Takuya Taniguchia,
Ayumi Kubotab,
Tatsuya Moritokic,
Toru Asahiad and
Hideko Koshima*d
aDepartment of Advanced Science and Engineering, Graduate School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
bDepartment of Life Science and Medical Bioscience, School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
cDepartment of Materials Science and Biotechnology, Graduate School of Science and Engineering, Ehime University, 3 Bunkyo-cho, Matsuyama, Ehime 790-8577, Japan
dResearch Organization for Nano & Life Innovation, Waseda University, 513 Wasedatsurumaki-cho, Shinjuku-ku, Tokyo 162-0041, Japan. E-mail: hkoshima@aoni.waseda.jp
First published on 5th October 2018
Photomechanical crystals are interesting from both basic and applied perspectives, and thus it is important to develop new examples. We investigated the photomechanical bending behaviour of a photochromic crystal of a dibenzobarrelene derivative. When a plate-like crystal was irradiated with ultraviolet (UV) light at 365 nm, two-step bending was observed. In the first step, the crystal quickly bent away from the light source, with an accompanying crystal colour change from colourless to purple. In the second step, under prolonged UV light, the bending returned slowly and then the crystal bent up towards the opposite direction, accompanied by an additional colour change to light yellow. Spectroscopic measurements and X-ray crystallographic analysis suggested that a long-lived biradical species is generated immediately upon UV light irradiation via a Norrish type II intramolecular hydrogen abstraction, and then the final photoproducts are formed under continuous UV exposure. X-ray crystallographic analysis before and after UV light irradiation for a few seconds revealed that the longitudinal axis (a axis) of the crystal elongated slightly after irradiation, which is consistent with the direction of the first-step bending. Based on these results, we propose that first-step bending could be induced by a biradical species, generated via a Norrish type II intramolecular hydrogen abstraction, and the second-step bending could originate from the formation of a mixture of final photoproducts under prolonged light irradiation.
Contrary to their appearance as solid but fragile objects, in the past decade molecular crystals have been attractive because they exhibit mechanical motion induced by external stimuli. Typical photochemical reactions, such as pericyclic reactions,3,4 trans–cis isomerisation,5,6 enol–keto isomerisation,7 [2 + 2] cycloaddition,8,9 and [4 + 4] dimerisation,10–12 induce various kinds of photomechanical motion in molecular crystals. Recent advances in photomechanical crystals have been summarised in several reviews.13–18 Although many photomechanical crystals have been developed so far, the types of photochemical reactions that have been used to achieve photo-induced motion are limited. Thus, there is a need to explore other photochemical reaction systems to evaluate a wider variety of mechanical behaviour.
Dibenzobarrelene derivatives are well known to partake in various photochemical reactions in solution, such as tri-π-methane rearrangement, di-π-methane rearrangement, Norrish type II reactions, and [2 + 2] cycloaddition.19–21 The solid state photochemistry of dibenzobarrelenes has also been well-studied, by Scheffer and co-workers.22,23 Photochromism in the solid state of a dibenzobarrelene derivative was reported in detail by Ramaiah and co-workers, in which the photochromism is caused by a long-lived triplet biradical, formed from a Norrish type II intramolecular hydrogen abstraction initiated by ultraviolet (UV) light irradiation.24 However, photochromism in other dibenzobarrelene derivatives has not been well-studied.
In this study, we investigated the photochromism and bending behaviour of a similar dibenzobarrelene derivative, namely, 11,12-dibenzoyl-9,10-dihydro-9,10-dimethyl-9,10-ethenoanthracene (1), which was reported to exhibit a colour change from colourless to purple after UV light irradiation (Fig. 1).24 In terms of photomechanical motion, the plate-like crystal of compound 1 was observed to bend quickly with the colour change (colourless to purple) initiated by UV light irradiation. Following this, the bent crystal slowly bent up towards the opposite direction, with additional colour changes from purple to yellow under prolonged UV light exposure. The mechanism of the two-step photomechanical bending motion of the compound 1 crystal is discussed with respect to spectral and crystal structure changes.
