Tetrakis(4-tert-butylphenyl) substituted and fused quinoidal porphyrins

Article abstract of DOI:10.1039/c2cc33728b

4-tert-Butylphenyl-substituted and fused quinoidal porphyrins 1 and 2 are prepared for the first time. They show (1) intense one-photon absorption in the far-red/near-infrared region, (2) enhanced two-photon absorption compared with aromatic porphyrin mon

Full text of DOI:10.1039/c2cc33728b

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COMMUNICATION
Tetrakis(4-tert-butylphenyl) substituted and fused quinoidal porphyrinswz
Wangdong Zeng,a Byung Sun Lee,b Young Mo Sung,b Kuo-Wei Huang,c Yuan Li,a
Dongho Kim*b and Jishan Wu*ad
Received 24th May 2012, Accepted 14th June 2012
DOI: 10.1039/c2cc33728b
4-tert-Butylphenyl-substituted and fused quinoidal porphyrins 1 and 2
are prepared for the first time. They show (1) intense one-photon
absorption in the far-red/near-infrared region, (2) enhanced
two-photon absorption compared with aromatic porphyrin
monomers, and (3) amphoteric redox behavior. Their geometry
and electronic structure are studied by DFT calculations.
Porphyrins with extended p-conjugation are of interest due to
their unique electronic and optical properties and promising
applications in organic optoelectronics.1 One general approach
is to fuse a porphyrin core to other aromatic systems including
porphyrin itself,2 boron dipyrromethene,3 and aromatic hydro-
carbons4 (e.g. naphthalene, anthracene, pyrene, azulene, and
perylene etc.). An alternative approach is to construct quinoidal
porphyrins which have proven to show different electronic
structure and optical properties compared with those of normal
aromatic porphyrins.5 For example, Anderson et al. reported
several tetracyano-substituted quinoidal pophyrin monomers
and dimers (including a fused dimer),5d,f a cumulenic quinoidal
porphyrin dimer5e and a push–pull type quinoidal porphyrin,5h
all of them showing intense absorption in the far-red and near
infrared (NIR) region. A combination of both approaches is
supposed to generate a new type of aromatics-fused quinoidal
porphyrins, which might show interesting opto-electronic
properties. In this communication, we report the first synthesis of
4-tert-butylphenyl substituted and fused quinoidal porphyins 1 and
2 (Scheme 1). Their geometry, electronic structure, redox behavior,
one-photon absorption (OPA) and two-photon absorption (TPA)
properties are investigated to afford a fundamental understanding
of this new type of hydrocarbon–quinoidal porphyrin hybrids.
Scheme 1 Reagents and conditions: (a) TTFA, CH2Cl2, TFA, rt,
42%; (b) CBr4, PPh3, toluene, 80 1C, 12 h, 55%; (c) (i) 2-(4-tert-
butylphenyl)-4,4,5,5-tetramethyl-1,3-dioxolane, Pd(PPh3)4, Na2CO3
(2 M), THF, reflux, 48 h, 68%; (ii) Ni(acac)2, toluene, reflux, 12 h,
99%; (d) DDQ, MeSO3H, CH2Cl2, rt, 20 min, 80%.
The key intermediate is compound 5, 21,21,22,22-tetrabromo-
10,20-bismesityl-5,15-porphyrinquinodimethane (Scheme 1).
Oxidation of 5,15-bismesitylporphyrin 3 by thallium trifluoro-
acetate (TTFA) in CH2Cl2 followed by demetalation with
trifluoroacetic acid (TFA) gave the 5,15-dioxo-10,20-bismesityl
porphodimethene 4 in 42% yield.6 Compound 4 reacted with
CBr4 and PPh3 in toluene (Corey–Fuchs reaction)7 to afford 5
in 55% yield. Compound 1 was then prepared in an overall
67% yield by four-fold Suzuki coupling reaction with 2-(4-tert-
butylphenyl)-4,4,5,5-tetramethyl-1,3-dioxolane8 followed by
metalation with Ni(acac)2. A Zn-analog of 1 was also prepared
by metalation with Zn(OAc)2, however, severe demetalation
and side reactions happened in the subsequent oxidative
cyclodehydrogenation reaction. The ring cyclization of 1 was
a Department of Chemistry, National University of Singapore, 3
Science Drive 3, 117543, Singapore. E-mail: chmwuj@nus.edu.sg;
Fax: +65-67791691; Tel: +65-65162677
9
first attempted by using mild oxidant FeCl3 in CH2Cl2 and
b Spectroscopy Laboratory for Functional p-Electronic Systems and
Department of Chemistry, Yonsei University, Seoul 120-749, Korea.
E-mail: dongho@yonsei.ac.kr; Fax: +82 2-2123-2434
c King Abdullah University of Science and Technology (KAUST),
Division of Chemical and Life Sciences and Engineering and KAUST
Catalysis Center, Thuwal 23955-6900, Saudi Arabia
only a partially cyclized product after loss of four hydrogen
atoms was obtained as indicated from its MALDI-TOF mass
spectrum. The structure, however, was not identified due to
the complex NMR spectrum and existence of isomers. Then,
photocyclization of 1 was tested in benzene with I2 as an
oxidant (irradiated by a 450 W medium-pressure mercury vapor
lamp) and a complicated mixture of partially cyclized products
together with some decomposed materials was obtained. We finally
found that cyclization of 1 by using 5,6-dicyano-1,4-benzoquinone
(DDQ) as an oxidant in the presence of CH3SO3H10 worked
smoothly and afforded the target compound 2 in 80% yield.
d Institute of Materials Research and Engineering, A*Star, 3 Research
Link, Singapore 117602. E-mail: wuj@imre.a-star.edu.sg
w This article is part of the ChemComm ‘Aromaticity’ web themed
issue.
z Electronic supplementary information (ESI) available: Synthetic
procedures and characterization data, general characterization
methods, DFT data, TA spectra and Z-scan curves. See DOI:
10.1039/c2cc33728b
c
7684 Chem. Commun., 2012, 48, 7684–7686
This journal is The Royal Society of Chemistry 2012

