The effect of addition of fluorescent moieties to dihydropyrenes: Enhancing photochromicity and fluorescence monitoring

Article abstract of DOI:10.1021/jo0712392

(Graph Presented) A series of dihydropyrenes with appending fluorescent moieties were synthesized with the objective of increasing the photochromic efficiency for this class of compounds and to establish how suitable fluorescence would be to follow their

Full text of DOI:10.1021/jo0712392

The Effect of Addition of Fluorescent Moieties to Dihydropyrenes:
Enhancing Photochromicity and Fluorescence Monitoring
Reginald H. Mitchell,* Cornelia Bohne,* Stephen G. Robinson, and Yanhong Yang
Department of Chemistry, UniVersity of Victoria, P.O. Box 3065, Victoria, BC, Canada V8W 3V6
regmitch@uVic.ca
ReceiVed June 11, 2007
A series of dihydropyrenes with appending fluorescent moieties were synthesized with the objective of
increasing the photochromic efficiency for this class of compounds and to establish how suitable
fluorescence would be to follow their photochromic behavior. The ring opening quantum yields of
dihydropyrenes with aroyl substituents at the 4-position showed increased ring opening quantum yields
without a decrease of the half-life for the thermal reversion of the less stable open isomer, the
metacyclophanediene to the dihydropyrene. The fluorescence of the appending naphthoyl or pyrenoyl
moieties was not suitable to follow the photochromic cycling of the dihydropyrenes. However, the emission
detected above 600 nm of the closed isomer of the dihydropyrene moiety was shown to be a good
monitoring method for the photochromic cycling.
Introduction
parent, 1-c, is actually not a very good photochrome because
the quantum yield of ring opening with visible light to yield
1-o is only 0.006.3 Moreover, the quantum yield for ring opening
of the synthetically more accessible4-6 di-tert-butyl derivative,
2-c, is four times less, 0.00153 (0.0018).7 Even in 1970,8 it was
known that introduction of a formyl group at the 2-position of
1 to give 3-c considerably enhanced (∼10 times) the photo-
chemical conversion to 3-o. Unfortunately, the thermal return
rate was also increased, ∼50 times, such that at room temper-
atures the photochromism of 3 was less useful. One objective
Dihydropyrenes (DHPs) such as 1 belong to the diarylethene
class of photochromes. Because the colorless open form (the
cyclophanediene, CPD), 1-o, reverts both photochemically and
thermally to the colored more stable closed form, 1-c, they are
called negative-T-photochromes.1 Such systems are less well-
known and studied than
(1) Bouas-Laurent, H.; Du¨rr, H. Organic Photochromism (IUPAC
Technical Report). Pure Appl. Chem. 2001, 73, 639-665.
(2) Irie, M. (Guest Editor) Chem. ReV. 2000, 100, 1683-1890.
(3) Sheepwash, M. A.; Mitchell, R. H.; Bohne, C. J. Am. Chem. Soc.
2002, 124, 4693-4700.
(4) Tashiro, M.; Yamato, T. J. Am. Chem. Soc. 1982, 104, 3701-3707.
(5) Mitchell, R. H. Eur. J. Org. Chem. 1999, 2695-2703.
(6) Mitchell, R. H.; Ward, T. R.; Chen, Y.; Wang, Y.; Weerawarna, S.
A.; Dibble, P. W.; Marsella, M. J.; Almutairi, A.; Wang, Z. Q. J. Am. Chem.
Soc. 2003, 125, 2974-2988.
(7) Murakami, S.; Tsutsui, T.; Saito, S.; Yamato, T.; Tashiro, M. Nippon
Kaguku Kaishi 1988, 2, 221-229; Chem. Abs. 1989, 110, 23163.
(8) Blattman, H. R.; Schmidt, W. Tetrahedron 1970, 26, 5885-5899.
the more popular positive-photochromes,2 where the thermo-
dynamically stable form is the colorless isomer. The green
* To whom correspondence should be addressed. Phone: 250-721-7159.
Fax: 250-721-7147. Address synthetic correspondence to regmitch@uvic.ca, and
address photochemistry correspondence to bohne@uvic.ca.
10.1021/jo0712392 CCC: $37.00 © 2007 American Chemical Society
Published on Web 09/19/2007
J. Org. Chem. 2007, 72, 7939-7946
7939

Mitchell et al.
of this paper was to find derivatives of 2 which had improved
ring opening quantum yields and reasonably slow thermal return
reactions since, frequently, an increase in the ring opening
quantum yield led to a decrease in the thermal stability of the
open isomer. A second objective was to investigate the use of
fluorescence to monitor the photochromic cycling, namely, the
state of the ring opening/closing reactions. Introduction of
fluorescence moieties is attractive because of the sensitivity of
fluorescence when compared to absorption measurements. A
further advantage of using fluorescence in the case of DHP
derivatives is that, as will be shown, the photochromic cycling
can be monitored at wavelengths where interference from
multiple absorptions is absent.
wavelengths similar to that of the annelated polyaromatic
hydrocarbons. In this work, we combined the use of a carbonyl
spacer with aromatic moieties (4-7) of which naphthalene,
anthracene, and pyrene are known to be fluorophores with high
fluorescence quantum yields. The thermal ring closing rate
constants for the synthesized compounds were initially studied,
and for the promising candidates, photophysical experiments
were performed.
Aroyl substituents seemed attractive because they lack the
easy oxidation of a formyl group and, based on previous reports,
looked promising: in 1967, Boekelheide9 reported the syntheses
of 2-benzoyl-1-c, and in 1970, Blattmann8 reported that this
compound had a photo-opening quantum yield similar to that
of the formyl compound 3-c. Similarly, Tashiro7 in 1988
reported quantum yields and kinetic data for a series of
derivatives of 2-c, including 4-benzoyl-2-c ()4-c below), but
without any synthetic details (then or since). The data for the
latter, however, indicated that the photo-opening quantum yield
was about 2.6 times higher than that of its parent 2-c, while the
thermal closing was 1.5 times faster.
Fluorescence has been used for different photochromic
systems,10a-c where the photochromic moiety itself provides the
fluorescent chromophore or a chromophore is appended to the
photochromic framework. The advantage of using a pendant
fluorescent moiety is that excitation of this reporter moiety may
not lead to switching of the photochrome when the fluorescent
and photochromic moieties are isolated, that is, no energy
transfer occurs. These two moieties can be isolated by using
nonconjugating spacers, such as, for example, the short bicyclo-
[1.1.1]pentane.10d Modulation of the fluorescence intensity and
spectra of a particular photochrome can be achieved by changing
the extent of conjugation or by changes in the efficiency of
energy or electron transfer between the various chromophores
in the molecule for each of the isomers.10b,c An additional
advantage of fluorescence is the ability to detect the emission
from single molecules. For example, the intramolecular quench-
ing by the closed isomer of a diarylethene of a fluorescent
moiety was employed to follow the photocycling by single
molecule detection.10e,f The ability of using fluorescence to
follow the photochromism of dihydropyrenes has not been
explored to date, despite the fact that 2-c was shown to be
weakly fluorescent (φ ) 0.002) with a very structured emission
spectrum above 600 nm.3 In the case of [e]-annelated dihydro-
pyrenes, both the closed and open isomers show weak fluores-
cence,11 where the closed isomer emits above 600 nm and the
open isomer shows, in some cases, an emission at shorter
Results and Discussion
Syntheses: Dihydropyrenes are sensitive to Lewis acid
conditions, and sometimes rearranged products, where the
internal groups have migrated, are obtained.12,13 With care,
however, reaction of the di-tert-butyl parent 2-c with the
appropriate acid chloride and AlCl3 in CH2Cl2 at 20 °C for 2-6
h gave the products 4-c-7-c as greenish-brown solids in 50-
97% yields. Full characterization for each compound is given
in the Experimental Section. In each case, new 1H NMR peaks
for the internal methyl protons were observed around δ -4,
which is characteristic of dihydropyrenes, in which the internal
protons are very shielded by the strong diatropic ring current.
