Shape-persistent poly-porphyrins assembled by a central truxene: Synthesis, structure, and singlet energy transfer behaviors

Article abstract of DOI:10.1142/S1088424612501271

Four dyad systems composed of a central truxene and either one or three β-substituted zinc(II) porphyrins (ZnP: TruZnP (7) and TruTriZnP (9)) or free-bases (H₂P: TruP (6) and TruTriP (8)) have been prepared. The presence of β-methyl groups minimizes π-conjugation through the quasi right angle made by the porphyrin and the truxene planes, and renders these dyads relatively rigid. The position of the absorption and emission 0-0 peaks confirms the role of the truxene and porphyrin as the energy donor and acceptor, respectively. Selective excitation of the truxene results in an efficient singlet energy transfer (S₁ ET) from the truxene to the porphyrin unit. The rates for S₁ ET (k_ET) are extracted from the change in the fluorescence lifetime of truxene in the presence and absence of the acceptor, and are temperature independent, (TruP (6), TruTriP (8), TruZnP (7) and TruTriZnP (9) are 5.0, 1.4, 1.0 and 1.4 at 298 K and 5.9, 1.3, 2.6, and 0.86 (ns)^-1 at 77 K, respectively), consistent with their relative rigidity. These k_ET's are similar to other related but more flexible systems reported by one of us (Inorg. Chem. 2011, 50, 11493-11505). The k _ET's time scale was assumed, based on modeling, to be related with hindered rotations about the truxene-porphyrin C-C bonds due to steric hexyl-hexyl interactions. This work confirms this earlier conclusion was correct. Copyright

Full text of DOI:10.1142/S1088424612501271

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2013; 17: 44–55
DOI: 10.1142/S1088424612501271
Published at http://www.worldscinet.com/jpp/
Shape-persistent poly-porphyrins assembled by a central
truxene: synthesis, structure, and singlet energy transfer
behaviors
Hai-Jun Xua, Bin Dub, Claude P. Gros*a◊, Philippe Richarda, Jean-Michel Barbea◊
and Pierre D. Harvey*b◊
a Université de Bourgogne, ICMUB (UMR 6302), 9, Avenue Alain Savary, BP 47870, 21078 Dijon Cedex, France
b Département de Chimie, Université de Sherbrooke, Sherbrooke, Québec J1K 2R1, Canada
Received 3 June 2012
Accepted 20 July 2012
ABSTRACT: Four dyad systems composed of a central truxene and either one or three b-substituted
zinc(II) porphyrins (ZnP: TruZnP (7) and TruTriZnP (9)) or free-bases (H2P: TruP (6) and TruTriP
(8)) have been prepared. The presence of b-methyl groups minimizes p-conjugation through the quasi
right angle made by the porphyrin and the truxene planes, and renders these dyads relatively rigid. The
position of the absorption and emission 0–0 peaks confirms the role of the truxene and porphyrin as the
energy donor and acceptor, respectively. Selective excitation of the truxene results in an efficient singlet
energy transfer (S1 ET) from the truxene to the porphyrin unit. The rates for S1 ET (kET) are extracted
from the change in the fluorescence lifetime of truxene in the presence and absence of the acceptor, and
are temperature independent, (TruP (6), TruTriP (8), TruZnP (7) and TruTriZnP (9) are 5.0, 1.4, 1.0
and 1.4 at 298 K and 5.9, 1.3, 2.6, and 0.86 (ns)-1 at 77 K, respectively), consistent with their relative
rigidity. These kET’s are similar to other related but more flexible systems reported by one of us (Inorg.
Chem. 2011, 50, 11493–11505). The kET’s time scale was assumed, based on modeling, to be related with
hindered rotations about the truxene-porphyrin C–C bonds due to steric hexyl–hexyl interactions. This
work confirms this earlier conclusion was correct.
