Sizeable red-shift of absorption and fluorescence of subporphyrazine induced by peripheral push and pull substitution

Article abstract of DOI:10.1039/c4cc05943c

Peripheral substitution with electron-donating (push) and electron-withdrawing (pull) substituents caused a sizeable red-shift of the Q band absorption and fluorescence of subporphyrazine, and the red-shift was controlled by the push substituents. Control

Full text of DOI:10.1039/c4cc05943c

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Sizeable red-shift of absorption and fluorescence
of subporphyrazine induced by peripheral push
and pull substitution†
Cite this: Chem. Commun., 2014,
50, 13781
Received 30th July 2014,
Accepted 16th September 2014
Xu Liang,a Soji Shimizu*b and Nagao Kobayashi*a
DOI: 10.1039/c4cc05943c
www.rsc.org/chemcomm
Peripheral substitution with electron-donating (push) and electron- trichloride at 140 1C. Recently the same group succeeded in the post-
withdrawing (pull) substituents caused a sizeable red-shift of the modification of the peripheral substituents with aryl groups using
Q band absorption and fluorescence of subporphyrazine, and the Pd-catalyzed copper(I) thiophene-2-carboxylate (CuTC)-mediated
red-shift was controlled by the push substituents. Control of coupling of boronic acids with heteroaromatic thioethers.8 With
the chromophore symmetry and inherent molecular chirality arising these peripherally substituted SubPzs in hand, they also revealed
from the pattern of substitution were also investigated.
a perturbed nature of absorption spectral morphologies by peri-
pheral aryl-substituents, which was mainly observed as shift and
Control of electronic absorption and fluorescence in the UV/visible intensification of intramolecular charge transfer (CT) bands at a
and near infrared regions in a simple manner such as introduction higher energy region than the Q band, whereas the changes in the
of substituents or changes in molecular symmetries has been central Q band absorption were modest. With the aim of controlling the
to the molecular design of functional dyes. Its importance has been Q band absorption and fluorescence of SubPzs, we envisioned
accelerated due to the recent growth of fields related to photo-energy introduction of both electron-donating (push) and electron-
conversion. As a result, attention has been refocused on conven- withdrawing (pull) substituents. Push and pull substituents,
tional chromophore molecules, for which these kinds of modifica- respectively, cause destabilization and stabilization of the HOMO
tion are possible. From this point of view, subporphyrazine (SubPz),1 and LUMO, and the synergetic effect of the push–pull substituents
a contracted analogue of tetraazaporphyrin (TAP)2 or in other words generally results in a decrease in the HOMO–LUMO energy gap
the meso-nitrogen-substituted congener of subporphyrin,3,4 has and hence a red-shift of the Q band absorption mainly comprising
become an attractive class of chromophore molecules because of the HOMO–LUMO transition. This push–pull strategy worked
its intense Q band absorption and fluorescence in the visible perfectly in the previous attempt at shifting the Q band absorption
region. Despite their potential control by peripheral substituents, of TAP molecules to the red.9,10 In addition to the perturbation by
it has been possible to introduce only a limited kind of substituents the push and pull substituents, molecular symmetries defined by
to the periphery of SubPz, mainly due to the poor stability of SubPz arrangement of the push and pull substituents also had a marked
molecules. The synthesis of SubPz was pioneered by three different effect on the morphologies of the Q band absorption of TAP
groups, Hanack,5 Torres,6 and us7 and unequivocally established molecules.10 In the case of SubPzs, formation of two positional
6
´
for the first time by Torres and Rodrıguez-Morgade et al. They isomers with C3 and C1 symmetry can be expected from the
reported the synthesis of several peripherally alkyl and alkylthio- reaction of a tricyanoethylene substituted with a push group.
