Aromatic Fused [30] Heteroannulenes with NIR Absorption and NIR Emission: Synthesis, Characterization, and Excited-State Dynamics

Article abstract of DOI:10.1002/chem.201600917

Two hitherto unknown planar aromatic [30] fused heterocyclic macrocycles (1.1.0.1.1.0), with NIR absorption in free-base form and protonation-induced enhanced NIR emission, have been synthesized from easy to make precursors. The induced correspondence of

Full text of DOI:10.1002/chem.201600917

DOI: 10.1002/chem.201600917
Communication
&
Fused Heterocycles
Aromatic Fused [30] Heteroannulenes with NIR Absorption and
NIR Emission: Synthesis, Characterization, and Excited-State
Dynamics
Abhijit Mallick,[a] Juwon Oh,[b] Dongho Kim,*[b] and Harapriya Rath*[a]
Dedicated to Professor Tavarekere K Chandrashekar
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Abstract: Two hitherto unknown planar aromatic [30]
fused heterocyclic macrocycles (1.1.0.1.1.0), with NIR ab-
sorption in free-base form and protonation-induced en-
hanced NIR emission, have been synthesized from easy to
make precursors. The induced correspondence of fusion
on the macrocyclic structure, electronic absorption, and
emission spectra have been highlighted.
Recent developments in many different fields of modern sci-
ence and medicine, such as molecular biology, material sci-
ence, drug discovery, and tissue diagnostics have fueled the
design of new chromophores that can absorb and/or emit
within the red or near infrared (NIR) region of the spectrum.[1]
In this context, expanded porphyrins have earned increasing
interest as being one of the more versatile heterocyclic skele-
ton chromophores.[2] These macrocycles possess electrical
properties and outstanding optical properties that make them
perfect building blocks for incorporation into multifunctional
materials. Under these fascinating attributes of expanded por-
phyrins, their aromatic nature plays a crucial role since aroma-
ticity controls stability, structure, and reactivity of expanded
porphyrins. However, the conformational flexibility and com-
plexity in the assessment of diatropicity becomes more per-
ceptible for larger expanded porphyrins.[3] In this context, con-
trolled modifications of the basic framework of expanded por-
phyrins results in systematic variation of the optical properties,
making them efficient NIR dyes in specific instances.[4] Effective
construction of aromatic macrocycles with extended conjuga-
tion requires a subtle balance of appropriate geometric confor-
mation fused to electronic stability derived from increased de-
localization.[5]
With the insight from our recent observation,[4a] where fused
ring expansion of the pyrrole moieties at the b positions and
fused thiophene moieties resulted in NIR absorption, efforts
were directed towards the synthesis of higher analogues
(Scheme 1) with free pyrrole moieties and fused thiophene
moieties in order for the constructs to be rigidly planar and
fully conjugated across the entirety of the porphyrinoid back-
bone. Based on our target macrocycles, the fused thieno [3, 2-
b] thiophene 1 has been synthesized following a literature
report.[6] Because the steric factor of the meso-aryl substituents
play a decisive role in cyclizing the precursors,[7] the desired
precursors 4 and 5 were synthesized in quantitative yields
Scheme 1. Synthesis of macrocycles 6 and 7.
from 2 and 3 (Scheme 1). Rational synthesis of macrocycles 6
and 7 entails self-coupling of 4 and 5 using trifluoroacetic acid
as the catalyst and chloranil as oxidant.[8] One equivalent of tri-
fluoroacetic acid gave the best yields for macrocycles 6 and 7.
