Guest-Induced Instant and Reversible
Crystal-to-Crystal Transformation of
1,4-bis(ferrocenylethynyl)anthraquinone
For decades, much attention has been focused on
crystallization from a mixture of electron donor (D) andacceptor (A) molecules, forming D- and/or A-stacked
column structures that exhibit unique physical properties
such as ferromagnetism, strong visible-NIR lightabsorption, and high electronic conductivity. The
tendency toward DA stacking in a crystal indicates that
D-A conjugated molecules can form a network structure,
which may manifest interesting physical properties. Weare interested in the properties of a new D-A conjugatedsystem [1-3] of ferrocenylethynylanthraquinones(FcAq's), from two viewpoints: the protonation-induced
intramolecular electron transfer reaction causing
valence tautomers, and the construction of D-A
stacked nanoporous network structures, which haveattracted much recent attention due to their highpotential for applications such as selective gas
adsorption, heterogeneous catalysts, and molecularrecognition. In the present study, we have found that a newT-shaped FcAq compound, 1,4-bis(ferrocenylethynyl)
anthraquinone (1,4-Fc 2 Aq), gives a facile single
crystal formation in the presence of guest solvent
molecules, and undergoes an instant and reversible
crystal-to-crystal transformation by heating and guest
vapor treatment [4].
The crystal structures of 1,4-Fc2Aq's obtained fromvarious solvents were confirmed by single-crystal Xray diffraction analysis. Most of the structures show

the inclusion of the solvent as the guest molecule.
The crystal structure of α-1,4-Fc2Aq•CH2Cl2 along theb axis is shown in Fig. 1(a) as an example. The
crystal was obtained by recrystallization fromdichloromethane-hexane at 263 K; under suchconditions, a large (25 × 1 × 1 mm3) crystal was grownwithin a week. α-1,4-Fc2Aq•CH2Cl2 has subnanoporeswith dimensions of 5.7 Å × 3.5 Å along the b axis, inwhich dichloromethane as guest molecules arecontained with a 1,4-Fc2Aq to dichloromethane moleratio of 1:1. An infinite one-dimensional columnar
structure is formed along the b axis by an alternating
arrangement of π-conjugated spacers and π-πstacking interactions between Fc (D) and Aq (A)moieties (Fig. 2(b)). The Aq and Cp planes lean about
45 degrees to the b axis. The two kinds of complex
columns are almost perpendicular to each other andare arranged alternately to construct the porous
framework. Crystals with the formula 1,4-Fc2Aq•X (X= guest molecule) were formed in various solvents.
Their crystal structures are similar to the structure of
α-1,4-Fc2Aq•CH2Cl2, but the size of the subnanoporeschanges according to the size of the guest molecules
included.
The recrystallization of 1,4-Fc 2 Aq from o dichlorobenzene-hexane at 293 K produced crystals
without pores and solvent molecules, the structure ofwhich is shown in Fig. 2(b). In the solvent-free form, acolumnar structure is maintained (Fig. 2), whereas thesyn conformation of the ferrocenes in 1,4-Fc 2 Aq
allows for the formation of pairs of adjacent moleculesin the crystal, and the pattern of the stacking
arrangement changes from D-A to D-A-A comparedwith that of the porous 1,4-Fc2Aq•X.
Thermal desorption of the guest molecules in 1,4Fc 2 Aq • X was examined by thermal gravimetricanalysis (TGA). The TG curve indicated thedesorption of guest molecules at T de = 346-380 K
without chemical decomposition. To clarify the crystal
structure after the desorption of guest molecules, thesynchrotron XRPD of a sample prepared by heating
1,4-Fc 2 Aq • THF at 420 K under vacuum wasmeasured, and the cell parameters of the sample
were determined by the Rietveld method. The XRPDderived cell parameters were consistent with thoseof solvent-free 1,4-Fc2Aq. Thus, it is confirmed thatguest desorption causes crystal-to-crystal
transformation from 1,4-Fc2Aq•X to solvent-free 1,4Fc2Aq.

