Steps to demarcate the effects of chromophore aggregation and planarization in poly(phenyleneethynylene)s. 2. The photophysics of 1,4-diethynyl-2-fluorobenzene in solution and in crystals

Article abstract of DOI:10.1021/jo015589e

Crystals of 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4 have a rich packing structure with four distinct molecules in the unit cell. A complex hydrogen bonding network results in the formation of cofacial trimers, cofacial dimers, and monomers

Full text of DOI:10.1021/jo015589e

3188
J . Org. Chem. 2001, 66, 3188-3195
Step s To Dem a r ca te th e Effects of Ch r om op h or e Aggr ega tion a n d
P la n a r iza tion in P oly(p h en ylen eeth yn ylen e)s. 2. Th e P h otop h ysics
of 1,4-Dieth yn yl-2-flu or oben zen e in Solu tion a n d in Cr ysta ls
Marcia Levitus, Gerardo Zepeda, Hung Dang, Carlos Godinez, Tinh-Alfredo V. Khuong,
Kelli Schmieder, and Miguel A. Garcia-Garibay*
Department of Chemistry and Biochemistry, The University of California,
Los Angeles, California 90095-1569
mgg@chem.ucla.edu
Received February 22, 2001
Crystals of 1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4 have a rich packing structure
with four distinct molecules in the unit cell. A complex hydrogen bonding network results in the
formation of cofacial trimers, cofacial dimers, and monomers within the same unit cell. Given a
remarkable opportunity to investigate the effect of aggregation on the photophysics of 1,4-
diethynylbenzenes, we analyzed the absorption, diffuse reflectance, and emission spectra of
compound 4 in solutions and in crystals. Diffuse reflectance and fluorescence excitation revealed
a red-shifted absorption that is absent in dilute solution but becomes observable at high
concentrations and low temperatures. The fluorescence emission in the solid state is dual with
components assigned to monomers and aggregates. The excitation and emission assigned to the
monomer are nearly identical in crystals and dilute solutions. The absorption and emission bands
assigned to aggregates are broad and red-shifted by 60-80 nm. As expected for a sample with
absorbers and emitters with different energies and incomplete equilibration, efficient monomer-
to-aggregate energy transfer was observed by a proper selection of excitation wavelengths. The
fluorescence quantum yield of 4 in solution is relatively low (ΦF ) 0.15) and the singlet lifetime
short (τF ) 3.8 ns). A lower limit for the triplet yield of ΦT ) 0.64 was determined indirectly in
1
solution by O2 sensitization, and a relatively strong and long-lived phosphorescence was observed
in low-temperature glasses and in crystals at 77 K.
Sch em e 1
In tr od u ction
Several poly(phenyleneethynylene)s (1)1,2 and other
arylethynyl fluorophores (e.g., 2-5, Scheme 1),3 have
become very attractive for a wide variety of photonics and
sensing applications due to their electron-transport abili-
ties and intense fluorescence emission. Among their
many interesting properties, red shifts in the absorption
and emission spectra are frequently observed upon
concentration and thin film formation4 and by addition
of nonsolvents.5 Although these spectral changes and the
observation of self-quenching have been primarily ex-
plained in terms of aggregation effects, possible contribu-
tions from coplanarization and twisting of the aryl groups
(1) (a) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605-1644. (b)
Levitsky, I. A.; Kim, J .; Swager, T. M. J . Am. Chem. Soc. 1999, 121,
1466-1472. (c) Sato, T.; J iang, D.-L.; Aida, T. J . Am. Chem. Soc. 1999,
121, 10658-10659. (d) Samori, P.; Francke, V.; Mu¨llen, K.; Rabe, J .
P. Chem. Eur, J . 1999, 5, 2312-2317.
have been recently suggested by Bunz et al. (Scheme 2).5,6
While it is reasonable that both planarization and
aggregation may have an effect on ground and excited
state properties of arylethynyl-containing compounds, the
separation of these effects has been difficult to factorize
and remains somewhat controversial.4a Ambiguities may
arise because spectroscopic manifestations of chro-
(2) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000,
100, 2537-2574, and references therein.
(3) (a) Hanhela, P. J .; B. D. P. Aust. J . Chem. 1984, 37, 553. (b)
Gisser, D. J .; J ohnson, B. S.; Ediger, M. D.; Von Meerwall, E. D.
Macromolecules 1993, 26, 512. (c) Lindasy, J . S.; Prahapan, S.;
J ohnson, T. E.; Wagner, R. W. Tetrahedron 1994, 50, 8941-8968.
(4) (a) McQuade, D. T.; Kim, J .; Swager, T. M. J . Am. Chem. Soc.,
2000, 122, 5885-5886. (b) Walters, K. A.; Ley, K. D.; Schanze, K. S.
Langmuir 1999, 15, 5676-5680. (c) Weder, C.; Wrighton, M. S.
Macromolecules 1996, 29, 5157-5165.
(5) (a) Miteva, T.; Palmer, L.; Kloppenburg, L.; Neher, D.; Bunz, U.
H. F. Macromolecules 2000, 33, 652-654. (b) Halkyard, C. E.; Rampey,
M. E.; Kloppenburg, L.; Studer-Martinez, S. L.; Bunz, U. H. F.
Macromolecules 1998, 31, 8655.
(6) Planarization effects on the electronic properties of conducting
polymers had been previously reported by Wudl and co-workers:
Rughooputh, S. D. D. V.; Hotta, S.; Heeger, A. J .; Wudl, F. J . Polym.
Sci. B 1987, 25, 1071.