substituted SubPzs in moderate yields by a one-pot reaction of the Furthermore, these push–pull SubPzs are inherently chiral due to
corresponding substituted maleonitriles in the presence of boron the absence of any symmetric mirror plane in the structures,
which is similar to the chirality of low-symmetry subphthalo-
cyanines, a peripherally benzene-annulated homologue of SubPz.1
a Department of Chemistry, Graduate School of Science, Tohoku University,
The inherent chirality of subphthalocyanines was disclosed for
Sendai 980-8578, Japan. E-mail: nagaok@m.tohoku.ac.jp; Fax: +81-22-795-7719;
the first time by Torres and Claessens11 and recently developed by
Tel: +81-22-795-7719
b Department of Chemistry and Biochemistry, Graduate School of Engineering,
Kyushu University, Fukuoka 819-0395, Japan. E-mail: ssoji@cstf.kyushu-u.ac.jp;
Fax: +81-92-802-2866; Tel: +81-92-802-2866
us12 to elucidate the relationship between the absolute structures
and the circular dichroism spectra. In this manuscript, the first
synthesis of push–pull SubPzs and their properties are described
with a special focus on correlation of the red-shift of the Q band
absorption and fluorescence to the donor ability of the push
substituents and on the inherent molecular chirality.
† Electronic supplementary information (ESI) available: Experimental procedure,
characterization of all the new compounds, Hammett plots, and theoretical
calculations. CCDC 1015544. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c4cc05943c
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A structural feature which should be emphasized is the high
co-planarity of the phenyl substituents with the core structure of
SubPzs. This results from small steric hindrance of the neigh-
bouring cyano groups so that a significant perturbation of the
optical properties by the push-substituents can be expected.
Compared to the Q band absorption of hexakis-alkyl-
substituted SubPzs at ca. 500 nm6,7 and hexakis-aryl-substituted
SubPzs at 560–580 nm,8 the push–pull SubPzs exhibited red-shift of
the Q band absorption; 2a: 633 nm, 2b: 594 nm, and 2c: 571 nm
(Fig. 2). The extent of the red-shift is proportional to the donor
ability of the push-substituent, which is broadly supported by a
linear correlation between the position of the Q band absorptions
and the Hammett sp parameters of the substituents of the aryl
groups (see the ESI†). Between the Q band and Soret band absorp-
tions at around 300 nm, a broad absorption was observed. The
absorption maxima of this broad band was also red-shifted in the
same order as observed for the Q band absorptions (2a: 435 nm, 2b:
419 nm, and 2c: 390 nm). In the magnetic circular dichroism (MCD)
spectra, a derivative-shaped Faraday A term with an inflection point
close to the maximum of the Q band absorption was observed for
these C3 symmetric compounds, which is indicative of degenerate
excited states due to the high molecular symmetry (Fig. 2). The C1
symmetric compound 3a exhibited its Q band absorption at a very
similar position to that of 2a at 626 nm. A derivative-shaped MCD
corresponding to the Q band absorption of 3a was assigned as a
pseudo Faraday A term, which can be observed if molecules with
less than three-fold molecular symmetry accidentally possess
nearly degenerate excited states. From these spectroscopic results,
insignificant perturbation of the Q band absorption by molecular
symmetries is inferred in the case of SubPzs. This is in marked
contrast to the push–pull TAPs.10 Despite the small difference in
the Q band region, significant changes in the absorption spectral
morphologies featured by two broad absorptions at 499 and
435 nm were observed in the higher energy region.
Scheme 1 Synthesis of push–pull SubPzs.
From the reaction of 1,1,2-tricyano-2-p-methoxyphenylethylene 1a
and BCl3 in xylene at 140 1C for 45 min, two diastereomers 2a and 3a
with C3 and C1 molecular symmetry were obtained in 3.3% and
0.2% isolated yields, respectively (Scheme 1). The discrepancy in the
ratio of the isolated yields from the statistical ratio (C3 isomer : C1
isomer = 1 : 3) is simply explained by the instability of the push–pull
SubPzs in solution. In particular, 3a is less stable than 2a. For this
reason, in the cases of p-tolyl or p-trifluoromethylphenyl-substituted
SubPz, only the C3 symmetric isomers (2b and 2c) were obtained in
5.6% and 7.2% yields, respectively, and formation of the C1 isomer
could not be observed (Scheme 1).