The absorption peak at 574 nm can be attributed to the extensive π-delocalisation of the biradical species between the triplet benzyl radical and the benzoyl groups, which are connected by the conjugated alkenyl double bond.24 The increase in peak intensity at 574 nm until 3 s after irradiation suggests that biradical 2 was produced immediately by a Norrish type II intramolecular hydrogen abstraction. The apparent half-life (τ1/2UV) of hydrogen abstraction from compound 1 to generate biradical 2 was estimated to be 0.12 s, based on the initial changes in peak intensity at 574 nm (Fig. S1b†). The peak at 430 nm can also be assigned to biradical 2, as reported previously.24,28,29
After stopping UV light irradiation for 3 s, the absorption peak at 574 nm decreased gradually with time, and its initial intensity returned completely within 3 days (Fig. 2e). However, the difference spectrum after 3 days indicates that spectral changes for the peaks at 340 and 400 nm clearly remained (Fig. 2f), especially at 340 nm, for which virtually no decrease was observed (Fig. S1g and h†); this suggests that final photoproducts were produced. This spectral behaviour indicates that biradical 2 largely reverted to initial compound 1, and that the remaining absorption peaks after 3 days should be assigned to π-conjugated final photoproducts produced by irreversible tri-π-methane and di-π-methane rearrangements, which are reported to occur in benzene solution (Fig. S3†).20 The photoproducts via the rearrangements in the crystals should be produced immediately upon UV irradiation based on the absorption increase at 340 nm by light irradiation (Fig. 2b). The half-life of thermal relaxation (τ1/2) from biradical 2 into compound 1 was estimated to be 30.6 min, based on the initial changes in peak intensity at 574 nm (Fig. S1c and d†); this is almost coincident with that (τ1/2 = 39.2 min) estimated at 430 nm (Fig. S1e and f†). The slow thermal relaxation of biradical 2 originates from the long lifetime of the triplet state biradical.
When the sample was irradiated for longer than 3 s, the peak intensity at 574 nm decreased gradually, but the intensities at 340 and 430 nm continued to increase with irradiation time (Fig. 2c). Difference spectra show these changes in the three peaks clearly (Fig. 2d). After irradiation for 60 s, the intensity at 430 nm increased the most, which is consistent with the observations of yellow-coloured material in the powder (Fig. S2b†).
The gradual decrease in the peak at 574 nm under prolonged UV light exposure might be due to the coupling of biradical 2 to produce cyclobutanol, which prohibits radical delocalisation (Fig. S3†). In terms of the photochemistry of benzophenone derivatives in the solid state, such cyclobutanol formation from biradical species has been well-documented.30–32 The continuous increase in the peak at 430 nm indicates that a biradical species is produced under light, but that the biradical cannot delocalise; this may be due to cyclisation. The continuous increase in the peak at 340 nm suggests that photoproducts of di-π-methane and tri-π-methane rearrangements are also continuously produced. Therefore, after prolonged light irradiation, the powdered sample could be a mixture of the initial compound, biradical 2, cyclobutanol, and the photoproducts from rearrangements (Fig. S3†).
The new peak at 1658 cm−1 might be attributed to biradical species 2. Here, the CO stretching vibrations of cyclobutanol and rearrangement photoproducts were not detected, probably due to low conversion rates or spectral overlap with the peaks of compound 1. These changes suggest that a Norrish type II intramolecular γ-hydrogen abstraction proceeded to decrease the CO bond strength of compound 1 to form biradical 2.
