Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Single-Crystal-to-Single-Crystal Transformation of HydrogenBonded Triple-Stranded Ladder Coordination Polymer via Photodimerization Reaction Hasan Ali Al-Mohsin,† Ammar AlMousa,† Sulayman A. Oladepo, Almaz S. Jalilov, Mohammed Fettouhi, and Abdul Malik P. Peedikakkal* Department of Chemistry, King Fahd University of Petroleum and Minerals, P.O. Box 5048, Dhahran 31261, Saudi Arabia Downloaded via NOTTINGHAM TRENT UNIV on July 18, 2019 at 14:58:00 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
S Supporting Information *
ABSTRACT: A one-dimensional hydrogen-bonded triplestranded ladder coordination polymer [Cd(bpe)1.5(NO3)2(H2O)] (1) (where bpe = trans-1,2-bis(4-pyridyl)ethylene) containing three parallel CC double bonds was synthesized. This compound undergoes photochemical [2 + 2] cycloaddition and produces rctt-tetrakis(4-pyridyl)cyclobutane (rctt-tpcb) in up to 67% yield via Single-Crystal-to-Single-Crystal (SCSC) transformation. Triple-stranded ladder-like structures have never before displayed such a kind of SCSC transformation. Furthermore, photoirradiation of ground 1 produces rctt-tpcb in up to 100% yield in the solid state. On the basis of the alignment of three CC olefinic bonds of bpe ligands in parallel, only two out of the three aligned bpe are expected to undergo [2 + 2] photodimerization. However, the quantitative yield from the solid-state photochemical [2 + 2] cycloaddition reaction has been achieved via grinding of crystals of 1 to a powder. The effects of grinding on photoreactivity of 1 were thoroughly studied using 1H NMR spectroscopy, thermogravimetric analysis (TGA), and Raman spectroscopy. These studies indicate that the molecular movements of the hydrogen-bonded ladders are reinforced due to the loss of coordinated water molecules and the further crystal repacking via bond-breaking/forming of the hydrogenbonded assemblies during mechanical grinding. The 100% photodimerization of ground 1 shows that the grinding accelerates internal molecular motions of ladder structures within the crystals lattice. The solid-state photoluminescence of 1, before and after UV irradiation, was investigated at room temperature, both indicative of interesting luminescent properties.
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INTRODUCTION Single-crystal-to-single-crystal (SCSC) structural transformations induced by light, heat, and guest removal have received considerable interest in recent years,1−3 particularly, light triggered SCSC transformations, which are displayed in several coordination polymers.4,5 Since Schmidt’s postulates on topochemical reactions,6,7 photochemists have been exploring the photochemical behavior of compounds with olefinic CC double bonds alignment that can undergo solid-state [2 + 2] cycloaddition reaction to unravel its mysteries.8−11 Several supramolecular interactions have been investigated for this purpose, such as hydrogen bonding, halogen bonding, π−π interactions, and coordination bonding in both discrete and infinite supramolecular structures.4,8,10−15 In the past decades, many photoreactive coordination polymers (CPs) have been synthesized and their photoreactivity has been extensively studied.4,8,11,14,15 However, molecular-ladder CPs have attracted chemists’ attention due to their unique optical,16,17 electronic, and magnetic properties.18−21 They have several classifications based on the connectivity of metal with ligands in the rails and rungs.22−24 Although one-dimensional ladder CP topology is uncommon, ladder CPs are considered some of the best-known assemblies © XXXX American Chemical Society
