Stealth fast photoswitching of negative photochromic naphthalene-bridged phenoxyl-imidazolyl radical complexes

Article abstract of DOI:10.1039/c6cc01534d

Naphthalene-bridged phenoxyl-imidazolyl radical complex (Np-PIC) is a novel fast switchable negative photochromic compound, which shows the thermal back reaction in the millisecond time scale. Upon UV light irradiation, Np-PIC shows the hypochromic effect

Full text of DOI:10.1039/c6cc01534d

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Stealth fast photoswitching of negative
photochromic naphthalene-bridged phenoxyl-
imidazolyl radical complexes†
Cite this: Chem. Commun., 2016,
2, 6797
5
Received 19th February 2016,
Accepted 20th April 2016
a
a
b
b
a
Katsuya Mutoh, Yoichi Kobayashi, Yasukazu Hirao, Takashi Kubo and Jiro Abe*
DOI: 10.1039/c6cc01534d
www.rsc.org/chemcomm
Naphthalene-bridged phenoxyl-imidazolyl radical complex (Np-PIC) coloration reactions of most reported examples of negative photo-
is a novel fast switchable negative photochromic compound, which chromism range from minutes to hours.
shows the thermal back reaction in the millisecond time scale. Upon
Recently, we have developed novel fast switchable negative
8
UV light irradiation, Np-PIC shows the hypochromic effect in the photochromic compounds based on the photochromism of
UVA region due to there being less conjugation in the transient the bridged imidazole dimer and the phenoxyl-imidazolyl radical
isomer. By replacing the phenoxyl unit with a naphthoxyl unit, the complex (PIC).
molecular structure has an asymmetric carbon, leading to fast Scheme 1a) shows a fast thermal back reaction with a half-life
chiroptical switching. This simple molecular design will be a good of 1.9 s. Upon irradiation with light, the stable colored isomer
candidate for the future development of negative photochromic generates the short-lived biradical species (t1/2 = 240 ns at 298 K).
2
a,9
0
The 1,1 -binaphthyl-bridged PIC (BN-PIC,
8
b
compounds.
The colorless isomer is kinetically generated after the thermal
recombination reaction of the biradical species since the activa-
Photochromism has been of interest not only in fundamental tion free energy required to isomerize into the colorless isomer is
1
studies but also for industrial applications. In general T-type lower than that into the initial colored isomer. The colorless
photochromic compounds, the stable colorless isomer photo- isomer is thermodynamically less stable as compared to the
isomerizes into the less stable colored isomer upon UV light colored isomer, leading to thermal isomerization into the colored
irradiation. Fast switchable T-type photochromic compounds isomer through the thermally accessible biradical species, in the
2
have potential applications in ophthalmic lenses, fluorescence second time scale. While this unprecedented fast switchable
3
4
switching, real-time holographic materials, as the photo-
5
trigger for light induced macroscopic structural changes, and
in the control of biological functions due to the instantaneous
6
changes in their electronic and molecular structures. However,
the excitation UV light cannot penetrate deeper into the inside
of the material because of the reabsorption of the excitation
light by the generated colored isomer. This issue limits the
application of fast T-type photochromism in bulk materials.
The negative photochromic reaction, which causes a decoloration
process upon irradiation with light in contrast to conventional
photochromic systems, is one of the appropriate approaches to
7
overcome this issue. However, the time scales for the thermal
a
Department of Chemistry, School of Science and Engineering, Aoyama Gakuin
University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara, Kanagawa 252-5258, Japan.
E-mail: jiro_abe@chem.aoyama.ac.jp; Fax: +81-42-759-6225; Tel: +81-42-759-6225
b
Department of Chemistry, Graduate School of Science, Osaka University,
1
-1 Machikaneyama, Toyonaka, Osaka 560-0043, Japan
Electronic supplementary information (ESI) available: Syntheses, characteriza-
tion data, spectroscopic measurements, and DFT calculations. CCDC 1454173 (1),
454174 (2), 1454175 (byproduct 1), 1454176 (byproduct 2). For ESI and crystallo-
†
1
graphic data in CIF or other electronic format see DOI: 10.1039/c6cc01534d
Scheme 1 Photochromic reaction schemes of (a) BN-PIC, (b) 1, (c) 2 and 3.