To analyse the intramolecular hydrogen abstraction reaction, we used four geometric parameters, d, Δ, θ, and ω (Fig. 4b), which were first introduced by Scheffer for the analysis of γ-hydrogen abstractions in the solid state.33 These parameters are defined as follows: d is the distance between the carbonyl oxygen and the γ-hydrogen atom, Δ is the CO⋯H angle, θ is the C–H⋯O angle, and ω is the angle by which the γ-hydrogen atom lies outside the mean plane of the carbonyl group (Fig. 4b). The previous paper reported that the favourable d is the sum of the van der Waals radii of hydrogen and oxygen atoms (2.72 Å), and that favourable angle geometries of ω, Δ, and θ are around 50°, 80°, and 115°, respectively.33
In Table 1, these parameters are summarised for compound 1. Here, only H1A and H2A (shown in Fig. 4a) were considered in the CO⋯H geometry, because the other methyl group hydrogen atoms are too far away from the carbonyl O1 and O2 atoms. The distances d of C1O1⋯H1A and C2O2⋯H2A are 2.656 and 2.874 Å, respectively, both of which are close to the favourable distance (2.72 Å). In terms of angle requirements, the values of ω, Δ, θ were 59.4°, 83.8°, and 111.3° for C1O1⋯H1A, and 63.6°, 72.7°, and 110.7° for C2O2⋯H2A, respectively. These angles are also similar to the favourable geometries. Based on this analysis, it is estimated that both carbonyl groups can abstract hydrogen atoms from methyl groups to produce biradical 2. The biradical species may cause further radical coupling, because the distances of both C1⋯C3 (2.92 Å) and C2⋯C4 (2.88 Å) allow for the formation of cyclobutanol, as reported in benzophenone crystals.30–32
d (Å) | ω (°) | Δ (°) | θ (°) | |
---|---|---|---|---|
C1O1⋯H1A | 2.656 | 59.4 | 83.8 | 111.3 |
C2O2⋯H2A | 2.874 | 63.6 | 72.7 | 110.7 |
Favorable geometrics33 | 2.72 | 50–60 | 80 | 115 |
After a second application of UV radiation for 1 s, the crystal bent up to 22.3° (Fig. 5f). The crystal bent slightly to 23.5° after the UV light was stopped (Fig. 5g). The bending returned gradually, reaching 12.2° in 24 h, with an accompanying colour change to colourless (Fig. 5h). Upon a third dose of UV exposure for 1 s, the crystal bent to 21.1°, and broke into two pieces in the next moment (Fig. 5i and j). The temporal profile of the tip displacement angle is summarised in Fig. S5b.† The tip displacement angle at maximum bending decreased with the increase of crystal thickness (Fig. S6†).
A two-step bending motion was observed when another thin plate-like crystal (length: 525 μm, width: 134 μm, thickness: 8.6 μm) was submitted to prolonged (1 min) UV light irradiation (Fig. 6 and Movie S2†). In the first step, the crystal underwent bending upon initial UV light irradiation for 1 s (Fig. 6a and b). Under prolonged UV irradiation, the crystal returned to the straight form with decreasing purple colour after 9 s (Fig. 6c), and then bent up in the opposite direction with an additional colour change to light yellow after 50 s (Fig. 6d). The yellow-coloured bent crystal did not change its colour or shape after the UV irradiation was stopped, indicating that the second-step bending is irreversible.
Fig. 6 Two-step bending of a plate-like crystal of compound 1 under continuous UV light exposure for 1 min. |
The molecular arrangement of the (001) plane before light irradiation is shown in Fig. 4c. The carbonyl CO group is aligned almost parallel to the a axis, which is the longitudinal direction of the crystal. After UV irradiation, biradical species 2 was not found, probably due to it not being present in sufficient quantities for structure determination, but the unit cell parameters changed slightly. The lengths of the a and b axes were elongated by 0.15% and 0.08%, respectively, and the c axis shortened by 0.19%. This lattice change could have been induced by biradical species 2, formed via an intramolecular γ-hydrogen abstraction, as reported by Hosoya and his co-worker.34 They determined the crystal structure of compound 1 after UV light irradiation at low temperature (90 K), and detected an electron density peak of the methylene radical (·CH2–) as the disorder of the methyl group (CH3) and the methylene radical. The abstracted hydrogen atom was also detected as a small electron density peak near the benzoyl oxygen atom with a distance of 0.967(2) Å, forming the benzyl-type radical. This result suggested that the γ-hydrogen atom transferred from the methyl group to the benzoyl oxygen atom, and that biradical 2 was produced during the hydrogen abstraction. Based on these results, biradical 2 might move away surrounding molecules along the a axis due to hydrogen transfer to the carbonyl CO, which is aligned almost parallel to the a axis. This molecular change should induce the first-step bending due to elongation of the a axis at the irradiated surface.
The second-step bending could have been induced by the formation of the photoproduct mixture via radical coupling of the biradical 2, di-π-methane rearrangement, and tri-π-methane rearrangement (Fig. S3†). The bending speed and colour change observed during the second-step bending are in reasonably good agreement with the measured changes in UV-vis and IR spectra.
Footnote |
† Electronic supplementary information (ESI) available. CCDC 1860420–1860422. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8ra06639f |
This journal is © The Royal Society of Chemistry 2018 |