Structure, photophysics, and photooxidation of crowded diethynyltetracenes

Article abstract of DOI:10.1039/c2jm16173g

This paper describes a previously unreported class of sterically crowded tetracene derivatives that have both phenyl and ethynyl substituents. The steric crowding above and below the tetracene core prevents overlap between the extended π-systems of the acenes. Substituent effects cause these tetra-substituted tetracenes to have absorbance and fluorescence spectra red shifted from either disubstituted derivatives or rubrenes, such that they have spectra similar to diarylpentacenes, but with higher quantum yields of fluorescence and greater photostability. These new molecules also undergo cycloaddition reactions with ¹O₂, giving regioisomeric mixtures of endoperoxides, and in contrast to longer acenes, the ethynyl substituents show only a modest stabilizing effect to photooxidation. Ethynylated tetracenes also exhibited photochromism, with their endoperoxides undergoing cycloreversion to yield the acene starting material at room temperature in the dark. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2jm16173g

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PAPER
Structure, photophysics, and photooxidation of crowded diethynyltetracenes†
a
Jingjing Zhang, Syena Sarrafpour, Terry E. Haas, Peter Muller and Samuel W. Thomas, III
a
a
b
a
*
€
Received 26th November 2011, Accepted 9th February 2012
DOI: 10.1039/c2jm16173g
This paper describes a previously unreported class of sterically crowded tetracene derivatives that have
both phenyl and ethynyl substituents. The steric crowding above and below the tetracene core prevents
overlap between the extended p-systems of the acenes. Substituent effects cause these tetra-substituted
tetracenes to have absorbance and fluorescence spectra red shifted from either disubstituted derivatives
or rubrenes, such that they have spectra similar to diarylpentacenes, but with higher quantum yields of
fluorescence and greater photostability. These new molecules also undergo cycloaddition reactions with
1O2, giving regioisomeric mixtures of endoperoxides, and in contrast to longer acenes, the ethynyl
substituents show only a modest stabilizing effect to photooxidation. Ethynylated tetracenes also
exhibited photochromism, with their endoperoxides undergoing cycloreversion to yield the acene
starting material at room temperature in the dark.
halogens,9,14 thioethers,15–17 sterically demanding arenes,9 and
heteroaromatics;18 ethynyl substitution, however, such as in
6,13-bis(triisopropylsilylethynyl)pentacene (TIPS pentacene) is
among the most popular.19–23 Silylethynyl groups have been
critical in stabilizing large acenes, such as hexacene,24,25 hepta-
cene,23,24 and nonacene26 that are otherwise too reactive to
isolate and characterize. Work on silylethynyl hexacenes has
shown that for these longer acenes, dimerization, and not
photooxidation, appears to be the primary mode of decompo-
sition.25 In addition, heptacenes that bear both silylethynyl and
phenyl substituents are reported to be the most stable such
derivatives.23,27
Introduction
This paper describes the synthesis, optical properties, and
cycloaddition reactivity of a previously unreported class of
substituted tetracenes: 5,12-diethynyl-6,11-diphenyltetracenes.
Linear acenes of three or more rings have important roles in
a number of applications. Acenes are among the materials that
have the best performance in organic electronics:1 thin films of
pentacene and single crystals of rubrene show efficient charge
transport properties. Rubrene and other acenes are common
luminescent materials in light-emitting devices2 and in chemilu-
minescent glowsticks. In addition to their useful properties in
devices, depending on their substitution, acenes can undergo
facile cycloaddition reactions with dieneophiles.3–6 Cycloaddition
reactions with singlet oxygen (1O2)7,8 have been identified as
a mechanism of acene degradation,9 and also play a key role in
Ethynyl substitution red-shifts absorbance and luminescence
spectra of acenes because of increased p-conjugation, and also
improves solubility, which enables solution processing to replace
vacuum deposition for constructing devices. Our group is inter-
ested in strategies for tuning both emission color and cycload-
schemes for sensing O2 both from our group10 and others.11,12
1
Acene-based cycloaddition reactions with other dieneophiles are
also useful for constructing structural scaffolds such as
iptycenes.13
1
dition reactivity of acenes for applications in O2 detection or
photo-tunable solid-state emitters. The goal of this study was to
develop soluble, red-emitting, 1O2-reactive acenes that are
significantly more stable than 6,13-diarylpentacenes, which are
too reactive for reliable handling of thin films under typical
laboratory conditions. Tetracenes are an interesting class of
molecules for this purpose because they have reactivities and
optical properties that are intermediate relative to similarly
substituted anthracene and pentacene derivatives. A number of
classes of substituted tetracenes have been reported, such as 5,12-
diethynyltetracenes,28,29 tetraalkyltetracenes,30–32 dialkoxyte-
tracenes,33 and substituted rubrene derivatives.34,35 Herein we
report the preparation, photophysical properties, and reactivity
of what we believe to be a new class of sterically crowded
substituted tetracenes.