KEYWORDS: truxene, porphyrin, fluorescence, singlet energy transfer.
variety of star-shaped oligomers and dendritic structures
with extended p-conjugations of the polyaromatic core,
as well as truxene-based donor–acceptor systems, have
INTRODUCTION
10,15-dihydro-5H-diindeno[1,2-a;1′,2′-c]fluorene
(Truxene), a C3h symmetric polycyclic hydrocarbon,
shown some potential applications such as in two-photon
has intensively been considered as a promising building
absorption [8], organic light-emitting diodes (OLED)
block to construct active materials in liquid crystals,
[9], NLO chromophores [10, 11] and organic fluorescent
supramolecular organizations and molecular energy
probes [12].
donors [1]. Moreover, the truxene unit can also easily be
To the best of our knowledge, there is only one
functionalized in three directions at the C-2, C-7 and C-12
report in which the truxene unit is covalently linked to
positions to serve as an excellent platform for the design
a porphyrin core (through a benzene bridge) [13] where
of large star-shaped architectures [2–7]. In addition,
evidence for energy transfer has qualitatively been
recently synthesized truxene derivatives including a
demonstrated. Recently, one of us has reported a series
of four dyads systems where the porphyrin central
cores are flanked by four truxene and tritruxene units
SPP full member in good standing
(see TetraTruP, TetraTruZnP, DodecaTruP and
DodecaTruZnP in Chart 1) [14]. Efficient quenching
of the truxene fluorescence and phosphorescence was
*Correspondence to: Pierre D. Harvey, email: Pierre.Harvey@
USherbrooke.ca and Claude P. Gros, email: Claude.Gros@
u-bourgogne.fr
Copyright © 2013 World Scientific Publishing Company

SHAPE-PERSISTENT POLY-PORPHYRINS ASSEMBLED BY A CENTRAL TRUXENE
45
12
R
15
R
10
R
R
N
N
N
N
M
N
2
5
N
R
M
N
R
7
N
6 (TruP): M = 2H
7 (TruZnP): M = Zn
R
R
R
R
N
N
N
N
M
R
R
N
N
M
N
N
M
N
N
N
8 (TruTriP): M = 2H, R= n-hexyl
N
9 (TruTriZnP): M = Zn,R= n-hexyl
R2
R2
Et4Me4PZn
R1
R1
R1
R1
R1
R1
R1
R1
R2
R1
R1
R2
R1
R1
(TetraTruP): M = 2H,
R1= n-hexyl, R2 = H
N
M
N
N
N
(TetraTruZnP): M = Zn,
R1= n-hexyl, R2 = H
R1
R1
R2
R1
R1
(DodecaTruP): M = 2H,
R1= n-hexyl, R2 = Truxene
R1
R1
R2
R1
R1
R1
R1
(DodecaTruZnP): M = Zn,
R1= n-hexyl, R2 = Truxene
R1
R1
R2
R2
Chart 1. Structures of the currently (up) and previously (down) studied dyads
noted and the rate for energy transfers (singlet and
triplet) was found somewhat placed mid-range with
respect to the closely related porphyrin-containing
dyads [15]. It was argued that the dihedral angle
between the truxene and the porphyrin planes was large
due to steric hindrance between the hexyl chains (based
on computer modeling) and consequently, the poor
p-orbital overlap and unfavorable relative orientation
of the transition moments of the donor and acceptor
chromophores, led to slower rates despite the direct
C–C bond linking both. In the absence of convincing
argument about this dihedral angle, such explanation
remains tentative. We now wish to report a new series
of truxene-porphyrin series where methyl groups are
introduced at the b-positions of the porphyrin unit
forcing the dihedral angle to be right (90°), hence
allowing to deduce the correct conclusion regarding the
energy transfer.
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 45–55

46
H.-J. XU ET AL.
5,5′,10,10′,15,15′-Hexahexyl-2-bromotruxene (4a).