Formation of the macrocycles at moderate concentration of tri-
fluoroacetic acid indicates the a–a self-coupling of precursors
4 and 5 with the formation of two direct pyrrole–pyrrole links
to form an intermediate porphyrinogen, which on subsequent
oxidation with chloranil yielded the desired [30]p conjugated
macrocycles 6 or 7. Column chromatographic separation over
basic alumina, followed by repeated silica gel (200–400 mesh)
chromatographic separation, yielded the macrocycles 6 and 7
in 5 and 6% yields, respectively, as dark green solid. Acid con-
centration above one equivalent of trifluoroacetic acid led to
acidolysis of the precursors 4 and 5 and yielded [22] heterocy-
clic macrocycles.[4a]
The new macrocycles have been characterized via spectro-
scopic and single-crystal X-ray diffraction analyses. Macrocycle
6 shows a parent ion peak at m/z 1058.641, whereas macrocy-
cle 7 shows a parent ion peak at 948.515 under positive ioniza-
tion conditions in MALDI-TOF mass spectrometry, thus confirm-
ing the proposed composition for the macrocycles. The ab-
sorption spectra of 6 and 7 showed typical characteristics of
aromatic porphyrinoids, with the intense B-like bands at
550 nm and weak Q-like bands in the range of 692–1088 nm
(Figure 1).[9] The NIR emission at 1089 and 1068 nm for 6 and
7, respectively, supports their aromatic character.[10a] Compared
to the weak and broad absorption and non-emissive behavior
of dithiahexaphyrin,[11] the spectral features of 6 and 7 are as-
cribed to the impact of fused thienothiophene moieties, which
[a] A. Mallick, Prof. Dr. H. Rath
Department of Inorganic Chemistry
Indian Association for the Cultivation of Science
2A/2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032 (India)
Fax: (+91)33-24732805
E-mail: ichr@iacs.res.in
[b] J. Oh, Prof. Dr. D. Kim
Department of Chemistry
Yonsei University, Seodaemun-gu, Seoul 120-749 (Korea)
E-mail: dongho@yonsei.ac.kr
Supporting information for this article, which includes full experimental de-
tails, is available on the WWW under http://dx.doi.org/10.1002/
chem.201600917.
Figure 1. UV/Vis Spectra and Emission Spectra of 6 (left) and 7 (right).
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induce relatively rigid and planar structures with a fully p-con-
jugated system, leading to the NIR absorption and emission.
Protonation of 6 and 7 results in further redshifts of B- and Q-
like bands with an increase of extinction coefficient of the
Soret maxima. Furthermore, their redshifted NIR emission was
enhanced almost 16- and 7 times, respectively, compared to
that of free base 6 and 7 with the reduced Stokes shifts
(174!118 cmÀ1 for 6 and 222!175 cmÀ1 for 7). These fea-
tures indicate that protonation provides the structural rigidity
and induces smoother p-conjugation by cut-off of intramolecu-
lar hydrogen-bonding interactions between pyrrole moieties.[10]
Here, compared to protonated 7, more enhanced NIR emission
and smaller Stokes shift of protonated 6 suggest that this has
a more rigid structure due to steric effects caused by the
meso-mesityl substituents.
broad signals at 10.14 and 10.94 ppm that exhibit scalar cou-
pling among themselves in 2D COSY spectra (Figure S21A in
the Supporting Information) have been unequivocally assigned
to outer b-CHs of pyrrolenic rings. With these observations, the
free base 6 can be gauged as [30] p electronic macrocycle
having no pyrrole ring inversion. The calculated Dd value from
chemical shift is found to be 17.77 ppm, thus suggesting aro-
1
maticity in this macrocycle.[12] The H NMR signals of the free-
base form of 6 are broadened beyond interpretation as we car-
ried out low temperature NMR studies (Figure S20 in the Sup-
porting Information), suggesting the dynamic behavior of the
macrocycle. We cannot exclude the presence of minor tauto-
mer 6B as a consequence of the failure to arrive at well-re-
solved peaks.[8a] This points to the presence of 6A as the only
tautomer or alternatively to a fast equilibrium between 6A
and 6B strongly shifted towards 6A (Scheme S2 in the Sup-
porting Information). In the next step, we tried to study the
impact of protonation on the aromaticity and conformational
change(s) in the macrocycle. Upon protonation, partially re-
solved spectra could be obtained due to aggregation but care-
ful addition of CD3OD (20 mL) gave a well-resolved spectra with
the missing signal for NH proton (Figure S25 in the Supporting
Information), which could be resolved only at low temperature
(Figure S26 in the Supporting Information). At 253 K, the NMR
spectrum of completely protonated 6 (Figure 2) displayed the
characteristic features of an aromatic macrocycle in solution
The 1H NMR spectral patterns provide ample evidence for
the macrocyclic aromaticity through a [30]p electronic delocali-
zation motif both in free base as well as in protonated form.
