Surface Self-Assembly of T rans -Substituted Porphyrin Double-Decker Complexes Exhibiting Slow Magnetic Relaxation (cid:3)

A novel single-molecule magnet (SMM) of trans -substituted Tb(III) porphyrin, { [Tb(BIPPP) 2 ](DBU-H) } (BIPPP: 5,15-bis(3,5-isopentoxyphenyl)porphyrin, DBU: 1,8-diazabicyclo[5.4.0]undec-7-ene), is reported here. The alternating current magnetic susceptibility measurements showed the frequency dependence on the in-phase and out-of-phase magnetic susceptibility under both zero and 2000 Oe direct current magnetic ﬁelds. The barrier height for the reversal of the magnetic moment at the high-temperature region was estimated to be 299 cm − 1 , which is the same order as that of the previously reported tetraphenylporphyrin-based Tb(III) double-decker complex. Atomic force microscopy revealed that { [Tb(BIPPP) 2 ](DBU-H) } formed self-assembled monolayers on highly oriented pyrolytic graphite using the simple drop-cast technique. The result proves the high design freedom of porphyrin-based double-decker SMMs. [DOI:


I. INTRODUCTION
Single molecule magnets (SMMs) are molecules with strong easy-axis magnetic anisotropy; they exhibit slow relaxation of magnetization and stable magnetization below the blocking temperature [1][2][3]. SMMs have attracted wide attention in the scientific community owing to their application to molecular spintronics [4]. So far, an interesting topic of research has been the design of new SMMs exhibiting a slow magnetic relaxation process. On the other hand, another interesting topic of research in the recent years has been the design of SMMs as the active units in spintronic devices [5]. The organization of SMMs on surfaces rather than in bulk phases is regarded as the first step along this direction.
The studies of SMMs on surfaces have been mainly based on the phthalocyanine Tb(III) double-decker compounds because of their considerably high energy barrier for magnetic moment reversal and planar struc- * This paper was presented at the 12th International Conference on Atomically Controlled Surfaces, Interfaces and Nanostructures   ture that is suitable for their organization on a surface [6][7][8][9].

A. General
All chemicals and solvents were of reagent grade and used as received without further purification. The elemental analyses were performed using a Yanaco CHN CORDER MT-5. Mass spectra (MS) were recorded using a Shimadzu AXIMA-CFR matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer. Ultraviolet-visible-near-infrared (UV-vis-NIR) absorption spectra were recorded using a Shimadzu UV-3150 double-beam spectrophotometer and 1.0 cm quartz cuvettes. The alternating current (ac) magnetic susceptibility and hysteresis loop measurements were carried out using a Quantum Design MPMS-XL7AC SQUID magnetometer. The atomic force microscopy (AFM) mea- surements were carried out using a Multimode 8 setup equipped with a NanoScope V controller (Bruker Nano Inc., Nano Surfaces Division, Santa Barbara, CA, USA). ScanAsyst-air cantilevers with k = 0.4 Nm −1 were used.