To a stirred solution of 3 (6.63 g, 7.82 mmol) in CH2Cl2
(50 mL) was added bromine (2.0 mL) over an hour
period at 0 °C in the dark. The solution was heated to
room temperature and stirred overnight. The reaction was
EXPERIMENTAL
Instruments
1H NMR spectra were recorded with a Bruker DRX-
300AVANCE transform spectrometer at the “Pôle Chimie
Moléculaire (Welience, UB-Filiale)”; chemical shifts are
expressed in ppm relative to chloroform. UV-vis spectra
were recorded with a Varian Cary 1 spectrophotometer.
Mass spectra were obtained with a Bruker Daltonics
Ultraflex II spectrometer in the MALDI-TOF reflectron
mode using dithranol as a matrix. The measurements
were made at the “Pôle Chimie Moléculaire (Welience,
UB-Filiale).” UV-vis spectra were recorded on a
Hewlett-Packard diode array model 8452A. Emission
and excitation spectra were obtained using a double
monochromator Fluorolog 2 instrument from Spex.
Fluorescence and phosphorescence lifetimes were
measured on a Timemaster Model TM-3/2003 apparatus
from PTI, incorporating a nitrogen laser as the source and
a high-resolution dye laser (fwhm = 1.4 ns). Fluorescence
lifetimes were obtained from high-quality decays and
deconvolution or distribution lifetime analysis. The
uncertainties ranged from 20 to 40 ps on the basis of
multiple measurements. Phosphorescence lifetimes were
determined using a PTI LS-100 incorporating a 1 μs
tungsten flash lamp (fwhm = 1 μs).
1
monitored by H NMR. After the reaction was finished,
excess bromine was removed by washing the reaction
mixture with a sodium dithionite solution. The residue
was extracted with CH2Cl2, and the solution was washed
with an aqueous solution of sodium carbonate and dried
over magnesium sulfate. The solvent was removed
under reduced pressure. The pure product was obtained
after purification by column chromatography on silica
gel (heptane) and recrystallization from ethanol. Yield:
7.06 g (83.2%) of a yellow crystalline compound was
1
obtained, mp 225–226 °C. H NMR (CDCl3): d, ppm
8.21 (1H, d, J = 8.4 Hz), 8.01 (1H, s, Ar-H), 7.90 (2H,
m, H-truxene), 7.61 (1H, d, J= 6.9 Hz), 7.56 (6H, dd, J =
8.4 Hz, J = 6.9 Hz), 3.00–2.78 (6H, m, -CH2-), 2.15–1.95
(6H, m, -CH2-), 1.06–0.80 (36H, m, -(CH2)3-), 0.60–0.56
(12H, m, -CH2-), 0.46 (18H, t, J = 7.0 Hz, -CH3). MS
(MALDI-TOF): m/z 924.57 [M]+•, 924.61 calcd. for
C63H89Br. UV-vis (CH2Cl2): lmax, nm (e × 10-3 M-1.cm-1)
280.1 (41.2), 296.2 (41.8), 307.0 (67.0).
5,5′,10,10′,15,15′-Hexahexyltruxene-2-carbalde-
hyde (5a). To a stirred solution of 4a (2.44 g, 2.25 mmol)
in anhydrous ethyl ether (120 mL) was added n-BuLi
(8.20 mL, 2.5 M in hexane, 20.3 mmol) at -78 °C under
nitrogen. The mixture was first stirred for 0.5 h at this
temperature, then allowed to warm to room temperature
and stirred for 0.5 h, cooled again to -78 °C, and DMF
(1.60 g, 22.0 mmol) was then added. The solution was
stirred overnight while returning to room temperature
gradually. HCl (2 M, 100 mL) was added and the mixture
was stirred for 2 h. The organic layer was separated, the
aqueous layer extracted with ethyl ether. The combined
organic extracts were dried over MgSO4. After removal
of the solvents under vacuum, the residue was purified by
chromatography on silica gel eluting first with petroleum
ether then increasing to petroleum ether/EtOAc (8:1 v/v)
to give the desired title product as a pale white solid (yield:
1.45 g, 74%). 1H NMR (CDCl3, 300 MHz): d, ppm 10.16
(1H, s, -CHO), 8.56 (1H, d, J = 9.0 Hz, H-truxene), 8.40
(2H, m, H-truxene), 8.01 (1H, s, Ar-H), 7.94 (1H, d, J =
6.0 Hz, H-truxene), 7.51–7.40 (6 H, m, H-truxene), 3.08–
2.88 (6H, m, -CH2-), 2.23–2.13 (6H, m, -CH2-), 0.94–
0.88 (36H, m, -(CH2)3-), 0.62–0.59 (12H, m, -CH2-),
0.51 (18H, t, J = 6.0 Hz, -CH3). MS (MALDI-TOF):
m/z 874.68 [M]+•, 874.69 calcd. for C64H90O1. UV-vis
(CH2Cl2): lmax, nm (e × 10-3 M-1.cm-1) 280.9 (41.5), 295.9
(42.3), 308.0 (66.0).