Specifically, the peaks appear slightly deshielded from those
of the parent 2-c (δ -4.06) and no longer identical to each other,
and for 4-c, they are at δ -3.83, -3.84; for 5-c, at δ -3.81,
-3.84; for 6-c, at δ -3.81, -3.86; and for 7-c, at δ -3.75,
-3.78. Also, new IR CdO stretching bands at 1630-1640 cm-1
were observed. Interestingly, the anthranoyl compound 6-c
showed restricted rotation, resulting in broad peaks for H-3,5,6
and the 2-t-Bu group protons of the dihydropyrenes at ambient
temperatures. All of these peaks sharpen on cooling to 210 K
or on heating to 380 K. In addition, the anthracenyl protons,
H-1′,8′, are a doublet at δ 7.98 at 325 K, where rotation is fast
on the NMR time scale, but collapse in to a broad singlet at
280 K ()Tc) and re-emerge as two doublets at 210 K, where
rotation is slow. The separation, ∆ν ) 36.3 Hz, suggests14
a
q
∆Gc of 14 kcal/mol for the rotational process.
While the purity of 4-c-7-c was suitable for basic photo-
chemical opening/closing and thermal closing studies, for
detailed photophysical measurements, very high purity com-
pounds were required. Compounds 5-c and 7-c were suitably
pure (g99.5%) by HPLC (see Experimental Section) to conduct
these latter experiments. The fluorescent impurities in 4-c and
6-c and their instability during extensive irradiation precluded
quantitative photophysical experiments, such as quantum yield
determination. Also, for comparison purposes, four additional
(9) Phillips, J. B.; Molyneux, R. J.; Sturm, E.; Boekelheide, V. J. Am.
Chem. Soc. 1967, 89, 1704-1709.
(10) (a) Matsuda, K.; Irie, M. J. Photochem. Photobiol. C 2004, 5, 169-
182 and references therein. (b) Raymo, F. M.; Tomasulo, M. Chem. Soc.
ReV. 2005, 34, 327-336 and references therein. (c) Raymo, F. M.;
Tomasulo, M. J. Phys. Chem. A 2005, 109, 7343-7352 and references
therein. (d) de Meijere, A.; Zhao, L.; Belov, V.; Bossi, M.; Noltemeyer,
M.; Hell, S. W. Chem.sEur. J. 2007, 13, 2503-2526. (e) Fukaminato, T.;
Sasaki, T.; Kawai, T.; Tamai, N.; Irie, M. J. Am. Chem. Soc. 2004, 126,
14843-14849. (f) Fukaminato, T.; Umemoto, T.; Iwata, Y.; Yokojima, S.;
Yoneyama, M.; Nakamura, S.; Irie, M. J. Am. Chem. Soc. 2007, 129, 5932-
5938.
(12) (a) Boekelheide, V.; Sturm, E. J. Am. Chem. Soc. 1969, 91, 902-
908. (b) Boekelheide, V.; Hylton, T. A. J. Am. Chem. Soc. 1970, 92, 3669-
3675.
(13) (a) Andes Hess, B.; Bailey, A. S.; Bartusek, B.; Boekelheide, V. J.
Am. Chem. Soc. 1969, 91, 1665-1671. (b) Andes Hess, B.; Boekelheide,
V. J. Am. Chem. Soc. 1969, 91, 1672-1678.
(14) See: Dixon, K. R.; Mitchell, R. H. Can. J. Chem. 1983, 61, 1598-
1602 and the references therein.
(11) Sheepwash, M. A.; Ward, T. R.; Wang, Y.; Bandyopadhyay, S.;
Mitchell, R. H.; Bohne, C. Photochem. Photobiol. Sci. 2003, 2, 104-122.
7940 J. Org. Chem., Vol. 72, No. 21, 2007

Addition of Fluorescent Moieties to Dihydropyrenes
chromophore and of the fluorophore (e.g., pyrene, naphthalene)
moieties appended to the DHP. The molar extinction coefficients
for all the DHP isomers were determined at several wavelengths
(see Supporting Information). The effect of substitution on the
DHP is apparent from the changes in the position and molar
absorptivity of the band with lowest energy. Parent 2-c has an
absorption maximum at 641 nm with a value of ꢀ ) 900 L
mol-1 cm-1, while the introduction of an acetyl substituent (12-
c) led to a red shift of the absorption to 666 nm and an increase
in ꢀ to 2740 L mol-1 cm-1. All compounds with a carbonyl
containing substituent (5-c, 7-c, and 11-c) have absorption
maxima between 663 and 671 nm, with molar absorptivity
values between 2230 and 2860 L mol-1 cm-1, while compounds
8-c and 9-c have an absorption maximum at 653 nm and a low
ꢀ value (744 and 780 L mol-1 cm-1, respectively). Attaching a
pyrene without the carbonyl spacer to the DHP moiety led to a
small shift of the absorption band (648 nm) and slight increase
in the ꢀ value (1150 L mol-1 cm-1). These results show that
the decrease for the energy of the first excited singlet state (S1)
of the closed isomers and the increase in the absorption
probability from S0 to S1 is primarily achieved with the
introduction of the carbonyl containing substituent. The addition
of the fluorescent moieties directly onto the ring or appended
as an aroyl group did not change significantly the absorption
properties of the DHPs.
FIGURE 1. Absorption spectra of the DHP isomer 7-c in cyclohexane
(black) and of the solution after 30 min irradiation above 500 nm (blue,
mainly the CPD isomer 7-o).
compounds 8-11 were prepared with similar purity to 5-c and
7-c. Wittig reaction of 5-c and 7-c with Ph3PdCH2 yielded 8
and 9 in about 70% yield each, and Suzuki coupling of 4-iodo-
2-c and 1-pyreneboronic acid with Pd(PPh4) yielded 73% of
10. Acetylation of the latter yielded 80% of an acetyl derivative,
probably 11.
The dark colored closed aroyldihydropyrenes 4-c-7-c were
all easily opened to the colorless cyclophanedienes 4-o-7-o by
irradiation of CDCl3 solutions (for NMR experiments) or
cyclohexane solutions (for UV-vis experiments) using ordinary
tungsten household lamps with a >490 nm cutoff filter.
Depending on concentration, 15-30 min irradiation gave >90%
opening. Continued irradiation led to 100% ring opening, which
The two pyrenyl compounds 10 and 11 were found to be a
mixture of two rotational isomers. For 10, this is evidenced by
four proton signals each for the t-butyl and internal methyl
protons and four each for the C(CH3)3, the internal bridge and
1
was cleanly evidenced by H NMR by the total disappearance
of the internal methyl protons at about δ -3.8, while new CPD
methyl proton signals appeared at about δ 1.5. This result was
supported by UV-vis absorption measurements where pro-
longed irradiation above 500 nm led to the complete disappear-
ance of the characteristic absorption above 600 nm for the closed
isomers of the DHPs.
1
internal methyl carbons. Variable-temperature H NMR over
the range of 190 to 355 K for 10 resulted in peak movement,
but no collapse of signals, so under these conditions, the isomers
do not interconvert. This complexity is also present in the spectra
of the acetyl derivative 11, and because of this peak overlap,
we are not certain whether 11 is mainly the 4-acetyl derivative
shown or its 5-isomer. For the purpose of this paper, which
isomer does not matter since observation of an opening rate
increase on introduction of the acetyl group was the main
objective. Full characterization of all of these compounds is
provided in the Experimental Section. For comparison purposes,
the acetyl derivative 12-c was also synthesized.15a,b Since these
two references report slightly different properties, we report our
recent high-field NMR data, as well as UV-vis data (Tashiro’s
UV data can be found in ref 7) in the Experimental Section.
Thermal Ring Closing Reactions. The absorption spectra
of five DHPs were characterized in order to use UV-vis
absorption measurements to determine rate constants for the
thermal closing reactions. The closed DHP isomers have
absorption bands throughout the spectrum (Figure 1). When the
DHP isomers were irradiated at long wavelengths (>500 nm),
the CPD isomer was formed with an enhanced absorption in
the UV (Figure 1). The absorption spectra of all compounds in
the 300-450 nm region include the absorption by the DHP
The kinetics for the thermal reaction of the open isomer back
to the closed isomer were measured with absorption experi-
ments. The kinetics were followed either for the changes of the
absorption above 600 nm or for the changes in the band close
to 500 nm. The kinetics followed a first-order behavior, and
for this reason, a small absorption for the open isomer in the
500 nm band does not influence the value recovered for the
rate constants of the thermal closing reactions. The rate constants
were measured at various temperatures between 46 and 70 °C.