Chem. Commun., 2016, 52, 6797--6800 | 6797
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negative photochromism has the attractive potential to be applied
to real-time holographic materials and photoresponsive chiral
10
dopants for liquid crystals, an increase in the rate of the thermal
back reaction to the time scale of milliseconds (video frame rate =
30 fps) is required for these applications.
In this study, we synthesized the fast switchable negative
photochromic compounds, naphthalene-bridged phenoxyl-
imidazolyl radial complex (Np-PIC, 1 in Scheme 1b) and
naphthalene-bridged naphthoxyl-imidazolyl radical complex Fig. 2 (a) Steady-state absorption spectra of 1, 2 and 3 in Me-THF at 153 K
(
Np-NIC, 2 and 3 in Scheme 1c). Compounds 1–3 show the (solid), and the absorption spectra at the PSS after 365 nm UV light
irradiation (25 mW) in Me-THF at 153 K. Each absorbance was normalized
hypochromic effect in the UVA region after UV light irradiation
and a fast thermal back reaction in the millisecond time-scale.
There are few reports on invisible photochromism. Invisible
ꢁ
5
by dividing with the concentration of each solution (1: 2.9 ꢀ 10 M, 2:
ꢁ5
ꢁ5
3
.9 ꢀ 10 M, 3: 2.5 ꢀ 10 M). (b) Time profiles of the transient absorbance
ꢁ5 ꢁ5 ꢁ5
1
1
at 380 nm (1: 7.4 ꢀ 10 M, 2: 5.9 ꢀ 10 M, 3: 4.9 ꢀ 10 M) in Ar-saturated
photochromism has advantages for use as a light-trigger benzene excited by a 355 nm laser pulse (5 ns, 7 mJ) at 298 K.
because the visible light absorption disturbs the performance
of opto- or electro-molecular devices used under room light,
and disturbs the spectroscopic analysis of combined chromo- the naphthalenone ring. The absorption band in 1–3 at around
phores in some cases. Moreover, Np-NIC possesses an asymmetric 360 nm is the S - S transition which can be attributed to the
0 2
carbon atom by replacing the phenoxyl unit with a naphthoxyl transition from the molecular orbital (MO) delocalized over the
unit. That is, fast chiroptical switching with a negative photo- diphenyl imidazole ring and the naphthalene ring to the MO
chromic reaction is also expected.
localized over the naphthalene ring, using TDDFT calculations
The target compounds were synthesized according to (Fig. S33, MPW1PW91/6-31+G(d)//MPW1PW91/6-31G(d), ESI†).
Schemes S1–S3 (ESI†). The molecular structures of 1 and 2 Fig. 2a shows the steady-state UV-vis absorption spectra of the
were determined using X-ray crystallographic analysis as shown Me-THF solutions of 1–3 and those at the photostationary state
in Fig. 1. Compounds 1 and 2 have an intramolecular C–N bond (PSS) under 365 nm UV light (25 mW) at 153 K. The UV light
between the 1-position of the imidazole ring and the 4-position irradiation leads to the disappearance of the CT absorption
of the cyclohexadienone or naphthalenone ring (1,4-isomer). bands at 360 nm. The absorption bands of 1–3 were thermally
The C–N bond lengths and the CQO bond lengths of 1 and 2 restored with half-lives of 499, 68, and 175 ms at 298 K in
were estimated to be dC–N(1) = 1.482 Å, dC–N(2) = 1.480 Å, dCQO(1)
=
benzene, respectively (Fig. 2b). The time profiles of the transient
1
.228 Å and dCQO(2) = 1.225 Å, respectively. These bond lengths are absorbance obey first-order reaction kinetics (Fig. S31, ESI†).