Due to the characteristic diatropic ring current effect, the pro-
tons inside the macrocycle resonate in the up-field region and
the protons on the periphery resonate in the deshielded
1
region. Figure 2 depicts representative H NMR spectra of free
base 6 and protonated 6. At 298 K, the free-base form of 6 ex-
hibits two signals in the shielded region at À2.43 and at
À6.15 ppm. The signal at À2.43 ppm has been unambiguously
assigned to the NH peak after a D2O exchange experiment
(see Figure S22 in the Supporting Information). The scalar cou-
pling between the signal at À2.43 ppm with the broad signals
at 11.06 and 10.08 ppm in 2D COSY spectra (Figure S21A in the
Supporting Information) confirms the later signals as b-CHs of
pyrrolic rings. Furthermore, the signal at À2.43 ppm exhibits
dipolar coupling with the signal at À6.15 ppm in 2D ROESY
spectra (Figure S21B in the Supporting Information) confirming
the latter signal as inner b-CH of thienothiophene ring. In the
deshielded zone, the broad signal at 11.62 ppm correlating
with the signal at 2.17 ppm in 2D ROESY spectra (Figure S21B
in the Supporting Information) clearly indicates the former
signal as outer b-CH of thienothiophene ring. Hence, the
1
state. The H NMR spectrum of protonated 6 at 253 K showed
two distinctly different broad signals for NH protons at À5.64
and À5.36 ppm, confirmed by D2O exchange spectra (Fig-
ure S29 in the Supporting Information). In the deshielded
region, there are two closely placed doublets at 11.63 and
11.61 ppm and another set of doublets at 10.70 and
10.76 ppm. As the second most conceptual step of analysis of
NMR spectra, 2D COSY and ROESY spectra were recorded. In
the 2D COSY spectra (Figure S27 in the Supporting Informa-
tion), the two sets of doublets at 11.61 and 11.63 ppm, corre-
sponding to one proton each, show correlation with doublets
at 10.76 and 10.70 ppm, respectively. Moreover, all four sets of
Figure 2. 1H NMR spectra of free-base 6 (top) in CDCl3 at 298 K and protonated 6 (bottom) in CF3COOH/CDCl3 at 253 K (* indicates residual solvent peak or
solvent impurity).
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doublets exhibit scalar coupling with the NH signals at À5.64
and À5.36 ppm that was informative enough for correct as-
signment of peaks as pyrrolic b-CHs. Furthermore, this inter-
pretation is reinforced by the observation of two sets of dipo-
lar couplings between the signal at 10.76 ppm with the signal
at 2.02 ppm and between signals at 10.70 ppm with that at
2.21 ppm in 2D ROESY spectra (Figure S28 in the Supporting
Information). The singlet at 12.08 ppm, which doesn’t exhibit
scalar coupling with any other signal(s) in 2D COSY spectra but
exhibit dipolar coupling with the signal at 2.02 ppm in ROESY
spectra (Figure S28 in the Supporting Information), has been
unequivocally assigned to the outer b-CH of the thienothio-
phene. The sharp singlet at À7.98 ppm exhibits dipolar cou-
pling with the NH signal at À5.34 ppm in 2D ROESY spectra,
confirming this to be the inner b-CH of the thienothiophene
moiety. With these observations, protonated 6 can be ascribed
as [30] p electronic macrocycle having no pyrrole ring inver-
sion similar to its free-base form. The calculated Dd value from
chemical shift is found to be 20.07 ppm, thus suggesting aro-
maticity in this macrocycle.[12] An enhanced aromaticity in the
protonated form of 6 compared to the free-base form of 6 is
in accordance with the absorption and emission spectral
changes upon protonation, indicating that protonation induces
a more rigid structure with enhanced p-electron delocalization.
For macrocycle 7, due to insolubility, it was difficult to arrive at
an assignable spectrum in the free-base form. However, upon
protonation using 10% CF3COOH/CD2Cl2, at 243 K, we could
arrive at well-assigned spectra (Figure S33 in the Supporting
Information). There are three signals at À3.74, À3.39, and
À3.06 ppm. The signals at À3.06 and À3.39 ppm have been
assigned to NHs of the pyrrole rings after D2O exchange ex-
periments (Figure S36 in the Supporting Information).
