Photochromic Crystals: Toward an Understanding of Color Development in the Solid State Janet L. Scott*,† and Koichi Tanaka‡ Centre for Green Chemistry, Monash University, Clayton, Victoria 3800, Australia, and Department of Applied Chemistry, Faculty of Engineering, Kansai University, Suita, Osaka, 564-8680, Japan Received December 2, 2004;
CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 3 1209-1213
Revised Manuscript Received January 26, 2005
ABSTRACT: Intermolecular interactions between fluorene groups of propargylallenes define the solid state photochromic response of these materials. Examination of crystal structures of both photoresponsive and nonresponsive forms (including solvates) indicates that all compounds that are photochromic in the solid state exhibit donor/acceptor π‚‚‚π interactions of fluorene aromatic systems that lead to infinite 1-D ribbons. Such interactions are absent in nonphotochromic crystalline forms. Introduction Photochromic compounds have potential for application in information storage, switching devices, and display systems, and large numbers of photoreactive solids have been documented. Many of these are organometallic1 or hybrid organic/inorganic materials.2 In purely organic photochromic solids, the photoresponse is often due to bond forming/breaking reactions, as in furofulgides,3 diarylethenes,4 or spiropyrans;5 keto/enol tautomerization, as in o-hydroxy Schiff bases6 and pyrazolone derivatives;7 or one of these accompanied by ionization8 or radical formation9 (to cite just a few examples of these fascinating compounds). Few photochromic crystalline materials, in which the photoresponse is not due to the specific formation of a new bond or isomerization, have been identified. Many photochromic organic compounds exhibit such behavior both in solution and in the solid state.10 We have previously reported solid-state photochromic behavior in crystals of various members of a group of propargylallene compounds 1.11,12 As solutions of these compounds do not exhibit a reversible photochromic response on exposure to visible light, while some crystals undergo rapid, dramatic color changes, it is clear that inter- or intramolecular interactions in the solid state must be examined to reveal the source of this effect. Herein we report a series of crystal structures of selected compounds from the group 1a-j and 2, which shed light on the specific packing requirements and intermolecular interactions required for a solid-state photochromic response in these propargylallenes.
* To whom correspondence should be addressed. E-mail: Janet.Scott@ sci.monash.edu.au. † Monash University. ‡ Kansai University.
Figure 1. Crystals of 1f: (left) prior to light exposure; (center) post 30 s exposure and (right) post 1 min of exposure to radiation from a UV lamp.
Results and Discussion Propargylallenes 1a-g crystallize from a variety of solvents in different crystal forms and as solvates. Some of these show a dramatic photoresponse, changing rapidly from colorless or pale yellow to green or green/blue upon exposure to light, Figure 1. Color development may be reversed by gentle heating (vigorous heating leads to thermal rearrangement13) and fades at differing rates once the source of illumination is removed.12 Preliminary analysis of a series of crystal structures of these compounds led to the conclusion that photochromism was due to the formation of charge-transfer complexes (possibly in low populations), facilitated by the overlap of fluorene moieties with π‚‚‚π interactions between relatively electron rich (allene terminus) and electron poor (alkyne terminus) fluorene moieties.12 To gain a general understanding of the donor/acceptor interactions required for photochromism in these systems, we compare a series of crystal structures of compounds that might be expected to display photochromism and examine the packing motifs and intermolecular interactions, particularly of the fluorene moieties. The crystal structures of two photochromic crystals, 1g and 1f, are compared with those of nonphotochromic crystals, 1d‚DMSO, 1c‚2DMSO, 1j, and 2, and a generalized description of the requirements for solid-state photoresponse in these compounds is presented. Crystal and refinement data are presented in Table 1, and molecular diagrams and representative numbering schemes in Figure 2. To verify the assumption that it is the fluorene group interactions that are critical to the development of charge-transfer complexes and hence color development,
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Figure 2. Molecular diagrams of (a) 2, (b) 1g, (c) 1f, (d) 1d‚DMSO solvate, (e) 1c‚2DMSO solvate, and (f) 1j. Ellipsoids are depicted at the 50% probability level, and disorder of solvent molecules is shown where appropriate. The numbering schemes of all compounds of type 1 mirror that of 1d. Both 1f and 1g have Z′ ) 2, and the independent molecules of the asymmetric unit adopt similar (but not identical) conformations (the same numbering system is followed for the second molecule and indicated C1′ etc.). This conformation, with opposite-end fluorene moieties approximately parallel and disposed opposite to each other, occurs in all complexes and solvate crystals, except that of 1j, where the fluorene moieties are twisted so that they are orthogonal with respect to each other and disposed on the same side of the molecule (f).