Primary pathways of acene decomposition include endoper-
oxide formation with singlet oxygen and photodimerization.
There have been a variety of substitution strategies to increase
the stability of acene derivatives, including substitution with
aDepartment of Chemistry, Tufts University, 62 Talbot Avenue, Medford,
MA, 02155, USA. E-mail: sam.thomas@tufts.edu; Fax: +1-617-627-3443;
Tel: +1-617-627-3771
bDepartment of Chemistry, Massachusetts Institute of Technology, 77
Massachusetts Ave, Cambridge, MA, 02139, USA
† Electronic supplementary information (ESI) available: kinetic data,
crystallographic information, and solid-state emission. CCDC reference
numbers 853371 and 853372. For ESI and crystallographic data in CIF
or other electronic format see DOI: 10.1039/c2jm16173g
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128.6, 128.2, 127.9, 127.7, 127.2, 126.9, 125.6, 119.2, 116.6, 113.7,
109.0, 88.7, 55.5. HRMS calcd for C48H32O2 (M + H)+, 641.2475;
found, 641.2463.
Experimental section
General considerations
3T (0.16 g, 16%) 1H NMR (500 MHz, CDCl3): d 8.56–8.54 (m,
2H), 7.60–7.57 (m, 10H), 7.56–7.47 (m, 6H), 7.40–7.39 (d, 4H),
7.33–7.31 (m, 4H). 13C NMR (125 MHz, CDCl3): d 141.2, 137.9,
134.3, 132.8, 131.8, 131.4, 128.5, 128.4, 127.5, 127.2, 126.1, 125.1,
118.9, 107.3, 91.8. HRMS calcd for C48H26F6 (M + H)+,
717.2011; found, 717.2028.
The following chemicals were purchased from commercial
sources and used as received: 1,3-diphenylisobenzofuran (Acros),
1,4-naphthoquinone
(Aldrich),
5,12-naphthacenequinone
(Acros), 6,13-pentacenedione (TCI), phenylacetylene (Aldrich),
4-methoxyphenylacetylene (Alfa Aesar), Sn(II) chloride dihy-
drate (Alfa Aesar) and poly(9,9-di-n-dodecylfluorenyl-2,7-diyl)
(Aldrich). Dry tetrahydrofuran (THF) was obtained using
a column purification system.
1
4T (0.18 g, 17%) H NMR (500 MHz, CDCl3): d 8.66–8.65
(m, 2H), 7.64–7.62 (m, 2H), 7.51–7.48 (m, 10H), 7.45–7.43
(m, 2H), 7.27–7.24 (m, 2H), 1.07 (s, 42H). 13C NMR (125 MHz,
CDCl3): d 141.0, 137.5, 135.0, 133.2, 130.9, 128.6, 128.1, 128.0,
127.0, 126.7, 125.6, 119.4, 112.1, 105.3, 19.1, 12.2. HRMS calcd
for C52H60Si2 (M + H)+, 741.4306; found, 741.4301.
All synthetic manipulations were performed under standard
air-free conditions under an atmosphere of argon gas with
magnetic stirring unless otherwise mentioned. Flash chroma-
tography was performed using silica gel (230–400 mesh) as the
stationary phase. NMR spectra were acquired on a Bruker
Avance III 500 or Bruker DPX-300 spectrometer. Chemical
shifts are reported relative to residual protonated solvent (7.27
ppm for CDCl3). High-resolution mass spectra (HRMS) were
obtained at the MIT Department of Chemistry Instrumentation
Facility using a peak-matching protocol to determine the mass
and error range of the molecular ion.