Redox properties of these new macrocycles have been stud-
ied with the help of cyclic voltammetry and differential pulse
voltammetry in dichloromethane using 0.1m tetrabutyl ammo-
nium hexafluorophosphate as a supporting electrolyte. Macro-
cycle 6 exhibits one irreversible oxidation peak at 0.30 V, fol-
lowed by two reversible oxidation peaks at 0.55 and 0.78 V,
and two reversible reduction peaks at À0.95 and À1.24 V with
HOMO–LUMO gap estimated at 1.25 V (Figure S38 in the Sup-
porting Information). This value is apparently much lower than
that of meso-tetraphenyl porphyrin (2.26 V)[13] and [26] tetrathia
rubyrin.[3a] This phenomenon may be due to fusion of thio-
phene rings into the core of the macrocycle, leading to rear-
rangement of HOMO–LUMO, which is an offshoot of such core
modifications that leads to decrease in Dredox value. For macro-
cycle 7, electrochemical studies could not be carried out due
to very low solubility in any organic solvent.
To gain further insights into the conformation of 6 and 7,
solid-state X-ray crystallographic analysis and density function-
al theory (DFT) calculations have been performed. For macro-
cycles 6 and 7, failure to obtain a good single crystal suitable
for X-ray diffraction in the free-base form forced us to resort to
1
geometry optimization based on H NMR spectra to arrive at
the proposed structure of the macrocycles on DFT-formal-
ism.[14] The optimized structures of 6 and 7 (Figure S39 in the
Supporting Information) showed planar geometries with mean
plane deviation (MPD) values of 0.057 and 0.144 Š, respective-
ly. On the other hand, single crystals suitable for X-ray diffrac-
tion were obtained by slow diffusion of hexane into a TFA/di-
chloromethane solution of 6. Macrocycle 6, upon protonation,
turned out to be of flat geometry (Figure 3).[15] The four meso-
Unfortunately, no scalar coupling between these two signals
with signals in the deshielded region were observed in 2D
COSY spectra. However, the dipolar coupling between the sig-
nals at À3.39 ppm with À3.74 ppm (Figure S35 in the Support-
ing Information) convinced us to assign the later signal as
inner b-CH of thienothiophene ring. In the 2D COSY spectra
(Figure S34 in the Supporting Information), the correlation be-
tween signals at 2.37 ppm with broad signals at 8.5–8.12 ppm
and 2.88 ppm with signals at 7.73 ppm clearly indicates that
the signals in the aromatic zone correspond to meso-tolyl CHs.
Hence, the signals in the deshielded region at 10.38 ppm,
which exhibit no correlation with any signal(s) in 2D COSY
spectra, have been unambiguously assigned to the outer b-CH
of the thienothiophene ring. The remaining broad signals from
9.7 to 10.80 ppm, which exhibit scalar couplings among them-
selves, unequivocally belong to the outer b-CHs of pyrrole
rings (Figure S34 in the Supporting Information). The calculat-
ed Dd value (difference in chemical shift between inner NH/CH
Figure 3. X-ray crystal structure of protonated 6. Top view (a) and side view
(b).
carbons lie in the macrocyclic plane. The dihedral angle for the
protonated pyrrole ring is 16.708 and for neutral pyrrole ring is
11.718 above and below the mean plane containing four meso
carbons. Based on the crystal structure of protonated 6, we ob-
tained optimized structures of protonated 6 and 7 (Figure S40
in the Supporting Information). Compared to freebase 6 and 7,
the protonated 6 and 7 exhibited more distorted structures
due to the steric repulsion between pyrrolic protons and inner
b-CH protons of thienothiophene moiety (MPD values of 0.228
and 0.304 Š for protonated 6 and 7, respectively). It was noted
that the dihedral angles of meso-mesityl substituents 76.58~
89.18 are larger than those of meso-tolyl substituents 51.38~
1
and outer NH/CH protons in H NMR spectrum) from chemical
shift is found to be 14.54 ppm, thus suggesting aromaticity in
this macrocycle. The smaller Dd value in the macrocycle 7
compared to 6 is due to the fact that meso-mesityl substitu-
ents impart more structural rigidity to the macrocycle 6 com-
pared to more flexible meso-tolyl in 7, as discussed in the ab-
sorption and emission spectra.