Figure 3. Aromatic rings of 2 corresponding to fluorene moieties in 1 are depicted in black. These, not being constrained by the five-membered central fluorene ring system, are no longer coplanar, with dihedral angles of 77.61(3)° and 74.06(5)° between pairs of Ar rings. Terminal Ar rings participate in a π‚‚‚π interaction C39-C44 to C32-C37(3/2-x,y-1/2,3/2-z) with interplanar d ) 3.6 Å.
compound 2 was synthesized and tested for photoresponse. As expected, crystals of 2 are not photochromic and analysis of the crystal structure reveals that the aromatic rings, corresponding to those of the fluorene moieties in 1, are no longer coplanar, Figure 2a. The phenyl groups are rotated to maximize CH‚‚‚π interactions, and the only face-to-face π‚‚‚π interactions are between terminal ArCdO groups corresponding to those in many crystal structures of compounds 1a-j, Figure 3. Both 1g and 1f have two independent molecules in the asymmetric unit and exhibit face-to-face π‚‚‚π interactions between fluorene moieties, Table 2, forming continuous chains or ribbons of molecules through the crystal. Highly photochromic 1g mirrors the “opposite end” over-
lap of electron rich and electron poor fluorene moieties and the formation of infinite 1-D ribbons, similar to those previously noted.12 In 1f, though also highly photochromic, a distinctly different packing mode of the propargylallene molecules exists. The molecules do not form the continuous ribbon motif of end-to-end, alternating electron rich/electron poor overlapping fluorene moieties, previously noted in all photochromic crystals, but instead, a side-to-side overlap occurs. Closer inspection reveals that this side-to-side mode of fluorene overlap nonetheless yields infinite ribbons in which each molecule is reversed with regard to the next, once again yielding alternating electron rich/electron poor fluorene interactions propagating in one dimension, Figure 4a. Compound 1f shows the longest bleaching time of any crystals of the compounds with structure 1, and so available crystal structures were examined for similar interactions. The crystal structure of compound 1d, previously reported, exhibits “opposite-end-to-end” molecular overlap and is photochromic, while crystallization of 1d as a DMSO solvate yields nonphotochromic crystals. While, upon cursory inspection, side-to-side overlap of pairs of molecules occurs (Figure 5), these do not form the continuous ribbons noted in the photochromic crystal forms. Instead, pairs of molecules are well separated from each other by guest DMSO molecules, accommodated in voids, Figure 6. The only extended interaction between host molecules is via π‚‚‚π interactions of alkyne terminus p-MeAr groups. Finally, neither of the nonphotochromic crystals, 1j or 1c‚2DMSO, exhibits packing that would allow for donor-acceptor interactions to propagate through the crystalline solid. Compound 1j shows “same-end” over-
Photochromic Crystals
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Figure 4. Overlap of the relatively electron rich (allene) fluorene moiety (green) with the electron poor (alkyne) fluorene moiety (red) forming infinite ribbons in the photochromic crystals of (a) 1f, side-to-side overlap, and (b) 1g, end-to-end overlap. Terminal ArCdO groups have been removed for clarity, and allene and alkyne moieties are represented as heavy bonds. Table 1. Crystal and Refinement Data photochromic response? empirical formula Mr crystal system space group a/Å b/Å c/Å R/deg β/deg γ/deg V/Å3 Z′; Z Dc/g cm-3 µ/mm-1 reflns unique reflns I > 2σ(I) R1/wR2 [I > 2σ(I)] R1/wR2 [all data] GoF on F2 parameters/restraints