to orient olefinic CC double bonds in infinite pairs for photochemical [2 + 2] cycloaddition reaction.15,25−30 The photoreactivity of three parallel aligned CC bonds in ladder CPs was rarely studied. We have studied the remarkable photoreactivity of triple-stranded CP, [Pb3(bpe)3(O2CCF3)4(O2CCH3)2],15 however, in which SCSC transformation was not successful due to the loss of crystallinity. Nevertheless, quantitative solid-state photodimerization might not occur because of unfavorable distances and/or the orientation of CC olefinic bonds in the crystal lattice. According to the Schmidt’s topochemical postulate, minimal internal molecular or atomic movement is required for photodimerization reaction to occur, where the distance between a couple of CC double bonds is between 3.5 and 4.2 Å.7 However, several solid-state photoreactions yield quantitative products, which still cannot be explained by a topochemical postulate.31 Such photoreactions require large internal molecular or atomic movements in the crystal lattice to achieve a quantitative yield photochemically.32,33 Such motions are induced by a variety of external forces, such as Received: May 1, 2019
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DOI: 10.1021/acs.inorgchem.9b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
C18H17N5O7Cd (527.77). Calcd C 40.96, H 3.25, N 13.27, Found: C 40.90, H 3.34, N 13.06%. Selected IR/cm−1 (KBr): 2426m, 1943w, 1767w, 1604s, 1558m, 1504s, 1379s, 1291s, 1220s, 1206s, 1095w, 1074m, 1034w, 1011s, 976s, 886w, 841s, 827s, 717w, 548s. 1H NMR (DMSO-d6, 500 MHz): δH 8.59 (d, 4H, δ-pyridyl proton), 7.60 (d, 4H, δ-pyridyl proton), 7.53(s, 2H, CHCH). 13C NMR (DMSO-d6, 500 MHz): δc 150.52 (δ-pyridyl carbon), 144.97 (δ-pyridyl carbon), 130.99 (CC), 121.80 (δ-pyridyl carbon). Synthesis of [Cd(rctt-tpcb)1.5(NO3)2(H2O)] (2). Manually ground samples of 1 in a pestle and mortar for 10 min were exposed to UV light for 5, 10, 20, 30, 40, and 50 h. The product 2 obtained after 50 h irradiation for ground 1 has Elemental Anal. for C18H17N5O7Cd·2.35 H2O. Calcd C 37.92, H 3.84, N 12.28, Found: C 37.73, H 3.15, N 11.60%. Selected IR/cm−1 (KBr): 2926w, 1612m, 1558w, 1422w, 1384s, 1224w, 1111w, 1069w, 1018w, 829w, 697w, 552w. 1H NMR (DMSO-d6, 500 MHz): δH 8.32 (d, 4H, δ-pyridyl proton), 7.23 (d, 4H, δ-pyridyl proton), 4.66 (s, 4H, CH-CH). 13C NMR (DMSO-d6, 500 MHz): δc 149.56 (δ-pyridyl carbon), 123.79 (δ-pyridyl carbon), 44.45 (CH-CH). UV Irradiation Experiments. The single crystal was placed on a Pyrex glass slide and UV irradiated in the photoreactor for 5, 10, 20, 30, 40, and 50 h. These samples were used for NMR studies, while another 50 h irradiated single crystals sample was used for TGA analysis. Powder samples were obtained by manually grinding single crystals of 1 for 10 min in a mortar and then exposing them to UV irradiation for 5, 10, 20, 30, 40, and 50 h to study the photoreactivity of bpe conversion into rctt-tpcb. Single crystals of 1 were manually ground for 20 and 30 min and were exposed to UV irradiation for 5 h. However, the 10 and 20 min ground samples of 1 were UV irradiated for 50 h for TGA analysis. For SCSC experiments, the single crystals were irradiated for 25 h and the single crystal X-ray analysis was conducted. NMR Studies. The 1H NMR spectra were used to determine the product distribution of the photodimerization reaction of 1 in several experimental conditions as shown in Table S1. The product percentages were calculated as the ratio of the reactant peaks to the peaks of the product. The values were acquired by integrating the intensities of two doublets and singlet peaks for each set of bpe, products and then averaged.
