure 4B. The symmetric CD3 stretch calculated at 2115 cm1 is assigned to bands observed at 2141, 2127, and 20792078 cm1. Bands for the CD3 group of 1 agree well with those reported previously for CD3 in deuterated propene.36 Except for the shoulder near 2233 cm1, which has an orientation factor of about 0.25, all of the bands in this region reduce identically with a corresponding orientation factor of 0.40 ( 0.01. Numerous bands appear in the region from 1650 to 900 cm1, and calculations suggest that many of these are of mixed origin (see Table 3). The most intense bands in the spectrum, overlapping bands calculated at 1667 and 1612 cm1 (observed at ∼1637 and ∼1610 cm1), are assigned to CdO stretching of the lactam group37 and to CC stretching of the N,N-dimethylaniline ring, respectively.38 Both bands appear to have identical orientation factors of 0.38 ( 0.01 as shown in Figure 4C. Weak calculated bands between 1602 and 1500 cm1 are assigned to skeletal ring breathing modes of the N,N-dimethylaniline, lactam, and phenyl rings (see Table 3).35c Of these bands, only those calculated at 1534 and 1518 cm1 (observed at 15511550 and 15281526 cm1), with orientation factors of 0.39 ( 0.01, are not obscured by the intense CdO and N,N-dimethylaniline CC stretches. Bands from 1400 to 900 cm1 (Figure 4D) generally involve deformations of the rings and/or the CH3, CD3, and/or N(CH3)2 groups and show a range of orientation factors from 0.41 ( 0.01 to 0.36 ( 0.01. Below 900 cm1, intense out-of-plane polarized bands due to aromatic CH bending are expected.35c Calculated bands at 805, 793, 759, and 690 cm1 match up well with bands observed near 806 (820 and 792 cm1 on Ge), 755, and 690 cm1. All of them reduce identically, giving orientation factors of 0.24 ( 0.01. The two higher energy bands are assigned to the N,N-dimethylaminosubstituted ring, while the two lower energy bands are assigned to the phenyl substituent. Below ∼675 cm1, band intensities are not reliable and are not considered further. Figure 4. DFT/B3LYP/6-31G(d,p) calculated transitions and IR spectra EZ (red line), EY (green line), EZ cEY (thin black line), and EZ dfEY (thick black line) of 1 in stretched polyethylene. (A) 0.55 e c e 1.85, 0.1 increments, df = 1.25 (upper) and 1.55 (lower) or (4) 0.85 e c e 1.85, 0.2 increments, df = 1.45. (B) 0.45 e c e 1.65, 0.15 increments, df = 1.35 (inset: 0.35 e c e 1.25, 0.15 increments, df = 0.65). (C) 0.8 e c e 1.7, increments of 0.1, df = 1.2 or (4) df = 1.3. (D) 0.90 e c e 1.65, increments of 0.15, df = 1.35 or (4) 1.0 e c e 1.8, increments of 0.2, df = 1.4. (E) 0.40 e c e 0.88, 0.08 increments, df = 0.64.

Starting at the high frequency end, eight bands calculated between 3110 and 3023 cm1 are assigned to CH stretching of the N,N-dimethylamino-substituted ring or the phenyl group in 1 as indicated in Table 3. While these transitions all fall in the region expected for aromatic CH stretching,35a correlation of

’ DISCUSSION Chromophore 1. The purpose of this work was to evaluate the chromophore 1 for use as a molecular rotator for optical studies. The most important requirements are that 1 should be a robust UVvis chromophore with a high fluorescence quantum yield, a ground state dipole moment perpendicular to the triple bond axle, and a purely polarized emission and absorption transition dipole moment, both oriented perpendicular to the triple bond axle (Figure 1). The presence of an additional absorption band purely polarized along the direction of the axle would also be helpful as a reference for characterizing rotation of the chromophore by fluorescence depolarization. Overall, the chromophore 1 looks very promising. Its ground state dipole moment is calculated to lie within 6 of the desired 11175