5,12-Diethynyltetracene derivatives (1D–4D). 1D–4D were
prepared according to the general synthesis procedure: 0.40 g (1.5
mmol) 5,12-naphthacenequinone, 3.0 equivalents of ethynyl
derivative, 2.8 ml (4.5 mmol) n-butyllithium in hexanes (1.6 M),
20 mL dry THF and 12 mL 10% HCl aqueous solution saturated
with SnCl2 dihydrate yielded the products after chromatography
and recrystallization as described above.
1D (0.32 g, 48%) 1H NMR (500 MHz, CDCl3): d 9.29 (s, 2H),
8.69–8.67 (m, 2H), 8.12–8.09 (m, 2H), 7.85–7.82 (m, 4H), 7.58–
7.55 (m, 2H), 7.49–7.46 (m, 8H).
2D (0.41 g, 54%) 1H NMR (500 MHz, CDCl3): d 9.28 (s, 2H),
8.70–8.69 (m, 2H), 8.10–8.08 (m, 2H), 7.80–7.78 (m, 4H), 7.57–
7.55 (m, 2H), 7.49–7.44 (m, 2H), 7.04–7.02 (m, 4H), 3.92 (s, 6H).
13C NMR (125 MHz, CDCl3): d 160.2, 133.4, 132.3, 132.2, 130.1,
128.8, 127.6, 126.6, 126.2, 126.0, 118.5, 116.0, 114.5, 103.5, 86.2,
55.6. HRMS calcd for C36H24O2 (M + H)+, 489.1849; found,
489.1853.
3D (0.45 g, 52%) 1H NMR (500 MHz, CDCl3): d 9.25 (s, 2H),
8.66–8.64 (m, 2H), 8.13–8.11 (m, 2H), 7.95–7.94 (m, 4H), 7.77–
7.75 (m, 4H), 7.62–7.60 (m, 2H), 7.53–7.51 (m, 2H). 13C NMR
(125 MHz, CDCl3): d 132.7, 132.5, 132.1, 130.1, 128.7, 127.4,
127.2, 126.6, 126.2, 125.8, 125.2, 123.1, 122.8, 118.4, 102.1, 89.5.
HRMS calcd for C36H18F6 (M + H)+, 565.1385; found, 565.1396.
4D28 (0.39 g, 43%) 1H NMR (500 MHz, CDCl3): d 9.31 (s, 2H),
8.64–8.62 (m, 2H), 8.03–8.01 (m, 2H), 7.56–7.54 (m, 2H), 7.48–
7.46 (m, 2H), 1.33 (s, 42H).
Synthesis
General procedure for the synthesis of acene derivatives. n-
Butyllithium (1.6 M) in hexanes was added to a solution of
acetylene derivative in dry THF dropwise at ꢀ78 ꢁC and stirred
vigorously at room temperature for 20 min. The reaction
mixture was then cooled to ꢀ78 ꢁC and solid quinone was
slowly added. The solution was then allowed to warm up to
room temperature and stirred for 75 min before treatment with
10% HCl aqueous solution saturated with Sn(II) chloride
dihydrate and extracted with dichloromethane. The crude
product was purified via flash chromatography on silica gel
using CHCl3 and recrystallized from methanol and chloroform.
6,11-diphenyltetracene-5,12-dione was prepared by condensa-
tion between 1,3-diphenylisobenzofuran and 1,4-naph-
thoquinone. 5,12-bis(4-methoxyphenyl)tetracene was prepared
as previously reported.10
5,12-Diethynyl-6,11-diphenyltetracene derivatives (1T–4T).
1T–4T were prepared according to the general synthesis proce-
dure: 0.6 g (1.45 mmol) 6, 11-diphenyltetracene-5,12-dione, 2.5
equivalents of ethynyl derivative, 2.2 mL (3.51 mmol) n-butyl-
lithium in hexanes (1.6 M), 30 mL dry THF and 15 mL 10% HCl
aqueous solution saturated with SnCl2 dihydrate yielded the
products after chromatography and recrystallization as
described above.