The values for the activation energies (Table 1) were determined
from Arrhenius plots (see Supporting Information), while the
values for the activation enthalpies and entropies (Table 1) were
determined from Eyring plots (see Supporting Information). The
half-life of the open forms was calculated from the linear
relationship determined in the Eyring plots (Table 1). Data for
the acetyl derivative7 12-o, the parent16 2-o, and the benzo
derivative6,17 13-o are included for comparison.
(16) Reference 7 gives Eact ) 21.8 kcal mol-1. Our best data, which
start from synthetic fully open 1-o, and follow the thermal closing using
NMR integrations, yield 20.4 kcal mol-1 (Ayub, K. Ph.D. Thesis to be
submitted, University of Victoria). Calculated t1/2 (46 °C) values are then
3.12 h (Ayub) and 3.11 h (ref 7).
(17) Williams, R. V.; Edwards, W. D.; Mitchell, R. H.; Robinson, S. G.
J. Am. Chem. Soc. 2005, 127, 16207-16214.
(15) (a) Tashiro, M.; Yamato, T. Org. Prep. Proc. Int. 1982, 14, 216-
219. (b) Miyazawa, A.; Yamato, T.; Tashiro, M. J. Org. Chem. 1991, 56,
1334-1337.
J. Org. Chem, Vol. 72, No. 21, 2007 7941

Mitchell et al.
TABLE 1. Values for the Activation Energy, Enthalpy, and
Entropy and the Calculated Half-Life at 46 °C Obtained from the
Arrhenius and Eyring Plotsa for the Thermal Reaction of the Open
to the Closed Isomer
Eact
∆Hq
∆Sq
t1/2
compound
kcal mol-1 kcal mol-1 cal K-1 mol-1 (46°C) h
4-o (benzoyl)b
5-o (naphthoyl)c
6-o (anthranoyl)d
7-o (pyrenoyl)e
24 ( 1
20.2 ( 0.8 19.6 ( 0.8 -15 ( 2
26 ( 1 26 ( 1 2 ( 4
23 ( 1
-6 ( 3
4.4
1.5
4.0
1.9
2.4
3.1
1.9
5.8
20.5 ( 0.2 19.8 ( 0.3 -14.8 ( 0.8
11-o (acetyl,pyrenyl)f 20 ( 2
19 ( 2
19.8
-17 ( 5
-15
2-o (parent)g
12-o (acetyl)h
13-o (benzo)i
20.4
24.4
24.6
23.9
4
a Errors correspond to statistical errors from the linear fits of the
Arrhenius or Eyring plots. For measurements at more than one wavelength,
rate constants were averaged. b Kinetics measured at 489 and 658 nm.
c Kinetics measured at 508 and 675 nm. d Kinetics measured at 489 and
658 nm. e Kinetics measured at 610 and 666 nm. f Kinetics measured at
671 nm. g See ref 16. h From ref 7. i From ref 6, and see also ref 17.
As can be seen from the data, there are notable differences
between Eact values for the various compounds. Clearly, the
activation energies for the pyrenoyl (7-o), naphthoyl (5-o), and
acetyl/pyrenyl (11-o) derivatives are similar to those of the
parent (2-o), while the values found for benzoyl (4-o), anthranoyl
(6-o), and acetyl (12-o) derivatives are more like the value for
the benzo derivative 13-o. Examination of the relative rates for
thermal closing at 46 °C (shown as half-lives in Table 1)
indicates them all to be acceptable. In contrast, introduction of
the formyl group in to 1-o to give 3-o improved the photo-
opening but also increased the thermal closing rate by about
50-fold,8 such that the half-life at room temperature was only
42 min. Our derivatives are thus much better and at room
temperature (20 °C) have half-lives from 1.1 days (naphthoyl,
5-o) to 6.7 days (anthranoyl, 6-o); for reference, the parent (2-
o) shows 2.3 days and benzo (13-o) 7.2 days. We are not yet in
a position to fully understand exactly what determines the barrier
for the thermal closing reaction,17 but it is interesting that the
anthranoyl compound 5-o has the highest barrier and is the one
compound in the series for which restricted rotation is observed
for the DHP form, 5-c. The open CPD form 5-o does not appear
to show any restricted rotation at room temperature, and possibly
the additional strain energy to achieve this rotation in the closed
form plays a roll in the barrier for the overall thermal reaction.
The negative activation entropies suggest a lower degree of
freedom in the transition state as expected for a unimolecular
process with partial bond formation. It is worth noting that
compounds with a lower activation enthalpy have a higher
entropy of activation when compared to the compounds with
higher activation enthalpies.
TABLE 2. Ring Opening Quantum Yieldsa Measured in
Cyclohexane Using Benzo-DHP 13 as a Standard with Quantum
Yield of 0.0423
compound
(φDHP f CPD)/10-3
2-c
1.5 ( 0.1b
13-c
12-c
5-c
7-c
8-c
9-c
10-c
11-c
42 ( 2b
3.8 ( 0.5b
9.2 ( 0.7 (4)
9.1 ( 0.8 (2)
3.0 ( 0.2 (2)
1.6 ( 0.2 (2)
0.7 ( 0.1 (2)
7.0 ( 0.4 (2)
a Errors correspond to standard deviations from independent experiments.
The number of experiments are indicated in parentheses. b From ref 3.
of the ring opening quantum yield (note that these experiments
were performed at single wavelength and to low conversion).
Direct attachment of the pyrene to the DHP ring (10-c) decreased
the opening quantum yield when compared to that for the parent
DHP 2-c. The higher quantum yield was recovered if an acetyl
group was added to compound 10-c to form 11-c. Within
experimental errors, the ring opening quantum yields were the
same for aerated solutions and solutions that were deoxygenated,
suggesting that the ring opening reaction occurs from the singlet
excited state as previously assigned.3
The photophysics of DHPs are different from the behavior
observed for other photochromic systems because the lifetimes
for the singlet excited states for the closed and open forms are
in the nanosecond time domain,3,11 while picosecond lifetimes
are observed for most photochromic systems, such as spiroox-
azines, fulgides, and diarylethenes.18 A recent theoretical study19
showed that the low efficiency for the ring opening reaction is
due to the complexity of the excited states involved. The reaction
channel leading to the ring opening process is quenched by
Irradiation with broad-band visible light of the non-carbonyl
compounds 8-10 did not show any significant conversion to
the open form, possibly because thermal and photochemical
closing was occurring as fast as opening and so no thermal
reactions were measured for these compounds.
Quantum Yields for Ring Opening Photoreactions. The
ring opening quantum yields for all compounds were measured
using Benzo-DHP 12-c as a standard (Table 2). This method
made it possible to irradiate the samples at long wavelengths,
a procedure previously shown to be necessary because of the
small absorption of the CPD isomers at shorter wavelengths.3,11
Previous work showed that the low ring opening quantum yield
of the parent DHP could be increased by a factor of 2.5 with
the addition of an acetyl group at the 4-position (12-c).3 For
this reason, we chose a carbonyl group as the bridging group
between the DHP and naphthalene or pyrene moieties. The ring
opening quantum yields of the naphthalene (5-c) and pyrene
(7-c) substituted compounds increased by a factor of about 2.4
compared to that of acetyl-DHP 12-c. Replacement of the
carbonyl group with an alkene (8-c and 9-c) led to a reduction
(18) (a) Wilkinson, F.; Worrall, D. R.; Hobley, J.; Jansen, L.; Willimas,
S. L.; Langley, A. J.; Matousek, P. J. Chem. Soc., Faraday Trans. 1996,
92, 1331-1336. (b) Tamai, N.; Masuhara, H. Chem. Phys. Lett. 1992, 191,
189-194. (c) Martin, S. C.; Singh, N.; Wallace, S. C. J. Phys. Chem. 1996,
100, 8066-8069. (d) Kurita, S.; Kashiwaga, A.; Kurita, Y.; Miyasaki, H.;
Mataga, N. Chem. Phys. Lett. 1990, 171, 553-557. (e) Tamai, N.; Saika,
T.; Shimidzu, T.; Irie, M. J. Phys. Chem. 1996, 100, 4689-4692. (f)
Miyasaka, H.; Araki, S.; Tabata, A.; Nobuto, T.; Mataga, N.; Irie, M. Chem.