mechanical grinding or light irradiation. Kaupp et al. investigated the mechanically prompted molecular migrations in the solid state by the AFM technique.34 The movements require void volume or empty space in the crystal lattice,34−37 as well as weak supramolecular interactions that can be fragmented to promote molecular migration in the solid state.35 While exploring the photoreactivity of coordination polymers of bpe, we found that the olefinic CC bonds of bpe in a known one-dimensional hydrogen-bonded triplestranded-like ladder CP, namely, [Cd(bpe)1.5(NO3)2(H2O)] (1)38 (where bpe is trans-1,2-bis(4-pyridyl)ethylene), aligned closely in CC nonbonded distances of 3.769 Å with a parallel orientation. When the single crystals of 1 were exposed to UV irradiation for 50 h, rctt-tetrakis(4-pyridyl)cyclobutane (rctt-tpcb) was achieved in 67% yield via SCSC transformation, and such SCSC transformation is observed for the first time in this triple-stranded ladder. However, the photoirradiation for 50 h of the sample that was ground for 10 min produces an rctt-tpcb photoproduct in 100% yield. The effects of grinding on the photoreactivity of the hydrogen-bonded triple-stranded ladder coordination polymer were thoroughly studied using thermal analysis, and 1H NMR and Raman spectroscopy. The quantitative photoreaction of ground 1 can only be explained due to the mechanically induced large molecular movement of ladders in the crystal lattice; further photoirradiation formed rctt-tpcb in up to 100% yield. We have also investigated the photoluminescence (PL) properties of 1 before and after dimerization. We present the details of our investigation below.
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EXPERIMENTAL SECTION
Materials and Methods. The ligand and metal salts were obtained commercially and used without further purification. Powder diffraction data were collected using a Rigaku Miniflex II diffractometer equipped with a Cu−Kα radiation source. Data were acquired over the 2θ range between 3° and 50°. UV irradiation data were collected by using a Luzchem LZC-DEV photoreactor, where 4−5 mg of sample was packed between two slides of Pyrex glass and kept standing on its side edge so it can be irradiated on both sides simultaneously by the UV lamps. The 1H NMR spectra were obtained by a JEOL JNM-LA500 spectrometer. The yields of UV irradiation experiments were calculated by integrating the relative spectral peaks of each product from 1H NMR. FT-IR spectra were obtained by a PerkinElmer 16F PC FT-IR spectrometer using KBr pellets. The grinding was performed manually with a pestle and mortar. Thermogravimetric analysis (TGA) was performed on a TA SDT 2960 thermal analyzer. The samples were ground right before TGA experiments to minimize exposure to moisture. About 3−5 mg of each sample was heated to 600 °C at a rate of 5 °C·min−1 while under a flow of nitrogen atmosphere at a rate of 50 mL/min. Raman measurements were carried out on a Horiba LabRAM HR Raman spectrometer using a 532 nm excitation laser with a 50× LWD microscope objective. Room temperature solid-state photoluminescence (PL) measurements were performed on the crystalline powder of 1 before and after irradiation, using a HORIBA Jobin Yvon NanoLog spectrofluorometer using a 450 W xenon lamp at 350 nm excitation wavelength. Synthetic Procedures. Caution! Cadmium compounds are toxic and should only be used in small amounts and handled with caution. Synthesis of [Cd(bpe)1.5(NO3)2(H2O)] (1). The single crystals of 1 were obtained based on a reported procedure with minor modification.38 A solution of bpe (13 mg, 0.075 mmol) in ethanol (0.5 mL) was carefully layered over an aqueous solution (0.5 mL) of Cd(NO3)2·4H2O (15 mg, 0.05 mmol). Colorless block crystals were obtained after 3 days, which were decanted and washed with diethyl ether and dried in air. Yield: 0.018 g, 69% Elemental Anal. for
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RESULTS AND DISCUSSION Single crystals of [Cd(bpe)1.5(NO3)2(H2O)] (1) were grown with slight modification from the reported procedure.38 The bulk characteristics of 1 were confirmed by powder X-ray diffraction (PXRD) measurements (Figure S1). The PXRD patterns of as-synthesized samples match well with the simulated patterns from crystal data, suggesting that the crystal is characteristic of the pure bulk compound. The X-ray structure of 1 reveals that the cadmium(II) ion is bonded to two nitrogen atoms, each belonging to a pyridyl donor of a bpe ligand, in addition to one oxygen atom of an aqua ligand, and three nitrato oxygen atoms. The resulting geometry is distorted octahedral. Two of the three nitrato ligands are related by a center of symmetry and adopt a μ2-O bridging coordination mode connecting two Cd(II) centers to form a centrosymmetric Cd2O2 core. The intracore interionic Cd(II)···Cd(II) distance is 4.278(6) Å. These bimetallic cores are bridged by one centrosymmetric bpe ligand connecting the metal ion center of the core to its adjacent core neighbor metal ion, hence generating a coordination polymeric stair-type chain, propagating in the ab direction. The second bpe ligand, coordinating in a terminal mode, is engaged by its uncoordinated pyridyl moiety in O−H···N [O1W−H1W··· N3, 2.758 Å] hydrogen bonding interactions with the aqua ligand of the adjacent bimetallic core neighbor. This interaction results in bpe-Cd-bpe-Cd-bpe linear structural units to form a hydrogen-bonded triple ladder as shown in B
DOI: 10.1021/acs.inorgchem.9b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Figure 1. As a result, the centrosymmetric bpe pairs in the triple strands are oriented closely with a distance of 3.753 Å
Figure 1. Illustration of a perspective view of the packing structure of two adjacent ladders. The C-H hydrogen atoms have been omitted for clarity.