10.1021/jo015589e CCC: $20.00 © 2001 American Chemical Society
Published on Web 04/04/2001

J . Org. Chem., Vol. 66, No. 9, 2001 3189
Sch em e 2
with these compounds. While only limited measurements
were carried out with 5, due to its rapid decomposition,
detailed results with compound 4 as a function of
concentration and in the solid state revealed several
differences between monomer and aggregate emission.
The aggregate emission was characterized by broad and
red-shifted spectra and by longer fluorescence lifetimes.
The results obtained in the solid state, including photo-
physical measurements and solid state 13C NMR, were
analyzed in terms of the unusually rich and complex
X-ray structure of compound 4.
Exp er im en ta l Section
Gen er a l. Reagents and solvents of the highest commercial
purity were purchased from Fisher or Aldrich and were used
as received. IR spectra were acquired on a Perkin-Elmer
Paragon 1000 FT-IR instrument. 1H(13C) NMR spectra were
obtained on Bruker ARX NMR spectrometer operating at 400
and 500 MHz for 1H and at 100 or 125 MHz for 13C in CDCl3
with TMS as internal standard. Gas chromatographic analyses
(GC) were conducted on a Hewlett-Packard 5890 Series II
capillary instrument equipped with a flame ionization detector
(FID) and a Hewlett-Packard 3396 Series II integrator. Melt-
ing points were determined with a Fisher-J ohns melting point
apparatus.
mophore aggregation are known to involve cofacial
interactions between molecules that are in planar con-
formations (Scheme 2a).2,7,8 To our knowledge, there are
no aggregates formed by twisted arylethynes that can be
detected by absorption or fluorescence changes, and it is
difficult to prevent the aggregation of planar aromatic
compounds. To prepare highly luminescent solid state
materials, synthetic strategies based on the use of bulky
substituents that minimize aggregation have encoun-
tered remarkable success.9 However, the results obtained
in those cases have not addressed the possible contribu-
tions from planarization effects.
In a recent study with 9,10-bis(phenylethynyl) an-
thracene 2 (Scheme 1), we noticed that conditions affect-
ing the planarization and conformational dynamics of the
three aryl rings led to significant changes in the absorp-
tion spectrum.10 A recently reported study with 1,4-bis-
(phenylethynyl)benzene,11 and work in progress on other
compounds,12 suggests that the conformation-dependent
photophysics of compound 2 may be a general property
of alkyne-conjugated aromatic chromophores. While pre-
paring a series of 1,4-diethynyl derivatives, we came
across samples of 1,4-diethynyl-2-fluorobenzene 5 and its
acetone-protected precursor 4 (Scheme 1). Recognizing
that 1,4-diethynylbenzenes would be the simplest aryl-
ethynyl chromophore with photophysical properties in-
dependent of phenyl group rotation, we decided to
investigate their properties in solution and in crystals.13
As illustrated in Scheme 2b, we reasoned that compounds
4 and 5 may illustrate the effects of aggregation without
complications from planarization of aryl groups linked
to the triple bonds. We report here the results obtained
1,4-Bis(2-h yd r oxy-2-m et h yl-3-b u t yn yl)-2-flu or ob en -
zen e (4). Compound 4 was prepared using a modification of
the method reported in the literature.14 1,4-Dibromo-2-fluo-
robenzene (1.15 g, 4.5 mmol), (PPh3)2PdCl2 (0.31 g, 0.44 mmol),
and 2-methyl-3-butyn-2-ol (0.93 g, 11.0 mmol) were dissolved
in 10 mL of piperidine in a three-neck round-bottom flask
equipped with a condenser and kept under an Ar atmosphere.
The mixture was refluxed for 4 h, and workup was carried
out by addition of 50 mL of ether before washing the organic
layer successively with 15 mL of water, dilute HCl, and brine.
The organic layer was dried over anhydrous MgSO4 and
evaporated to near dryness at low temperature avoiding high
vacuum. The product was purified by column chromatography
(hexanes:ethyl acetate 4:1 v/v) to afford 0.67 g, 56% yield, of a
pale yellow crystalline solid: mp 134.5-135.0 °C; 1H NMR (500
MHz, CDCl3) δ 1.61 (s, 6H, -CH3), 1.63 (s, 6H, CH3), 2.32
(broad, 2H, OH), 7.10 (m, 1H, Ar), 7.12 (m, 1H, Ar), 7.31 (m,
1H, Ar); 13C NMR (125 MHz, CDCl3) δ 31.26, 31.28, 65.44,
65.62 (d, J CF ) 56.5 Hz), 75.23, 80.67 (d, J CF ) 12.5 Hz), 96.28,
100.51, 111.49 (d, J CF ) 63.5 Hz), 118.41 (d, J CF ) 90.5 Hz),
124.39 (d, J CF ) 37.5 Hz), 127.24 (d, J CF ) 14 Hz), 133.18 (d,
J CF ) 7.5 Hz),162.05 (d, J CF ) 1001 Hz); IR (KBr) cm-1
)
3365.6, 3087.5, 3087.5, 2982.3, 1615.6, 1363.6, 1124.0; MS (70
eV) m/z (rel intensity) ) 260 (65, M+), 245 (100), 227, (40),
187 (20). Details of the crystal structure of 4 are included in
the Supporting Information section.
