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A R T I C L E S
Cozzolino et al.
morphs of 6. In each ribbon structure there are two distinct
Te-N SBI distances; while in one case (ꢀ) the distances
alternate from one [Te-N]2 supramolecular synthon to the next,
in the other (R) both the short and the long distances are
observed in the same supramolecular synthon. Interestingly the
long Te-N SBI distances in each phase are equal within 3σ
and the same is observed for the short distances. However, as
it can be inferred from Figure 6, the HOMO-LUMO transition
(bg r bg) in a centrosymmetric dimer is symmetry forbidden.
In the R phase, breaking the symmetry of the [Te-N]2 ring by
a noncentrosymmetric distortion would result in more excitations
being allowed, including the HOMO-LUMO transition. In
addition, electron delocalization along the supramolecular
ribbons also influences the HOMO-LUMO gap; optimized
tetramer models of the ribbons showed decreases of 0.65 eV
for the R phase and only 0.51 eV for the ꢀ phase, with respect
to the monomer. Because of such an extended conjugation, the
TD-DFT method could not be applied to model the electronic
spectra of these materials more accurately.
Figure 5. Ultraviolet/visible absorption spectra of R-6 (s), ꢀ-6 (- - -),
and 62 ·Py2 (· · · · ) in the solid state obtained from diffuse reflectance
measurements. Inset: photograph of ꢀ-6 before (yellow) and after (red) the
phase change to R-6 induced by heating.
DFT calculations were also used to compare the stability of
the crystalline phases of 6. The calculated total association
energy of the supramolecule 62 ·Py2 is -169.8 kJ/mol; of this
-82.2 kJ/mol corresponds to the [Te-N]2 supramolecular
synthon and -43.8 kJ/mol to each of the Te--Npy SBIs. For the
ribbon structures, the estimated association energies are -36.8
kJ/mol for ꢀ-6 and -63.8 kJ/mol for R-6, which is consistent
with the small enthalpy change that is experimentally observed.
Conclusions
The three crystalline phases formed by 6 were structurally
characterized; all were shown to contain the [Te-N]2 supramo-
lecular synthon. In one case pyridine caps the tellurium atoms
preventing the association of molecules beyond a dimer; in this
phase the [Te-N]2 supramolecular synthon is symmetric. In the
other, nonsolvated structures, the steric interaction between
neighboring fluorine atoms distorts the [Te-N]2 supramolecular
synthon and the ribbon chain. Two distinct polymorphs arise
from in-plane and out-of-plane distortions which leave cen-
trosymmetric and noncentrosymmetric supramolecular synthons,
respectively. The two phases are related by a monotropic change
that is accompanied by a color change from yellow to red.
Although previous computational studies had concluded that
the contribution of π orbitals to the stabilization of the [Te-N]2
supramolecular synthon is negligible, the present case demon-
strates that their interactions can have an important effect in
the overall electronic structure and optical properties of a
material. Although the irreversibility of the phase transition
between its two polymorphs discounts the application of 6 as a
thermochromic material, the results of these investigations do
suggest that materials featuring switchable SBIs could find
application in optical-chemical sensors and electronic devices.
Figure 6. Correlation of frontier orbitals for a centrosymmetric dimer (C2h)
of 6.
HOMO (a2 under C2V) is a π orbital with the largest contributions
from the C6 ring, the HOMO-1 (b1) mostly corresponds to the
lone pair of tellurium with π orientation, the LUMO (b1) has a
π*Te-N character, and the LUMO+1 (b2) consists of a combina-
tion of σ*Te-N orbitals. With the exception of b2 r b1, the
electron excitations between these orbitals are symmetry al-
lowed. Comparable calculations (PW91, TZP) for 6 yielded
equivalent frontier orbitals but a HOMO-LUMO gap that is
smaller than that of 2 by 0.276 eV. A TD-DFT calculation
(SAOP, QZ4P) gave the first excitation band on the individual
molecule (b1 r a2) at 456 nm with an oscillator strength f )
0.014 and the second (b1 r b1) at 367 nm and f ) 0.207.
Intermolecular association through the SBIs does cause a
perturbation of the electronic structure. Figure 6 displays the
evolution of the frontier orbitals of a centrosymmetric dimer of
6 as a function of the Te--N distance from 2.2 to 3.2 Å. Within
the range studied, there is a change of 0.6 eV in the
HOMO-LUMO gap that results from mixing of orbitals in the
π manifold. In contrast, appending two molecules of pyridine
(as observed in the solvated structure) had a negligible effect
on the frontier orbital energies of the model dimer because all
the relevant orbital interactions are confined to the σ framework.
Although the calculations show that the HOMO-LUMO gap
decreases with the Te--N SBI length, this fact alone is not
sufficient to explain the difference of color of the two poly-
Experimental Section
Computational Method. DFT calculations in this study were
performed using the ADF DFT package (SCM, versions 2008.01
and 2009.01).12-14 The Adiabatic Local Density Approximation
(ALDA) was used for the exchange-correlation kernel,15,16 and the
differentiated static LDA expression was used with the Vosko-
(12) Te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Fonseca Guerra,
C.; Van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comp. Chem.
2001, 22, 931–967.
(13) Guerra, C. F.; Snijders, J. G.; Te Velde, G.; Baerends, E. J. Theor.
Chem. Acc. 1998, 99, 391.
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17268 J. AM. CHEM. SOC. VOL. 132, NO. 48, 2010