1T (0.26 g, 31%) 1H NMR (500 MHz, CDCl3): d 8.58–8.57 (m,
2H), 7.56–7.54 (d, 6H), 7.46–7.43 (t, 6H), 7.30–7.27 (m, 10H),
7.23 (s, 5H). 13C NMR (125 MHz, CDCl3): d 141.3, 137.9, 134.3,
132.8, 131.7, 131.2, 128.6, 128.3, 128.1, 128.1, 128.0, 127.7, 127.2,
127.1, 125.8, 124.3, 119.1, 108.9, 89.7. HRMS calcd for C46H28
(M + H)+, 581.2264; found, 581.2280.
6,13-Diethynylpentacenes (1P–4P). 1P–4P were prepared
according to the general synthesis procedure: 0.40 g (1.3 mmol)
6,13-pentacenedione, 3.0 equivalents of ethynyl derivative, 2.4 ml
(3.8 mmol) n-butyllithium in hexanes (1.6 M), 18 mL dry THF
and 10 mL 10% HCl aqueous solution saturated with SnCl2
dihydrate yielded the products after chromatography and
recrystallization as described above.
1P36 (0.33 g, 53%) 1H NMR (500 MHz, CDCl3): d 9.31 (s, 4H),
8.07–8.05 (m, 4H), 7.93–7.91 (m, 4H), 7.55–7.52 (m, 4H), 7.50–
7.48 (m, 2H), 7.44–7.42 (m, 4H).
2P (0.32 g, 43%) 1H NMR (500 MHz, CDCl3): d 9.27 (s, 4H),
8.05–8.02 (m, 4H), 7.85–7.82 (m, 4H), 7.41–7.38 (m, 4H), 7.06–
7.03 (m, 4H), 3.91 (s, 6H). 13C NMR (125 MHz, CDCl3) d 160.3,
133.4, 132.3, 130.37, 128.8, 126.3, 126.0, 118.2, 116.0, 114.5,
104.7, 87.0, 55.6. HRMS calcd for C40H26O2 M+, 538.1927;
found, 538.1924.
1
2T (0.33 g, 35%) H NMR (500 MHz, CDCl3): d 8.57–8.56
(m, 2H), 7.55–7.54 (d, 6H), 7.50–7.43 (m, 6H), 7.35–7.31 (m, 6H),
7.22–7.20 (m, 6H), 6.82–6.80 (d, 4H), 3.82 (s, 6H). 13C NMR (125
MHz, CDCl3): d 159.6, 141.5, 137.9, 134.3, 133.2, 132.8, 131.2,
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3P37 (0.35 g, 44%) 1H NMR (500 MHz, CDCl3) d 9.29 (s, 4H),
8.10–8.08 (m, 4H), 8.03–8.02 (m, 4H), 7.81–7.80 (m, 4H), 7.48–
7.46 (m, 4H).
Results and discussion
Synthesis of acenes
4P38 (0.33 g, 40%) 1H NMR (500 MHz, CDCl3): d 9.31 (s, 4H),
Fig. 1 shows the four derivatives of the diethynyldiphenylte-
tracene core (tetra-substituted tetracenes, compounds with suffix
‘‘T’’) prepared, each with different terminal groups bound to the
alkyne substituents. Cycloaddition between 1,3-diphenyliso-
benzofuran and napthoquinone yielded the common diphenyl-
tetracenequinone intermediate.34,35 Addition of appropriately
substituted acetylide anions, prepared by deprotonation of the
terminal acetylene with n-BuLi, followed by reduction of the
resulting diols with SnCl2 gave the target acenes 1–4T. Similar
strategies, using commercially available tetracenequinone or
pentacenequinone, gave analogously substituted 5,12-dieth-
ynyltetracenes (1D–4D) and 6,13-diethynylpentacenes (1P–4P).
Yields of the tetrasubstituted compounds were 15–35%, whereas
those of the disubstituted compounds were somewhat higher. We
ascribe the lower yield of the T-series compounds to steric
hindrance presented by the terphenyl moiety on the quinone.