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64.48, reflecting the larger steric effect of mesityl substituents
on the macrocycle along with their MPD values.
porting Information). Furthermore, the excited-state lifetime of
the lowest singlet states were increased to 370 and 200 ps for
protonated 6 and 7, respectively. Along with the enhanced NIR
absorption and emission, as well as 1H NMR results, these
changes in the excited-state dynamics are attributable to the
more rigid structure and enhanced aromaticity upon protona-
tion.[10] It was noted that the excited-state lifetime of protonat-
ed 6 is longer than that of protonated 7, indicating that the
steric effect of meso-mesityl substituents affords more structur-
al rigidity to the macrocycle.
To gain deeper insights into the aromaticity of 6 and 7, we
conducted nucleus-independent chemical shift (NICS)[16] calcu-
1
lations. In line with the H NMR spectra of 6 and 7, their NICS
values at the center of the macrocycles were estimated to be
À14.4 and À14.3 ppm (Figures S41 and S42 in the Supporting
Information), indicating aromatic character. This aromatic char-
acter was clearly illustrated by Anisotropy of the Induced Cur-
rent Density (ACID) plots,[17] where the distinct clockwise ring
currents of free-base and protonated 6 and 7 visualized their
aromatic nature (Figures S43 and S44 in the Supporting Infor-
mation). Furthermore, the estimated Harmonic Oscillator
Model of Aromaticity (HOMA)[18] values (Figures S41 and S42 in
the Supporting Information) and degenerate frontier molecular
orbital (FMOs) with energy level diagrams (Figures S45 and S46
in the Supporting Information) are well matched with their ar-
omatic nature.
In conclusion, we have synthesized two new fused heterocy-
clic macrocycles through easily available precursors, which will
allow further exploitation of their rich chemistry in terms of
their coordination behavior towards transition metals and their
use as catalysts for organic transformation. Moreover, as elec-
tronically attractive as these macrocycles are due to NIR ab-
sorption and NIR emission, they may be potent candidates for
biomedical applications. Such thriving research activity is cur-
rently underway in our laboratory.
The excited-state dynamics of free-base and protonated 6
and 7 were investigated by femtosecond transient absorption
(TA) measurements. The TA spectra of freebase 6 and 7 exhibit-
ed intense ground-state bleaching (GSB) signals, corresponding
to their absorption spectral features, with relatively weak excit-
ed-state absorption on both sides of the GSB signals (Figure 4).
The decays of these TA spectra were fitted with two time con-
stants; one is in hundreds ps time scales and another is longer
than 10 ns, where the shorter time constants arise from inter-
system crossing process and the deactivation of lowest singlet
excited state. The longer time constants are assigned as the
lifetime of triplet state. These TA spectral features are similar to
those of typical aromatic expanded porphyrins, being well-
matched with the aromatic nature of 6 and 7.[10a] The protona-
tion of 6 and 7 caused the change of their excited-state dy-
namics. In their TA spectra, the GSB signals became redshifted,
sharpened, and prominently intensified (Figure S47 in the Sup-
Acknowledgements
The work at IACS, Kolkata was supported by DST-SERB (SR/S1/
IC-37/2012), CSIR (02(0120)/13/EMR-II), New Delhi, India and
DST-SERB Ramanujan Fellowship (SR/S2/RJN-93/2011), India.
A.M. thanks CSIR for senior research fellowship. The work at
Yonsei University was supported by the Global Research Labo-
ratory Program (2013K1A1A2A02050183) funded by the Minis-
try of Science, ICT & Future, Korea. The quantum calculations
were performed using the supercomputing resources of the
Korea Institute of Science and Technology Information (KISTI).
We are indebted to the Single Crystal X-Ray Diffractometer fa-
cility in IISER, Pune, India. Our sincere thanks to Dr. R. Natara-
jan, IICB, Kolkata for final refinement of crystal data.
Figure 4. The femtosecond transient absorption (fs-TA) spectra (left) and decay profiles (right) of free-base (a) 6 and (b) 7 with photoexcitation at 530 nm.
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Keywords: aromaticity
heteroannulenes · NIR absorption · NIR emission
·
fused
heterocycles
·
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