Modulating the photoisomerization of N,C-chelate organoboranes with triplet acceptors

Article abstract of DOI:10.1021/ol302742g

Triplet acceptors such as naphthalene, pyrene, and anthracene have been found to be highly effective in controlling the photoisomerization efficiency of N,C-chelate boryl chromophores, establishing the involvement of a photoactive triplet state in the isomerization of this class of photochromic compounds.

Full text of DOI:10.1021/ol302742g

ORGANIC
LETTERS
2012
Vol. 14, No. 21
5610–5613
Modulating the Photoisomerization
of N,C-Chelate Organoboranes with
Triplet Acceptors
Zachary M. Hudson,† Soo-Byung Ko,† Shigehiro Yamaguchi,*,‡ and Suning Wang*,†
Department of Chemistry, Queen’s University, Kingston, Ontario, K7L 3N6, Canada,
and Department of Chemistry, Graduate School of Science, Nagoya University, Furo,
Chikusa, Nagoya 464-8602, Japan
wangs@chem.queensu.ca; yamaguchi@chem.nagoya-u.ac.jp
Received October 4, 2012
ABSTRACT
Triplet acceptors such as naphthalene, pyrene, and anthracene have been found to be highly effective in controlling the photoisomerization
efficiency of N,C-chelate boryl chromophores, establishing the involvement of a photoactive triplet state in the isomerization of this class of
photochromic compounds.
Photoresponsive materials are attractive for a wide
range of uses in molecular electronics, including optical
data storage, molecular switching, and logic technologies.
Thermally reversible photochromic dyes in particular have
generatedconsiderable interestasmaterialsfor ophthalmic
glasses and photoresponsive windows.1 While modification
of existing systems such as spirooxazines, azobenzenes,
and diarylethenes allows for the fine-tuning of their optical
and electronic properties, the investigation of new classes
of photoresponsive materials may lead to new applications
in molecular switching technology.
photochemical reaction proceeds by formation of a three-
membered boracycle with dearomatization of one of the
pendant mesityl rings (Scheme 1). In solution or doped
polymer films, this transformation results in a pronounced
color change, typically from light yellow to deep blue or
green, as well as fluorescent quenching. These compounds
are attractive photochromic materials for several reasons,
characterized by thermal reversibility, quantitative photo-
isomerization, and drastic color changes even at low
conversion. With this system, diverse chemical structures
based on organic π-conjugated materials,3 metal coordi-
nation complexes,4 and even N-heterocyclic carbenes5
have been recently investigated. Despite this research
activity however, a more detailed understanding of the
mechanism of this reaction has remained elusive and is
critical to the rational design of these materials.
We recently reported the unusual photochromism of
N,C-chelate dimesitylboranes, which undergo thermally
reversible isomerization on exposure to UV light.2
Driven by the steric congestion of the mesityl groups, this
† Queen's University.
‡ Nagoya University.
(3) (a) Baik, C.; Hudson, Z. M.; Amarne, H.; Wang, S. J. Am. Chem.
Soc. 2009, 131, 14549. (b) Baik, C.; Murphy, S. K.; Wang, S. Angew.
Chem., Int. Ed. 2010, 49, 8224. (c) Rao, Y.; Amarne, H.; Lu, J.-S.; Wang,
S. Dalton Trans. 2012, ASAP (DOI: 10.1039/C2DT31370G).
(4) Rao, Y.-L.; Wang, S. Organometallics 2011, 30, 4453.
(1) For example, see: (a) Berkovic, G.; Krongauz, V.; Weiss, V.
Chem. Rev. 2000, 100, 1741. (b) Irie, M. Chem. Rev. 2000, 100, 1685.
(c) Ko, C.-C.; Yam, V. W.-W. J. Mater. Chem. 2010, 20, 2063 and
references therein. (d) Feringa, B. L. Molecular Switches; Wiley-VCH:
Weinheim, Germany, 2001.
(5) (a) Rao, Y.-L.; Chen, L.; Mosey, N. J.; Wang, S. J. Am. Chem.
€
ꢀ
(2) Rao, Y.-L.; Amarne, H.; Zhao, S. B.; McCormick, T. M.; Martic,
Soc. 2012, 134, 11026. (b) Nagura, K.; Saito, S.; Frohlich, R.; Glorius,
S.; Sun, Y.; Wang, R. Y.; Wang, S. J. Am. Chem. Soc. 2008, 130, 12898.
F.; Yamaguchi, S. Angew. Chem., Int. Ed. 2012, 51, 7762.
r
10.1021/ol302742g
Published on Web 10/23/2012
2012 American Chemical Society