9
almost identical with those of PIC derivatives. The diphenyl That is, the initial 1,4-isomers isomerize into the thermally
imidazole ring is coplanar to the naphthyl moiety, while these two unstable transient species upon UV light irradiation. The activa-
parts are perpendicular to the cyclohexadienone or naphthalenone tion energies for the thermal back reactions of the transient
ring. As discussed later, the Cotton effect is induced in the CD species generated from 1, 2, and 3 were estimated to be 72.2,
ꢁ
1
spectrum of 2 by this chiral molecular structure. It is noteworthy 67.3, and 69.6 kJ mol at 298 K, respectively (Fig. S36, ESI†).
that the chiral crystal can be obtained via recrystallization from the These compounds show excellent durability against repeated
racemic mixture.
exposure to 355 nm laser pulses (10 Hz, 5 mJ) in spite of the
The steady-state UV-vis absorption spectra of 1–3 in 2-methyl absence of tert-butyl groups in 1 and 2 (Fig. S32, ESI†). As shown
tetrahydrofuran (Me-THF) at 298 K are shown in Fig. S30 (ESI†). in Fig. S34 (ESI†), the absorption spectrum at the PSS of 1 can be
All of the compounds show the characteristic absorption bands well explained by the TDDFT calculations (MPW1PW91/6-31+G(d)//
at around 360 nm. The molar extinction coefficients of 1–3 MPW1PW91/6-31G(d)) for the optimized structure of the 2,4-isomer,
4
4
at 360 nm were estimated to be 1.3 ꢀ 10 , 1.5 ꢀ 10 and 1.5 ꢀ in which the 2-position of the imidazole ring connects with the
4
ꢁ1
ꢁ1
0
1
0 M cm , respectively. The increase in the molar extinction 4-position of the cyclohexadienone ring (1 , Scheme 1b). In
coefficients of 2 and 3, compared with that of 1, is due to the contrast to the 1,4-isomer, the permitted transition in the UVA
extension of the aromatic system from the cyclohexadienone to region is not observed due to there being less conjugation
between the imidazole and naphthalene rings in the 2,4-isomer.
Here, we tentatively assign the molecular structure of the transient
species as the 2,4-isomer, and carried out NMR and time-resolved
infrared (TRIR) spectroscopy to determine the structure of the
transient species.
1
13
The H and C NMR characterizations for the transient
species of 1 were carried out using a 400 MHz NMR spectro-
meter coupled with in situ UV light irradiation (Fig. 3). To
inhibit the thermal back reaction of the transient species, the
solution was cooled to 200 K. After UV light irradiation, the
NMR signal assigned to 1 completely disappeared and the new
Fig. 1 ORTEP representations of the molecular structures of 1 and 2 with
thermal ellipsoids (50% probability), where nitrogen and oxygen atoms are
highlighted in blue and red, respectively.
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Fig. 4 Reaction scheme to generate the byproducts and the ORTEP
representations of the molecular structures of the byproducts generated
from 1 with thermal ellipsoids (50% probability), where nitrogen, oxygen
and chlorine atoms are highlighted in blue, red, and green, respectively.
1
13
Fig. 3 H and C NMR spectra of the Me-THF solution of 1 and those at
the PSS under UV light irradiation (365 nm, 4 mW) at 200 K. The theoretical
1
3
C peak was calculated for the optimized molecular geometry of the
,4-isomer.
of 1 in CD
CH Cl /AcOEt = 20/1). The X-ray crystallographic analysis of
byproduct 1 revealed that the imidazole ring was decomposed
2 2 2
Cl at 200 K, using SiO column chromatography
2
(
2
2
signals appeared. The two doublet peaks assigned to the by the reaction of the photogenerated imidazolyl radical with
protons of the cyclohexadienone ring were shifted after the molecular oxygen (Fig. 4) because the lifetime of the photo-
photo-isomerization reaction, indicating that there was a change generated biradical species of 1 would increase enough to react
in the binding mode between the imidazole ring and the with molecular oxygen at 200 K. To inhibit the reaction of
1
3
hexacyclodienone ring. The C NMR spectrum after UV light the biradical species with the molecular oxygen, the CD
irradiation shows a single characteristic peak at 117.19 ppm. The solution was degassed using freeze–pump–thaw cycles. How-
calculated chemical shift, subtracted from the shielding value ever, 1 quickly reacts with CD Cl upon UV light irradiation at
obtained for TMS by the GIAO-DFT (MPW1PW91/6-31+G(d,p)// 193 K. The X-ray crystallographic analysis for another byproduct 2
2 2
Cl
2
2
1
3
MPW1PW91/6-31G(d) level of the theory), indicates that the
NMR peak at 117 ppm can be attributed to the sp carbon on the moiety is substituted with a chlorine atom (Fig. 4), which is also
C
indicates that a hydrogen atom at the 4-position of the naphthalene
3
imidazole ring of the 2,4-isomer. The colorless isomer of BN-PIC supported by HR-ESI-TOF-MS (Fig. S25, ESI†). These results show
3
which has a similar sp carbon on the imidazole ring also shows the generation of the biradical species as short lived intermediates
1
3
8b
a single C NMR peak at around 110 ppm. We also carried out upon UV light irradiation as shown in Scheme 1.
