Photoisomerization of 1-phenyl-2-(pyridin-2-yl)indole BMes2: The dark isomer

Article abstract of DOI:10.1021/om101111p

A new member of photochromic N,C-chelate organoboron compounds, BMes2(Py-N-Ph-In) (1), where Mes = mesityl, Py-N-Ph-In = 1-Ph-2-(2-Py)-indolyl, has been synthesized, and its photo-thermal isomerization properties have been investigated. Upon irradiation b

Full text of DOI:10.1021/om101111p

Organometallics 2011, 30, 665–668 665
DOI: 10.1021/om101111p
Photoisomerization of 1-Phenyl-2-(pyridin-2-yl)indole BMes2:
The Dark Isomer
Hazem Amarne, Chul Baik, Rui-Yao Wang, and Suning Wang*
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6, Canada
Received November 26, 2010
Summary: A new member ofphotochromic N,C-chelate organo-
Scheme 1
boron compounds, BMes2(Py-N-Ph-In) (1), where Mes =
mesityl, Py-N-Ph-In = 1-Ph-2-(2-Py)-indolyl, has been synthe-
sized, and its photo-thermal isomerization properties have been
investigated. Upon irradiation by UV light (365 nm), compound
1 isomerizes to compound 1a, changing color from light yellow to
dark turquoise-green with the disappearance of the green fluo-
rescence of 1. This process involves the breaking/formation of a
B-C bond and a C-C bond and is thermally reversible.
Compound 1a finally allowed us to obtain suitable crystals. The
crystals of the dark isomer 1a were isolated, and its structure was
determined by a single-crystal X-ray diffraction analysis, which
confirms the formation of a C-C bond between the indolyl ring
and a mesityl ring, the dearomatization of the mesityl ring, and
the presence of a BC2 triangle in 1a. NMR and DFT computa-
tional data further support that 1a is an analogue of the dark
isomers generated from previously reported N,C-chelate BMes2
compounds.
is facile (λex, 350 - 450 nm) and thermally reversible, involving
the formation/breaking of a B-C bond and a C-C bond, in
addition to a reversible dearomatization/aromatization of a
mesityl group, as suggested by the proposed structure A. The
N,C-chelate boron compounds distinguish themselves from
previously known organic photochromic compounds,4,5 by
displaying highly tunable fluorescent color in the light colored
state and the absorption color in the dark colored state, in
addition to the fact that the forward photoisomerization is
driven by steric congestion imposed by the mesityl groups.3
Thus, this class of compounds have the potential to be effective
photochromic materials. The key for further studies of both
photochromic switching and photochemical reactivity of the
N,C-chelate boron system is to understand the properties of
the dark isomer. The structure of the dark isomer A was
established by NMR and computational studies.3 Our earlier
attempts to obtain direct crystal structural evidence that
corroborates the proposed structure A in the phenyl-pyridyl
(ppy) chelate series have not been successful. This failure was
caused by several features of the ppy-based molecules: (1) the
high solubility of the dark isomer in organic solvents including
hexanes, (2) the competing thermal reversal process that
generates the much less soluble light-colored isomer during
the recrystallization of the dark isomer, and (3) the high
oxygen sensitivity of the dark isomer in the solid state that
transforms it spontaneously to the C-C coupled product B,
making the manipulation of the dark crystals very difficult.
In search of new and thermally more stable photochromic
organoboron compounds that are more amenable for crys-
tallizing the dark isomer, we extended our investigation to
Organoboron compounds are known to display rich
photophysical properties that are responsible for their var-
ious applications in materials and organic devices.1 Earlier
investigations on photochemical reactivity have revealed
many interesting and unusual transformations of organo-
boron molecules when irradiated by UV light.2 We have
recently reported an unprecedented photochemical transfor-
mation phenomenon involving N,C-chelate four-coordinate
boron chromphores (Scheme 1).3 In contrast to previously
reported irreversible C-C coupling reactions on a boron
center that requires the use of high-energy UV light (λex
e
250 nm), the transformation of N,C-chelate BMes2 compounds
*To whom correspondence should be addressed. E-mail: wangs@
chem.queensu.ca.
€
€
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Chem., Int. Ed. 2002, 41, 2927. (d) Entwistle, C. D.; Marder, T. B. Chem.
Mater. 2004, 16, 4574. (e) Wade, C. R.; Broomsgrove, A. E. J.; Aldridge,
S.; Gabbaı, F. P. Chem. Rev. 2010, 110, 3958. (f) Elbing, M.; Bazan,
G. C. Angew. Chem., Int. Ed. 2008, 47, 834. (g) Fukazawa, A.;
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r
2011 American Chemical Society
Published on Web 01/25/2011
pubs.acs.org/Organometallics