1,4-Dieth yn yl-2-flu or oben zen e (5). Compound 4 (0.2 mg,
0.768 mmol), potassium hydroxide (0.3 g, 5.36 mmol), tetra-
butylammonium iodide (0.5 g, 1.35 mmol), and 10 mL of
benzene were added to a round-bottom flask equipped with a
condenser and heated to 50 °C under an Ar atmosphere. The
yellow-orange suspension was stirred for 12 h and monitored
by TLC. Workup was carried out by adding 50 mL of diethyl
ether before washing the organic layer successively with 15
mL of water, dilute hydrochloric acid, and brine. The organic
layer was dried over anhydrous MgSO4 and evaporated to near
dryness at low temperature avoiding high vacuum. The
product was purified by column chromatography using pentane
as the eluant to yield 80 mg (56% isolated yield) of the pure
product as a pale yellow solid which exhibited a high vapor
pressure as well as high thermal and light sensitivity: 1H
NMR (400 MHz, CDCl3) δ 3.21 (s, 1H), 3.39 (s, 1H), 7.19-
7.23 (m, 2H), 7.40-7.44 (m, 1H); 13C NMR (100 MHz, CDCl3)
δ 76.5, 80.1, 81.8 (d, J CF ) 12.4 Hz), 84.2 (d, J CF ) 13.6 Hz),
111.5 (d, J CF ) 63.2 Hz), 119.1 (d, J CF ) 90.4 Hz), 124.4 (d,
(7) Li, H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules
1998, 31, 52-58.
(8) (a) Bredas, J . L.; Cornil, J .; Bejonne, D.; dos Santos, D. A.; Shuai,
Z. Acc. Chem. Res. 1999, 32, 267. (b) Cornil, J .; Dos Santos, D. A.;
Crispin, X.; Silbey, R.; Bredas, J . L. J . Am. Chem. Soc. 1998, 120,
1289-1299. (c) Beljonne, D.; Cornil, J .; Silbey, R.; Millie´, P.; Bre´das,
J . L. J . Chem. Phys. 2000, 112, 4749-4758.
(9) (a) Williams, V. E.; Swager, T. M. Macromolecules 2000, 33,
4069-4073. (b) Yang, J .-S.; Swager, T. M. J . Am. Chem. Soc. 1998,
120, 11864-11873.
(10) Levitus, M.; Garcia-Garibay, M. A. J . Phys. Chem. A 2000, 104,
8632-8637.
(11) Levitus, M.; Schmieder, J .; Ricks, H.; Bunz, U. H. F.; Garcia-
Garibay, M. A. Manuscript in preparation.
(12) Levitus, M.; Schmieder, K.; Garcia-Garibay, M. A. Manuscript
in preparation.
(13) We were surprised to find out that the photophysics of 1,4-
diethynylbenzene have been only scarcely investigated, see: (a) Laposa,
J . D. J . Lumin. 1979, 20, 67-72. (b) Sazhnikov, V. A.; Razumov, V.
F.; Alfimov, M. V. Opt. Spektrosk. 1978, 44, 196-198. (c) Razumov, V.
F.; Sazhnikov, V. A.; Alfimov, M. V.; Kotlyarevskii, I. L.; Bardamova,
M. I.; Vasilevskii, S. F. Izv. Akad. Nauk SSSR, Ser. Khim. 1979, 2,
358-362.
(14) Pugh, C.; Percec, V. Polym. Bull. 1990, 23, 177-184.

3190 J . Org. Chem., Vol. 66, No. 9, 2001
Levitus et al.
J CF ) 37.2 Hz), 127.8 (d, J CF ) 14.4 Hz), 133.8 (d, J CF ) 7.6
Hz), 162.7 (d, J CF ) 1006.4 Hz).
UV-Vis a n d F lu or escen ce. Absorption spectra were
recorded in a Hewlett-Packard 8453 UV-vis or a Shimadzu
3101-PC UV-Vis-NIR spectrometer. A diffuse reflectance ac-
cessory was attached to the Shimadzu spectrometer for diffuse
reflectance measurements, which were carried out with prop-
erly characterized crystalline powdered samples using BaSO4
as a reference. Fluorescence spectra were recorded in a Spex-
Fluorolog II spectrometer and were corrected for nonlinear
detector response. Compounds 4 and 5 were purified by column
chromatography and by recrystallization until their fluores-
cence emission in solution was independent of the excitation
wavelength. Fluorescence decays were measured with a time-
correlated single photon counting fluorimeter (Edinburgh
Instruments, model FL900CDT) equipped with a pulsed H2
discharge lamp operating at 0.4 bar (fwhm ∼0.7 ns). Signal
intensities were attenuated to detect photons at a rate of 1%
of the excitation source repetition rate operating at 40 kHz.
The instrumental response recorded with scattered light from
a LUDOX suspension was used to deconvolute the short-lived
fluorescence signals from the excitation pulse intensity profile.
Measurements with solid samples and with amorphous glasses
at 77 K were carried out by exciting the sample with vertically
polarized light and setting the emission polarizer at the magic
angle to eliminate polarization effects.
F igu r e 1. Fluorescence excitation and emission of 1,4-
diethynyl-2-fluorobenzene 4 in dilute and concentrated solu-
tions in methylcyclohexane at 298 K (dotted lines) and at 77
K (filled lines). The absorption spectrum is marked with circles
and goes with the scale on the right.
