meso-(4-(N,N-dialkylamino)phenyl)-substituted subporphyrins: Remarkably perturbed absorption spectra and enhanced fluorescence by intramolecular charge transfer interactions

Article abstract of DOI:10.1021/ja804846v

A series of meso-(4-(N,N-dibenzylamino)phenyl)-substituted subporphyrins was synthesized by means of Buchwald-Hartwig amination protocol. Substitution of the amino group at the 4-position of the meso-phenyl substituent resulted in a remarkable red shift in the absorption spectra and drastic enhancement of fluorescence intensity probably as a consequence of intramolecular CT interaction. These characteristics have been utilized to construct a cation-sensing system by appending a 1-aza-15-crown-5 unit to subporphyrin that displays large spectral changes upon cation binding. Copyright

Full text of DOI:10.1021/ja804846v

Published on Web 08/23/2008
meso-(4-(N,N-Dialkylamino)phenyl)-Substituted Subporphyrins: Remarkably
Perturbed Absorption Spectra and Enhanced Fluorescence by Intramolecular
Charge Transfer Interactions
Yasuhide Inokuma,† Shanmugam Easwaramoorthi,‡ Zin Seok Yoon,‡ Dongho Kim,*,‡ and
Atsuhiro Osuka*,†
Department of Chemistry, Graduate School of Science, Kyoto UniVersity, Sakyo-ku, Kyoto 606-8502, Japan and
Department of Chemistry, Yonsei UniVersity, Seoul 120-749, Korea
Received July 2, 2008; E-mail: osuka@kuchem.kyoto-u.ac.jp; dongho@yonsei.ac.kr
Since our synthesis in 2006,1 subporphyrin, a ring contracted
congener of porphyrin, has emerged as a promising macrocycle in
light of the bowl-shaped triangular structures, distinct aromaticity
along its 14π-electronic circuit, bright green fluorescence, and
theoretical interest.2-4 Despite this progress, the chemistry of
subporphyrin still remains in its rudimentary stage compared to
that of subphthalocyanine.5 One of the remarkable characteristics
of subporphyrin is the large substituent effects of the meso-aryl
group on the electronic property of subporphyrin, which are arising
Figure 1. X-Ray crystal structure of 1. Selected bond lengths and dihedral
from its small rotational barrier. An interesting example is meso-
tri(4-nitrophenyl)-substituted subporphyrin that exhibits strongly
solvent-dependent optical properties due to the intramolecular
charge transfer (CT) interaction with the subporphyrin core as an
electron donor.4a Here we report the synthesis and full characteriza-
tions of meso-(4-(N,N-dibenzylamino)phenyl)-substituted subpor-
phyrins that exhibit unique absorption properties and remarkably
enhanced fluorescence emission due to the intramolecular CT
interaction in the opposite direction.
angles are shown.
Figure 2. UV-vis absorption (solid lines) and fluorescence (dashed lines;
excited at 394, 401, 401, and 373 nm for 1, 2, 3, and 4, respectively) spectra
of subporphyrins 1-4 in CH2Cl2.
exhibits a slight but distinct distortion toward a quinodimethene-
like form. Namely, the bond lengths of C2-C3 and C5-C6 are
shorter than the other bonds in the 4-aminophenyl substituent in 1
(Figure 1) and all the 4-aminophenyl substituents in 3 (SI). These
features are similar to those of donor-acceptor molecules such as
4-cyano- and 4-nitro-substituted N,N-dimethylanilines.8 Such a
structural distortion is trivial in corresponding meso-(4-aminophe-
nyl)porphyrins.
The UV-vis absorption spectra of 1-3 are clearly different
depending on the number of 4-aminophenyl substituents (Figure
2). Subporphyrin 1 exhibits a split Soret band at 359 and 394 nm
and a red-shifted Q-like band at 505 nm compared to that of
triphenylsubporphyrin 4. Subporphyrin 2 also displays a split Soret
band, but its shape is changed; a high-energy band at 343 nm is
attenuated, and a low-energy band at 401 nm is intensified.
Interestingly, subporphyrin 3 shows a nonsplit sharp Soret band at
401 nm. To understand these spectral features, MO calculations
were performed at the B3LYP/6-31G* level. Among the four
frontier orbitals of subporphyrin 4, HOMO (a1) and HOMO-1 (a2)
are energetically close to the HOMO of the N,N-dialkylaminophenyl
group but only the a1 orbital can be well hybridized with the HOMO
Subporphyrins 1-3 were prepared by means of a palladium
catalyzed Buchwald-Hartwig amination protocol6 from corre-
sponding 4-bromophenyl-substituted subporphyrins and dibenzyl-
amine (Supporting Information, SI). The 1H NMR spectrum of
subporphyrin 1 indicates Cs molecular symmetry, featuring a singlet
at 8.08 ppm and a couple of doublets at 8.16 and 8.07 ppm due to
the peripheral ꢀ-protons. Appearance of a pair of doublets at 7.92
and 7.07 ppm due to the protons of the aminophenyl substituent
indicates its free rotation. Similar molecular symmetry was
confirmed for 2 by its 1H NMR spectrum (SI). In contrast, 3 exhibits
C3-symmetry by displaying a singlet at 8.08 ppm due to the
ꢀ-protons and a single set of signals due to the meso-aryl-
substituents.
The solid-state structures of 1 and 3-OEt, a B-OEt analogue of
3, were unambiguously elucidated by X-ray diffraction analysis
(Figure 1 and SI).7 The dihedral angles between the subporphyrin
mean plane and N,N-dibenzylaniline moiety are 40.9° for 1 and
42.7°-48.6° for 3-OEt. Interestingly, each 4-aminophenyl group
† Kyoto University.
‡ Yonsei University.
9
12234 J. AM. CHEM. SOC. 2008, 130, 12234–12235
10.1021/ja804846v CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Binding Constants of 5 with Alkali and Alkaline Earth
of the N,N-dimethylaniline moiety, since the meso-positions of the
a2 orbital are nodes. In subporphyrin 1, such orbital hybridization
results in the formation of stabilized HOMO-2 and destabilized