7.99–7.97 (m, 4H), 7.43–7.41 (m, 4H), 1.38 (s, 42H).
Optical experiments
Electronic absorbance spectra were acquired with a Varian Cary-
100 instrument in double-beam mode using a solvent-containing
cuvette for background subtraction spectra. Irradiation of the
1
methylene blue photosensitizer to generate O2 was performed
with a 200W Hg/Xe lamp (Newport-Oriel) equipped with
a condensing lens, recirculating water filter, manual shutter,
a 665 nm high-pass filter and a focusing plano-convex lens (focal
length 15 cm) in the light path.
Kinetics
A stock solution of methylene blue was prepared in CHCl3 to
give an absorbance of 1.0 at its peak. Ethynyl derivatives (1T–4T,
1D–4D, 4P), 5,12-bis(4-methoxyphenyl) tetracene 5, or 6,13-bis
(4-methoxyphenyl) pentacene 6 was dissolved in CHCl3 at
a concentration of 0.005 M. 5,12-Bis(4-methoxyphenyl)tetracene
5 was used as reference for 1T–4T, 1D–4D, and 6,13-bis(4-
methoxyphenyl) pentacene 6 was used as the reference for 4P.
When the 0.005 M solution was diluted 100-fold with the
methylene blue stock solution, the final concentration of the
acene in the sample was 0.05 mM. With the sample placed 15 cm
from the light source, the solution was irradiated using a 665 nm
high-pass filter and convex lens in increments of 15 s for 120 s
(1T–4T) or increments of 45 s for 360 s (1D–4D, 4P); the
absorbance spectrum was measured after each interval of
irradiation.
X-ray crystal structures
Single crystal X-ray structures of compounds 2T and 4T high-
light the influence of the substituents on the structures of this new
class of substituted acenes (Fig. 2 and 3). Both of these sterically
crowded molecules relieve intramolecular congestion through
structural distortions. In addition to all the phenyl substituents
on the terphenyl moieties being rotated away from a conforma-
tion coplanar with the tetracene backbone, the substituents are
bent away from each other, which has been observed in other
tetra-substituted tetracenes.28,35 The arylethynyl groups of 2T are
rotated out of plane by 73ꢁ and 66ꢁ in opposite directions, in
contrast to less congested 5,12-diarylethynyltetracenes, in which
the aromatic groups bound to the sp-hybridized carbons are
coplanar with the acene backbone.29 The bulkier triisopropylsilyl
groups of 4T force even greater distortions, with the tetracene
backbone of 4T having a significant twist of 31ꢁ (Fig. 3). This
type of twisting is known to occur in single crystals of sterically
congested acenes.42 We suspect that in solution the acene back-
bone of 2T may also be twisted, but that crystal packing forces
planarize the tetracene in the single crystal, as is known for
rubrene.43 4T crystallizes in the chiral space group P212121 with
Synthetic photooxidation
The acene was dissolved in CDCl3 with methylene blue and
bubbled with O2. The solution was irradiated with the Hg/Xe
lamp using a high-pass filter and plano-convex lens. When all
starting material has reacted, methylene blue was removed by
quick filtration with silica gel in a filter funnel.
4T endoperoxides: HRMS calcd for C52H60O2Si2 (M + H)+,
773.4205; found, 773.4190. 3D endoperoxides: HRMS calcd for
C36H18F6O2 (M + H)+, 597.1284; found, 597.1298.
Crystal structure determination
Low-temperature diffraction data (4-and u-scans) were
collected on a Bruker-AXS X8 Kappa Duo diffractometer
coupled to a Smart Apex2 CCD detector with Mo-Ka radiation
ꢀ
(l ¼ 0.71073 A) from an ImS microsource. All structures were
solved by direct methods using SHELXS39 and refined against F2
on all data by full-matrix least squares with SHELXL-9740 using
established refinement techniques.41 For details of data quality
and structure refinement, see ESI.
Fig. 1 Structures of acenes studied in this work.
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Fig. 2 X-ray crystal structure of 2T with atomic labeling scheme.
Hydrogen atoms omitted for clarity. Thermal ellipsoids shown at 50%
probability. Packing diagram visualized in a projection along tetracene
cores.
