A Molecular Nanotube with Three-Dimensional π-Conjugation**

A π-conjugated twelve-porphyrin tube is synthesized in 32 % yield by a template-directed coupling reaction that joins together six porphyrin dimers, forming twelve new C=C bonds. The nanotube has two bound templates, enclosing an internal volume of approximately 4.5 nm3. Its UV/Vis/NIR absorption and fluorescence spectra resemble those of a previously reported six-porphyrin ring, but are red-shifted by approximately 300 cm−1, reflecting increased conjugation. Ultrafast fluorescence spectroscopy demonstrates extensive excited-state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs, resulting in a negative fluorescence anisotropy on excitation at 742 nm.

Abstract: A p-conjugated twelve-porphyrin tube is synthesized in 32 %y ield by at emplate-directed coupling reaction that joins together six porphyrin dimers,f orming twelve new C À C bonds.T he nanotube has two bound templates,e nclosing an internal volume of approximately 4.5 nm 3 .I ts UV/Vis/NIR absorption and fluorescence spectra resemble those of ap reviously reported six-porphyrin ring, but are red-shifted by approximately 300 cm À1 ,r eflecting increased conjugation. Ultrafast fluorescence spectroscopyd emonstrates extensive excited-state delocalization. Transfer of electronic excitation from an initially formed state polarized in the direction of the nanotube axis (z axis) to an excited state polarized in the xy plane occurs within 200 fs,resulting in anegative fluorescence anisotropyo nexcitation at 742 nm.
The synthesis of p-conjugated belts,b arrels,a nd tubes has become af ocus of intense interest. [1][2][3][4] Increasing the dimensionality of a p-system is expected to enhance the electronic delocalization, [5] perhaps leading to exotic cooperative electronic phenomena, such as Aharonov-Bohm oscillations, [6,7] which occur in single-walled carbon nanotubes (CNTs), but have not yet been detected in molecular materials.T he bottom-up "total synthesis" of structurally defined CNTs or analogous structures could bring many technological benefits, because CNTs have highly desirable optoelectronic properties.H owever,t he current methods for preparing them yield mixtures of species with different chiralities,d iameters,a nd lengths,r esulting in broad distributions of properties. [8] Recent efforts towards the rational chemical synthesis of CNTs focused on the use of cycloparaphenylenes as precursors or templates. [1][2][3][9][10][11] Herein, we present the synthesis and photophysical behavior of ad iscrete molecular tube t-P12 (Figure 1a), consisting of twelve porphyrins,each of which is directly p-conjugated to its three neighbors.T his tube can be viewed as arim-to-rim dimer of the cyclic porphyrin hexamer c-P6,s hown as its complex with the T6 template in Figure 1b; [12] t-P12 is also obtained as at emplate complex, t-P12·(T6) 2 ,with the geometry of ahexagonal prism. Awide variety of synthetic molecular and supramolecular prisms and nanotubes have been reported previously, [13][14][15][16][17][18][19] but these structures lacked a p-conjugated cylindrical surface.C obalt porphyrins have been polymerized on CNT templates to give am aterial with some similarity to t-P12,a lthough that polymer had ap oorly defined multilayer structure. [20] The monodisperse tube t-P12 can be prepared by templatedirected synthesis from areadily accessible porphyrin dimer. This work is as ignificant step towards the bottom-up synthesis of longer well-defined p-conjugated nanotubes.
Thep orphyrin nanotube t-P12·(T6) 2 was synthesized as shown in Scheme 1. Thek ey intermediate is a5 ,15-A 2 BC porphyrin, P1 b,w hich was prepared from P1 a [21] using aS enge arylation [22] to introduce ab ulky solubilizing group, followed by metalation with zinc, bromination at the remaining free meso position, Sonogashira coupling with trimethylsilylacetylene,a nd selective removal of the TMS group. Palladium-catalyzed homo-coupling of P1 b followed by removal of the four triisopropylsilyl groups gave the precursor P2 for cyclo-oligomerization to form t-P12·(T6) 2 .
When we planned the route shown in Scheme 1, it was not clear whether the final step would be feasible.Inthis step,six molecules of porphyrin dimer P2 are brought together by two T6 templates to form t-P12.Itisknown that butadiyne-linked zinc porphyrin dimers such as P2 bind T6 to form 3:1 complexes of the type (P2) 3 ·T6, [12] which would have the wrong spatial arrangement to form t-P12 (Scheme 2). The6:2 complex, (P2) 6 ·(T6) 2 ,w ith the correct arrangement for tube formation is entropically disfavored relative to (P2) 3 ·T6, although repulsion between the bulky aryl substituents may destabilize (P2) 3 ·T6 relative to (P2) 6 ·(T6) 2 .W ee nvisioned that coupling of P2 molecules would give short belt-like oligomers (such as P6 in Scheme 2), which would bind to the template in the correct orientation to form the t-P12 nanotube.Inpractice,this template-directed coupling reaction was remarkably efficient, as is shown by the GPC chromatogram of the crude reaction mixture ( Figure 2a); the desired product is the dominant peak, and t-P12·(T6) 2 was isolated in 32 % yield, in ar eaction involving the formation of twelve new C À Cb onds.T he 24 trihexylsilyl groups on the rims of the nanotube provide excellent solubility in solvents such as chloroform, and the compound was readily characterized by mass spectrometry (MALDI-TOF) and NMR spectroscopy ( Figure 2).