The Journal of Physical Chemistry A direction (84 from the triple bond axle) and at 5.7 D could be useful for future studies of driven rotation. The fluorescence quantum yield of 1 varies significantly as a function of solvent polarity, with the highest obtainable yield (∼1) observed in the moderately polar solvent EPA and Φ ∼ 0.13 in cyclohexane. The observed decrease in Φ with decreasing solvent polarity is somewhat unusual for an intramolecular charge transfer compound; however, the same trend has been reported for 2.31 It remains to be seen whether the fluorescence quantum yield of the rotator will be more analogous to its polar or nonpolar solvent value when assembled in a monolayer. From the steady state fluorescence anisotropy and fluorescence excitation anisotropy of 1 shown in Figure 2, it is clear that the fluorescence and lowest energy absorption are almost purely polarized, with transition dipole moments essentially parallel to each other. These transitions occur in a favorable spectral region for time-resolved fluorescence depolarization studies, and the remaining question is whether their transition moments lie in the desired direction, as suggested by the results of TD-DFT calculations. We shall first use the observed orientation factors Kf of the electronic transitions to obtain the orientation factors Ku of the orientation axes of 1 in stretched polyethylene and the angles jfz that the observed electronic transition moments f make with the principal orientation axis z. Then, we shall use the IR transition moment directions of a few vibrations that are known without reasonable doubt to deduce the absolute direction of the orientation axis z in the molecular frame. Relative Transition Moment Directions. Since 1 has Cs symmetry, its transition moments must either be oriented perpendicular to the symmetry plane, along x, or lie at some direction within the yz plane. It is safe to assume that all intense electronic transitions will be of ππ* nature and thus in the ring plane, and this is supported by the TD-DFT results. All out-ofplane polarized CH bending vibrations share the same orientation factor, 0.24 ( 0.01, and it is straightforward to assign this value to the orientation factor of the out-of-plane axis, Kx. Then, Ky = 0.76 Kz, and we are left with a single unknown orientation factor Kz. From IR dichroism, we know that Kz g 0.44 ( 0.01. The value of Kz can be determined by taking advantage of the already mentioned fortunate circumstance that the first UVvis absorption at 25 250 cm1 and the next absorption at 31 350 cm1 are both purely polarized. The r0 value for the first transition (f = 1) is 0.38 ( 0.01, and we shall assume that its 5% deviation from the theoretically expected value of 0.40 is due to experimental imperfections. For the second transition (f = 2), the observed value of 0.16 ( 0.01 would become 0.17 if similarly corrected for experimental imperfections and, according to eq 3, would correspond to cos2 β2 = 0.05 and an angle β12 = 77 (taken positive by definition) between the transition moments of transitions f = 1 and f = 2. We shall assume the true value of r0 to be 0.17 ( 0.01, and the resulting angle β12 then is 77 ( 2. (2) Next, the angles j(1) z and jz between the principal orientation axis z and the moments of the first (f = 1) and second (f = 2) transition, respectively, are evaluated using eq 8 and the Kf values from Table 2 (K1 = 0.371 ( 0.004, K2 = 0.432 ( 0.003). This is done for a series of assumed values of Kz and Ky, keeping their sum equal to 1 Kx = 0.76. The angle j(1) z decreases slowly, and the angle j(2) z increases somewhat faster as the assumed value of Kz is increased from 0.44. During this process, the difference (2) |j(1) z | |jz | varies between ∼15 and ∼35 and never comes