Fig. 3 X-ray crystal structure of 4T. Methyl groups omitted from lower
structure for clarity. Ellipsoids shown at 50% probability.
and 6,13-dianisylpentacene (6)—in dilute dichloromethane
solution. Relative to the disubstituted tetracene 2D that does not
have pendant phenyl rings, compound 2T shows a 30 nm red
shift (587 nm vs. 557 nm, 0.12 eV lower energy) in the peak of the
lowest energy vibronic band of absorbance, and a ꢂ50 nm (0.16
only one of the two possible twisted enantiomers present in the
crystal we analyzed.
Although 2T shows interactions between the anisyl rings in
neighboring columns along the crystallographic a-axis, neither
2T nor 4T exhibits significant p–p interactions between the acene
backbones in their crystal structures. Steric hindrance presented
above and below the acene plane by the twisted arene rings
prevents such interactions. The closest to such an interaction is
a distance of 4.63 Angstroms between parallel mean planes
defined by the tetracene ring systems of 2T. The pitch between
these molecules is so large, however, that the distance between
Table 1 Key properties of tetracene derivatives described in this papera
c
d
l
max (ab)b
l
max (em)
FF
krel
1T
2T
3T
4T
1D
2D
3D
4D
578 nm
586 nm
577 nm
575 nm
552 nm
559 nm
556 nm
534 nm
616 nm
626 nm
625 nm
602 nm
565 nm
575 nm
571 nm
542 nm
0.73
0.63
0.71
0.75
0.76
0.85
0.87
0.67
0.39
0.30
0.26
0.40
0.31
0.40
0.18
0.23
ꢀ
centroids of the tetracenes is 7.02 A (Fig. 2).
Photophysical properties
Table 1 summarizes absorbance and fluorescence parameters for
all tetracenes investigated; the trends discussed below comparing
2T and 2D are consistent with the other pairs of similarly
substituted acenes. Fig. 4 contrasts the absorbance and fluores-
cence spectra of tetrasubstituted tetracene 2T and with two
related substituted acenes—5,12-dianisylethynyl tetracene (2D)
a
All photophysical data collected in CH2Cl2; kinetics data collected in
CHCl3. Wavelength of maximum absorbance of the 0,0 band.
b
c
Measured relative to cresyl violet in methanol (FF ¼ 0.54) or
d
Coumarin 6 in ethanol (FF ¼ 0.78). Rate of reaction, relative to 5,
with 1O2 prepared by irradiation of methylene blue in the presence of O2.
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structures that have arylethynyl substituents, consistent with
reduced delocalization in molecules with fewer conjugated p-
bonds. Increased conjugation from ethynyl groups also accounts
for the red shifted absorbance and fluorescence that all of the T-
series compounds described here have relative to rubrene.43 All
four of the tetrasubstituted tetracenes described here are highly
fluorescent, with quantum yields of fluorescence (FF) > 0.6, while
FF of 6 is 0.24. When doped into a host thin film of poly(9,9-
didodecylfluorene) at 20% loading (w/w), 2T accepted energy
from the photoexcited host to give a ratio of emission intensities
of 6 : 1 (guest:host, see ESI). The spectrum of the emissive acene
in this film was nearly identical to that observed in solution.
1O2-mediated oxidation of tetracenes
Ethynyl substituents have been the most popular method for
stabilizing pentacenes and higher-order acenes that are otherwise
susceptible to photooxidation. To quantitatively evaluate the
effect of alkynes on the photooxidation of these tetracenes, we
determined the relative rates of reaction of these acenes with 1O2.
Each acene, at a concentration of 5.0 ꢃ 10ꢀ5 M in air-equili-
brated CHCl3, was exposed to 1O2 by irradiation of the sensitizer
methylene blue (MB) with a peak absorbance of 1.0 using a 200
W Hg/Xe lamp and a 665 nm high-pass filter. These conditions
insured that MB was the only chromophore in solution
absorbing any incident light. Control experiments in the absence
of MB showed no reaction of acenes under otherwise identical
irradiation conditions.