1d‚DMSO
1c‚2DMSO
1f
1g
1j
2
N C48H36O3S 692.83 monoclinic P21/c 12.1597(2) 19.8671(4) 15.6274(2) 90 93.923(1) 90 3766.39(11) 1; 4 1.222 0.128 9142 4702 0.0644/0.1362 0.1686/0.1756 0.980 482/0
N C50H42O4S2 770.96 triclinic P1 h 8.9035(2) 13.1202(3) 18.6112(6) 102.770(1) 97.995(1) 102.662(1) 2028.16(9) 1; 2 1.262 0.177 8254 4831 0.0549/0.0993 0.1243/0.1212 1.009 511/0
Y C92H60O8 1293.40 monoclinic C2/c 40.5767(8) 13.9347(4) 27.1523(8) 90 120.821(1) 90 13184.3(6) 2; 8 1.303 0.082 16135 5965 0.0945/0.1724 0.2749/0.2378 1.000 905/0
Y C92H60O8 1293.40 triclinic P1 h 9.7057(1) 17.5950(2) 20.9577(3) 72.196(1) 81.517(1) 77.065(1) 3309.03(7) 2; 2 1.298 0.082 13675 10304 0.0465/0.1060 0.0713/0.1186 1.024 919/0
N C44H24Cl2O2 655.53 monoclinic P21/n 15.3320(3) 12.9255(4) 17.0902(4) 90 103.060(3) 90 3299.16(14) 1; 4 1.320 0.235 7944 3504 0.0728/0.1487 0.1987/0.1900 0.988 433/0
N C44H30O2 590.68 monoclinic P21/n 12.0801(2) 18.5467(3) 14.5798(3) 90 108.928(1) 90 3089.91(10) 1; 4 1.270 0.076 7507 4251 0.0579/0.0989 0.1392/0.1230 1.021 415/0
Table 2. Geometrical Parameters Describing π‚‚‚π Interactions of Fluorene Moieties 1d‚DMSO 1c‚2DMSO 1fd
1g 1j
π‚‚‚π interaction
slip ∠a/deg
atom-atom
d/Å
∠ between fluorene moietiesb/deg
C1 > C13‚‚‚C16 > C28(1-x,-y,1-z) C16 > C28‚‚‚C16 > C28(-x,2-y,1-z) C2 > C7‚‚‚C23 > C28(1/2-x,1/2-y,1-z) C8 > C13‚‚‚C17 > C22(1/2-x,3/2-y,1-z) C2′ > C7′‚‚‚C23′ > C28′(-x,1-y,1-z) C8′ > C13′‚‚‚C17′ > C22′(-x,-y,1-z) C1 > C13‚‚‚C16 > C28(1+x,y,z) C1′ > C13′‚‚‚C16′ > C28′(1+x,y,z) C1 > C13‚‚‚C1 > C13(1-x,2-y,1-z)
6.18(7) coplanarc 5.8(3) 2.2(3) 1.4(3) 5.3(3) 6.80(4) 5.69(5) coplanarc
C4‚‚‚C27(1-x,-y,1-z) C22‚‚‚C24(-x,2-y,1-z) C11‚‚‚C21(1/2-x,1/2-y,1-z) C5‚‚‚C25(1/2-x,3/2-y,1-z) C11′‚‚‚C18′(-x,1-y,1-z) C3′‚‚‚C26′(-x,-y,1-z) C13‚‚‚C20(1+x,y,z) C2′‚‚‚C25′(1+x,y,z) C5‚‚‚C7(1-x,2-y,1-z)
3.42 3.56 3.47 3.45 3.45 3.43 3.37 3.45 3.38
6.20(7) 10.61(4) 4.4(2) 3.7(2) (′) 6.81(4) 5.70(5) (′) 58.45(6)
a Angle between planes of overlapped fluorene moieties. b Within a molecule. c Related by center of symmetry. side overlap; hence individual six-membered Ar rings of fluorene moieties are considered.
lap of fluorene moieties (as noted too in its acetone solvate crystals12), Figure 7a,b. However, dissimilar to the previously reported solvate crystals, there is a lack of overlap of the alkyne termini. Instead, the molecular conformation is quite different from that occurring in all other crystals of these compounds, with the fluorene moieties almost orthogonal to each other as illustrated in Figures 2f and 7b. In all other crystalline complexes analyzed, fluorene moieties within the molecule approach coplanarity, Table 2. Photochromic crystals of 1c have not been isolated from any solvent, though the (weakly) electron donating Me substituent of the Ar
d
1f exhibits side-to-
Figure 5. Side-to-side overlapping pairs in 1d‚DMSO solvate, a nonphotochromic crystal.
groups should not militate against photochromism.12 1c‚ 2DMSO, in common with 1j, exhibits isolated pairs of
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Figure 6. Packing diagram of 1d‚DMSO solvate viewed down c. Pairs of 1d molecules (shown in stick mode with light and dark bonds to differentiate molecules in a pair) do not overlap as they are separated by disordered DMSO molecules (shown in spacefill mode) and no extended ribbons of molecules with overlapping fluorene moieties occur.