Phys. Lett. 1994, 230, 249-254.
(19) Boggio-Pasqua, M.; Bearpark, M. J.; Robb, M. A. J. Org. Chem.
2007, 72, 4497-4503.
7942 J. Org. Chem., Vol. 72, No. 21, 2007

Addition of Fluorescent Moieties to Dihydropyrenes
internal conversion to the lower excited state, which is nonre-
active. The complex trends observed for the ring opening
quantum yields for substituted DHPs3,11 are probably due to
subtle changes in the energies of the various excited states
involved. Addition of the naphthoyl (5-c) and pyrenoyl (7-c)
substituents enhanced the ring opening quantum yield without
a decrease in the half-life for the thermal back reaction from
the open to the closed form, leading to more efficient photo-
switching. It is interesting to note that attachment of an aromatic
moiety directly onto the DHP framework is counter productive,
leading to a marked decrease of the ring opening quantum yield
for 10-c, which can be partially recovered by adding an acetyl
substituent at the opposite site of the DHP ring (11-c). These
results show that the rational design of substituted DHPs to
increase their photochromicity is at this point not possible due
to the limited understanding of the mechanism for photoswitch-
ing.
FIGURE 2. Emission of the DHP 5-c in cyclohexane (λex ) 470 nm
and λem ) 676 nm, top spectrum, black) and the residual emission when
the solution was irradiated above 590 nm (bottom spectrum, blue). The
inset shows the cycling of the DHP emission of 5-c after irradiation
with visible light followed by irradiation with UV light.
Fluorescence. Two excitation wavelengths are of relevance
for the fluorescence experiments. DHPs have been shown to
have a sharp fluorescence above 600 nm,3,11 and excitation at
470 nm has sufficient energy to excite only the S1 state of the
DHP without exciting either the pyrene or naphthalene moieties.
The excited states of naphthalene and pyrene can be reached
when the molecules are excited at shorter wavelengths (260-
350 nm).
photoswitch. In addition, most materials, which maybe used as
devices, such as polymers or films, are unlikely to have any
appreciable absorption in the 600-700 nm region.
Emission was also monitored by exciting 5-c and 7-c at
shorter wavelengths. No emission was observed for the naph-
thoyl-DHP 5-c, but an emission was observed for the pyrenoyl-
DHP 7-c. This emission occurred in the same spectral region
as the fluorescence observed for pyrenecarboxylic acid in
cyclohexane. Cycling between the DHP-CPD isomers of 7 led
to a gradual and constant increase of the emission monitored at
383 nm. In addition, the intensity of the emission of 7 for
solutions of equal concentrations was different for compounds
prepared in different syntheses. These results suggest that a very
fluorescent pyrene impurity is present in the samples of 7.
Attempts to identify this impurity by HPLC failed. From the
analysis of the absorption spectra in the HPLC experiments,
the samples seem pure (>99.5%). However, from fluorescence
detection on the HPLC, a small shift in the retention time is
observed, which could indicate the presence of an impurity that
is highly fluorescent. In addition, the gradual increase of the
emission at 383 nm during the cycling between the DHP and
CPD isomers suggests that 7 is not stable to irradiation. No
formation of decomposition products was observed by monitor-
ing the NMR spectra of irradiated samples. It is important to
note that fluorescence is much more sensitive than NMR, and
the fact that decomposition is observed with the former
technique suggests that the products formed are in small
quantities but highly fluorescent.
The design feature for the DHPs with fluorescent moieties
was the following: When the molecules are excited at 470 nm,
only the emission of the DHP above 600 nm can be observed.
In contrast, the emission of pyrene or naphthalene may be
observable when the compounds are excited at short wave-
lengths. In the case of the DHP isomers, we would not expect
any emission from naphthalene or pyrene because the S1 state
of DHP (<45 kcal/mol) is lower in energy when compared to
the S1 states of naphthalene (92 kcal/mol) and pyrene (77 kcal/
mol)20 and fast intramolecular energy transfer would be expected
from the excited states of naphthalene and pyrene to the lower
energy level of the DHP. However, the energy of the CPD
isomers is much higher (e.g., 70-84 kcal/mol for [e]-annelated
CPDs)11 and is comparable to the excited state energies for the
fluorescent moieties. In this case, an emission characteristic for
naphthalene and pyrene could be observed if the CPD moiety
did not act as a quencher. Therefore, the expectation was that
on-off cycling for the emission above 600 nm would be
mirrored by an off-on cycling for the emission of either the
naphthalene or pyrene moieties.
Switching of the DHP emission above 600 nm when the DHP
isomers were opened to the CPD isomers was observed (Figure
2 for 5-c). The conversion from the DHP isomer to the CPD
isomer was not complete, leading to a residual emission from
the DHPs that were not opened. The range for the emission
change between the completely closed form and the residual
emission for the photostationary state depends on factors such
as the ring opening quantum yields, the magnitude of the thermal
reversion rate constants, and the time of irradiation used in the
experiment. The changes observed for the emission above 600
nm show that fluorescence is a useful diagnostic tool to follow
switching for DHPs. The major advantage in following the
emission of the DHP closed form is that the fluorescence occurs
at wavelengths where the photoswitch does not absorb, avoiding
the possibility of reabsorption of the emitted light by the
These results show that the use of naphthalene and pyrene
as a fluorescence moiety to follow the switching of the DHPs
is not suitable, despite both substituents leading to an increase
in the ring opening quantum yields without a slowdown of the
thermal reversion reaction. One of the problems is that very
small amounts of impurities (e0.5%) overwhelmed the emission
from the switch. These impurities are formed by a decomposi-
tion of the DHP, leading to a nonidentified photoproduct in very
small yields. A second problem with using these chromophores
is that their emission spectrum overlaps with the absorption
spectrum of the closed form of the DHPs, leading to self-
absorption of the emission and potential switching of the DHP.
The design for future work with fluorescence DHPs will include
fluorescent moieties that emit between 500 and 600 nm where
the closed form of the DHP has minimal absorption.
(20) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochem-
istry, 2nd revised and expanded ed.; Marcel Dekker, Inc.: New York, 1993.
J. Org. Chem, Vol. 72, No. 21, 2007 7943

Mitchell et al.
2,7-Di-tert-butyl-4-(1-naphthoyl)-trans-10b,10c-dimethyl-10b,-
10c-dihydropyrene 5-c. Using the procedure for 4-c, from 1-naph-
thoyl chloride (0.3 mL, 2 mmol) and dihydropyrene 2-c (57 mg,
0.17 mmol) for 2 h was obtained 81 mg (97%) of 5-c as dark brown
crystals from methanol, mp 196-197 °C: 1H NMR δ 9.39 (d, J )
1.0 Hz, 1H, H-3), 8.61 (br s (b ) half-width ∼ 2.57 Hz), 1H, H-5),
8.57 (br s, 1H, H-6), 8.56 (br s, 1H, H-1), 8.54 (d, 1H, J ) 7.9 Hz,
H-10), 8.48 (d, 1H, J ) 7.9 Hz, H-9), 8.47 (br s, 1H. H-8), 8.44
(dm, 1H, Jd ) 8.3 Hz, H-8′), 8.06 (dm, 1H, Jd ) 8.3 Hz, H-2′),
7.98 (dm, 1H, Jd ) 8.3 Hz, H-5′), 7.63 (dd, J ) 7.0, 1.3 Hz, 1H,
H-4′), 7.54 (td, J ) 6.8, 1.2 Hz, 1H, H-6′), 7.49 (dd, J ) ∼7 Hz,
1H, H-3′), 7.47 (td, J ) 6.8, 1.4 Hz, 1H, H-7′), 1.61 (s, 9H,
7-C(CH3)3), 1.55 (s, 9H, 2-C(CH3)3), -3.81 (s, 3H, 10b-CH3),
-3.84 (s, 3H, 10c-CH3); 13C NMR δ 201.0 (CdO), 150.4 (C-2),
146.3 (C-7), 140.4 (C-1′), 139.8 (C-10a), 137.0 (C-3a), 136.8 (C-
5a), 134.5 (C-10d), 134.1 (C-8b′), 131.5 (C8a′), 131.4 (C-2′), 129.9
(C-3b), 128.9 (C-4′), 128.6 (C-5′), 127.5 (C-7′), 126.6 (C-6′), 126.3
(C-8′), 126.1 (C-5), 125.3 (C-10), 124.9 (C-3′), 123.9 (C-8), 123.8
(C-9), 123.4 (C-6), 121.6 (C-1), 120.4 (C-3), 36.7 (2-C(CH3)3), 36.1
(7-C(CH3)3), 32.0 (7-C(CH3)3), 31.9 (2-C(CH3)3), 31.2 (C-10b), 29.4
(C-10c), 15.1 (10c-CH3), 14.9 (10b-CH3); IR (KBr) ν 2961 (s),
1640 (s), 1506 (m), 1460 (m), 1343 (m), 1228 (s), 1196 (m), 1108
(m), 886 (s), 781 (s), 672 (m) cm-1; UV-vis (cyclohexane) λmax
(ꢀmax, L mol-1 cm-1, nm) 289 (9920), 343 (41100), 399 (35700),
492 (7200), 607 (541), 663 (2230); CI MS m/z 499 (MH+); HRMS
m/z calcd for C37H38O 498.2923, found 498.2922. Anal. Calcd: C,
89.11; H, 7.68. Found: C, 88.69, H, 7.94.