Table 1 summarizes the results of these experiments. Each
reaction with photogenerated 1O2 followed pseudo first-order
kinetics; the slopes of best-fit lines to plots of ln(absorbance) vs.
time (Fig. 5) gave relative rate constants krel. The previously
reported 5,12-dianisyltetracene (5) was the standard (krel ¼ 1) for
these kinetics experiments.10 All of the ethynylated tetracenes (T
and D) derivatives react with 1O2 with similar relative rate
constants that are 2.5–5 times slower than diaryltetracene 5.
Trifluoromethyl-substituted derivatives 3T and 3D were the least
reactive of each of their respective classes (T or D) of tetracenes.27
This structure-property relationship is consistent with the
Fig. 4 Absorbance (top) and photoluminescence (bottom) spectra of 2T
(red solid lines), 2D (blue dashed lines), and 6 (black dotted lines) in
CH2Cl2. Phenyl substitution on 2T red-shifts the absorption and emission
spectra such that they resemble that of pentacene 6.
eV) shift in the onset of absorbance. This red shift, resulting from
the two additional phenyl rings substituted on the tetracene
backbone, is analogous to that between 5,12-diphenyltetracene
(494 nm) and rubrene (528 nm). Recent theoretical calculations
on tetracene and rubrene have attributed this bathochromic shift
upon substitution with phenyl rings to a destabilization of the
HOMO by inductive effects from the phenyl rings and twisting of
the tetracene backbone.44 In addition, similar to rubrene, the
absorbance spectrum of 2T shows less vibronic resolution than
2D, and a larger Stokes Shift, consistent with additional vibra-
tional modes involving rotations of the phenyl substituents.44
The optical HOMO–LUMO gap of 2T is similar to 6 (1.9 eV),
a highly reactive pentacene derivative, derivatives of which have
been used as red emitters in light-emitting devices.45
The fluorescence spectrum of 2T and related acenes in CH2Cl2
follows a similar pattern to that found in absorbance (Fig. 4): it is
strongly red-shifted from most other previously reported tetra-
cene derivatives, including 2D, and has a maximum wavelength
of emission (626 nm) similar to 6 (620 nm). We attribute these
effects on the optical properties of 2T to a combination of the
extended p-conjugation from the phenylethynyl substituents and
the inductive effects of the phenyl rings, which because of steric
interactions are nearly orthogonal to the tetracene core. The
emission spectra of 4T and 4D, which do not have aromatic rings
bound to the alkyne substituents, are blue shifted from similar
Fig. 5 Semilogarithmic kinetic plots showing relative rates of reaction of
5 (red diamonds) and 2T (black circles) as a function of irradiation time
of MB in air-equilibrated CHCl3.
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findings of both Chi and Ong, who used 4-trifluoromethylphenyl
substituents to improve the stability of a heptacene and a pen-
tacene, respectively, and attributed these observations to
a lowering of the energy of the HOMO.27,37
Although there was some reduction in reactivity between these
ethynylated tetracenes compared to non-ethynylated tetracene 5,
consistent with the electron withdrawing alkyne groups
decreasing the reactivity of acenes towards 1O2, the magnitude of
1
this difference in stability towards photogenerated O2 is one
order of magnitude less than that reported for the difference in
photolytic stability between 6,13-diphenylpentacene and TIPS-
pentacene (4P).9 Therefore, the ethynyl groups on TIPS-penta-
cene have a greater stabilizing effect than the ethynyl groups on
these tetracenes. In addition, there was no statistically significant
stabilization of any of the ethynyl-substituted tetracenes
described here compared to 5 upon direct irradiation at l > 400
nm in the absence of MB.
These observations are consistent with the conclusions of
Maliakal, who found that the principal reason for stabilization of
ethynylated pentacenes is the selective stabilization of the pen-
tacene LUMO. These lower energy LUMOs result in low triplet
energies, making them slow to donate energy to O2, and make
photoinduced electron transfer from the excited acene to O2 less
energetically favorable.46,47 Tetracene derivatives have a larger
band gap and higher energy LUMO than larger acenes. We
therefore propose that unlike similarly substituted pentacene
derivatives, the triplet energies of these ethynylated tetracenes are
not of low enough energies to either prevent 1O2 sensitization or
1
accept energy from O2, resulting in only a modest stabilization
towards photooxidation relative to non-ethynylated tetracene 5.