9,10-diphenylanthracene (3) display singlet energies of
3.93, 3.33, and 3.05 eV, respectively,9ꢀ11 in all cases higher
than that of the boryl unit (2.77 eV). These chromophores
have been widely used as triplet acceptors for photoche-
mical reactions and tripletꢀtriplet upconversion and pro-
vide a considerable range in triplet energy level.9ꢀ11
Scheme 1. Efficiency of Photoisomerization (ΦPI) in This System
Can BeModulated byControlling the Energy Transfer(ET) from
the Photoactive Borane to a Triplet Acceptor Chromophore
Covalent attachment of the photoswitch to the acceptor
serves to strengthen the interaction between the two. The
use of a nonconjugated linker is critical, as the energies of
the excited states on the boron chromophore can be kept
consistent while different acceptors are employed. Further-
more, previous work has shown that increases in the extent
of π-conjugation about the boron chromophore can dra-
matically slow or even inhibit the photoreaction, presum-
ably due to the presence of low-lying πfπ* excited states
that compete with the photoactive charge transfer state.3c,5b
Compounds 1ꢀ3 are prepared by Negishi cross-coupling
of di-(p-iodophenyl)dimethylsilane with the appropriate
bromo- or iodoarene, followed by attachment of the N,C-
chelate pyridylindole by copper-catalyzed Ullman conden-
sation (Scheme 2). The boron center may then be installed
by deprotonation of the product with n-butyllithium at the
indole 3-position, followed by addition of FBMes2.
In particular, elucidating the nature of the photoactive
excited state is of key importance if the kinetics of the
forward and reverse reactions are to be controlled.
A number of photochromic materials, including spiro-
pyrans, spirooxazines, and diarylethenes, have been shown
to isomerize via photoactive triplet states.6 Based on this
principle, it has been possible to sensitize photoisomeriza-
tion reactions using heavy atoms, which facilitate inter-
systemcrossing(ISC) toatripletstate by strongspinꢀorbit
coupling.7 These prior studies have inspired us to examine
if the triplet state also plays a role in photoisomerization of
organoboron compounds.
Scheme 2. Synthetic Proceduresa
To determine the nature of the photoactive excited state
inthese organoboranes, wedesignedaseriesof compounds
incorporating photochromic boron units and acceptor
chromophores of varying triplet energy. Herein we provide
evidence that the isomerization in this system proceeds
via a photoactive triplet state and that the quantum yield
of photoisomerization can be modulated in a controlled
manner using the triplet energy of the acceptor (Scheme 1).
Three nonconjugated compounds 1ꢀ3 were prepared
incorporating a previously investigated photochromic boryl
chromophore A18 as well as aromatic acceptors. The A1
chromophore in these systems is based on a 2-(2-pyridyl)-
indole chelate, exhibiting a moderate photoisomerization
quantum yield (ΦPI) of 0.09 as well as green fluorescence
with λmax = 490 nm in toluene.8 The acceptor chromo-
phores 1-phenylnaphthalene (1), 1-phenylpyrene (2), and
a Ar = 1-naphthalenyl (1); 1-pyrenyl (2); or 10-phenyl-9-anthryl (3).
Single crystals of 1 were obtained by slow evaporation of
its hexanes/CH2Cl2 solution (Figure 1).12 The two chro-
mophores are closely spaced, with a separation distance of
˚
7.02 A between the pyridyl and naphthyl rings (C(16) and
C(28)) in 1. Both chromophores lie out-of-plane with the
phenyl groups of the linker, exhibiting dihedral angles
of 59.7° for the naphthyl ring and 76.1° for the indole,
indicative of minimal extension of conjugation through
the diphenylsilane bridge. As anticipated, compounds 1ꢀ3
all display photophysical and electrochemical properties
characteristic of the boryl chromophore A1 (Table 1).
(6) (a) Tamai, N.; Miyasaka, H. Chem. Rev. 2000, 100, 1875 and
references therein. (b) Battal, E. D. M.; Cusido, J.; Sortino, S.; Raymo,
F. M. Phys. Chem. Chem. Phys. 2012, 14, 10300.
(7) (a) Indelli, M. T.; Carli, S.; Ghirotti, M.; Chiorboli, C.; Ravaglia,
M.; Garavelli, M.; Scandola, F. J. Am. Chem. Soc. 2008, 130, 7286.
(b) Yam, V. W.-W.; Ko, C.-C.; Zhu, N. J. Am. Chem. Soc. 2004, 126,
12734. (c) Roberts, M. N.; Nagle, J. K.; Finden, J. G.; Branda, N. R.;
Wolf, M. O. Inorg. Chem. 2009, 48, 19.
(8) Amarne, H.; Baik, C.; Wang, R.-Y.; Wang, S. Organometallics
2011, 30, 665.
(9) (a) Jockusch, S.; Koptyug, I. V.; McGarry, P. F.; Sluggett, G. W.;
Turro, N. J.; Watkins, D. M. J. Am. Chem. Soc. 1997, 119, 11495. (b)
Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photochemistry,
2nd ed.; Marcel Dekker: New York, 1993.
(10) (a) Herbrich, R. P.; Schmidt, R. J. Photochem. Photobiol. A
ꢀ
ꢀ
ꢀ
ꢀ
2000, 133, 149. (b) Perez-Prieto, J.; Perez, L. P.; Gonzalez-Bejar, M.;
Miranda, M. A.; Stiriba, S. -E. Chem. Commun. 2005, 5569.
(11) Li, Q.; Zhang, C.; Zheng, J. Y.; Zhao, Y. S; Yao, J. Chem.
Commun. 2012, 48, 85.
(12) Crystal data for 1 have been deposited at at the Cambridge
Crystallographic Data Centre (CCDC 900063).
Org. Lett., Vol. 14, No. 21, 2012
5611