These push–pull SubPzs were characterized by high-resolution
MALDI-TOF mass analysis, and the molecular symmetries were
1
confirmed by H NMR spectroscopy (see the ESI†). In the case of
the C3 symmetric compounds 2a–c, a pair of doublets was observed
at 8.88 and 7.24 ppm for 2a, 8.71 and 7.55 ppm for 2b, and 8.68 and
7.39 ppm for 2c, while a singlet due to the methoxy and methyl
substituents at para-positions of the push substituents in 2a and 2b
was observed at 4.00 and 2.63 ppm, respectively (see the ESI†).
In contrast to the symmetric patterns of the proton signals
observed for 2a–c, 3a exhibited three pairs of doublets of the phenyl
substituents at 8.91, 8.78, 8.71, 7.30, 7.23, and 7.20 ppm and three
singlets of methoxy groups at 4.01, 3.98, and 3.96 ppm due to the
C1 molecular symmetry.
The structure of the C3 symmetric isomer was unambiguously
elucidated by X-ray structural analysis on single crystals of 2c
obtained from slow diffusion of hexane into a toluene solution of
a racemic mixture of 2c (Fig. 1).‡ The crystal structure of 2c
exhibited C3 symmetric arrangement of cyano and p-trifluoro-
methylphenyl substituents. The structures of both enantiomers with
clockwise and anticlockwise arrangement of these substituents were
also confirmed (see the ESI†). In the crystal packing, 2c formed
slipped-stack columns in a convex–concave manner, in which
solvent toluene molecules resided between each 2c molecule.
Fig. 2 UV/vis absorption (bottom) and MCD (middle) spectra of 2a (red
line), 2b (green line), 2c (blue line), and 3a (orange line) in toluene and CD
spectra of enantiomers of 2b in toluene (top).
Fig. 1 X-ray single crystal structure of 2c, top view (left) and side view
(right). The thermal ellipsoids were scaled to the 50% probability level.
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considered to be composed of both intramolecular CT transitions
from the HOMO À 1 and HOMO À 2 to the LUMO and p–p*
transitions between the SubPz-centred HOMO À 3 and LUMO. The
peripherally aryl-substituted SubPzs8 also exhibit similar CT absorp-
tion in this region. Since a Faraday A term like MCD signal was
observed for the band at 435 nm for 2a, 419 nm for 2b, and 390 nm
for 2c in the MCD spectra, this band can be assigned as the p–p*
transition, which accompanies angular momentum change (Fig. 2).
Based on the TDDFT calculations, which indicates that the p–p*
transitions follow the CT transitions, the CT transitions contribute
broad absorption between the Q band and the p–p* transitions
(ca. 575–450 nm for 2a, 550–450 nm for 2b, and 525–425 nm for 2c).
The broadening of the observed Q band absorption upon increasing
the donor ability of the push-substituents from 2c to 2b and to 2a
can be explained in terms of enhanced contribution of the CT
transition to the absorption in the Q band region. This also
illustrates the low R square value of 0.8 for the regression line of
the Hammett plot (see the ESI†). In the case of 3a, based on the
TDDFT calculation, the absorption spectral morphologies in the
Q band and the higher energy regions can be similarly explained:
the Q band absorption comprises transitions from the HOMO to
the non-degenerate LUMO and LUMO + 1, whereas the higher-
energy broad band at 499 and 435 nm can be assigned as
intramolecular CT-like transitions and p–p* transitions between
the SubPz-centred orbitals.