Figure 2. Illustration of a perspective view of SCSC transformation of 1 to 2a. The disordered cyclobutane carbons are given in yellow color. The hydrogen atoms have been omitted for clarity.
(d1) between the centers of CC bonds within the strand. The olefinic CC bonds of this three bpe are aligned in parallel in the ladder, and they are also reinforced by strong π···π interactions between the pyridyl rings. The adjacent ladders interact via O−H···O [O1W−H2W···O4, 2.825 Å] hydrogen bonding interactions with one of the hydrogen atoms of the adjacent coordinated water molecules and the terminal nitrate anions. As a result, 1 forms a supramolecular hydrogen-bonded network. The hydrogen-bonded triple-stranded-like ladder CPs are assembled along the ac plane as presented in Figure 1 with a minimum nonbonded Cd−Cd distance of 6.881(9) Å between the two interstrands. This confirms that the bpe linkers in the neighboring 1D ladder are oriented in parallel but then slipstacked with a centroid distance of 4.654 Å (d2) among the olefinic CC bonds of bpe in the neighboring 1D ladders. Thus, it is not expected for a pair of CC double bonds from adjacent 1D ladders to undergo photochemical reaction to form a cyclobutane derivative in the solid state. Single-Crystal-to-Single-Crystal Transformation. Guided by the fact that the assembly of three almost parallel aligned olefinic CC bonds of bpe ligands in the triple ladder motif obeys the distance criterion for the Schmidt photochemical [2 + 2] cycloaddition reaction in the solid state, photoreaction experiments were therefore carried out. When the single crystals of 1 were subjected to UV exposure for 5 h, the crystals of 1 underwent photoreaction to yield 40% rctt-tpcb, as observed from 1H NMR spectroscopy (Table S1). Further UV exposure of the single crystals of 1 for 50 h produced 67% rctt-tpcb as shown in Figure S2. This reaction was achieved through SCSC transformation that yields the crystals of [Cd(rctt-tpcb)(bpe)0.5(NO3)2(H2O)] (2a) with two bpe reacting to produce rctt-tpcb out of three bpe ligands. The product, rctt-tpcb ligand, maintains its coordination to the cadmium centers after the cycloaddition process, as shown in Figure 2. Single-crystal X-ray analysis shows that 2a is similar to 1; it belongs to the same triclinic system with similar P1̅ in the volume and an increase in unit cell length along a and a decrease along b. A randomly directional photochemical dimerization took place and involved the central bpe molecule of the triple ladder motif of 1 with one of its two adjacent bpe neighbors. Accordingly, a 50% disordered double site occupancy was found for the internal ethylenic cyclobutane carbons of the rctt-tpcb product molecule in 2a. Furthermore, the intracore distance between the two nitratobridged Cd(II) centers increased from 4.279 to 4.309 Å (see
the Supporting Information). The separation between the Cd(II) centers of adjacent ladders decreased from 6.882 to 6.804 Å after dimerization via SCSC transformation. The distances between the pyridyl rings in the adjacent ladders were reduced after topochemical photodimerization (from 3.770 to 3.614 Å), probably due to the steric influence experienced during the formation of cyclobutane rings. The observed 67% of conversion in the formation of rctttpcb is consistent with the illustrated crystal structure of 1 since only one bpe pair is predicted to go through a dimerization reaction and the third coordinated bpe ligand is expected to remain unreacted in the crystal lattice. When the crystals of compound 1 were crushed to a powder for 10 min and exposed to UV light for a time of 5 h, 65% dimerization was observed. However, when the powdered 1 was UV exposed for a time of 50 h, 100% photoreaction was achieved as shown in Table S1 and Figure S2. The 1H NMR spectrum of the UV dimerized product dissolved in DMSO-d6 displays the whole disappearance of the olefinic protons at 7.53 ppm, the presence of the cyclobutane protons at 4.65 ppm, and the consistent movement in the bipyridyl protons from 8.60 and 7.61 ppm to 8.32 and 7.22 ppm, respectively. Quantitative photoreactivity of the crushed sample related