Angewandte Chemie
The 1 HNMR spectrum of t-P12·(T6) 2 is strikingly simple for am olecule of its size owing to the D 6h symmetry.T he assignment of the aromatic protons shown in Figure 2c is supported by COSY and NOESY spectra (see the Supporting Information), including the observation of NOEs from proton dofthe template to protons i, g, and hofthe porphyrin. The signals from the template are strongly shielded by the porphyrin ring currents.There are two signals for the protons of the aryl side groups that are ortho to the porphyrin (proton ia nd j, Figure 2c)b ecause the inside and the outside of the tube provide different environments.T he NMR data are consistent with the geometry of t-P12·(T6) 2 from molecular mechanics calculations (Figure 3a), and the dimensions of the tube can be compared with crystal structures of its components. [12b,23] TheZ n-Zn diameter of the tube is 2.43 nm, and the distance between the two T6 templates is 1.35 , resulting in an internal cavity volume of approximately 4.5 nm 3 .T he length of the nanotube is 2.3 nm measured to the van der Waals surfaces of the outer b-pyrrole hydrogen atoms,o r 3.2 nm measured to the para hydrogen atoms of the aryl groups.A lthough the length of t-P12 is very short when compared with aC NT,t ot he best of our knowledge,i ti s longer than all previously synthesized p-conjugated molecular belts,b arrels,ornanotubes. [1][2][3][4] TheU V/Vis/NIR absorption and fluorescence spectra of t-P12·(T6) 2 are compared with those of c-P6·T6 in Figure 4. Both compounds show similar split absorption and emission bands,indicating that they have similar electronic structures, with emission from ad ipole-forbidden first excited state becoming partially allowed through Herzberg-Teller coupling with higher excited states. [12b] Thea bsorption and emission maxima of t-P12·(T6) 2 are red-shifted by approximately 300 cm À1 relative to those of c-P6·T6,r eflecting the greater conjugation in the larger p-system. DFT calculations (BLYP/ 6-31G(d)) indicate that the HOMO of t-P12·(T6) 2 is distributed over the entire p-system (Figure 3b)w hereas the LUMO is spread over both rings,with negligible coefficients on the butadiyne "staves" connecting the two rings in the z direction. Comparison of these frontier orbitals with the four Gouterman orbitals of as imple porphyrin unit shows that the HOMO and LUMO of t-P12 are derived from the a 2u and e gx orbitals of the monomer,r espectively, [24] which explains the presence of an ode along each stave in the LUMO.
Ultrafast time-resolved fluorescence spectroscopy reveals strong coupling between the components of the nanotube. Figure 5a shows the fluorescence dynamics of t-P12·(T6) 2 , excited at 830 nm and 742 nm, with excitation polarized perpendicular or parallel to the detection polarization. The time-dependent fluorescence anisotropies (g)a tt hese two wavelengths,d erived from the data in Figure 5a,s how no change over 0-15 ps after excitation (Figure 5b). Theaverage anisotropy on excitation at 830 nm is 0.07, which is similar to that recorded for c-P6·T6 [12b,25] and close to the theoretical valueo f0 .1 for an excited state delocalized over at wodimensional ring. [26] This indicates that in t-P12·(T6) 2 , absorption at 830 nm is associated with transition dipoles in the plane of the six-porphyrin rings (xy plane,F igure 3), and that emission occurs from an excited state that is delocalized over this same xy plane.H owever,a ts horter excitation wavelengths,t he anisotropy decreases,b ecoming negative and reaching av alue of À0.04 at 742 nm, whereas the fluorescence anisotropy of c-P6·T6 is independent of the wavelength (Figure 5c). Them ost likely explanation for this behavior is that at 742 nm, light is absorbed by transitions associated with the z-polarized butadiyne-linked porphyrin dimer staves of the barrel to former higher excited states that emit from the xy-polarized state.T his interpretation is supported by the observation that porphyrin dimer 5a has aQband at 705 nm and by the report that ar elated square  . . butadiyne-linked porphyrin tetramer exhibits ad imer-like absorption spectrum (Q band at 659 nm). [27] If the absorption by t-P12·(T6) 2 at 742 nm were entirely z-polarized, with emission entirely from astate polarized in the xy plane,then the anisotropy would be À0.2. [28] Thev alue of À0.04 reflects the presence of overlapping bands at 742 nm, with different polarizations.M igration of electronic excitation from the z-polarized staves to the xy-polarized state takes place faster than the 200 fs time resolution of our experiment.
In summary,w eh ave synthesized af ully p-conjugated three-dimensional porphyrin nanotube in 32 %y ield by an alkyne homo-coupling reaction involving the formation of twelve new C À Cb onds.T he preparation of this nanotube represents as urprising new type of cooperative templatedirected synthesis,inwhich the binding mode of the templates switches as the reaction progresses;t his contrasts with other types of cooperative template-directed syntheses in which the templates assemble the starting materials in the correct geometry for product formation. [29] Thea bsorption and emission spectra of the nanotube are red-shifted by approximately 300 cm À1 compared to the corresponding six-porphyrin nanoring.T ime-resolved fluorescence anisotropy measurements show that there is ultra-fast energy migration from excited states associated with the staves of the barrel to the six-porphyrin ring regions.F urther studies of the electronic delocalization in the radical cations of these molecules are currently underway.W ea re also investigating the possibility of preparing longer nanotubes using this synthetic strategy.