Quantum yield measurements
For measurements at 298 K, all samples were
prepared in a glovebox, under argon (O2 < 12 ppm),
by dissolution of the compounds in 2MeTHF, using
1 cm3 quartz cells with a septum. Three different
measurements (i.e. different solutions) were performed
for each set of photophysical data (quantum yield). The
sample concentrations were chosen to correspond to an
absorbance of 0.05 at the excitation wavelength. Each
absorbance value was measured five times for better
accuracy in the measurements of emission quantum
yield. The references were tetraphenylporphyrin free-
base (0.10 in THF) [16] or tetraphenylporphyrin zinc(II)
(0.033 in THF) [17, 18].
Chemicals and reagents
Unlessotherwisestated,allchemicalsandsolventswere
of analytical reagent grade and used as received.Absolute
dichloromethane (CH2Cl2) was obtained from Carlo Erba.
Silica gel (Merck; 70–120 mm) was used for column
chromatography. Analytical thin-layer chromatography
was performed with Merck 60 F254 silica gel (precoated
sheets, 0.2 mm thick). Reactions were monitored by thin-
layer chromatography, UV-vis spectroscopy and MALDI/
TOF mass spectrometry. Truxene [19], hexahexyltruxene
(3) [4, 12], tribromohexahexyltruxene (4b) [4, 12], and
tricarbaldehydehexahexyltruxene (5b) [20] were synthe-
sized as previously reported in the literature.
2-(13,17-Diethyl-2,3,7,8,12,18-hexamethylpor-
phyrin-5-yl)-5,5′,10,10′,15,15′-hexahexyltruxene
TruP (6). A solution of 5a (175 mg, 0.20 mmol) and
1,19-dideoxy-8,12-diethyl-2,3,7,13,17,18-hexamethyl-
a,c-biladien 2 (120 mg, 0.20 mmol) in ethanol (100 mL)
was refluxed, and nitrogen was bubbled through the
Copyright © 2013 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2013; 17: 46–55

SHAPE-PERSISTENT POLY-PORPHYRINS ASSEMBLED BY A CENTRAL TRUXENE
47
system. Then, 10.0 mL of a solution of para-toluene
2.51 (s, 6H, -CH3), 2.21–2.09 (m, 6H, -CH2(CH2)4CH3),
1.79 (t, 6H, J = 7.8 Hz, -CH2CH3), 1.11–0.87 (m, 36H,
-CH2(CH2)4CH3), 0.76–0.58 (m, 30H, -(CH2)4CH2CH3).
