Anti-[2.2](1,4)phthalocyaninophane: Spectroscopic evidence for transannular interaction in the excited states

Article abstract of DOI:10.1021/ja068528c

The design and synthesis of a novel type of bisphthalocyanines, anti-[2.2](1,4)phthalocyaninophanes, were reported. The phthalocyanine dimer exhibited significantly split and red-shifted absorption in the near-IR region and maintained the fluorescence properties of phthalocyanines. The origin of the electronic communication between the two phthalocyanines was rationalized on the basis of the MO model analysis. Copyright

Full text of DOI:10.1021/ja068528c

Published on Web 03/27/2007
anti-[2.2](1,4)Phthalocyaninophane: Spectroscopic Evidence for Transannular
Interaction in the Excited States
Yoshiaki Asano,† Atsuya Muranaka,† Akira Fukasawa,† Terutaka Hatano,‡,§
Masanobu Uchiyama,‡,§ and Nagao Kobayashi*,†
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan,
Graduate School of Pharmaceutical Sciences, UniVersity of Tokyo 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033,
Japan, and The Institute of Physical and Chemical Research, RIKEN, Wako-shi, Saitama 351-0198, Japan
Received November 28, 2006; E-mail: nagaok@mail.tains.tohoku.ac.jp
An enhanced understanding of the electronic communication
between chromophores is of great importance in designing organic
materials for applications in optoelectronic technologies. In this
respect, bis- and multiphthalocyanines have attracted considerable
interest because of their high stability, the presence of intense π-π*
transitions in the visible region (Q-band), and their redox ability.1
Since the relative orientation and distance between phthalocyanine
(Pc) chromophores are essential to their electronic excited states, a
number of Pc dimers and oligomers have been explored.2 Herein
we report a novel type of bisphthalocyanines, anti-[2.2](1,4)-
phthalocyaninophanes (1Zn and 1Cu), in which two Pc units are
linked by ethano bridges at the para positions, to create a [2.2]-
paracyclophane moiety. The cyclophane framework could allow
for a well-defined slipped-stack arrangement of the two Pc units.
Construction of this kind of a slipped-stack dimer has been
stimulated by the fact that the special pair and antenna subunits in
photosynthetic light-harvesting antenna consisting of bacteriochlo-
rophyll dimers have a similar spatial arrangement to fine-tune the
electronic excited states,3 and that the fluorescence properties of a
Pc chromophore preserve, which is in contrast to nonfluorescent
Pc oligomers with face-to-face geometry.2,4
Figure 1. MCD (a), absorption (b), and fluorescence (c) spectra of 1Zn
(red), 2Zn (blue), and 3Zn (black) in benzene. ZINDO/S calculations (d).
Gaussian bands with half-bandwidth of 400 cm-1 were used; x and y indicate
transition polarizations.
between those of the Pc dimer without π-conjugation (J-type Pc
dimer, 700 nm)6 and π-conjugated planar Pc dimer (852 nm).7 The
magnetic circular dichroism (MCD) signals of 1Zn and 2Zn
associated with the two lowest-energy transitions have negative
{[θ]M ) -6.23 × 105 (1Zn), -1.05 × 106 (2Zn)} and positive
signs {[θ]M ) 7.52 × 105 (1Zn), 7.21 × 105 (2Zn)}, respectively.
The Q absorption peak positions are almost the same as the observed
trough and peak, so that these signals are assigned to coupled
Faraday B terms, which arise from magnetically induced mixing
of nondegenerate excited states.8 Since the spectral pattern of 1Zn
was unchanged upon addition of pyridine and a linear relationship
between absorbance and concentration was confirmed in Beer’s
law experiments, aggregation behavior should be negligible under
the experimental conditions. The spectroscopic properties of the
Cu complexes (1Cu and 2Cu) were almost identical to those of
the Zn complexes. From these spectroscopic data, it is evident that
significant through-space π-π interactions are involved in the
excited singlet states of the present system. This is quite different
from the bisphthalocyanine system consisting of a [2.2]paracyclo-
phane bridge reported by Torres et al.9
The anti-[2.2](1,4)-zinc(II)-9(10),16(17),23(24)-tri-tert-butylph-
thalocyaninophane (1Zn) was synthesized from [2.2]paracyclophane
in three steps (see Supporting Information). The anti-[2.2](3,6)-
phthalonitrilo(1,4)zinc(II)-9(10),16(17),23(24)-tri-tert-butylphtha-
locyaninophane (2Zn) was also obtained in the synthetic procedure.
The Cu complexes (1Cu and 2Cu) were synthesized similarly to
the Zn complexes. Figure 1 shows experimental and calculated
spectra of 1Zn, 2Zn, and reference zinc(II) tetra-tert-butylated Pc
(3Zn). The 678 nm absorption band of 3Zn splits into two well-
resolved bands (683 and 706 nm) in the case of 2Zn. The splitting
of the Q-band appears to be similar to that of conventional low-
symmetrical Pc derivatives reported previously.5 In contrast, 1Zn
exhibited significantly split and red-shifted absorption bands,
observed at 753 and 690 nm. It should be noted that, when
compared to the dimeric Pcs with a similar geometrical arrangement,
the lowest-energy Q-band position of the dimer lies midway