Fig. 1 Absorption spectra of compounds 1 and 2 in CH2Cl2.
The structures of 1 and 2 were unambiguously identified by
NMR spectroscopy and mass spectrometry (see ESIz). The
free-base of 1 exhibited a singlet at 12.80 ppm (–NH) and two
resonances at 5.67 and 6.03 ppm (pyrrole rings), indicating a
typical quinoidal porphyrin structure. A singlet at 6.83 ppm for
the pyrrole rings in 2 was observed, suggesting an enhanced
aromatic character of the porphyrin core due to the fusion of
four aromatic sextet rings.
Fig. 2 Calculated geometry and frontier molecular orbital profiles of
1 (left) and 2 (right). Hydrogen atoms are omitted for clarity.
and the other at 770.5 nm (H + 0 - L + 0, f = 0.395) (Fig. S2
and Table S2 in ESIz). A very weak H ꢀ 1 - L + 0 transition
was also predicted at 866.2 nm (f = 0.007), which could be
correlated with the observed weak band at 897 nm (Fig. 2). The
band shape and the trend of the absorption maximum are in
good agreement with the experimental data.
Compound 1 shows two distinct absorption bands at 450
and 635 nm in CH2Cl2 while 2 displays a red-shifted absorption
spectrum with two major bands at 533 and 780 nm together with
two weak bands at 669 and 897 nm (Fig. 1) due to extended
p-conjugation. The absorption spectra of both 1 and 2 are
obviously different from the normal aromatic porphyrins which
usually exhibit an intense Soret band and a weak Q-band. The
optical energy gaps were determined to be 1.69 eV and 1.33 eV
for 1 and 2, respectively, from the onset of the lowest energy
absorption band.
Cyclic voltammetry and differential pulse voltammetry
measurements of compound 1 in CH2Cl2 showed two quasi-
reversible oxidation waves with half-wave potentials (E1o/x2) of
0.21 and 0.95 V and two quasi-reversible reduction waves with
half-wave potentials (E1re/d2) of ꢀ1.55 and ꢀ1.82 V (vs. Fc+/Fc)
The geometry, electronic structure and optical properties of
1 and 2 were studied by time-dependent density function
theory (TDDFT at B3LYP/6-31G*). The optimized structure
of 1 shows a highly contorted, saddle-shaped conformation
due to repulsion between the b-H of the porphyrin ring and
the 4-tert-butylphenyl groups (Fig. 2), which is similar to
Anderson’s tetracyano-quinoidal porphyrin.5d The bond
length of the exo methylene bond is 1.368 A (labeled in
Fig. 2), indicating a typical quinoidal structure. 2 exhibits a
more planar conformation but there is still distortion from
planarity due to the steric congestion between the fused phenyl
rings (Fig. 2). The length of the same exo methylene bond now
becomes 1.414 A, indicating that fusion of four aromatic
sextet rings weakens the quinoidal character of the porphyrin
core.11 The HOMO and LUMO are mainly delocalized at the
porphyrinquinodimethane unit in 1, while they are more
delocalized in 2 by extension to the extra phenyl rings
(Fig. 2). Upon ring fusion, there is almost no change in the
HOMO energy level, but the LUMO energy level of 2 is
0.52 eV lower than that of 1. TDDFT calculations also
predicted two distinct absorption bands for 1 above 400 nm,
one at 419.2 nm (H ꢀ 1 - L + 1, oscillator strength f =
0.8364, H for HOMO, L for LUMO) and another band at
613 nm is a combination of two transitions (605.4 nm, H + 0 -
L + 0, f = 0.2162; 628.4 nm, H ꢀ 1 - L + 0, f = 0.105)
(Fig. S1 and Table S1 in ESIz). 2 was predicted to show three
distinct bands above 400 nm, one at 474.6 nm (H ꢀ 1 - L + 1,
f = 1.3032), one at 629.0 nm (H ꢀ 2 - L + 0, f = 0.1818)
(Fig. 3, Fig. S3 and Table S3 in ESIz). Similarly, compound 2
1/2
exhibited two quasi-reversible oxidation waves with E of
ox
0.35 and 0.85 V and two quasi-reversible reduction waves with
1/2
E
of ꢀ1.08 and ꢀ1.50 V. The HOMO and LUMO energy
red
levels were evaluated to be ꢀ4.93 and ꢀ3.60 eV for 1, and
ꢀ5.06 and ꢀ3.86 eV for 2, respectively, from the onset of the
first oxidation and reduction wave. Accordingly, the electro-
chemical energy gaps (EEg C) of 1 and 2 were determined to be
1.33 and 1.20 eV, respectively. Interestingly, the first oxidation
1/2
potential E of 1 is 0.14 V negatively shifted compared with 2
ox
and the EEg C of 1 is also significantly different from its optical
energy gap. Such an unusual phenomenon could be explained
by the large strain in the neutral 1, which can be released upon
Fig. 3 Cyclic voltammograms of 1 and 2 in CH2Cl2 with 0.1 M
Bu4NPF6 as a supporting electrolyte, AgCl/Ag as a reference electrode,
a Au disk as a working electrode, a Pt wire as a counter electrode, and a
scan rate of 50 mV sꢀ1
.
c
This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 7684–7686 7685