to the behavior of a single crystal is relatively unpredicted based on the crystal packing in 1. In order to unravel the rare photoreactivity of 1, the photodimerization of single crystals and powdered single crystals (physically ground for 10 min) was examined by 1H NMR spectroscopy (Figures S3 and S4) at a number of time intervals as reported earlier for the triple-stranded ladder.15 A percentage conversion versus time plot presented in Figure 3a evidently shows that the photoreaction takes place in one step in single crystals, whereas two steps are involved in the powdered samples. A prolonged UV irradiation of a single crystal in 1 did not produce higher than 67% dimer, which indicates that the molecular movements were not initiated by further exposure to UV light. However, such molecular movements introduced by UV exposure that result in quantitative conversion of bpe was observed in the Pb(II) triple-stranded ladder CP.15 Although the PXRD of the samples ground for 5, 10, 20, and 30 min (Figure S5) does not show any possibilities of notable phase transition, the intensities of the peaks became weaker than that of the crystalline powder perhaps due to microstrain or crystal defects. The PXRD of the unground single crystals irradiated C
DOI: 10.1021/acs.inorgchem.9b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 1. Possible Photoreaction Pathway in 1a
a The bpe linkers in the neighboring ladders are given in 1. Parts: (a) Structural orientation of ladders in 1. (b) Common dimerization of olefinic CC bonds of bpe within the strands via SCSC. (c) Restructuring/repacking at the end of the first step and realignment of remaining CC bonds of bpe. (d) 100% photodimerized product, 2.
powdered single crystals (physically ground for 20 and 30 min) and examined by 1H NMR and Raman spectroscopy. A 1H NMR spectrum presented in Figure S6 clearly reveals that the photoreaction produces quantitative yields of 2 with increasing grinding time. This indicates that the molecular movements of ladders were reinforced by the mechanical motion. The driving force for these large internal molecular movements is expected due to the establishment of strong molecular interactions within the crystal. Initially, each of these ladder structures contains three repeated aligned bpe ligands, which is derived from π···π stacking of the pyridyl rings that produce four π···π interactions in total for the three aligned bpe, one between each two pyridyl rings after mechanical grinding. Moreover, adjacent ladder structures are held together by two kinds of hydrogen bonding such as O−H···N and O−H···O hydrogen bonds as discussed earlier. When crystals are ground, the adjacent ladder structures likely slide to opposite directions and alignment of bpe ligands between the strands might occur. Then, four new π···π stackings of the pyridyl rings may take place between the two-adjacent ladder structures, which is a one-forth enhancement in the π···π stacking. Such interactions may partially reinforce the internal molecular movement and lattice repacking. However, a small percentage of rtct-tpcb, the thermodynamically favored product, was observed possibly due to the isomerization of rctt-tpcb in the solid solution.35 The single crystals were dehydrated and opaque single crystals were obtained by heating 1 to 175 °C for 1 h. The dehydrated crystals were exposed to UV irradiation for 10 and 40 h. The 1 H NMR spectrum (Figures S7 and S8) showed 70% conversion with a slight increase in reactivity that may be due to the rearrangement of the crystal lattice during the loss of coordinated aqua molecules. Raman spectra show that the CC bands around 1650 cm−1 decrease in intensity with increasing grinding time as shown in Figure 4a. This appears to suggest that grinding progressively creates structural defects in the crystal lattice and/or a reduction in the crystal particle size, which leads to a decreasing signal intensity of CC of the ligand. Similarly, we
Figure 3. (a) Plot of the rctt-tpcb product obtained versus time for single crystal and 10 min ground single crystal. (b) PXRD patterns of single crystals irradiated for different times.