HOMO, while HOMO-1 (a2) remains at the same energy level.
According to Gouterman’s four orbital theory,9 the Soret band of
porphyrin consists of energetically degenerated transitions from
HOMO (a1) or HOMO-1 (a2) to LUMO (e) or LUMO+1 (e) (i.e.,
Bx and By transitions). In the case of 1, Bx and By are no longer
equivalent, since the a1 level is split into two different energy levels.
This situation leads to the split Soret band. The similar feature was
predicted and was actually observed as a split Soret band for 2.
Importantly, the degeneracy of the HOMO is recovered for 3, due
to its C3 molecular symmetry. These spectral changes in the Soret
band underscore the characteristic substituent effects of subpor-
phyrin. The calculations also indicate that HOMOs of 1-3 are
progressively destabilized in this order through the orbital interac-
tions with the dimethylaminophenyl moiety, which is in good
agreement with the first oxidation potentials measured by cyclic
voltammetry; 1 (0.38 V), 2 (0.30 V), 3 (0.26 V), and 4 (0.71 V).
This trend leads to a larger energy difference between HOMO (a1)
and HOMO-1 (a2), which may account for the gradual increase
and bathochromic shift in the Q-like bands of 1-3. Therefore, these
spectral characteristics are still understandable within Gourterman’s
four orbital theory.
Subporphyrins 1-3 emit reddish-orange fluorescence tailing over
800 nm as mirror images of their Q-like bands. Remarkably, the
fluorescence quantum yields of 1-3 recorded in CH2Cl2 are
drastically enhanced; Φ ) 59, 60, and 58%, respectively, which
are more than 4-fold that of 4 (ΦF ) 13%). The fluorescence
lifetimes of 1-3 determined by a time-correlated single photon
counting method are considerably longer than those of 4 in solvents
examined (SI), hence suggesting suppression of a nonradiative
decaying route in the singlet excited state. Subporphyrins 1-3 also
exhibit solvatochromic behaviors in the absorption and fluorescence
spectra. These fluorescence spectral changes were analyzed by a
Lippert-Mataga plot,10 which provided large slopes of 2210, 1550,
and 1510 cm-1 for 1, 2, and 3 (SI), suggesting substantial CT
characters for the excited singlet states. On the contrary, the
fluorescence quantum yields and lifetimes remain insensitive to the
solvent polarity, implying that the fluorescence comes not from an
ion-pair state but from a singlet excited state with considerable CT
character.
Metal Perchlorates Measured by Absorption Titration in Acetonitrile
cation
Li+
Na+
Mg2+
Ca2+
Sr2+
Ba2+
log K
2.80
2.30
3.54
4.51
3.69
3.68
were confirmed with other metal cations examined (Table 1). As
an advantageous propensity, the fluorescence spectra of solutions
of cation-bound 5 were found to vary among orange, yellow, and
green, depending upon the binding cation (SI). The determined
binding constants are roughly similar to the previously reported
trend of 1-aza-15-crown-5 attached chromoionophores,11 and it can
be concluded that cation-induced spectral changes in the absorption
and fluorescence spectra are attributed to the suppression of the
electron-donating character of the amine nitrogen atom by coor-
dination to metal cation. Protonation of the amino group in 5 with
trifluoroacetic acid resulted in a fluorescence color change to green.
While the cation selectivity is low for this prototype, coordination-
induced changes in the absorption and fluorescence are large enough
to be distinguishable by the naked eye and thus promising.
In summary, we have demonstrated tunable characteristics of
subporphyrins by incorporating electron donating 4-aminophenyl
substituents. Distinct CT interactions in 1-3 lead to large changes
in the absorption spectra and drastic enhancement of the fluores-
cence intensity, which has been utilized to explore a new chemosen-
sor. Further tuning and fabrications of subporphyrins through meso-
aryl substituents are actively in progress in our laboratories.
Acknowledgment. The work was partly supported by a Grand-
in-Aid for Scientific Research from MEXT. Y.I. thanks the JSPS
Research fellowships for Young Scientists. The work at Yonsei
was supported by the Star Faculty of Ministry of Education, Science,
and Technology of Korea.
Supporting Information Available: Detailed experimental data.
This material is available free of charge via the Internet at http://
pubs.acs.org.
Strong electronic interaction between the 4-aminophenyl sub-
stituent and the subporphyrin core is attractive in view of a chemical
sensing system. Along this line, aza-crown-substituted subporphyrin
5 was designed and prepared via the similar Pd-catalyzed amination
route (SI). Similarly to 1, the subporphyrin 5 shows a split Soret
band at 358 and 394 nm and a Q-band at 508 nm in acetonitrile
due to the electron-donating character of the nitrogen atom
embedded in the crown ether. The cation binding ability of 5 was
examined by UV-vis absorption and fluorescence titration in
acetonitrile using perchlorate salts (Table 1 and SI). Upon addition
of Ca(ClO4)2, the perturbed absorption spectrum of 5 was gradually
changed to an unperturbed one that was quite similar to that of
triphenylsubporphyrin 4 with several clear isosbestic points (SI),
allowing accurate determination of the binding constants (Table
1). This Ca2+ binding can be monitored by a vivid color change
of solution from orange to yellow. During the titration, the perturbed
reddish-yellow fluorescence of 5 was changed to a normal sub-
porphyrin-like green yellow fluorescence. The similar binding events
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