All compounds exhibit a broad absorption peak at λmax
=
their dark isomers, with photoisomerization quantum
yields of ΦPI = 0.12 and 0.03 respectively. Similar to A1,
this reaction results in the appearance of a broad peak at
612 nm in their absorption spectra (Figure 2). The conver-
sion of 1 is complete in 5 min using a hand-held UV lamp
while the reaction of 2 is much slower, requiring 30 min of
irradiation under the same conditions.
417 nm with similar extinction coefficients (see Supporting
Information (SI)), attributable to a transition of mixed
πꢀπ* and charge-transfer character on the boryl unit,
giving the compounds a bright yellow color in the solid
state and solution.
Table 1. Photophysical and Electrochemical Properties
red
λmax
,
λem.
[nm]a Φfl
E1/2
HOMO LUMO
abs
[nm]a
[V]c
[eV]d
[eV]
b
ΦPI
1
2
3
323, 351,
417
498
501
500
0.88 0.12 ꢀ2.31
0.82 0.03 ꢀ2.31
ꢀ5.13
ꢀ2.49
329, 343,
417
ꢀ5.16
ꢀ5.15
ꢀ2.49
ꢀ2.48
352, 373,
393, 417
0.69
ꢀ
ꢀ2.32
a Measured at 10ꢀ5 M in toluene at 298 K. b Measured using an
integration sphere in degassed toluene under N2, excited at 417 nm.
c Relative to FeCp2
optical energy gap.
.
d Calculated from the LUMO level and the
0/þ
Figure 1. Crystal structure of compound 1 with 35% thermal
ellipsoids. Important bond lengths and angles: B(1)ꢀC(31)
1.621(6), B(1)ꢀC(38) 1.626(6), B(1)ꢀC(51) 1.637(6), B(1)ꢀN(2)
In the case of 3 the isomerization appears completely
suppressed, showing no observable spectral changes on
extended irradiation. These pronounced differences in
reaction rate are further highlighted in Figure 3, which
shows the percent conversion of each species to the dark
isomer over time, as well as the visual color changes
observable for solutions of 1ꢀ3 on exposure to UV light.
These changes were also monitored by fluorescence mea-
surements, which show 1 and 2 to undergo fluorescent
quenching upon isomerization, while the emission of 3
remains essentially constant (see SI).
˚
1.688(6) A; C(31)ꢀB(1)ꢀN(2) 94.3(3), C(38)ꢀB(1)ꢀC(51)
115.4(4)°.
These compounds show very bright greenish-yellow
fluorescence (λmax = 498ꢀ501 nm) with emission profiles
and quantum efficiencies similar to those of A1 (relative
quantum efficiencies obtained using A1 as the standard
under N2 are 1.00, 1.02, 0.88, and 1.12 for A1, 1, 2, and 3,
respectively). These data indicate that the singlet emission
from the boryl chromophore is largely unperturbed by the
presence of the aromatic groups in all three compounds.
However, the presence of the acceptor chromophores in
1ꢀ3 results in dramatically different photoisomerization
behavior. Upon irradiation at 419 nm in toluene (10ꢀ5 M),
compounds 1 and 2 undergo quantitative conversion to
1
The isomerization process was also monitored by H
NMR spectroscopy, by irradiating solutions of 1ꢀ3 in
C6D6 using a UV reactor at 419 nm. The NMR data show
that compounds 1 and 2 reach 85 and 38% conversion res-
pectively after 2 h of irradiation, with only trace amounts
of product observed for 3 (see SI). As with the parent A1
Figure 2. Changes in the absorption spectra of 1 (left), 2 (middle), and 3 (right) at 10ꢀ5 M in toluene on irradiation with a hand-held UV
lamp at 365 nm.
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Org. Lett., Vol. 14, No. 21, 2012