HR-MS (MALDI-TOF): m/z 1356.8793 [M]+•, 1356.8804
calcd. for C93H120N4Zn. UV-vis (CH2Cl2): lmax, nm
(e × 10-3 M-1.cm-1) 280.0 (47.7), 308.1 (76.0), 406.0
(263.8), 534.0 (14.3), 570 (11.3).
sulfonic acid (PTSA, 1.50 g) in ethanol was slowly
added during 18 h. The deep red solution was refluxed
for 48 h under N2. The organic solvent was removed
under reduced pressure. The residue was dissolved
in CHCl2, washed with saturated NaHCO3 aqueous
solution. The organic solution was separated by use of
a separatory funnel. A saturated methanolic solution of
Zn(OAc)2·2H2O (10.0 mL) was added, and the solution
was stirred for 4 h at room temperature. The solvent was
removed under vacuum and the remaining residue was
purified by repeated column chromatography on silica
gel with 60% CH2Cl2-heptane as eluents. The bright
red band eluted was collected, dried, and redissolved in
CHCl3 (100 mL). The solution was vigorously stirred
with 10% HCl (10.0 mL) for 0.5 h. The solution was
neutralized with a saturated aqueous sodium carbonate
solution, and the reaction mixture was stirred for an
additional 15 min. The organic phase was separated,
washed with water (3 × 50 mL), and dried over MgSO4.
The organic solvent was removed. The remaining residue
was purified by repeated column chromatography on
silica gel with CH2Cl2. The pure title compound 6 was
obtained as a purple solid after recrystallization from
CH2Cl2/methanol. Yield: 75 mg, 29%. 1H NMR (CDCl3):
d, ppm 10.21 (s, 2H, H-meso), 9.99 (s, 1H, H-meso),
8.75 (d, 1H, J = 8.7 Hz, H-truxene), 8.50–8.43 (m, 2H,
H-truxene), 8.26 (d, 1H, J = 1.8 Hz, H-truxene), 8.08–
8.04 (m, 1H, H-truxene), 7.55–7.42 (m, 6H, H-truxene),
4.11 (q, 4H, -CH2-CH3), 3.68 (s, 6H, -CH3), 3.56 (s, 6H,
-CH3), 3.25 (m, 2H, -CH2-(CH2)5CH3), 3.08 (m, 4H,
-CH2-(CH2)5CH3), 2.60 (s, 6H, -CH3), 2.22 (m, 6H,
-CH2-(CH2)5CH3), 1.92 (t, 6H, J = 7.5 Hz, -CH2CH3),
1.17–0.94 (m, 36H, -CH2(CH2)3CH2CH3), 0.84–0.68
(m, 30H, -(CH2)4CH2CH3), -3.04 (s, 1H, NH), -3.20
(s, 1H, NH). HR-MS (MALDI-TOF): m/z 1294.9615
[M]+•, 1294.9664 calcd. for C93H122N4. UV-vis (CH2Cl2):
2,7,12-Tri-(13,17-diethyl-2,3,7,8,12,18-hexamethy-
lporphyrin-5-yl)-5,5′,10,10′,15,15′-hexahexyltruxene
TruTriP (8). A solution of 5b (373 mg, 0.40 mmol) and
1,19-dideoxy-8,12-diethyl-2,3,7,13,17,18-hexamethyl-
a,c-biladien 2 (723 mg, 1.20 mmol) in ethanol (100 mL)
was heated to reflux, and nitrogen was bubbled through
the system. Then, 10.0 mL of a solution of PTSA (1.50
g) in ethanol was added slowly during 18 h. The deep
red solution was refluxed for 48 h under N2. The organic
solventwasremovedunderreducedpressure.Theresidues
were dissolved in CH2Cl2, washed with saturated NaHCO3
aqueous solution. The organic solution was separated.
A saturated methanolic solution of Zn(OAc)2·2H2O
(10.0 mL) was added, and the solution was stirred for
4 h at room temperature. The solvent was removed and
the remaining residue was purified by repeated column
chromatography on silica gel with 70% CHCl3-heptane
as eluents. The bright red band that eluted was collected,
dried, and redissolved in CHCl3 (100 mL). The solution
was vigorously stirred with 10% HCl (10.0 mL) for 0.5 h.