Conclusions
Several dihydropyrenes with fluorescent moieties were syn-
thesized with the dual objective of enhancing the photochromic
properties of these compounds and to establish the suitability
of using fluorescence to follow the photoswitching between the
open and closed forms of dihydropyrenes. The photochromicity
of dihydropyrenes was enhanced with the addition of a carbonyl
substituent to the 4-position of the dihydropyrene framework
without decreasing the half-life for the thermal isomerization
of the open isomer of these compounds. The aroyl substituent
was mostly responsible for the enhanced ring opening quantum
yields. The addition of fluorescent naphthyl or pyrenyl substit-
uents was not suitable for the detection of the photoswitching
between the open and closed forms of the dihydropyrenes
because of the formation of very low level, but highly emissive,
impurities. However, the emission of the closed form of the
dihydropyrenes in the spectral region above 600 nm was shown
to be an excellent monitoring method for the photoswitching.
Therefore, fluorescence of dihydropyrenes even in the absence
of appending emissive moieties will be the method of choice
to follow the photoswitching of dihydropyrenes because of the
sensitivity of fluorescence when compared to absorption mea-
surements.
Experimental Section
The Open Isomer 5-o. Irradiation of an NMR sample in CDCl3
with visible light (see general photochemistry section) until the color
disappeared gave a solution of 5-o: 1H NMR δ 8.32 (d, J ) 7.7
Hz, 1H), 7.95 (d, J ) 8.3 Hz, 1H), 7.90 (d, J ) 7.7 Hz, 1H), 7.70
(∼d, J ) 7.3 Hz, 1H), 7.60-7.48 (m, ∼3H), 7.19 (s, 1H), 6.77,
6.73, 6.72, 6.65 (4s, 4H), 6.40 and 6.38 (AB, J ) 11.2 Hz, 2H),
1.51 and 1.50 (s, 3H each, 10b,c-CH3), 1.23 and 1.14 (s, 9H each,
2,7-C(CH3)3); 13CNMR δ 199.8, 152.1, 150.6, 145.9, 145.3, 144.0,
141.0, 138.0, 137.7, 137.4, 136.6, 136.0, 134.0, 133.4, 132.6, 131.2,
128.7, 127.5, 127.4, 126.6, 125.8, 124.98, 124.96, 124.65, 124.56,
124.47, 123.8, 34.28, 34.26, 31.4, 31.3, 20.2, 19.3.
Syntheses: General conditions and spectral assignment methods
are given in the Supporting Information, as well as compound
numbering for the NMR assignments.
4-Benzoyl-2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-di-
hydropyrene 4-c. Benzoyl chloride (0.5 mL, 4 mmol) and then
anhydrous AlCl3 (ca. 20 mg) were added under Ar to a stirred
solution of dihydropyrene 2-c6 (50 mg, 0.16 mmol) in dry CH2Cl2
(25 mL) at 20 °C. After stirring for 6 h, ice-water was added to
the deep blue solution, and then the organic layer was washed with
0.25 M aq KOH and water and then was dried and evaporated.
The residue was chromatographed over neutral alumina using
hexane/CH2Cl2 (1:1) as eluant. Eluted first was green unchanged
2-c. Eluted second (a brown band) was product 4-c: 35 mg (50%),
dark greenish brown crystals from methanol, mp 192-194 °C (no
lit. mp given7); 1H NMR δ 8.95 (d, J ) 1.0 Hz, 1H, H-3), 8.61 (s,
1H, H-5), 8.58 (s, 1H, H-6), 8.56 (s, 1H, H-8), 8.55 (s, 1H, H-1),
8.54 (AB, J ) 7.7 Hz, 1H, H-9), 8.49 (AB, 1H, H-10), 7.91-7.89
(m, 2H, H-2′,6′), 7.62-7.59 (m, 1H, H-4′), 7.50-7.47 (m, 2H,
H-3′,5′), 1.66 (s, 9H, 7-C(CH3)3), 1.55 (s, 9H, 2-C(CH3)3), -3.83
(s, 3H, 10b-CH3), -3.84 (s, 3H, 10c-CH3); 13C NMR δ 199.5, 148.9
(C-2), 146.5 (C-7), 141.0 (C-1′), 139.0 (C-10a), 136.8 (C-10d),
136.1 (C-3a), 134.6 (C-5a), 132.5 (C-4′), 130.6 (C-2′,6′), 129.6 (C-
4), 128.5 (C-3′,5′), 124.9 (C-9), 124.7 (C-5), 123.8 (C-10), 123.2
(C-6), 122.9 (C-8), 121.5 (C-1), 120.3 (C-3), 36.5 (2-C(CH3)3), 36.2
(7-C(CH3)3), 32.1 (7-C(CH3)3), 31.9 (2-C(CH3)3), 30.9 (C-10b), 29.5
(C-10c), 15.0 and 14.9 (10b,c-CH3); UV-vis (cyclohexane) λmax
(ꢀmax, L mol-1 cm-1, nm) 343 (62300), 394 (43200), 489 (9070),
657 (2090) [lit.7 ꢀ ) 53000, 35000, 8600, 1900]; HRMS m/z calcd
for C33H36O 448.2777, found 448.2766. Anal. Calcd: C, 88.35; H,
8.09. Found: C, 87.99; H, 8.23.
4-(9-Anthranoyl)-2,7-di-tert-butyl-trans-10b,10c-dimethyl-
10b,10c-dihydropyrene 6-c. 9-Anthracenecarboxylic acid (129 mg,
0.58 mmol) and oxalyl chloride (0.2 mL, 2 mmol) in dry CH2Cl2
(30 mL) were stirred under reflux under argon for 6 h. The solvent
and excess oxalyl chloride were removed under vacuum. Dry CH2-
Cl2 (30 mL) was then added to the yellow residue, followed by
dihydropyrene 2-c (50 mg, 0.14 mmol) and anhyd AlCl3 (∼20 mg).
The deep blue solution was stirred for 4 h at 20 °C, and then ice-
water was added. The organic layer was separated, washed with
aq 10% KOH, water, and then was dried and evaporated. The
residue was chromatographed over alumina (neutral) using hexane/
dichloromethane (1:1) as eluant. Eluted first was any unchanged
2-c. Eluted second was 61 mg (80%) of 6-c as dark brown crystals
1
from methanol: mp 222-224 °C; H NMR δ 10.5 (br s ∼30 Hz
width, 1H, H-3), 8.64 (s, 2H, H-1,10′), 8.60 (br s ∼30 Hz width,
1H, H-5), 8.53 and 8.48 (AB, J ) 8 Hz, H-9,10), 8.52 (s, 1H, H-1),
8.24 (br s ∼30 Hz width, 1H, H-6), 8.11 (d, J ) 8.6 Hz, 2H,
H-4′,5′), 7.98 (d, J ) 9.0 Hz, 2H, H-1′,8′), 7.46-7.43 (m, 2H,
H-3′,6′), 7.28-7.25 (m, 2H, H-2′,7′), 1.75 (v br s, 9H, 2-C(CH3)3),
1.56 (s, 9H, 7-C(CH3)3), -3.81 and -3.86 (s, 3H each, 10b,c-CH3).