X-r a y Str u ctu r e Deter m in a tion . Crystals of compound
4 (C16H17FO2) were grown by slow evaporation from mixtures
of benzene and hexanes. A pale yellow needle with ap-
proximate dimensions of 0.6 mm × 0.2 mm × 0.1 mm was used
for X-ray crystallographic analysis. The X-ray intensity data
were measured at 298 K on a Bruker SMART 1000 CCD-based
X-ray diffractometer system equipped with a Mo-target X-ray
tube (λ ) 0.71073 Å) operated at 2250 W power. The detector
was placed at a distance of 4.986 cm from the crystal. A total
of 1321 frames were collected with a scan width of 0.3° in ω,
with an exposure time of 30 s/frame. The frames were
integrated with the Bruker SAINT software package using a
narrow-frame integration algorithm. The integration of the
data using a triclinic unit cell yielded a total of 14183
reflections to a maximum θ angle of 28.31°, of which 9779 were
independent and 3440 (35%) were greater than 2σ(I). The final
unit cell contents of a ) 6.0599(10) Å, b ) 17.450(3) Å, c )
21.186(3) Å, R ) 101.422(3)°, â ) 95.053(3)°, γ ) 92.618(3)°,
and volume ) 2182.7(6) Å3 are based upon the refinement of
the xyz centroids of 870 reflections above 20σ(I). Analysis of
the data showed negligible decay during data collection. The
structure was refined on the basis of the space group P1h with
Z ) 6, using the Bruker SHELXTL (Version 5.3) software
package. The final anisotropic full-matrix least-squares refine-
ment on F2 converged at R1 ) 5.78% [I > 2σ(I)], wR2 ) 16.21%,
and a goodness of fit of 0.818. The largest peak on the final
difference map was 0.378 e/Å3. The calculated density for
of the parent 1,4-diethynylbenzene13 (Figure 1). In agree-
ment with the well-documented chemical instability of
1,4-diethynylbenzene,16 the terminal alkyne 5 was found
to decompose within a few hours at ambient temperature,
or after a few days when stored at -20 °C. The absorption
spectrum of the protected compound 4 in the UV region
consists of three overlapping bands centered at 270 (ꢀ )
4 × 104 M-1 cm-1), 290, and 300 nm (ꢀ ) 1.2 × 104 M-1
cm-1). The emission spectrum in methylcyclohexane
solution at ambient temperature extends from 300 to 400
nm with a λmax at ca. 312 nm. The emission quantum
yields determined for 4 and 5 were the same, with a value
of ΦF ) 0.15((0.02). Fluorescence decays in cyclohexane
solutions were determined by time-correlated single
photon counting and were fit to single exponentials which
gave lifetimes of 2.0 ((0.1) ns and 3.8 ((0.1) ns for
compounds 4 and 5, respectively. Given its higher stabil-
ity and interesting solid state structure (see below), most
detailed measurements were carried out with compound
4.
Absor p tion a n d Em ission Sp ectr a of Com p ou n d
4 in Con cen tr a ted Solu tion a t 298 a n d 77 K. In
search of experimental evidence for intermolecular ag-
gregation in the 1,4-diethynyl-2-fluorobenzene chro-
mophore, we carried out a series of measurements as a
function of concentration. We reasoned that compound
4 may take advantage of weak hydrogen bonding in
nonpolar solvents to form spectroscopically observable
cofacial dimers (Scheme 3).
C
16H17FO2 is 1.188 Mg/m3 and F(000) is 828e.
Solid Sta te 13C CP MAS NMR. Solid state 13C NMR
spectra were acquired at ambient temperature on a Bruker
Avance 300 NMR instrument operating at 75 MHz by cross-
polarization and magic angle spinning (CPMAS). Powdered
samples packed in 4 mm i.d. rotors were spun at 10 kHz.
Hydrogens were polarized with 4 µs 90° pulses and cross-
polarization was carried out with contact times of 4 ms.
Relaxation delays were set to 5 s. Quaternary signals were
assigned by using a 50 µs dipolar-dephasing delay immediately
after cross-polarization.15
As illustrated by the spectra shown with dotted lines
in Figure 1, measurements in methylcyclohexane at 298
K resulted in relatively small changes on either excitation
or emission signals with samples at concentrations
ranging from 7 × 10-2 to 1.7 × 10-6 M (Table 1). Although
the fluorescence lifetimes of concentrated samples at 298
K were slightly longer (τ ) 2.2 ( 0.1 ns) than those
obtained in dilute solutions (τ ) 2.0 ( 0.1 ns), no evidence
Resu lts a n d Discu ssion
Absor p tion a n d Em ission Sp ectr a in Dilu te Solu -
tion . We were able to show that the UV-vis and
fluorescence spectra of 4 and 5 in dilute cyclohexane
solutions at ambient temperature are essentially identi-
cal to each other and in agreement with published spectra
(16) (a) Gottfried, C.; Walter, R. Chem. Ber. 1975, 108, 528-537.
(b) Petrus, A. A. Khim. Vis. Energ. 1974, 8, 280-281; Chem. Abstr.
1974, 81, 78329.
(15) Opella, S. J .; Frey, M. H. J . Am. Chem. Soc. 1979, 101, 5854.

J . Org. Chem., Vol. 66, No. 9, 2001 3191
Sch em e 3
Ta ble 1. P h otop h ysica l P r op er ties of Com p ou n d 4
u n d er Va r iou s Exp er im en ta l Con d ition sa
solvent
concn [M]
temp (K)
λmax (nm)
τF (ns)b
MCH
MCH
MCH
MCH
crystal
crystal
1.7 × 106
7 × 10-2
1.7 × 106
7 × 10-2
300
300
77
77
300
300
310
310
310
332
330
430
2.0
2.2
3.0, 0.4c
F igu r e 3. Electronic absorption spectra of 1,4-diethynyl-2-
fluorobenzene 4 obtained under different experimental condi-
tions: (a) UV-vis absorption spectrum in hexane solution, (b)
diffuse reflectance spectrum in the crystalline solid state (given
in Kubelka-Monk units), and (c) fluorescence excitation in the
solid state (in arbitrary units).