for 10, 20, 30, 40, 50 h (Figure 3b) indicates that the crystallinity of 1 was maintained during the course of photodimerization reaction. The formation of 100% dimers of ground 1 after the photoirradiation was due to the molecular movements and repacking of its lattice contents. The reassembling of adjacent ladders may be essential to shrink the stress triggered by the partial development of the cyclobutane derivative in 1. On the other hand, the mechanical forces of grinding likely caused the slight misalignment of ladder structures within 1 to move in opposite directions (Scheme 1b) and realign in the solid state. Therefore, it enables a structure with an infinite number of parallel-aligned bpe ligands to undergo photoreaction via large internal molecular movements. Such initiation of large internal molecular movements via grinding has been documented previously.31,32 The distance between CC double bonds from two adjacent ladder structures might be reduced to 4.2 Å or less, which allows photodimerization reaction to occur between the two neighboring ladder structures (Scheme 1c,d). Therefore, complete photodimerization (100% conversion) of all bpe ligands within 1 can be achieved as reported for a similar triple-stranded ladder CP.15 In order to understand the effect of grinding on the photoreaction of 1, the photoreaction was conducted on D
DOI: 10.1021/acs.inorgchem.9b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
water migration may require the rotation of the coordination bond of terminal water and change of its direction toward the metal center of the adjacent ladder in 1. TGA experiments were conducted to investigate the behavior of water molecules during grinding and their impact on the photoreactivity of compound 1. TGA was conducted on three samples before and after UV irradiation experiments. These three samples are single crystals that were not ground and samples that were ground for 10 and 20 min. Figures S15−S20 show the TGA for each sample before and after UV irradiation. Table 1 shows a summary of percent weight loss of water molecules obtained from TGA analysis. Table 1. Percent Weight Loss of H2O Molecules in 1 before and after Photoreaction from TGA
a
compound 1a
% wt loss
single crystals UV irradiated single crystals for 50 h 10 min ground crystals UV irradiated, 10 min ground crystals for 50 h 20 min ground crystals UV irradiated, 20 min ground crystals for 50 h
3.42 3.45 4.15 6.27 4.18 6.28
Calculated weight loss for a water molecule in 1 is 3.41%.
Ground crystals show a slightly higher percentage of weight loss for water molecules, which might be due to the higher surface area. UV irradiated samples also have a higher percentage of weight loss, which might be due to long exposure of the ground sample to the atmosphere while irradiating for 50 h in the photoreactor. The TGA indicates that there is not much difference between 10 and 20 min of ground samples as their curves are almost identical. The TGA also indicates that the temperature of water loss has not changed between pre- and postirradiation experiments. However, loss of water molecules of single crystals is above 100 °C, while it is near 50 °C for ground crystal samples. The change in water loss temperature between single and ground crystals indicates these water molecules are coordinated in single crystals, while they are uncoordinated (become free water molecules) within the lattice structure of ground crystal samples. These observations show that grinding caused the coordinated water molecules to break from the Cd(II) coordination sphere and the movement of adjacent ladders may have resulted in 2D sheets. Such 1D → 2D structural transformation prior to photoreaction via desolvation has been observed in CPs.40 The water loss due to the mechanical grinding provides a better medium for the orientation of the olefinic CC bonds within the crystal lattice prior to further photoreaction that has caused enhancement in photoreactivity in the solid state. This could be attributed to the lattice water molecules occupying the space between the dimerized pyridyl rings to form hydrogen bonds and thus stabilizing the photoproducts. It is also well-known that these mechanical forces enhance the reactivity due to an increase in surface area, as well as the formation of crystal defects,41,42 and overcoming the detrimental effect of static disorder.43 Photoluminescence Studies. The solid-state photoluminescence properties of 1 before irradiation and 10 min ground 1 irradiated for 50 h were examined at room temperature upon excitation at 350 nm (Figure 4b). Compound 1 shows a strong emission in comparison to the photodimerized product. It exhibits a major peak around 450
Figure 4. (a) Raman spectra of 1 at different grinding times. (b) Luminescent emission spectra of 1 (λex = 350 nm) at room temperature before irradiation and of 1 irradiated for 50 h.