time-resolved FT-IR spectroscopy (see ESI†). We observed a shift
Because it is difficult to estimate the extinction coefficient of
of the CQO stretching vibration of the cyclohexadienone ring the short-lived biradical species, the quantum yield for the
upon UV light excitation, indicating a change in the binding photogeneration of the biradical species could not be definitely
mode between the imidazole ring and the cyclohexadienone determined. In contrast, the conversion efficiencies of 1, 2, and
ring as well as the result of the NMR spectroscopy. Moreover, 3 from the 1,4-isomer to the 2,4-isomer can be experimentally
ꢁ
1
15
the characteristic band at 1557 cm in the transient IR spectrum estimated using laser actinometry. The conversion efficiencies of
can be ascribed to the C–N stretching vibration on the imidazole 1, 2, and 3 were estimated to be 0.18, 0.06, and 0.04, respectively,
ring of the 2,4-isomer. Therefore, we concluded that the by using benzophenone as a standard (see experimental details in
2,4-isomer is generated by UV light irradiation of the 1,4-isomer. ESI†). DFT calculations suggest that the activation free energy
The nanosecond laser flash photolysis measurements of 1–3 barrier for the thermal recombination reaction from the biradical
did not show any absorption bands assigned to the biradical species into the 2,4-isomer is smaller than that into the 1,4-isomer
species. However, the biradical species should be generated (Fig. S51, ESI†). This result supports the idea that the 2,4-isomer is
just after the C–N bond cleavage reaction of 1–3 as we have kinetically generated through the thermal recombination reaction
definitely demonstrated the generation of the biradical species of the photogenerated biradical species upon UV light irradiation.
9
in the photochromic reactions of the PIC derivatives. Efficient The thermodynamics and reaction kinetics of the negative photo-
demonstration of the generation of the radical species is chromism are discussed in ESI.†
1
2
necessary to reveal the specific reaction of the radical species.
As previously reported, the triphenylimidazolyl radical reacts negative photochromism of 2 which has chirality originating
with molecular oxygen, resulting in the generation of O from the asymmetric carbon. Both enantiomers of 2 were
The typical reaction between the imid- optically resolved using chiral HPLC and recrystallization.
Finally, we demonstrated chiroptical switching with the
2
1
3,14
adducted products.
azolyl radical and molecular oxygen produces a byproduct The optical purities of the R- and S-enantiomers were up to
which has a dioxetane structure, which demonstrates the 99% ee (Fig. S29, ESI†). Fig. 5 shows the circular dichroism (CD)
1
4
thermal degradation of the imidazole ring. Therefore, we spectra of the Me-THF solution of 2 and those at the PSS upon
isolated byproduct 1 generated after 365 nm light irradiation UV light irradiation at 153 K. The absolute configuration of
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This work was supported partly by the Core Research for
Evolutionary Science and Technology (CREST) program of the
Japan Science and Technology Agency (JST), a Grant-in-Aid for
Scientific Research on Innovative Areas ‘‘Photosynergetics’’
(No. 26107010) from MEXT, Japan, and the MEXT-Supported
Program for the Strategic Research Foundation at Private Uni-
versities, 2013–2017.
Notes and references
Fig. 5 CD spectra of the Me-THF solution of 2 (solid line) and those at the
PSS (dashed line) under 365 nm light irradiation (10 mW) at 153 K.
1
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Cotton effect with the configuration estimated from the TDDFT
calculation (Fig. S48, ESI†). The large Cotton effect mainly
originates from the electronic transition between the diphenyl-
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switching of the fluorescence and for holographic materials.
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