8.5, 2.4c
d
d
a
All samples were thoroughly deoxygenated before measure-
ments. Estimated errors in lifetimes are within 0.1 ns. c Decays
were satisfactorily fit to double exponential functions. Decays
b
d
were too complex to fit to one or two exponentials.
F igu r e 4. Fluorescence excitation (dotted line) and emission
(filled line) of compound 4 in a polycrystalline sample. Bands
assigned to monomer and aggregate are labeled on the top.
F igu r e 2. Lamp profile (a) and double exponential fluores-
cence decay (b) of compound 4 in dilute (τ1 ) 3.0 and τ2 ) 0.4
ns) and (c) concentrated (τ1 ) 8.5 and τ2 ) 2.4 ns) methyl-
cyclohexane glasses at 77 K.
Diffu se Reflecta n ce a n d Em ission in Cr ysta ls of
4. Since formation of spectroscopically detectable ag-
gregates in the solid state is relatively common, we
carried out diffuse reflectance and fluorescence measure-
ments with crystals of compound 4. As illustrated in
Figure 3, we found with satisfaction that the diffuse
reflectance spectra from fine-powder samples revealed a
broad band between 320 and 400 nm that is significantly
red-shifted with respect to the lowest energy absorption
band that we had measured in dilute solutions.17 The
formation of a new chromophore in the solid state was
also evident from the fluorescence excitation and emis-
sion measurements shown in Figure 4. Two excitation
and emission band systems were clearly distinguished
by measurements carried out by front-face illumination
and detection. The position of one of the excitation and
emission band systems in the solid state resembles the
position of the excitation and emission spectra in dilute
solution, suggesting a monomeric species. The second
band system is more intense and red-shifted by ap-
proximately 60-80 nm (Figure 4), and it has an excita-
tion spectrum that is in excellent agreement with the red-
shifted absorption determined by diffuse reflectance
methods.
for aggregation was obtained from 1H NMR spectra
recorded in CDCl3 with samples prepared over a 100-
fold concentration range. The combined effects of high
sample concentration and low temperatures gave rise to
observable aggregation effects on the fluorescence spectra
(Figure 1, top spectra). While fluorescence measurements
with dilute samples at 77 K in MCH (methylcyclohexane)
showed only small changes as compared to those at 298
K, measurements carried out with concentrated samples
at the same low temperature resulted in severe broaden-
ing and a 30-40 nm red shift in the λmax of the emission
spectrum. We interpret these results as an indication of
a very weak cofacial association that gives rise to weakly
interacting aggregates. Although relatively small changes
were observed in the excitation spectra, lifetime mea-
surements for the aggregated and monomeric species at
77 K were significantly different (Table 1). Decays at 77
K were fit to double-exponential functions with lifetimes
of 3.0 and 0.4 ns for the monomer and 8.5 and 2.4 ns for
the aggregate (Figure 2). It may be noted that a small
increase in the lifetime of the monomer at low temper-
ature suggests that contributions of thermal decay at
ambient temperature should be relatively small.
(17) Selected spectra, X-ray diagrams, and X-ray tables can be found
in the Supporting Information.

3192 J . Org. Chem., Vol. 66, No. 9, 2001
Levitus et al.
Sch em e 4
of 6.06 Å (not shown). It is also worth noting that all the
hydroxyl groups in the structure of 4 are involved in a
complex three-dimensional H-bonding network that is
schematically illustrated in Scheme 4. The observed
H-bonding pattern can be described in terms of a hep-
tameric cyclic array that encloses molecule A2. The
enclosure is formed by molecules C-A1-B-C-A1-B,
and molecules A2 are held inside the “box” by hydrogen
bonding to the hydroxyl oxygen of molecules B located
at opposite corners of the box. As indicated in Figure 5
and Scheme 4, molecules C experience simultaneous
H-bond to B and A1 of adjacent “boxes” while molecules
B hydrogen bond to A1, A2, and C.
F igu r e 5. Packing motif of molecules of 1,4-bis(2-hydroxy-2-
methyl-3-butynyl)-2-fluorobenzene 4 illustrating its association
in offset trimers (A), dimers (B), and monomers (C). Fluorine
atoms are evenly disordered through the four aromatic groups.
See text for more detailed description.
X-r a y Cr ysta l Str u ctu r e. The packing structure of 4
is remarkably complex and structurally rich and accounts
satisfactorily for the photophysical observations illus-
trated in Figures 3 and 4. Slow evaporation of hexanes-
benzene solutions of 4 yielded triclinic crystals with space
group P1h having the unusual number of six molecules
per unit cell.17 Even more remarkable is the fact that
there are four structurally different molecules in the
crystal. Two molecules sit in general crystallographic
positions and two centrosymmetric molecules sit at
crystallographic inversion centers. This arrangement
gives a total of three independent molecules per unit cell
(i.e., two full molecules plus two halves). From the four
structurally different molecules in the crystal, two are
essentially planar and the other two have alkyne groups
bent away from the plane of their phenyl rings. The
fluorine substituents are disordered through the four
positions of the phenyl rings, creating a space-disordered
packing structure. The structure was refined by a full-
matrix least-squares procedures on F2, giving R1 ) 0.0578
for I > 2σI and wR2 ) 0.1621 for all the diffraction data.
The packing structure of 4 is shown in Figure 5 from
a view that illustrates layers of molecules on the (100)
plane. Molecules labeled A, B, and C have their long
molecular axes roughly parallel to the (100) plane and
the planes of their phenyl groups tilted from the normal.