666 Organometallics, Vol. 30, No. 4, 2011
Amarne et al.
˚
Figure 1. Crystal structures of 1 (left) and 1a (right) with 30% ellipsoids. Important bond lengths (A) and angles (deg) for 1 (the second
number is for the second independent molecule in the asymmetric unit): B(1)-C(1) = 1.647(8)/1.641(7), B(1)-C(10) = 1.634(7)/
1.652(7), B(1)-C(25) = 1.609(7)/1.615(7), B(1)-N(1) = 1.690(7)/1.668(7); C(10)-B(1)-C(1) = 116.0(4)/115.0(4), C(25)-B(1)-
N(1) = 94.6(4)/95.1(4). For 1a: B(1)-C(1) = 1.626(7), B(1)-C(2) = 1.602(7), B(1)-C(10) = 1.593(5), B(1)-N(1) = 1.592(5),
C(1)-C(2) = 1.574(7); C(10)-B(1)-C(1) = 126.7(6), C(2)-B(1)-C(10) = 121.9(7), C(1)-B(1)-C(2) = 58.4(3), C(10)-B(1)-
N(1) = 110.8(6), C(1)-B(1)-N(1) = 112.3(6).
new N,C-chelate systems, where the phenyl ring in ppy is
replaced by a heterocyclic group such as thienyl, furyl, or
indolyl. One member of the new heterocyclic N,C-chelate
boron compounds, BMes2(py-N-ph-IN) (1), has finally al-
lowed us to establish the crystal structure of the elusive dark
isomer. The details are reported herein.
Compound 1 is brightly green fluorescent in solution and
the solid state (λem = 490 nm, Φ = 0.32 in toluene). Upon
irradiation by UV light (350 or 365 nm), 1 undergoes photo-
isomerization in the same manner as BMes2(ppy) does, chang-
ing color from light yellow to dark turquoise-green with the
rise of a broad low-energy absorption band (λmax = 606 nm)
and the diminishment of fluorescence, as shown in Figure 2.
11B chemical shift in the NMR spectrum of 1 changes from
8.35 ppm to -4.55 ppm with irradiation, similar to the 11B
NMR spectral change of BMes2(ppy) upon the formation of
A.3a Compared to BMes2(ppy), photoisomerization of 1 to the
dark isomer 1a is much less efficient. Using BMes2(ppy) as an
internal standard, the relative rate constant of BMes2(ppy) vs 1
for the isomerization process at 298 K with 350 nm irradiation
was determined to be ∼14 by NMR spectroscopic methods
(see SI), despite the much greater molar extinction coefficient
of 1 at 350 nm (9022 M-1 cm-1), compared to that of BMes2-
(ppy) (3600 M-1 cm-1). The quantum efficiency of photoi-
somerization by 1 to 1a was determined to be 0.09 in toluene at
298 K, with λex = 360 nm (under the same conditions, QE for
BMes2(ppy) was determined to be 0.85), using ferrioxalate
actinometry.10 The low quantum efficiency of 1 to 1a isomer-
ization can be attributed mostly to the benzo ring of the indolyl
group, which impedes the transformation by steric blocking. In
fact, related molecules B(benzothienyl-pyridine)Mes2 and
B(benzofuryl-pyridine)Mes2 display a similar phenomenon.
The details will be reported in due course.
Compound 1 was synthesized in an overall 39% yield by
first lithiating 1-phenyl-2-(pyridin-2-yl)indole6 using n-BuLi
at -78 °C, followed by the addition of BMes2F in THF and
purification by column chromatography. This compound
was fully characterized by NMR, elemental analysis, and
X-ray diffraction analysis (see SI).7 There are two indepen-
dent molecules of 1 in the asymmetric unit, and one of them is
shown in Figure 1. These two independent molecules display
similar bond lengths and angles except the dihedral angle
between the N-phenyl ring and the indolyl ring, which is
62.2(4)° and 72.7(4)°, respectively, for the two molecules.
The B-C and B-N bond lengths of 1 are similar to those of
recently reported sterically congested N,C-chelate BMes2
compounds.3,8,9 A noteworthy feature of 1 that is also
common for all N,C-chelate BMes2 molecules reported by
us is the asymmetric arrangement of the two mesityl groups
with respect to the chelate ring. One mesityl(C(1)) is much
closer to the carbon atom (C(25)) of the chelate C(1)
˚
C(25) = 2.595(5)/2.574(5) A than the other one (C(10)
˚
C(25) = 2.856(5)/2.876(5) A), which may be attributed to
steric congestion of the mesityls.
3 3 3
3 3 3
1a can be reversed back to 1 thermally with a t1/2 of ∼9.5 h
at 50 °C, which is much slower than BMes2(ppy),3a thus
supporting the relatively high thermal stability of 1a. The 1H
NMR spectrum of 1a in d6-benzene shares many common
features with that3a of the dark isomer A of BMes2(ppy). For
example, there are two distinct singlet peaks at 6.20 and
5.55 ppm that can be assigned to the Ha and Hb protons of 1a
shown in Figure 2. In addition, the chemical shifts of the six
methyls from the two mesityls are all well resolved in the 1H
NMR spectrum with one of the methyl peaks appearing at
(6) McCormick, T. M.; Liu, Q.; Wang, S. Org. Lett. 2007, 9, 4087.
(7) The crystal data of 1 and 1a have been deposited at the Cambridge
Crystallographic Data Centre (CCDC 793402 and 793403).
(8) Wakamiya, A.; Taniguchi, T.; Yamaguchi, S. Angew. Chem., Int.
Ed. 2006, 45, 3170.
(9) Garcia-Hernandez, Z.; Gabbai, F. P. Z. Naturforsch. 2009, 64b,
1381.
(10) (a) Parker, C. A. Proc. R. Soc. London A 1953, 220, 104. (b)
Abdallah, D.; Whelan, J.; Dust, J. M.; Hoz, S.; Buncel, E. J. Phys. Chem.
A 2009, 113, 6640.