ARTICLE (2) close to 77. In contrast, the sum |j(1) z | þ |jz | starts a little below 70 and increases steadily. It becomes equal to 75 at Kz = 0.46 and reaches 79 at Kz = 0.50. Clearly, one of the transition moments deviates clockwise and the other counterclockwise from the positive direction of z (Figure 1). We conclude that compatibility between the observed r0 value and the LD measurements requires Kz = 0.48 ( 0.02 and thus Ky = 0.28 ( 0.02, given that Kx = 0.24 ( 0.01 is known from IR LD. It is also possible to write a general algebraic solution to a set of three (2) equations for the three unknowns, Kz, j(1) z , and jz , using eq 8 once with f = 1 and once with f = 2, plus the equation β12 = j(1) z j(2) z , but the resulting expression is quite complicated unless β12 = 90, in which case (Kz K1)(Kz K2) = (K1 Ky)(K2 Ky) holds. It is now possible to use eq 8 and combine the values and error limits of the orientation factors of the molecular orientation axes with the Kf value measured for each transition to obtain the absolute value of the deviation of its transition moment from z. The results have been collected in Table 2 for electronic and in Table 3 for vibrational transitions. The uncertainty reported for |jfz| in Tables 2 and 3 reflects the maximum deviation from the average angle given the range of possible values for Ky and Kz. Absolute Transition Moment Directions. The only unknown that remains to be determined is the direction of the principal orientation axis z in the molecular yz plane. To do this unambiguously, we would need to know the absolute directions of two nonparallel transition moments in the plane and their Kf values, which would yield the absolute values of the angles that these moments make with z. Such transition moments are sometimes encountered in localized vibrations of IR chromophores. There are two such vibrations in the present instance, the symmetric CD3 stretch and the triple bond stretch. These vibrations occur in a frequency region that contains few other fundamentals, and although they may appear as multiple peaks due to anharmonic coupling with the numerous overtones and combination transitions that commonly occur at these frequencies, these fundamentals provide the only significant absorption intensity and hence polarization. These vibrations are expected to be polarized along the direction of the triple bond axle, and this belief is reinforced by their identical orientation factors, Kf = 0.40 ( 0.01. The DFT calculations also agree, predicting that the transition moments for the CtC and symmetric CD3 stretch lie 4 apart, at 15 and 11, respectively, from the CtC bond axis. Given the molecular orientation factors and their uncertainties, the Kf value observed for all these peaks corresponds to an absolute value of 38 ( 4 for the angle Rz between z and the triple bond axle. If we had another localized vibration in the molecule with a differently oriented and also well-known transition moment, we might be able to make an unambiguous choice of sign for this angle. Unfortunately, this is not so, and we chose the sign of Rz and hence direction of z indicated in Figure 1 based on the general observation that the principal orientation axis of samples of aromatics in stretched polyethylene tends to be the “longest” molecular axis. This choice is supported by the results of DFT calculations discussed below. Having made an assignment of the direction of z in the molecular yz plane, we can now use eq 8 to convert the absolute values of the angles |jfz| deduced from the observed Kf values of each electronic or vibrational transition into a choice of two absolute directions in the molecular frame. In general, there is no reason to favor one or the other sign for jfz. However, for certain electronic transitions the value of fluorescence anisotropy r0

11176



ARTICLE

allows us to eliminate one of the possibilities, and we have already discussed the evidence for the opposite signs of the angles j(1) z and j(2) z . The results will be discussed briefly below. To describe the absolute polarization directions relative to the triple bond axis, we use the notation f

Rexptl ¼ Rz ( jjfz j

ð10Þ

Electronic Transition Moments. The determination of electronic polarization directions is the main goal of the present study. Table 2 presents the principal results and compares them with results of TD-DFT calculations. When two absolute polarization directions are possible after both the LD and the fluorescence anisotropy results have been considered, both are given for each transition, and the one that agrees better with the calculated direction is shown in bold. The agreement between the calculated and experimental transition moment directions is excellent. Most important, the purely polarized lowest energy transition is polarized within a few degrees of a direction that is perpendicular to the triple bond axle of this intended molecular rotator, as hoped for. In the crowded region between 30 000 and 40 000 cm1, transition moments for the two lowest energy bands are approximately parallel to the triple bond axle, as also hoped for. Only the calculated polarization direction of the highest energy transition in this region lies well outside the experimental error. At higher energies, fluorescence polarization data are not available, and the ambiguity due to uncertain angle sign remains. Vibrational Transition Moments. Although the determination of vibrational transition moment directions is only secondary to the main purpose of this study, the results will be of value for characterizing orientation in films containing 1 and may be of interest for future efforts to improve the methods of calculation of these quantities. Agreement between the calculated and experimental vibrational frequencies appears to be quite reasonable for the three frequency scaling factors chosen (see Table 3 and Figures 3 and 4). Calculated band assignments given in Table 3 suggest that many bands in the spectrum of 1 are of mixed origin and cannot be assigned to a single functional group in the molecule. Three notable and useful exceptions have already been taken advantage of: the CtC stretch which appears around 2210 cm1 (calculated at 2230 cm1) and the symmetric and asymmetric CD3 stretches which appear at 2233 cm1 (calculated at 2227 cm1) as a shoulder on the CtC band and at 2141, 2127, and 2078 cm1 (calculated at 2115 cm1), respectively, in an otherwise uncrowded region of the spectrum (Figure 4B). The three distinct bands observed for the symmetric CD3 stretch are attributable to anharmonic coupling of the fundamental with overtone and combination bands, which borrow intensity and polarization from the fundamental and have identical reduction factors. The orientation factor obtained for the out-of-plane polarized asymmetric CD3 stretch is identical, within the experimental uncertainty of (0.01, to those obtained for aromatic out-ofplane CH bending modes associated with the phenyl- and dimethylamino-substituted rings between 850 and 650 cm1 (Figure 4E). The common orientation factor for all of these modes supports our initial assumption of Cs symmetry and allowed us to immediately derive the value of the orientation factor Kx using eq 9, which gives Kx = 0.24 ( 0.01. The accessible