molecules with same-end overlap though here it is the alkyne terminus fluorene groups that participate in offset face-to-face π‚‚‚π interactions. No face-to-face fluorene/fluorene π interactions of the allene termini occur, and each host is separated from the next by DMSO guest molecules, Figure 7d. Thus, some of the prerequisite intermolecular interactions that allow the development of colored chargetransfer complexes and lead to solid-state photochromism in crystals of 1 may be summarized schematically as in Figure 8. Examination of a range of crystal structures of this class of propargylallenes leads to the conclusion that while these crystal structures all show quite disparate packing motifs, all photochromic crystals exhibit some degree of face-to-face overlap of relatively electron rich π systems with relatively electron poor π systems and form continuous 1-D ribbons. It is postulated that this allows for development of colored charge-transfer complexes, while in nonphotochromic crystals isolated opposite-end overlap, same-end interactions, or no interactions of the fluorene π systems are noted. Experimental Section The synthesis and characterization of 1a-j have been described previously,12 and 2 was prepared by the previously reported method.14
Scott and Tanaka
Figure 8. Cartoon of modes of fluorene overlap in photochromic and nonphotochromic crystals. (a) Donor-acceptor interactions forming infinite 1-D ribbons: photochromic, e.g. 1g. (b) Same-end overlap with infinite 1-D ribbons: not photochromic, e.g. 1j acetone solvate (ref 12). (c) Opposite-end overlap but no 1-D ribbon formation: not photochromic, e.g. 1d DMSO solvate. (d) Side-to-side donor-acceptor interactions forming infinite 1-D ribbons: photochromic, e.g. 1f. (e) Antiparallel fluorene groups with same-end overlap, e.g. 1j, and parallel fluorene groups with same-end overlap, e.g. 1c DMSO solvate: not photochromic. Crystal Structure Analysis. Crystals of 1d‚DMSO and 1c‚2DMSO were prepared by slow cooling of supersaturated DMSO solutions, and crystals of 1j and guest-free 1f, 1g, and 2 by slow evaporation of solutions of the relevant compound in chloroform or ethyl acetate. Phase purity was checked by two methods, namely, multiple cell determinations on a minimum of three randomly chosen single crystals and photoresponse; in all cases the crystals formed in a single crystallization experiment exhibited the same photoresponse, with respect to both color change and rate of response, and crystals selected for cell determination exhibited similar cell parameters. Data were collected on an Enraf-Nonius Kappa CCD diffractometer at 123 K using graphite monochromated Mo KR radiation (λ ) 0.71073 Å, φ and ω scans). Structures were solved by direct methods using the program SHELXS9715 and refined by full matrix least-squares refinement on F2 using the programs SHELXL-9716 and XSeed.17 All nonhydrogen atoms were refined anisotropically. Hydrogen atoms were inserted in geometrically determined positions with temperature factors fixed at 1.2 times that of the parent atom (1.5 times for methyl group hydrogen atoms). DMSO guest molecules in 1d‚DMSO were refined with the S atom disordered over two positions, sof ) 70% for the major component and 30% for the minor. An MeO group of one p-MeO-phenyl
Figure 7. Nonphotochromic crystals exhibit no donor-acceptor interactions. (a) Solvent-free crystals of 1j; the molecules pack such that isolated same-end overlap results. (b) Overlap of allene terminus fluorene groups (green) in 1j (ArCdO groups have been removed for clarity, and allene and alkyne moieties are indicated with heavy bonds). (c) Overlap of alkyne terminus fluorene moieties occurs in 1c‚2DMSO yielding dimers which are isolated from each other (d) and do not yield photochromic crystals.
Photochromic Crystals group of 1g (two molecules in the ASU) was modeled with the O atom disordered over two sites (60 and 40% sof for each component). Supporting Information Available: Crystallographic information files (CIF) for all crystal structures. This material is available free of charge via the Internet at http:// pubs.acs.org.
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Crystal Growth & Design, Vol. 5, No. 3, 2005 1213 (7) Liu, L.; Jia, D.-Z.; Ji, Y.-L.; Yu, K.-B. J. Photochem. Photobiol. A 2003, 154, 117-122. (8) Vancˇik, H.; Sˇ imunic-Mezˇnaric, V.; C Ä aleta, I.; Mesˇtrovic´, E.; Milovac, S.; Mlinaric´-Majerski, K.; Veljkovic´, J. J. Phys. Chem. B 2002, 106, 1576-1580. (9) Ishida, T.; Murakami, M.; Yoshikawa, H.; Nishikiori, S.-I. Internet Electron. J. Mol. Des. 2003, 2, 14-23; http:// www.biochempress.com. (10) Suh, H.-J.; Lim, W.-T.; Cui, J.-Z.; Lee, H.-Soo; Kim, G.-H.; Heo, N.-H.; Kim, S.-H. Dyes Pigm. 2003, 57, 149-159. (11) Tanaka, K.; Tomomori, A.; Scott, J. L. CrystEngComm 2003, 5, 147-149. (12) Tanaka, K.; Tomomori, A.; Scott, J. L. Bull. Chem. Soc. Jpn. 2005, 78, 294-299. (13) Tanaka, K.; Tomomori, A.; Scott, J. L. Eur. J. Org. Chem. 2003, 2035-2038. (14) Toda, F.; Yamamoto, M.; Tanaka, K.; Mak, T. C. W. Tetrahedron Lett. 1985, 26, 631-634. (15) Sheldrick, G. M. SHELXS-97; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (16) Sheldrick, G. M. SHELXL-97; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (17) Barbour, L. J. J. Supramol. Chem. 2001, 1, 189.
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