In d8-toluene, at 380 K, the broad signals for H-3, H-5, H-6, and
the 2-t-Bu all sharpen and likewise in CDCl3 at 210 K: 13C NMR
δ ∼203, 152.4 (br), 146.1, 140.8, 138.33, 137.1, 136.7, 134.7, 131.6,
129.3, 128.8, 128.2, 127.5 (br), 126.5, 126.1, 125.9, 125.6, 124.8,
124.1, 123.8, 121.7, 120.7 (br), 36.9 (br, -C(CH3)3), 36.0 (-C(CH3)3),
32.1 (br,-C(CH3)3), 32.0 (-C(CH3)3), 31.6 and 29.4 (C-10b,c), 15.3
and 15.0 (10b,c-CH3); IR (KBr) ν 2961 (s), 1639 (s), 1444 (m),
1347 (m), 1233 (m), 1196 (s), 1113 (m), 883 (s), 846 (w), 731 (s),
The Open Isomer 4-o. Irradiation of an NMR sample in CDCl3
with visible light (see general photochemistry section) until the color
disappeared gave a solution of 4-o: 1H NMR δ 7.87 (∼d, J ) 7.3
Hz, 2H), 7.51 (∼t, J ) 7.3 Hz, 1H), 7.42 (∼t, J ) 7.3 Hz, 2H),
7.16 (s, 1H), 6.78, 6.76, and 6.73 (3s, 3H), 6.50 (s, 1H), 6.41 and
6.40 (AB, J ) 11.3 Hz, 2H), 1.56 and 1.50 (s, 3H each, 10b,c-
CH3), 1.25 and 1.14 (s, 9H each, 2,7-C(CH3)3); 13CNMR δ 197.9,
152.2, 150.7, 144.0, 143.8, 143.0, 140.7, 138.5, 138.1, 137.7, 137.4,
136.0, 133.3, 132.7, 132.4, 129.7, 128.5, 124.8, 124.6, 123.49,
123.46, 34.3, 31.54, 31.47, 20.1, 19.4.
672 (m) cm-1; UV-vis (cyclohexane) λmax (ꢀmax, L mol-1 cm-1
nm) 255 (96800), 347 (22500), 367 (21000), 388 (20800), 406
,
7944 J. Org. Chem., Vol. 72, No. 21, 2007

Addition of Fluorescent Moieties to Dihydropyrenes
(31000), 493 sh (5200), 513 (5540), 676 (3050); CI MS m/z 549
(MH+); HRMS m/z calcd for C41H40O 548.3079, found 548.3083.
Anal. Calcd: C, 89.74; H, 7.35. Found: C, 88.79, H, 7.48. This
compound is the least stable of 4-c-7-c (especially in solution)
and was not possible to free completely of oxidized impurity.
The Open Isomer 6-o. Irradiation of an NMR sample in CDCl3
with visible light (see general photochemistry section) until the color
disappeared gave a solution of 6-o: 1H NMR δ 8.55-8.53 (m,
∼1H), 8.10-7.97 (m, ∼4H), 7.57-7.43 (m, ∼5H), 7.14-6.73 (m,
∼3H), 6.50-6.30 (m, ∼3H), 1.47 and 1.46 (s, 3H each, 10b,c-
CH3), 1.18 and 1.17 (s, 9H each, 2,7-C(CH3)3); 13CNMR δ 201.3,
152.1, 150.4, 145.2, 144.3, 141.0, 140.8, 138.1, 137.2, 136.0, 135.0,
133.4, 132.9, 132.5, 131.4, 129.7, 129.1, 129.0(*), 128.8, 128.5,
125.8, 125.7(*), 125.4, 125.3, 123.5, 123.2. 34.4, 34.2, 31.4 (both
C(CH3)3), 20.4, 19.2; (*) ) small peaks also underneath.
2,7-Di-tert-butyl-4-(1-pyrenoyl)-trans-10b,10c-dimethyl-10b,-
10c-dihydropyrene 7-c. Using the procedure described for 6-c
above, 1-pyrenecarboxylic acid (286 mg, 1.16 mmol), oxalyl
chloride (0.2 mL, 2 mmol) in dry CH2Cl2 (30 mL), and then
dihydropyrene 2-c (100 mg, 0.291 mmol) and AlCl3 (∼40 mg) in
CH2Cl2 (30 mL) at 20 °C for 4 h yielded 133 mg (80%) of brown
crystals of 7-c: mp 213-214 °C; 1H NMR δ 9.52 (d, J ) 1.1 Hz,
1H, H-3), 8.65 (d, J ) 9.3 Hz, 1H, H-10′), 8.59 (s, 1H, H-1), 8.575
(s, 1H, H-6), 8.570 (s, 1H, H-5), 8.56 (d, J ) 7.9 Hz, 1H, H-10),
8.51 (d, 1H, J ) 7.9 Hz, H-9), 8.40 (s, 1H, H-8), 8.27 (dd, J )
7.7, 1.0 Hz, 1H, H-6′), 8.21 (d, J ) 8.9 Hz, 1H, H-5′), 8.20 (d, J
) 7.85 Hz, 2H, H-3′,8′), 8.16 (d, J ) 7.85 Hz, 1H, H-2′), 8.15 (d,
J ) 9.9 Hz, 1H, H-4′), 8.05 (dd, J ) 8.05 Hz, 1H, H-7′), 8.04 (d,
J ) 9.3 Hz, 1H, H-9′), 1.57 (s, 9H, 7-C(CH3)3), 1.55 (s, 9H,
2-C(CH3)3), -3.75 (s, 3H, 10b-CH3), -3.78 (s, 3H, 10c-CH3).;13C
NMR δ 201.4 (CdO), 150.6 (C-2), 146.3 (C-7), 139.9 (C-3a/10a),
137.2, 137.1 (C-10a/3a), 136.8 (C-5a/10d), 134.6 (C-10d/5a), 133.3,
131.5, 131.1, 130.2 (C-4), 130.1, 129.3 C-3′/5′/8′), 129.0 (C-7′),
128.1 (C-2′), 127.6 (C-1′), 126.54 (c-5/6,9′), 126.53 (c-5/6,9′), 126.2
(C-6′), 126.0 (C-3′/5′/8′), 125.39 (C-10/10′), 125.37 (C-10′/10),
125.2, 124.9, 124.3 (C-3′/8′), 124.0 (C-8), 123.8 (C-9), 123.5 (C-
6/5), 121.6 (C-1), 120.6 (C-3), 36.7 (2-C(CH3)3), 36.1 (7-C(CH3)3),
32.0 (7-C(CH3)3), 31.9 (2-C(CH3)3), 31.3 (C-10b), 29.4 (C-10c),
15.2 (10c-CH3), 15.0 (10b-CH3); IR (KBr) ν 2962 (s), 1633 (s),
1595 (m), 1505 (m), 1383 (m), 1343 (m), 1249 (m), 1220 (s), 1114
(m), 1066 (m), 1016 (m), 940 (w), 887 (s), 840 (s), 781 (m) 729
(s) cm-1; UV-vis (cyclohexane) λmax (ꢀmax, L mol-1 cm-1, nm)
234 (50600), 240 (50700), 274 (26900), 341 (57500), 401 (44900),
498 (7820), 665 (2,600); HRMS m/z calcd for C43H40O 572.3079,
found 572.3085.