in-plane polarized transitions listed in Table 3 show orientation factors ranging from 0.34 ( 0.01 to 0.44 ( 0.01. The two possible values of Rfexptl for each transition, based on eq 10, are given in Table 3 along with the DFT/B3LYP/6-31G(d,p) calculated transition moment directions with respect to the CtC bond axis. In general, agreement between the experimental and calculated values of R is quite poor. Of the 26 accessible in-plane polarized bands, only 8 agree within the calculated experimental error, and 12 show differences in excess of 20. Better agreement was obtained for higher frequency (>1600 cm1) transitions with the exception of the CdO stretch calculated at 1667 cm1. The calculation suggests the transition moment for the CdO stretch lies about 13 from the CdO bond axis, inclined toward the NCD3 bond axis, while experiment indicates that it lies 7 from the NCD3 bond (the preferred value based on the determined location of the principal orientation axis and our anticipation that the transition moment lies between the CdO and NCD3 bonds), a difference of 40. In contrast, the calculated transition moment for the most intense band in the spectrum (calculated at 1612 cm1) is identical, within the uncertainty of (6, to the experimentally determined value. Errors of the magnitude observed for the CdO transition are not entirely uncommon. The difficulty of calculation of IR transition moment directions in molecules of low symmetry has been recognized before, and the problems were attributed primarily to the neglect of anharmonic mode coupling in the calculations.36 Results such as those presented here may be useful as benchmarks for the testing of better computational procedures.

’ CONCLUSIONS Steady state fluorescence and excitation anisotropy of 1 in a glassy matrix, combined with its IR and UVvis linear dichroism in stretched polyethylene, show that the lowest energy UV absorption band and the emission band of 1 are purely polarized with transition dipoles essentially perpendicular to the triple bond axle of the rotator, while the transition dipole of the second UV band lies very close to the direction of the axle. The chromophore 1 thus represents a useful addition to the molecular rotor Tinkertoy kit and can be used to characterize rotation of surface-mounted molecular rotors. ’ ACKNOWLEDGMENT The authors are grateful to Dr. Xudong Chen for his contributions to the synthetic work and Dr. Brian Stepp for help with UVvis measurements. The work was supported by the DOE (DE-FG02-08ER15959). ’ REFERENCES (1) Vacek, J.; Michl, J. New J. Chem. 1997, 21, 1259. (2) Gimzewski, J. K.; Joachim, C.; Schlitter, R. R.; Langlais, V.; Tang, H.; Johannsen, I. Science 1998, 281, 531. (3) Hersam, M. C.; Guisinger, N. P.; Lyding, J. W. Nanotechnology 2000, 11, 70. (4) Yasuda, R.; Noji, H.; Kinosita, K., Jr.; Yoshida, M. Cell 1998, 93, 1117. (5) Yasuda, R.; Noji, H.; Kinosita, K., Jr.; Itoh, H. Nature 2001, 410, 898. (6) Vacek, J.; Michl, J. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 5481. (7) Clarke, L. I.; Horinek, D.; Kottas, G. S.; Varaska, N.; Magnera, T. F.; Hinderer, T. P.; Horansky, R. D.; Michl, J.; Price, J. C. Nanotechnology 2002, 13, 533. 11177



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