Compound 2 represents the first aromatic hydrocarbon-fused
quinoidal porphyin and the new synthetic strategy can likely be
applied to the synthesis of other extended quinoidal porphyrins.
DFT calculation predicted their different geometry and electronic
structure. Both compounds showed intense absorption in the
far-red/NIR region and enhanced TPA response compared with
the aromatic porphyrin monomer. An unusual negative-shift of
the first oxidation potential was observed for the highly contorted
molecule 1. Our research provided a preliminary study on a new
type of aromatics-fused quinoidal porphyrin molecules.
J. W. acknowledges the financial support from the BMRC-
NMRC grant (no. 10/1/21/19/642), MOE Tier 2 grant
(MOE2011-T2-2-130) and IMRE Core Funding (IMRE/
10-1P0509). The work at Yonsei Univ. was supported by WCU
(World Class University) programs (R32-2010-10217-0) and an
AFSOR/APARD grant (no. FA2386-09-1-4092). K.-W. H.
acknowledges the financial support from KAUST.
Notes and references
Fig. 4 OPA (black solid line and left vertical axis) and TPA spectra
(blue symbols and right vertical axis) of 1 (a) and 2 (b) in CH2Cl2. TPA
spectra are plotted at lex/2.
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those of most ethynyl-linked and aromatics-fused porphyrins,
and a better understanding of this difference is needed in future
work by designing different quinoidal porphyrins.
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In summary, two 4-tert-butylphenyl-substituted and fused
quinoidal porphyrins 1 and 2 were prepared for the first time.
c
7686 Chem. Commun., 2012, 48, 7684–7686
This journal is The Royal Society of Chemistry 2012