The reported observation of the endoperoxide of silylethyne-
substituted anthradithiophenes (ADTs),48 together with an only
slight increase in stability to photooxidation that these substit-
uents provide to the ADT backbone,36 is consistent with the
proposed model.
1
Regioselective reversibility of O2 cycloaddition
Analytical data of products of MB-mediated photooxidation of
the tetracenes were consistent with mixtures of endoperoxides.
For D-series tetracenes, 1H NMR spectra showed clean mixtures
of two endoperoxides, with characteristic singlets from bridge-
head (d 6.6 ppm) and aromatic protons (d 8.2 ppm) corre-
sponding to 1O2 cycloaddition across the unsubstituted and
ethynylated positions, respectively (Fig. 6).49 Therefore, although
there was a modest 2 : 1 regiochemical preference for oxidation
of the unsubstituted position, silyl acetylenes did not protect
ethynylated positions from photooxidation.23 T-series
compounds did not have these clearly identifiable characteristic
NMR signals because of the high degree of substitution on the
acenes; nevertheless, high resolution mass spectra (HRMS) of
their products of oxidation by photosensitization with methylene
blue were consistent with endoperoxides.
Cycloreversion of endoperoxides of tetracene derivatives
typically requires heating or irradiation with short wavelengths
Fig. 6 NMR spectrum of 4D (top), its products of photooxidation by
1
methylene blue-mediated O2 photosensitization (middle) in CDCl3, and
endoperoxide substituted across the ethynylated positions; vertical
the same sample three weeks later, showing cycloreversion of the
arrows signify NMR signals assigned to 4D formed via cycloreversion.
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of light.50,51 In contrast, isolated mixtures of ethynylated tetra-
cene endoperoxides developed the red color of the acene starting
material upon storage in the dark, neat, under otherwise ambient
conditions. In addition, for the one acene that we investigated in
detail (4D, selected for the ease of NMR peak assignments), there
was a large difference in the rate of the cycloreversion for the two
endoperoxide regioisomers: the endoperoxoide on the ethyny-
lated positions underwent cycloreversion, while it appeared that
the other regioisomer was unreactive. The 1H NMR spectrum of
the endoperoxides of 4D, which initially showed a 2 : 1 molar
ratio of the two endoperoxides (as shown in Fig. 6), showed
approximately 10% conversion to the acene and a 3 : 1 ratio of
endoperoxides after four days in the dark. After three weeks, the
ratio of endoperoxides was 20 : 1, with the endoperoxide on
the unsubstituted positions the major regioisomer present, and
the unoxidized acene representing about 25% of the molecules
in the sample. No peaks other than those readily assigned to the
endoperoxides or the acene were observed. This observation is
consistent with a recent report by Fudickar and Linker, who
found that endoperoxides of 9,10-diethynylanthracenes gave 1O2
through cycloreversion on a short time scale.52 We suspect that
a transition state with propargyl radical character is responsible
for the cycloreversion of the reactive regioisomer.50,52,53
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We have prepared new sterically congested tetracene derivatives
with absorbance and fluorescence similar to diarylpentacenes,
but that are more efficient emitters and more resistant to
1
photooxidation while still retaining sensitivity to O2. We antic-
ipate that the steric congestion around the tetracene core of the T
series compounds, as reflected in the lack of acene-acene inter-
actions in their crystal structures, should prevent ‘‘butterfly’’
dimerization,25,54 which would otherwise be a source of false
positives in 1O2-responsive materials. Research into applications
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1
of these acenes in O2-responsive materials, including reversibly
responsive materials enabled by their photochromism, is
currently under way in our laboratory.
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Acknowledgements
The authors acknowledge DARPA for a Young Faculty Award
(N66001-09-1-2116) and Tufts University for support. X-ray
diffraction instrumentation was purchased with the help of
funding from the National Science Foundation (CHE-0946721).
37 Y. Li, Y. Wu, P. Liu, Z. Prostran, S. Gardner and B. Ong, Chem.
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