1Zn exhibited a fluorescence peak at 764 nm longer than those
of 2Zn (710 nm) and 3Zn (684 nm), which is consistent with the
formation of J-type aggregate of organic dyes.10 The fluorescence
quantum yields in benzene were determined to be 0.12 (1Zn) and
0.19 (2Zn) using 3Zn as a standard.11 The value of 1Zn is smaller
than that of a J-type Pc dimer,6 but 1 order higher than those of
† Tohoku University.
‡ University of Tokyo.
§ RIKEN.
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10.1021/ja068528c CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
pyrrole π orbitals on the other Pc unit contribute to the stabilization
of the LUMO energy, although the interaction between the π
orbitals on the nearest carbons is slightly antibonding. In contrast,
since the relative size of the π orbitals on the cyclophane moiety
of the LUMO+1 is considerably small, the through-space π-π
interactions between the benzene and pyrrole units are weak, which
results in moderate stabilization of the LUMO+1.17 The small
Q-band splitting observed for 2Zn can also be rationalized by
considering the relative size of the π orbitals in the cyclophane
moiety (see Supporting Information).
In summary, we have demonstrated the design and synthesis of
a novel bisphthalocyanine system (1Zn and 1Cu) that exhibited
significantly split and red-shifted absorption in the near-IR region.
It was found that 1Zn maintained the fluorescence properties of
Pcs. The MO model analysis provides a clear-cut picture for the
nature of the electronic communication between the two Pc
chromophores, which could offer a new approach for understanding
transannular interactions of cyclophanes. In addition, since the
phthalocyaninophanes can mimic the slipped-stack arrangement
with significant interchromophore coupling, the present system has
potential application as a novel model of the special pair and
subunits in photosynthetic systems.
Figure 2. (a) Frontier MOs of the B3LYP-optimized 1Zn. (b) Energy
correlation diagram of zinc dimethyl Pc (left) and 1Zn (right). (c) Schematic
representation of the origin of the π-π interaction. The relative π orbital
size is based on the CI calculations. Bonding and antibonding interactions
are indicated by solid and dotted arrows, respectively.
Acknowledgment. This research was partially supported by a
Grant-in-Aid (17350063) for Scientific Research and the COE
project, Giant Molecules and Complex Systems, 2006, from the
Ministry of Education, Science, Sports and Culture, Japan. A.M.
is indebted to the ERYS (Tohoku University) for a research grant.
Most calculations were performed by using of RIKEN Super
Combined Cluster (RSCC). The authors thank these computational
facilities for generous allotment of computer time.
coplanar Pc dimers.12 The Stokes shift of 1Zn was fairly small
(ca. 190 cm-1), thus indicative of a rigid structure of the molecule.
According to the B3LYP/6-31G* optimized structures, the
average distance between the facing benzene planes is 2.98 Å. The
geometrical parameters of the cyclophane moiety of 1Zn and 2Zn
are close to those of [2.2]paracyclophane calculated using the
B3LYP functional,13 suggesting that a considerable electron
exchange within the π units should be taken into account.14 The
calculated spectral features (ZINDO/S) are in good agreement with
the experimental spectra. It is clearly seen from Figure 2a that the
MOs of 1Zn consist of linear combinations of the monomeric MOs.
The lowest and second lowest energy absorption bands of both 1Zn
and 2Zn can be attributed to long- and short-axis polarized Q
transitions, respectively, since these transitions were formed
predominantly through combinations of MOs derived from Gout-
erman’s orbitals.15 These results agree with the observed coupled
Faraday B terms since transitions of different polarizations generally
have differently signed MCD signals.8 In view of the assignment
of the polarization of the Q-bands, the MO calculation is consistent
with Kasha’s exciton coupling theory.16
The origin of the spectral features of 1Zn was rationalized as
shown in Figure 2b. The HOMO and LUMO are destabilized and
stabilized, respectively, compared with a corresponding Pc monomer
due to the significant orbital interactions, which leads to the spectral
red shift (Qx). Since the stabilization of the LUMO+1 is modest,
the energy difference between the LUMO and LUMO+1 becomes
large, which results in the marked Qx-Qy splitting. The orbital
interaction energies of 1Zn are calculated to lie midway between
those of the corresponding Pc dimer with and without π-conjugation
(see Supporting Information). Figure 2c illustrates a schematic
representation of the π orbitals of the cyclophane moiety in the
LUMO and LUMO+1. In the case of the LUMO, bonding
interactions between the benzene π orbitals on one Pc unit and the
Supporting Information Available: Experimental and computa-
tional details. This material is available free of charge via the Internet
at http://pubs.acs.org.
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