At the center of the cluster shown is a sandwich-like
trimeric aggregate with molecules labeled A1-A2-A1
(Figure 5). The long molecular axis and the plane of
molecule A2 are offset from the axes, and the planes of
the two molecules A1 on the outside of the trimer.
Although the center-to-center distance between the middle
molecule and either of the two outside molecules is 4.58
Å, close contacts between alkyne and aryl carbons are
as small as 3.087 Å. The central trimer in Figure 5 is
flanked on the sides by two dimers of bent molecules
having cofacial phenyl rings with intermolecular contacts
as short as 3.51 Å and aromatic center-to-center distances
of 4.38 Å. These are labeled B in Figure 5.
Solid Sta te 13C CP MAS NMR. The solid state 13C
NMR of compound 4 was obtained with cross-polarization
and magic angle spinning (CPMAS) to document the
strain and intermolecular interactions that are present
in the crystal. The isotropic 13C NMR solution spectrum
of 4 consists of 14 signals spanning from 30 to 162 ppm,
with 13C-19F scalar coupling constants for aromatic and
alkyne carbons ranging between 7.5 and 1001 Hz.
Although three nonequivalent molecules in the unit cell
of 4 may be expected to give as many as 42 distinguish-
able signals, only 12 groups of broad or partially resolved
signals were observable in the solid state spectrum
(Figure 7). An easily distinguishable 13C-19F scalar
coupling in the solid state was observed only for the ipso
C-F carbon at 162 ppm. Alkyne signals near 78 and 90
ppm and aromatic signals between 110 and 165 ppm are
heterogeneously broadened by the relative large number
of nonequivalent nuclei from crystallographically differ-
ent molecules and by residual scalar coupling (see inset
in Figure 7). It is interesting that the dispersion in
chemical shifts falls within ca. 5-6 ppm (∼450 Hz) for
most carbon signals. This is a relatively narrow range
given the presence of molecules with severe distortions
from planarity and molecules with a variety of hydrogen
bonding and aromatic stacking interactions.
Solid Sta te P h otop h ysics. It is well-known that
aggregation may cause large spectral shifts and band
shape changes. Several investigators have correlated
X-ray structural data with photophysical properties in
the crystalline solid state to document the nature of inter-
chromophore interactions.18 Qualitatively, molecular ex-
citon theory predicts that spectral shifts will depend on
the electrostatic interaction between the transition dipole
moments of neighboring molecules.19,20 These interactions
(18) (a) Lewis, F. D.; Yang, J .-S.; Stern, C. L. J . Am. Chem. Soc.
1996, 118, 2772-2773. (b) Lewis, F. D.; Yang, J .-S. J . Phys. Chem. B
1997, 101, 1775-1781. (c) Singh, A. K.; Krishna, T. S. R. J . Phys. Chem.
A 1997, 101, 3066-3069. (d) Brinkman, M.; Gadret, G.; Muccini, M.;
Taliani, C.; Masciochi, N.; Sironi, A. J . Am. Chem. Soc. 2000, 122,
5147-5157.
(19) (a) McRae, E. G.; Kasha, M. J . Chem. Phys. 1958, 28, 721-
722. (b) Hochstrasser, R. M.; Kasha, M. Photochem. Photobiol. 1964,
3, 317-331.
(20) (a) Siddiqui, S.; Spano, F. C. Chem. Phys. Lett. 1999, 308, 99-
105. (b) Burshtein, K. Y.; Bagatu’yants, A. A.; Alfimov, M. V. Chem.
Phys. Lett. 1995, 239, 195-200.
Edge-on views illustrating the bending, cofacial inter-
action, and relative displacement in molecules A and B
are shown in Figure 6. Also shown is an edge-on view of
a molecule C, which has no close cofacial contacts.
Molecules A, B, and C translate down the a-axis in a
staircase-like arrangement with intermolecular distances

J . Org. Chem., Vol. 66, No. 9, 2001 3193
F igu r e 6. Ortep diagrams of molecules 4 illustrating their interaction in trimeric (left), dimeric (center), and monomeric
arrangements (right). All molecules translate along similar staircase-like chains.
F igu r e 7. 13C CPMAS NMR spectrum of compound 4.
Protonated aromatic signals identified by dipolar decoupling
F igu r e 8. Scatter plot of the three lowest energy transitions
are indicated with an asterisk. Signals corresponding to alkyne
and oscilator strength in circles, squares, and triangles,
carbons attached to the phenyl group shown in expansion.
respectively, calculated with the ZINDO/S method for mol-
ecules A1, A2, and B with fluorine atoms in different positions.
are strongly dependent on aggregate size and geometry,
leading to blue-shifts (H-aggregates), red-shifts (J -ag-
method using the 201 lowest, singly excited configura-
gregates), and/or spectral broadening, depending on
which of the aggregate transitions are symmetry allowed.
It has been shown that close cofacial interactions are
among the strongest.7,21 In the case of compound 4,
discrete aggregates and monomers were found by single-
crystal X-ray diffraction analysis. In analogy with other
examples reported in the literature, the red-shifted
absorption (300-380 nm) and emission (360-550 nm) is
probably determined by the strongly interacting dimeric
(B-B) and trimeric aggregates (A1-A2-A1) and the
blue-shifted bands by the weakly interacting monomeric
species (C).18 Several studies suggest that red shifts
require coplanar molecules at distances of 4 Å or less,
which must be displaced from a perfect parallel align-
ment.7,19,20 An additional aspect to consider in the case
of compound 4 comes from the distortion of molecules
constituting the dimer (B) and the central molecule in
the trimer (A2) and from the positional disorder of the
fluorine atom.22 As shown in Figure 8, results from
semiempirical spectroscopic calculations suggest that
these factors may cause only small changes to the
electronic spectra of isolated molecules. Single-point
ground state energies were calculated with the AM1
method using the X-ray coordinates of molecules A, B,
and C as defined in Figure 3, and their electronic
transition energies were calculated with the ZINDO/S
tions to compute the CI matrix.23 Shown in Figure 8 with
triangles, squares, and circles are the three lowest energy
transitions and oscillator strength calculated for isolated
molecules having the fluorine atom changed over the four
nonequivalent crystallographic positions. The results in
Figure 8 suggest that molecular strain and fluorine
disorder alone may account for some of the spectral
dispersion, but there seems to be no indication of molec-
ular structural factors that may be responsible for the
observed red shift.