Communication
Organometallics, Vol. 30, No. 4, 2011 667
Figure 2. UV-vis spectral change of 1 in toluene upon irradiation by UV light (365 nm) under N2 at 298 K, recorded at ∼5.0 ꢀ 10-5
concentration with 5 to 20 s intervals. Inset: Fluorescence spectral change of 1 upon irradiation under the same conditions.
M
0.60 ppm, corresponding to Mea (see SI). Thus, the NMR
data support that 1a has a structure similar to that of the
dark isomer A shown in Scheme 1.
increased s-character for exocyclic bonds as well-known in
cyclopropane derivatives.12 The six-membered ring compris-
ing B(1), C(1), C(25), C(24), C(23), and N(1) is coplanar with
the py-indolyl ring. In the crystal lattice, there are extensive
intermolecular π-stacking interactions between py-indolyl
rings of adjacent molecules, with the shortest atomic separa-
Bulk conversion of 1 to 1a was achieved by placing solid 1
in hexanes and irradiating the hexanes solution in a UV
reactor (350 nm) at ambient temperature. The colorless solid
of 1 has a poor solubility in hexanes. It slowly disappeared as
the dark isomer 1a was enriched in solution with irradiation
time. After irradiating the solution for about 2 days, the dark
turquoise-green solution was concentrated in vacuo and kept
at -50 °C. Dark green-black crystals were obtained and
isolated from this solution after about one month. These
crystals are very soluble in hexanes and paraffin oil. For-
tunately they are stable in epoxy glue under air for a few
hours, which allowed us to select and mount a suitable
crystal on the diffractometer and collect the diffraction data
at 180 K successfully. The diffraction intensity of the crystal
is low and the quality of the data is poor due to the poor
quality of the crystal and the partial loss of hexane solvent
molecules in the crystal lattice (see SI). Nonetheless, the key
structural feature of 1a was established.
˚
tion distance being 3.41 A. The N-phenyl group is oriented
toward the channel in the crystal lattice, where disordered
hexane molecules (∼0.5 hexane per molecule of 1a) reside
(see SI). Structures of chelate borabicyclo[4.1.0]hepta-2,
4-diene compounds such as 1a are previously unknown,
except the few rare examples of structurally characterized
compounds with a BC2 ring similar to that in 1a reported by
Schuster and co-workers.2 The N,C-chelate group around
the B center in 1a clearly plays a critical “anchoring” role in
thermally reversing the isomerization process of the com-
pound.
DFT computational results11 also confirmed that 1a
shares the same electronic properties with the dark isomers
generated from other N,C-chelate BMes2 compounds. For
example, the HOMO level of 1a is dominated by the two π
bonds of the cyclohexadienyl ring and the “bent” bonds12 of
the BC2 ring, while the LUMO involves mostly the π* orbital
of the N,C-chelate ring (Figure 3). DFT computations
further show that the low-energy absorption peak of the
dark isomer 1a is caused by destabilization of the HOMO
level by ∼1 eV (see SI), thus a drastic decrease of the
HOMO-LUMO gap, compared to that of 1. This is in
agreement with other dark isomers of N,C-chelate BMes2
compounds and accounts for their instability toward
oxygen.3 In the CV diagram of 1, upon irradiation by a
hand-held UV lamp (365 nm), an oxidation peak appears
at -0.56 V (vs FeCp20/þ) and grows with irradiation
time, attributable to the oxidation of 1a.
As shown in Figure 1, compound 1a indeed has a chelate
borabicyclo[4.1.0]hepta-2,4-diene structure similar to what
we proposed previously for A using 2D NMR and computa-
tional data.3 The B(1)-C(1) and B(1)-C(2) bond lengths of
˚
the BC2 ring in 1a are 1.626(7) and 1.602(7) A, respectively.
Within the transformed mesityl group, the C(1)-C(2) bond
length is 1.574(7) A, similar to that calculated by DFT (1.567
˚
˚
A) and that in a related BC2 triangle reported by Schuster
2
˚
and co-worker (1.58(5) A). In addition, C(1)-C(6)
˚
˚
(1.448(10) A), C(2)-C(3) (1.446(10) A), and C(4)-C(5)
˚
(1.472(11) A) are much longer than C(3)-C(4) (1.312(10)
˚
˚
A) and C(5)-C(6) (1.356(10) A). These are all consistent
with the transformation of this mesityl group to a cyclohexa-
dienyl group. The B(1)-N(1) and B(1)-C(10) bonds are
significantly shorter than those in 1, perhaps due to the
In benzene or toluene solution, compound 1a reacts
rapidly with oxygen molecules, forming the C-C coupled
colorless product 1b, an analogue of B shown in Scheme 1. In
(11) Frisch, M. J.; et al. et al. Gaussian 03, Revision A.9; Gaussian,
Inc.: Wallingford, CT, 2004.
(12) Wiberg, K. Acc. Chem. Res. 1996, 29, 229.