B. Synthesis of {[Tb(BIPPP)2](DBU-H)}
The free-base porphyrin, 5,15-bis(3,5isopentoxyphenyl)porphyrin (H 2 BIPPP), was prepared according to the reported procedure [12]. The   (Fig. 2). The absorption spectrum exhibited a characteristic Soret band at 398 nm and broad Q bands in the 500-750-nm range, which are usually observed in the typical spectra of porphyrin double-decker complexes [10]. Notably, the spectrum does not exhibit the characteristic NIR absorption band of oxidized double-decker complexes in the 800-1200nm range [13]. This result strongly indicates that pure anionic complexes were isolated, and no neutral π radical complex was formed.
It is known that ac magnetic susceptibility measurements monitor the response of a magnetic moment to an applied oscillating magnetic field, and are suitable to detect the slow relaxation of magnetization of SMMs. To estimate the magnetization relaxation behavior of Oe dc magnetic field with ac frequencies of 10, 100, and 1000 Hz, respectively. The χ M ′′ peak temperature for an AC field of 1000 Hz (22 K) showed almost the same value as that of the previously reported Tb(III) tetraphenylporphyrin (TPP) double-decker complex (24 K) [10]. The application of dc magnetic field can change the relaxation dynamics, particularly for measure- ments at a low frequency; the χ M ′′ signal showed sharper peaks than in the absence of dc magnetic field, indicating that the tunneling process is the dominant relaxation process at the low-temperature region. The barrier height for the reversal of magnetic moment at the high-temperature region was estimated to be 299 cm −1 with a frequency factor (τ 0 ) of 4.2×10 −13 s; these were estimated from the Arrhenius plot using the χ M ′′ peak tops of 10, 100, and 1000 Hz under 2000 Oe magnetic field. The value obtained for the barrier height was of the same order as that of the previously reported TPP double-decker complex (283 cm −1 ) [10].
To investigate the SMM property in more detail, the variable-frequency ac magnetic susceptibility measurements under 2000 Oe dc magnetic fields were performed (Figs. 4(a) and (b)). In the measurement temperature range (12, 14, and 16 K), clear frequency dependency of χ M ′ , and χ M ′′ peaks was observed. The Argand plots (i.e., χ M ′′ vs. χ M ′ plot) are shown in Fig. 4(c), which can be fitted by the generalized Debye model [14]. The α parameters, which quantifies the width of the relaxation time (τ ) distribution and are obtained in the fitting of the generalized Debye model, are in the 0.28-0.30 range, proving that the distribution is sufficiently small.
When the dc magnetization of {[Tb(BIPPP) 2 ](DBU-H)} was measured at 1.8 K within ±20 kOe, a butterfly shape was observed (Fig. 5). The butterfly-shaped loop can be attributed to the fast magnetization tunneling process occurring near the zero field assisted by nuclear spin I = 3/2 of Tb, which was typically observed in other Tb double-decker complexes [15].
To investigate the self-assembly properties of {[Tb(BIPPP) 2 ](DBU-H)} on a surface, a solution of the complex in CH 2 Cl 2 , which has an absorbance of 0.3 for the Soret peak at 398 nm, was drop cast onto a freshly cleaved highly oriented pyrolytic graphite (HOPG) under ambient conditions. The solvent was allowed to evaporate; the films obtained were observed by AFM. Figure 6(a) shows the AFM image of the complex on the HOPG. The molecules on the HOPG were observed as thin films with porous structures. The height or thickness of the thin film layer is approximately 1.6 nm, as shown in Fig. 6(b). The surface of the films were relatively smooth, indicating that well-ordered structures were formed. To investigate the assembled structure, a simple molecular model was constructed, which was based on the crystal structure of the TPP double-decker complex. In this model, the methoxy groups were geometrically located at the meta positions of the phenyl rings, as shown in Fig. 6(c). The distance between the two outer hydrogen atoms of the methoxy groups was added to two times the van der Waals radius of hydrogen, resulting in an ideal thickness of the core structure of {[Tb(BIPPP) 2 ](DBU-H)}, which is 1.56 nm. This is in good agreement with the film thickness observed by the AFM. In Fig. 6(b), there are stripes running from upper-left to lower-right direction. They would be attributed to the step-terrace structure of HOPG substrate because all the height of the film from the bottom substrate are consistent with the molecular height. The estimated dimension from the simple model strongly indicates that {[Tb(BIPPP) 2 ](DBU-H)} in the film would lay flat on the HOPG surface and form self-assembled monolayers.

IV. CONCLUSIONS
In conclusion, a novel porphyrin double-decker complex, {[Tb(BIPPP) 2 ](DBU-H)} was synthesized. The UV-vis-NIR absorption spectrum showed that the isolated complex exists in an anionic form.
The ac magnetic susceptibility measurements showed that {[Tb(BIPPP) 2 ](DBU-H)} behaves as an SMM. This is the first report on an SMM of trans-substituted porphyrins. The results show that the low-symmetry porphyrins can also be a component of SMMs, proving that the porphyrin-based SMMs may have significantly high design freedom. Furthermore, the AFM study showed that {[Tb(BIPPP) 2 ](DBU-H)} forms self-assembled monolayers on HOPG by the simple drop-cast technique. We expect that porphyrin-based SMM systems will provide a new design strategy for SMM-based molecular devices.