In tr a cr ysta llin e En er gy Tr a n sfer . Dual emission
coming from different conformers or different environ-
ments in pure and mixed crystalline solids is not rare.24
Selective excitation of surface sites with wavelengths that
have low transmission and preferential excitation of bulk
molecules with wavelengths that are near the red-edge
of the absorption spectrum can also lead to different
spectra from the surface and from the bulk.18a,b Dual
emission from bulk sites requires incomplete energy
transfer and equilibration. For that reason, it is more
likely to occur with short-lived species with high radiative
rates and/or low rates of energy transfer. As expected for
(23) Hypercube, Inc., Gainesville, FL.
(24) (a) Lecuiller, R.; Berrehar, J .; Lapersonne-Meyer, C.; Schott,
M. Phys. Rev. Lett. 1998, 80, 4068-4071. (b) Brejc, K.; Sixma, T. K.;
Kitts, P. A.; Kain, S. R.; Tsien, R. Y.; Ormo, M.; Remington, S. J . Proc.
Natl. Acad. Sci. U.S.A. 1997, 94, 2306-2311. (c) Vanden Bout, D. A.;
Kerimo, J .; Higgins, D. A.; Barbara, P. F. C. D. J . Phys. Chem. 1996,
100, 11843-11849. (d) Yasuda, M.; Kuwamura, G.; Nakazono, T.;
Shima, K.; Inoue, Y.; Yamasaki, N.; Tai, A. Bull. Chem. Soc. J pn. 1994,
67, 505-510. (e) Masahide, Y.; Goro, K.; Takumi, N.; Kensuke, S.;
Yoshihisa, I.; Noritsugu, Y.; Akira, T. Bull. Chem. Soc. J pn. 1994, 67,
505-510.
(21) (a) Wang, S.; Bazan, G. C.; Tretiak, S.; Mukamel, S. J . Am.
Chem. Soc. 2000, 122, 1289-1297.
(22) (a) Yamasaki, N.; Inoue, Y.; Yokoyama, T.; Tai, A.; Ishida, A.;
Takamuku, S. J . Am. Chem. Soc. 1991, 113, 1933-1941. (b) Masahide,
Y.; Goro, K.; Takumi, N.; Kensuke, S.; Yoshihisa, I.; Noritsugu, Y.;
Akira, T. Bull. Chem. Soc. J pn. 1994, 67, 505-510.

3194 J . Org. Chem., Vol. 66, No. 9, 2001
Levitus et al.
F igu r e 9. Time dependence of the emission of crystalline 4
under the following conditions: (1) detection of the monomer
at 320 nm upon excitation at 300 nm; (2) detection of the
aggregate at 430 nm upon direct excitation at 360 nm; (3)
detection of the aggregate at 430 nm upon excitation of the
monomer.
F igu r e 10. Phosphorescence spectrum of 4 in methylcyclo-
hexane at 77 K and phosphorescence decay in a 20 s window
(inset)
gap of 35 kcal/mol calculated from spectra for 4 and 5
compares well with a gap of 32.4 kcal/mol reported for
diphenylacetylene. The most significant difference be-
tween compound 4, tolan, and other phenylacetylenes
came from the high triplet yields of the former. Whereas
tolan has a triplet yield of only ΦT ) 0.03, we determined
a lower limit of ΦT ) 0.64 for compound 4 in solution at
ambient temperature. The lower limit for the triplet yield
of 4 was estimated by measuring the photosensitized
generation of singlet oxygen26 with concentrations that
were low enough to ensure that only the triplet state
could act as a sensitizer. The high triplet yield and
intense emission of compound 4 are significantly higher
than those of other arylacetylenes.27 This property seems
to originate from the presence of the fluorine substituent.
It is known that a single fluorine increases the triplet
yield of benzene from ΦT ) 0.25 up to ΦT ) 0.8 in
fluorobenzene.28 It may be noted that the high triplet
yields and relatively strong triplet emission for com-
pounds 4 and 5 bode well for electroluminescence and
magnetooptic applications of fluoro-substituted aryl-
ethynes and poly(phenylene-ethynylenes).
a sample with absorbers and emitters having different
excitation energies and slow equilibration rates, the
relative intensities of monomer and excimer emission in
the spectrum of 4 (Figure 4) are dependent on the
excitation wavelength. We also noticed that fluorescence
decay is too complex to be described by simple exponen-
tial functions. However, the wavelength dependence of
the decay conforms well to the expected energy transfer
between species that may undergo exothermic energy
transfer. Thus, the time dependence of the aggregate
emission detected at 430 nm depends on whether it is
excited directly or indirectly by energy transfer from the
monomer (Figure 9). Direct excitation of the aggregate
at 360 nm gave a time profile with a growth that is
parallel to that of the lamp pulse followed by its complex
decay (Figure 9, signal 2). In contrast, preferential
excitation of the monomer at 300 nm resulted in a ca.