668 Organometallics, Vol. 30, No. 4, 2011
Amarne et al.
Figure 3. HOMO and LUMO orbitals of 1a with an iso-contour value of 0.045 for all surfaces.
the solid state, however, 1a is much more stable toward
oxygen than A, as evident by the fact that solid A rapidly
loses its dark color and changes to B upon exposure to air,
while the crystals of 1a remain unchanged for at least an hour
under air. This surprising stability of 1a toward oxygen may
be attributed to the extensive intermolecular π-π stacking
interactions and the tight crystal lattice packing facilitated by
the N-phenyl ring.
In summary, a new photochromic organoboron com-
pound based on a pyridyl-indolyl chelate has been achieved,
which allowed us to establish the structure of a rare N,
C-chelate borabicyclo[4.1.0]hepta-2,4-diene molecule by single-
crystal X-ray diffraction analysis. NMR, electrochemical, and
computational studies support that 1a is an analogue of the
dark isomers of previously reported N,C-chelate BMes2 com-
pounds with similar structural features and electronic proper-
ties. Thus, the mystery of the elusive dark isomer involved in
photoisomerization of N,C-chelate BMes2 compounds has
been finally resolved.
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for financial
support.
Supporting Information Available: Synthetic details, charac-
terization data, photoisomerization experiments, DFT compu-
tational data, and X-ray crystal structural data. This material is
available free of charge via the Internet at http://pubs.acs.org.