J ) ∼8 Hz, 1H, H-5′), 7.82 (dm, J ) ∼8 Hz, 1H, H-4′), 7.62 (dd,
J ) 7.1, 1.5 Hz, 1H, H-2′), 7.48 (dd, J ) 8.3, 7.1 Hz, 1H, H-3′),
7.34 (ddd, J ) 8.6, 6.8, 1.2 Hz, 1H, H-6′), 7.14 (ddd, J ) 8.4, 6.8,
1.3 Hz, 1H, H-7′), 6.065 and 6.063 (d, J ) 2.4 Hz, 1H each, >Cd
CH2), 1.62 (s, 9H, 7-C(CH3)3), 1.41 (s, 9H, 2-C(CH3)3), -3.89 (s,
3H, 10c-CH3), -3.93 (s, 3H, 10b-CH3). 13C NMR δ 149.6 (>Cd
CH2), 145.99 (C-2), 145.98 (C-7), 142.8 (C-1′), 137.3 (C-10a),
137.0 (C-10d), 136.3 (C-5a), 135.9 (C-4), 134.2 (C-8b), 132.7 (C-
3a), 132.0 (C-8a), 128.5 (C-5′), 128.0 (C-4′), 127.2 (C-2′), 126.5
(C-8′), 125.9 C-7′), 125.74 (C-6′), 125.68 C-3′), 124.4 (C-5), 123.1
and 122.9 (C-9,10), 122.0 (dCH2), 121.3 (C-8), 120.9 (C-6), 120.8
(C-1), 120.4 (C-3), 36.2 (2-C(CH3)3), 36.1 (7-C(CH3)3), 32.1 (7-
C(CH3)3), 31.9 (2-C(CH3)3), 30.5 (C-10b), 29.9 (C-10c), 15.1 (10c-
CH3), 14.2 (10b-CH3); IR (KBr) ν 2946 (s), 1435 (m), 1358 (m),
1230 (m), 1119 (m), 886 (m), 802 (m), 777 (s), 721 (m), 542 (m)
cm-1; UV-vis (cyclohexane) λmax (ꢀmax, L mol-1 cm-1, nm) 288
(80400), 350 (43800), 389 (31900), 484 (6500), 653 (744); CI MS
m/z 496 (M+); HRMS m/z calcd for C38H40 496.3130, found
496.3126. Anal. Calcd: C, 91.88; H, 8.12. Found: C, 91.57; H,
8.23.
1-[4-(2,7-Di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihy-
dropyrenyl)]-1-(1-pyrenyl)ethene 9-c. This compound was pre-
pared from butyllithium (0.6 mmol), methyltriphenylphosphonium
bromide (Aldrich, 98%) (160 mg, 0.45 mmol), dry THF (25 mL),
and then dihydropyrene 7-c (50 mg, 0.087 mmol) in dry THF (10
mL) exactly as described for 8-c above. After chromatography, 35
mg (70%) of yellowish green crystals of 9-c was obtained: mp
1
187-189 °C; H NMR δ 8.92 (d, J ) 1.2 Hz, 1H, H-3), 8.61 (d,
J ) 9.3 Hz, 1H, H-10′), 8.45 (d, J ) 1.2 Hz, 1H, H-8), 8.42 (d, J
) 1.2 Hz, 1H, H-1), 8.41 (s, 1H, H-5), 8.39 (s, 2H, H-9,10), 8.37
(br s, 1H, H-6), 8.15 (d, J ) 7.9 Hz, 1H, H-3′), 8.14 (dd, J ) 7.6,
1.1 Hz, 1H, H-6′), 8.10 (d, J ) 7.9 Hz, 1H, H-2′), 8.06 and 8.05
(AB, J ) 8.95 Hz, 2H, H-4′,5′), 8.03 (dd, J ) 7.7, 1.0 Hz, 1H,
H-8′), 7.93 (t, J ) 7.7 Hz, 1H, H-7′), 7.80 (d, J ) 9.3 Hz, 1H,
H-9′), 6.22 (d, J ) 2.0 Hz, 1H, >CdCHaHb), 6.18 (d, J ) 2.0 Hz,
1H, >CdCHaHb), 1.60 (s, 9H, 7-C(CH3)3), 1.37 (s, 9H, 2-C(CH3)3),
-3.85 (s, 3H, 10c-CH3), -3.88 (s, 3H, 10b-CH3); 13C NMR δ 149.8
(>CdCH2), 146.1 (C-2), 146.0 (C-7), 140.4 (C-1′), 137.3 and 137.0
(C-10a,10d), 136.3 (C-5a), 136.2 (C-4), 132.9 (C-3a), 131.7 (C-
5a′), 131.2 (C-10d′), 130.8 (C-3a′), 129.0 (C-10a′), 127.72 (C-2′),
127.67 and 127.47 (C-4′,5′), 127.35 (C-9′), 126.1 (C-7′), 125.9 (C-
10′), 125.4 (C-10b′), 125.21 (C-10c′), 125.19 (C-6′), 124.94 (C-
3′), 124.93 (C-8′), 124.7 (C-5′), 123.2 and 123.0 (C-9,10), 122.8
(>CdCH2), 121.4 (C-6), 121.0 (C-8), 120.9 (C-1), 120.5 (C-3),
36.2 (2-C(CH3)3), 36.1 (7-C(CH3)3), 32.1 (7-C(CH3)3), 31.9 (2-
C(CH3)3), 30.5 (C-10b), 29.9 (C-10c), 15.1 (10c-CH3), 14.3 (10b-
CH3); IR (KBr) ν 3042 (w), 2959 (s), 1437 (m), 1231 (m), 1119
The Open Isomer 7-o. Irradiation of an NMR sample of 7-c in
CDCl3 with visible light (see general photochemistry section) until
the color disappeared gave a solution of 7-o: 1H NMR δ 8.60 (d,
J ) 9.3 Hz, 1H), 8.27-8.05 (m, 8H), 7.20 (s, 1H), 6.82, 6.78, 6.73,
6.64 (4s, 4H), 6.41 and 6.40 (AB, J ) 8.7 Hz, 2H), 1.58 and 1.56
(s, 10b,c-CH3), 1.22 and 1.12 (s, 2,7-C(CH3)3); 13C NMR (CDCl3)
δ 200.2, 152.2, 150.6, 146.1, 145.7, 144.0, 141.0, 138.1, 137.4,
136.7, 136.1, 134.5, 133.4, 133.1, 132.6, 131.4, 131.0, 129.9, 129.2,
129.1, 127.5, 126.7, 126.6, 126.2, 126.1, 125.14, 125.08, 125.0,
124.9, 124.7, 124.2, 123.4, 123.4, 34.3, 34.3, 31.3, 27.1, 20.3, 19.3.
1-[4-(2,7-Di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-dihy-
dropyrenyl)]-1-(1-naphthyl)ethene 8-c. Butyllithium (0.28 mL,
0.7 mmol in hexane) was added under argon to a stirred suspension
of methyltriphenylphosphonium bromide (Aldrich, 98%) (179 mg,
0.5 mmol) in dry THF (25 mL). The yellow solution was stirred
for 5 min at 20 °C, and then dihydropyrene 5-c (50 mg, 0.10 mmol)
in dry THF (10 mL) was added. The solution was refluxed for 1 h
under argon, cooled, and then water was added. The product was
extracted using dichloromethane and was then washed, dried, and
chromatographed to give 35 mg (70%) of yellowish green crystals
(m), 845 (s) cm-1; UV-vis (cyclohexane) λmax (ꢀmax, L mol-1 cm-1
,
nm) 235 (37600), 242 (37600), 268 (20100), 278 (26100), 348
(59300), 391 (41600), 484 (6750), 653 (782); CI MS m/z 570 (M+);
HRMS m/z calcd for C44H42 570.3287, found 570.3300.
2,7-Di-tert-butyl-4-(1-pyrenyl)-trans-10b,10c-dimethyl-10b,-
10c-dihydropyrene10-c.4-Iodo-2-c3 (46mg,0.1mmol),1-pyrenebo-
ronic acid (Aldrich, 50 mg, 0.2 mmol), and tetrakis(triphenylphos-
phine)palladium(0) (Aldrich, 5 mg) in THF (20 mL) were refluxed
under argon for 2 h, cooled to room temperature, and then hexane
was added. The organic layer was washed, dried, and evaporated.
The residue was chromatographed on neutral alumina, using first
hexane, which eluted a small amount of unchanged green iododi-
hydropyrene. Hexane/benzene (6:1) then eluted the second bright
green band to give 40 mg (73%) of the product 10-c: mp 208-
209 °C, which by NMR is a mixture of two approximately equal
1
rotational isomers; H NMR δ 8.60-7.75 (m, 16H), 1.70, 1.54,
1.40, and 1.35 (s, 9H each), -3.58, -3.59, -3.71 and -3.74 (s,
3H each); 13C NMR δ 146.44, 146.37, 146.24, 146.19, 138.8, 137.9,
137.3, 137.1 (×2), 136.4, 136.0, 135.3, 134.6, 133.9, 133.7, 131.83,
131.82, 131.42, 131.40, 131.0, 130.9, 130.8, 130.2, 130.0, 129.5,
127.91, 127.88, 127.7, 127.5, 127.34, 127.27, 127.0, 126.5, 126.28,
126.25, 126.1, 125.3, 125.2 (>×2), 125.1, 125.0, 124.6, 124.3,
1
of 8-c: mp 176-177 °C; H NMR δ 8.86 (d, J ) 1.3 Hz, 1H,
H-3), 8.45 (d, J ) 1.3 Hz, 1H, H-6), 8.42 (d, J ) 1.3 Hz, 1H,
H-1), 8.41 (s, 1H, H-5), 8.40 (d, J ) 1.3 Hz, 1H, H-8), 8.37 (s,
2H, H-9,10), 8.30 (ddd, J ) 8.6, 1.9, 0.9 Hz, 1H, H-8′), 7.83 (dm,
J. Org. Chem, Vol. 72, No. 21, 2007 7945

Mitchell et al.