0.3-0.5 delayed growth of the aggregate emission (Figure
9, signal 3). Also shown in Figure 9 is the time depen-
dence of the “quenched” monomer emission excited at 300
nm, which grows with the excitation pulse and decays
by fast emission and by energy transfer to the aggregate.
Although the lifetimes of the aggregate and monomer
emission were too complex to be fitted by two or three
exponentials, their decays are consistent with the pos-
tulated energy transfer processes.
P h osp h or escen ce Em ission a n d Tr ip let Yield .
Measurements carried out at 77 K in dilute methyl-
cyclohexane glasses revealed a remarkably intense, long-
lived, and highly structured phosphorescence between
450 and 600 nm (Figure 10). Phosphorescence decays
were fit to a single-exponential function over most of their
decays to give lifetimes of 1.4 and 1.0 s for 4 (Figure 10,
inset) and 5, respectively. Measurements carried out at
high concentration, under conditions where only ag-
gregate fluorescence was observed, yielded no phospho-
rescence signal. Phosphorescence emission in the crys-
talline solid was significantly weaker, less resolved, and
red-shifted by about 25 nm with respect to the one
observed in solution.17 The triplet state properties of 4
and 5 are similar to those reported for dilute solutions
of diphenylacetylene (tolan) in terms of triplet energies
and S1-T1 energy gap. The onset of the phosphorescence
emission of compounds 4 and 5 corresponds to triplet
energies of 63.5 kcal/mol, as compared to 63.4 kcal/mol
that was reported previously for tolan.25 A S1-T1 energy
Con clu sion s
In this paper, we have analyzed the solution and solid
state photophysics of 1,4-diethynyl-2-fluorobenzene 5 and
1,4-bis(2-hydroxy-2-methyl-3-butynyl)-2-fluorobenzene 4.
The singlet state of compound 4 in dilute solution was
characterized by a relatively strong fluorescence emission
with a λmax ) 312, a quantum yield of ΦF ) 0.15, a
lifetime of τF ) 2.0 ns, and a remarkably high intersystem
crossing quantum yield of ΦISC > 0.64. Dilute samples
in methylcyclohexane at 77 K gave excitation and emis-
sion spectra that were similar to those observed at
ambient temperatures and a fluorescence lifetime of 3.1
ns. Concentrated solutions in the same solvent at 77 K
gave rise to aggregate emission with a λmax ) 340 and a
lifetime of 8.8 ns. Consistent with relatively high triplet
(25) (a) Evans, D. F. J . Chem. Soc. 1957, 1351-1357. (b) Beer, M.
J . Chem. Phys. 1956, 25, 745-750.
(26) Foote, C. S.; Clennan, E. L. In Active Oxygen in Chemistry;
Foote, C. S., Valentine, J . S., Greenberg, A., Liebman, J . F., Eds.;
Blackie Academic and Professional: New York, 1995.
(27) (a) Walters, K. A.; Ley, K. D.; Schanze, K. S. Chem. Commun.
1998, 115-116.
(28) Sandros, K. Acta Chem. Scand. 1969, 23, 2815-2829.

J . Org. Chem., Vol. 66, No. 9, 2001 3195
yields estimated by singlet oxygen sensitization was a
strong, well resolved, and long-lived phosphorescence
which was observed only for the monomer in dilute
methylcyclohexane solutions cooled to 77 K. The photo-
physics of concentrated solutions at low temperatures
were qualitatively consistent with results obtained in the
solid state. Using diffuse reflectance and fluorescence
measurements in the solid state, we documented the
effects of aggregation on the photophysics of 1,4-diethyn-
ylbenzene chromophore for the first time. Spectral bands
associated with aggregates were characterized by red
shifts of 60-80 nm and by the loss of vibrational
structure in both absorption and emission. Along with
aggregate emission, a weak fluorescence band assigned
to the monomer was also observed in the solid state.
Single-crystal X-ray analysis revealed a remarkably rich
packing structure with four distinct molecules per asym-
metric unit with the fluorine substituent disordered over
all the crystallographically nonequivalent aromatic posi-
tions. The positional disorder is manifested in terms of
multiple overlapping signals in the solid state CPMAS
13C NMR spectra. Among the packing interactions that
may be responsible for the aggregate emission, close
cofacial contacts were observed in discrete dimeric and
trimeric units. We speculated that monomer emission
may originate from molecules with no close cofacial
contacts. We were able to document intracrystalline
singlet energy transfer from the excited state monomers
to the lower excited energy aggregates. We hope that
results reported here will prove useful in distinguishing
the nature and magnitudes of aggregation and planariza-
tion on the photophysics of compounds with alkyne-
conjugated aromatic luminophores. Work in progress
with 1,4-bis(arylethynyl)arenes and oligo(phenyleneethy-
nylenes) is consistent with results presented here.
Ack n ow led gm en t. This work was supported by the
NSF (Grants DMR9988439 and CHE0073431). Solid
state NMR spectra were acquired with equipment
purchased with NSF Grant DMR9975975. We thank
Prof. Uwe Bunz from the University of South Carolina
and Prof. Chris Foote from UCLA for advice. One of us
(G.Z.) would like to thank COFAA-IPN (Cre´dito Edu-
cativo No.9911220454), CONACyT (Ref. 990205), and
the US-Mexico Commission for Educational and Cul-
tural Exchange (Fulbright-Garc´ıa Robles fellowship).
Su p p or tin g In for m a tion Ava ila ble: Phosphorescence
spectra, phosphorescence decay, stereoscopic views, and X-ray
tables of 3. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O015589E