2.10, and 1.77 eV, respectively, for 1-phenylnaphthalene,
1-phenylpyrene, and 9,10-diphenylanthracene.
Using TD-DFT calculations, the lowest triplet state
localized on the boryl unit was found to lie consistently
at 2.13 eV for species A1 and 1ꢀ3 (see SI) representing the
T1 state for A1. However, the lowest 3πfπ* excited states
on the acceptor chromophores in 1ꢀ3 were calculated
at 2.63, 2.05, and 1.73 eV, respectively, similar to those
observed experimentally for the independent chromo-
phores.9ꢀ11 Relative to the lowest triplet state on the boryl
unit, the lowest triplet energy of the acceptor is thus much
higher in 1, much lower in 3, and approximately equal in 2.
This is consistent with the presence of a photoactive triplet
state in these compounds, as energy transfer to the triplet
excited state of the acceptor would be expected to be
negligible in 1, weak in 2, and dominant in 3 (Figure 4).
This may also explain the thermal irreversibility of com-
pounds 2 and 3 in which the low-lying triplet state of
the acceptor may be close to the activation energy of the
thermal reversal, thus suppressing the process. The experi-
mental and computational data therefore support that the
photoisomerization quantum efficiency may be effectively
controlled by varying the triplet energies of appended
triplet acceptors.
Figure 3. Left: Conversion of 1ꢀ3 to their dark isomers over a
20 min period under irradiation at 419 nm (10ꢀ5 M in toluene).
Right: Photos of 1ꢀ3 respectively after 10 min of irradiation
(10ꢀ4 M in toluene).
In summary, using a series of two-chromophore materi-
als, we have shown that the photoisomerization of N,C-
chelate dimesitylboranes likely proceeds via a triplet state.
In addition, we have shown that the photoreactivity of the
sample can be effectively modulated by controlling the
tripletꢀtriplet energy gap between the photochromic unit
and the triplet acceptor chromophore. This finding has
important implications for the design of photochromic
N,C-chelate boron compounds. With isomerization pro-
ceeding via a triplet state, it should be possible to either
sensitize or quench the photoisomerization using appro-
priate triplet sensitizers or acceptors.
Figure 4. A diagram showing the relative energy levels of the
3
photoactive triplet state (Tp) on the boryl unit and the πfπ*
state of the acceptor in 1ꢀ3. Energy transfer from Tp to 3πfπ*
partially quenches the photoisomerization in 2 and efficiently
suppresses it in 3, but does not interfere in 1.
chromophore, the thermal reversal of 1 is slow, achieving
only 33% conversion back to the light isomer after heating
at 40 °C for 20 h. In contrast, the thermal reversal of the
dark isomers of 2 and 3 is suppressed, with no significant
conversion back to their light isomers upon extended
heating under N2.
Consistent with their fluorescence spectra, TD-DFT
calculations indicate that 1ꢀ3 share a common S1 state
located at 2.77eV, centeredonthe boron chromophore. As
noted in literature, the singlet levels of 1-phenylnaphtha-
lene, 1-phenylpyrene, and 9,10-diphenylanthracene all lie
at higher energy (>3 eV).9ꢀ11
Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada and the Japan
Society for the Promotion of Science for financial support,
and Jia-Sheng Lu at Queen’s University for his assistance
with the crystal structural work for 1.
Supporting Information Available. Synthetic procedures,
fluorescence spectra, NMR isomerization experiments, and
TD-DFT data for 1ꢀ3. This material is available free of
charge via the Internet at http://pubs.acs.org.
The triplet energies of the acceptors, however, vary con-
siderably with respect to that of the borane, lying at 2.55,
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 21, 2012
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