In order to give an insight into the perturbation of the
electronic structures by the push–pull substituents, the energy
levels of the HOMO and LUMO of 2b were compared with those
of 4. The LUMO is significantly stabilized, whereas the HOMO
remains at the same energy probably due to cancellation of
stabilization by the pull substituents with destabilization by the
push substituents. Changing the substituents at the p-positions
from methyl to methoxy and trifluoromethyl results in destabiliza-
tion and stabilization of the HOMO and LUMO, respectively. In
both cases, the extent of stabilization or destabilization is more
significant for the HOMO. The MO diagram, therefore, clearly
reproduces the trend of the red-shift of the Q band absorption.
In the case of 3a, the energy difference between the LUMO and
LUMO + 1 is not large. This is also in good agreement with the
broad single Q band absorption of 3a (Fig. 2).
Fig. 3 Fluorescence spectra of 2a (red line), 2b (green line), 2c (blue line),
and 3a (orange line) in toluene.
Similar to regular SubPzs and subphthalocyanines,1 the push–
pull SubPzs also exhibited moderately intense fluorescence with a
similar trend in the red-shift as observed for the Q band absorp-
tions (Fig. 3, 2a: 662 nm, FF = 0.16, 2b: 620 nm, FF = 0.16, 2c:
593 nm, FF = 0.14, 3a: 668 nm, FF = 0.18). The larger Stokes shift
with respect to those of SubPzs and subphthalocyanines reflects
structural dynamics in the excited states mainly due to the
peripheral aryl groups. The circular dichroism spectra of enantio-
mers of 2b separated using HPLC equipped with a chiral column
exhibited a mirror imaged spectra, clearly indicating the inherent
molecular chirality of the push–pull SubPzs (Fig. 2).
DFT and time-dependent (TD) DFT calculations at the B3LYP/
6-31G(d) level provided a detailed insight into the perturbed
electronic structures of the push–pull SubPzs as well as spectral
morphologies in the Q band region. As a reference compound,
calculations on unsubstituted SubPz 4 were also carried out
(Fig. 4). The TDDFT calculations demonstrated that in the cases
of 2a–c, the transitions from the HOMO to the degenerate LUMO
mainly contribute the Q band absorption, which were followed
by transitions from the HOMO À 1 and HOMO À 2 to the LUMO
and transitions from the HOMO À 3 to the LUMO (Fig. 4 and see
the ESI†). The distribution patterns of the molecular orbitals
in the HOMO and HOMO À 3 indicate that these orbitals are
derived from the same a2 HOMO of 4 upon linear combination
with molecular orbitals of the peripheral push and pull substituents,
whereas the HOMO À 1 and HOMO À 2 localize on the push–pull
substituents. The broad absorption observed between the Q band
and Soret band in the absorption spectra of 2a–c, is, therefore,
In summary, a series of push–pull SubPzs were synthesized for
the first time, and their diastereomers and enantiomers related to
the arrangement of the peripheral substituents were successfully
separated. The electronic absorption spectra revealed a perturba-
tion of the spectral morphologies and positions of the Q band
absorption by the push–pull substituents. The red-shift of absorp-
tion and fluorescence were clearly dependent on the donor ability
of the push substituents, which affects the HOMO–LUMO energy
gaps based on the DFT calculations. The TDDFT calculations
also demonstrated that the broad absorption in the energy
region higher than the Q band absorption is characteristic of
the electronic structures of these push–pull SubPzs. With this
information on the push–pull SubPzs in hand, it is anticipated
that the absorption, fluorescence, and chiroptical properties
can be further controlled, and the research along this direction
is currently being pursued in our laboratory.
Fig. 4 Partial frontier MO diagrams, in which MOs of 2a and 4 are shown.
For the complete partial frontier MO diagrams, see the ESI.†
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´
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%
space group P1 (no. 2), a = 7.441(2) Å, b = 17.725(5) Å, c = 30.886(9) Å,
a = 104.188(4)1, b = 91.911(4)1, g = 91.436(4)1, V = 3945(2) Å3, Z = 2, rcalcd
=
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,
CCDC 1015544.
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13784 | Chem. Commun., 2014, 50, 13781--13784
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