observed that the intensity of the three bands centered around 1030 cm−1, which are ascribed to NO3− stretch, decreases with increasing grinding time (Figure S9). In addition, the central band at 1030 cm−1 appears to shift slightly to higher wavenumber, while the shoulder band around 1050 cm−1 also shifts slightly to lower wavenumber (Figure S9), suggesting changes to the bonding of the NO3− oxygens. Overall, these observations suggest increasing structural defects in the crystal lattice with progressive grinding, which predisposes the ladders for more enhanced photodimerization. Thus, our Raman results appear to corroborate NMR data, in the sense that grinding appears to create structural defects that present the CP ladders in a favorable position for enhanced photodimerization. Interestingly, we found that the overall decrease in Raman band intensities is a linear function of grinding time. Thus, when the observed Raman intensities were plotted against grinding time, a linear curve was obtained (data not shown). The lattice rearrangements may also be reinforced due to the bond breaking/forming of hydrogen-bonding due to the water molecules. Such a rearrangement occurred due to water loss or migration in several CPs during the photoreactions.4,39 Such E
DOI: 10.1021/acs.inorgchem.9b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
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nm and a shoulder at 520 nm. Taking into account the fact that the pure bpe ligand, in the solid state, emits around 450 nm upon excitation at 369 nm at room temperature, attributable to n → π* and π → π* transitions,44 the 450 nm emission of 1 is assigned to a ligand-centered transition. The shoulder at 520 nm is likely due to a metal-to-ligand charge transfer (MLCT) excited state. The photodimerized product shows a weaker emission, with a broadband centered at 420 nm, likely due to rctt-tpcb pyridyl-centered transitions.45 The shoulder peak is almost completely absent in the dimerized product due to the change in the coordination environment around the Cd(II) metal ion.
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Almaz S. Jalilov: 0000-0002-8932-2107 Abdul Malik P. Peedikakkal: 0000-0002-4745-2843 Author Contributions †
The first two authors contributed equally to this research work.
Notes
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The authors declare no competing financial interest.
CONCLUSION The photoreactivity of a one-dimensional hydrogen-bonded triple-stranded ladder 1D coordination polymer has been studied in the solid state. The CP undergoes solid-state photodimerization reaction to yield 67% of the rctt-tpcb in crystals; however, a yield of 100% of rctt-tpcb was obtained when ground to a powder. The photoreaction was found to take place in one step in the crystal, whereas two steps are involved in the ground crystals. 1H NMR and Raman results point to structural rearrangement in the crystal lattice due to grinding that predisposes the CP ladders for enhanced photodimerization. The mechanical force successfully enhanced the initiation of large internal molecular movements in the crystals. The CP contains slightly misaligned ladders, where each one of them exists as sets of three aligned CC double bonds in three bpe ligands. The grinding has exerted an internal pressure on these ladders and executed a large internal molecular motion that causes the ladders to move to opposite directions and bpe ligands to align. The driving force for such prealignment during mechanical motion is expected due to the reinforcement of π···π stacking between the pyridyl rings from adjacent ladders. The TGA study showed that crystal grinding induces dissociation or weakening of the coordination bond of the aqua ligands within the crystal lattice. Such a phenomenon allows the ladders to achieve intermolecular distances and orientations favoring the photodimerization reaction. The current investigation has a significant impact in the field of molecular solid-state chemistry, particularly the solid-state reactivity of coordination polymers.
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ACKNOWLEDGMENTS
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REFERENCES
A.M.P.P. would like to acknowledge the support provided by the Deanship of Scientific Research (DSR) at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project No. IN131005.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01276. Product distribution data of [2 + 2] cycloaddition reaction, experimental and calculated PXRD patterns, 1 H NMR spectra, Raman spectra, X-ray crystallographic analysis, crystallograpic data, ORTEP views, illustrations of packing structure, TGA analysis, IR spectra, and reference list (PDF) Accession Codes
CCDC 1893340 and 1893341 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
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DOI: 10.1021/acs.inorgchem.9b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.9b01276 Inorg. Chem. XXXX, XXX, XXX−XXX