123.24, 123.22, 123.15, 123.11, 121.2, 121.1 (>×2), 120.9, 36.3
(×2), 36.2, 36.1, 32.2, 32.0, 30.6, 30.4, 30.1, 30.0, 15.3, 15.1, 14.9,
14.8; IR (KBr) ν 2962 (s), 1506 (m), 1460 (m), 1382 (m), 1360-
Photophysics and Kinetic Experiments. Details on solvent
sources, HPLC procedures, and equipment description and settings
for absorption and fluorescence measurements are described in the
Supporting Information.
(m), 1343 (m), 1231 (m), 884 (m), 843 (s), 755 (m), 668(m) cm-1
;
UV-vis (cyclohexane) λmax (ꢀmax, L mol-1 cm-1, nm) 233 (51200),
241 (68300), 264 (24900), 274 (29400), 346 (75500), 389 (49600),
483 (9930), 648 (1150); CI MS m/z 544 (M+); HRMS m/z calcd
for C42H40 544.3130, found 544.3143. Anal. Calcd: C, 92.60; H,
7.40. Found: C, 92.34, H, 7.58.
Irradiation Procedures for the Interconversion between the
DHP and CPD Isomers. The photochemical conversion of the
DHP isomer to the CPD isomer was achieved by using as the light
source a 500 W household tungsten-halogen lamp (8500 lumens).
A cutoff filter (λ > 490 nm or λ > 590 nm) and a 1 L beaker
containing ice-water were placed between the light source and
the fluorescence cell. The container with water was used to filter
infrared emission from the light source, and ice was used to cool
the water in the container. The distance between the light source
and the sample cell was ca. 27 cm. The irradiation time with visible
light was usually between 20 and 30 min.
Irradiation of the CPD isomers was performed by placing samples
in a black box and irradiating them with a 3 W low-pressure Hg
pencil light source (254 nm). The distance between the UV light
source and the samples was ca. 5 cm. The UV light irradiation
time was usually between 10 and 15 min.
Determination of the Thermal Decay Rate Constants for the
CPD Isomers. DHP samples were irradiated by visible light to
achieve a significant conversion to the CPD isomer. The kinetics
for the thermal conversion of the CPD isomer into the DHP isomer
were followed by absorption measurements either for the absorption
band at the wavelengths above 600 nm or for the absorption close
to 500 nm. The molar fraction (X) of CPD was calculated, and the
first-order rate constants at various temperatures were obtained from
plots of ln(Xt) versus time.
Determination of Molar Absorptivity Coefficients and Ring
Opening Quantum Yield. Molar absorptivity coefficients were
determined from absorption values for solutions with at least three
different DHP concentrations (see Supporting Information for
details). The ring opening quantum yields were determined using
benzo-DHP 13-c as a standard (0.042),3 using a method adapted3
from previous literature measurements21 (see Supporting Informa-
tion for details).
4-Acetyl-2,7-di-tert-butyl-9-(1-pyrenyl)-trans-10b,10c-dimethyl-
10b,10c-dihydropyrene 11-c. Acetic anhydride (1 mL) was added
under argon to the pyrenyldihydropyrene 10-c (33 mg, 0.061 mmol)
in dried dichloromethane (25 mL). The solution was stirred at room
temperature for 10 min, then boron trifluoride diethyl etherate (1
mL) was added. The solution was stirred under argon at room
temperature for another 3 h, and then ice-water (20 mL) was added.
The organic layer was separated, washed with aqueous 10%
potassium hydroxide, water, and then was dried and concentrated.
The residue was chromatographed over neutral aluminum oxide
with hexane/dichloromethane (1:1) as eluant. Eluted first was a
green band containing a small amount of starting 10-c. Eluted
second was a deep brown band which yielded 28 mg (80%) of
dark brown crystals of 11-c: mp 148-150 °C, which by NMR is
1
a mixture of rotational isomers (see 10-c); H NMR δ 9.86-7.73
(m, 15H), 3.126, 3.124, 3.119, 3.115 (s, 3H total, -COCH3), 1.71,
1.70, 1.35, 1.34 (s, 18H total, -C(CH3)3), -3.44 to -3.61 (8s, 6H
total, 10c,10d-CH3); 13C NMR δ 202.6 and 202.4 (CdO), 151.7-
120.8 (60 peaks), 36.9-36.1 (6 peaks), 33.5-29.5 (16 peaks),
15.9-14.3 (8 peaks); IR (KBr) ν 2962 (s), 2922 (s) 1661 (s), 846
(s) cm-1; UV-vis (cyclohexane) λmax (ꢀmax, L mol-1 cm-1, nm)
273 (30100), 348 (41300), 404 (55100), 487 (82300), 671 (2860);
EI MS m/z 586 (M+); HRMS calcd for C44H42O 586.3236, found
586.3234.
4-Acetyl-2,7-di-tert-butyl-trans-10b,10c-dimethyl-10b,10c-di-
hydropyrene 12-c. This is a modified procedure from refs 15a,b.
BF3‚Et2O (1.2 mL) was added to a solution of 2-c (345 mg, 1.00
mmol) in acetic anhydride (3 mL) at 20 °C, and the mixture was
stirred for about 30 min (monitored by the disappearance of the
fast moving DHP spot on TLC). The mixture was then poured on
to crushed ice and extracted with dichloromethane. The organic
layer was washed with satd NaHCO3 soln, water, and then was
dried and evaporated. The residue was chromatographed on silica
gel using hexane to elute unchanged 2-c and then hexane/ethyl
acetate (1:1) to elute product, which yielded 180 mg (49%) of
brownish-green crystals: mp 184-185 °C (lit. mp 185-187 °C,15a
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada and the University
of Victoria for support of this work.
Supporting Information Available: General synthetic experi-
mental conditions, photophysical experimental conditions, com-
pound numbering; molar absorption coefficients for compounds 5-c,
7-c, 8-c, 9-c, 10-c, and 11-c; thermal closing data for 4-o, 5-o, 6-o,
7-o, and 11-o; VT NMR spectra for 6-c and 10-c; 1H and 13C NMR
spectra for 5-c, 7-c, 8-c, 9-c, 10-c, and 11-c. This material is
available free of charge via the Internet at http://pubs.acs.org.
1
183-185 °C15b); H NMR (360 MHz) δ 9.79 (d, J ) 1.0 Hz, 1H,
H-3), 8.92 (s, 1H, H-5), 8.64 (s, 1H, H-6), 8.54 (s, 1H, H-1), 8.45
and 8.51 (AB, J ) 7.8 Hz, 1H, H-9,10), 3.07 (s, 3H, 4-COCH3),
1.69 and 1.68 (s, 9H each, 2,7-C(CH3)3), -3.93 and -3.94 (s, 3H
each, 10b,c-CH3); 13C NMR (90.6 MHz) δ 202.1, 150.8, 146.0,
139.7, 136.5, 135.6, 134.8, 127.9, 125.1, 124.3, 123.6, 123.4, 123.1,
121.4, 120.6, 36.6, 35.9, 31.9, 30.84, 30.78, 28.1, 15.0, 14.5; UV-
vis λmax (ꢀmax, L mol-1 cm-1, nm) 346 (37800), 395 (39500), 495
(7240), 666 (2740).
JO0712392
(21) (a) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A
1956, 518-536. (b) Parker, C. A. Proc. R. Soc. London, Ser. A 1953, 220,
104.
7946 J. Org. Chem., Vol. 72, No. 21, 2007