Fig. 1. Crystal structure of α -1,4-Fc2Aq⋅CH2Cl2 along
the b axis (a) and that of solvent-free 1,4-Fc2Aq along
the b axis (b). Hydrogen atoms and disordered guest
molecules (CH2Cl2) are omitted for clarity.

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400 K. XRPD patterns were measured at every 50 K,
and the X-ray exposure time was 5 min for every
measurement. By heating the 1,4-Fc2Aq•THF sample,
THF molecules were released and the XRPD patternsof solvent-free 1,4-Fc2Aq emerged. At 400 K, patterns
corresponded perfectly with the pattern of solvent-free
1,4-Fc2Aq (Fig. 3(b), iii). When the capillary was filledwith THF vapor by increasing Tg, THF molecules wereadsorbed on the host, and XRPD patterns arising from
1,4-Fc2Aq•THF were observed (Fig. 3(b), iv-vii). WhenTs and Tg were kept at 300 K, guest adsorption wascompleted within 5 min. These results indicate a rapid
and reversible crystal-to-crystal transformation
between 1,4-Fc2Aq•THF and solvent-free 1,4-Fc2Aq.
In view of the conformational change required, thisfast transformation is particularly remarkable. Thesefeatures are derived from the crystal structure
flexibility caused by the D-A interaction, which isweaker than other chemical bonds, and from thehigher stability of 1,4-Fc 2 Aq • THF compared withsolvent-free 1,4-Fc2Aq.
In conclusion, a D-A conjugated compound, 1,4Fc2Aq, was newly synthesized. Crystals of 1,4-Fc2Aq
have either a solvent-incorporated porous structure ora solvent-free nonporous one, which can be reversed
instantly by the desorption and absorption of guest
molecules, causing the alternation of the D-A
arrangement in a one-dimensional columnar structure.

Fig. 2. One-dimensional columnar structure of 1,4Fc2Aq⋅X (a) and that of solvent-free 1,4-Fc2Aq (b).

The reversibility of the crystal-to-crystal
transformation by guest desorption and adsorption of1,4-Fc 2 Aq were studied by variable-temperature
XRPD (VT-XRPD) at beamline BL02B2. For thismeasurement, we prepared a vacuum-sealed cell,shown in Fig. 3(a). After the tube stood for 3 hours atroom temperature, the XRPD pattern completelychanged from that of solvent-free 1,4-Fc2Aq to that of1,4-Fc2Aq•THF (Fig. 3(b), from i to ii). Then, THF wasfrozen at 90 K and the sample was heated from 300 K
to 420 K, followed by maintenance of the temperatureof the sample capillary, Ts, at 300 K and increasing
the temperature of the THF capillary, Tg, from 90 K to

Mio Kondoa, Hiroshi Nishiharaa,* and Eiji Nishiboriba Department of Chemistry, The University of Tokyob Department of Applied Physics, Nagoya University*E-mail: nisihara@chem.s.u-tokyo.ac.jpReferences[1] M. Murata et al.: Chem Lett. (2000) 1328.
[2] M. Murata et al.: J. Am. Chem. Soc. 123 (2001) 12903.
[3] M. Kondo et al.: Angew. Chem. Int. Ed. – in press.
[4] M. Kondo, M. Murata, H. Nishihara, E. Nishibori,S. Aoyagi, M. Yoshida, Y. Kinoshita and M. Sakata,Angew. Chem. Int. Ed. 118 (2006) 5587.

Fig. 3. (a) Illustration of capillary used in VT-XRPD
experiment. (b) XRPD pattern of solvent-free 1,4-Fc2Aq (i)and results of VT-XRPD measurement (ii-vii). The
temperatures of the sample (Ts) and guest (Tg) are (ii) Ts =350 K, Tg = 90 K, (iii) Ts = 400 K, Tg = 90 K, (iv) Ts = 300
K, Tg = 200 K, (v) Ts = 300 K, Tg = 300 K, (vi) Ts = 300 K,Tg 350 K, and (vii) Ts = 300 K, Tg = 400 K, respectively.

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