The solution was neutralized with a saturated aqueous
sodium carbonate solution, and the reaction mixture was
stirred for an additional 15 min. The organic phase was
then separated, washed with water (3 × 50 mL), dried
over MgSO4. The organic solvent was removed under
vacuum. The remaining residue was purified by repeated
column chromatography on silica gel with CH2Cl2, and
CHCl3. The pure title compound 8 was obtained as a
purple solid (57 mg, 13%) by recrystallization from
CH2Cl2/methanol. 1H NMR (CDCl3): d, ppm 10.13
(s, 6H, H-meso), 9.89 (s, 3H, H-meso), 8.79 (d, 3H, J
= 8.4 Hz, H-truxene), 8.31 (s, 3H, H-truxene), 8.03 (d,
3H, J = 8.4 Hz, H-truxene), 4.01 (q, 12H, -CH2CH3),
3.59 (s, 18H, -CH3), 3.52 (s, 18H, -CH3), 3.36 (m, 6H,
-CH2(CH2)4CH3), 2.62 (s, 18H, -CH3), 2.41 (m, 6H,
-CH2(CH2)4CH3), 1.82 (t, 18H, J = 7.5 Hz, -CH2CH3),
1.28–1.25 (m, 48H, -CH2(CH2)4CH3), 0.93–0.87 (t, 18H,
J = 7.5 Hz, -CH2(CH2)4CH3), -3.00 (s, 3H, NH), -3.19
(s, 3H, NH). HR-MS (MALDI-TOF): m/z 2191.5042
[M]+•, 2191.4923 calcd. for C153H186N12. UV-vis (CH2Cl2):
l
max, nm (e × 10-3 M-1.cm-1) 281.1 (40.5), 296.0 (42.3),
308.1 (65.5), 404.0 (201.9), 501.0 (15.7), 535.0 (6.1),
570.0 (6.6).
Zn-2-(13,17-diethyl-2,3,7,8,12,18-hexamethyl-
porphyrin-5-yl)-5,5′,10,10′,15,15′- hexahexyltruxene
TruZnP (7). A saturated methanolic solution of
Zn(OAc)2·2H2O (5.0 mL) was added to a solution of 6
(30 mg, 0.023 mmol) in CHCl3 (30 mL). The reaction
mixture was stirred overnight and the solvent was
removed. The solid residue was purified by flash column
chromatography on silica gel with a 60% CH2Cl2-
heptane mixture followed by recrystallization from
CH2Cl2/methanol to afford pure 7, as bright red crystals,
l
max, nm (e × 10-3 M-1.cm-1) 284.0 (51.1), 311.0 (67.0),
407.1 (641.5), 502.0 (51.8), 535.9 (19.9), 571.0 (21.7),
625.1 (7.1).
1
in quantitative yield. H NMR (CDCl3): d, ppm 10.02
Zn-2,7,12-tri(13,17-diethyl-2,3,7,8,12,18-hexame-
thylporphyrin-5-yl)-5,5′,10,10′,15,15′-hexahexyltru-
xene TruTriZnP (9). A saturated methanolic solution of
Zn(OAc)2·2H2O (3.0 mL) was added to a solution of 8
(10 mg, 0.0046 mmol) in CHCl3 (30 mL). The reaction
mixture was stirred overnight and the solvent was
removed. The solid residue was purified by flash column
(s, 2H, H-meso), 9.83 (s, 1H, H-meso), 8.66 (d, 1H, J
= 7.8 Hz, H-truxene), 8.40–8.33 (m, 2 H, H-truxene),
8.17 (d, 1H, J = 1.8 Hz, H-truxene), 8.02–7.99 (m, 1H,
H-truxene), 7.49–7.30 (m, 6H, H-truxene), 3.97 (q, 4H,
-CH2CH3), 3.53 (s, 6H, -CH3), 3.46 (s, 6H, -CH3), 3.18
(m, 2H, -CH2(CH2)4CH3), 2.98 (m, 4H, -CH2(CH2)4CH3),
Copyright © 2013 World Scientific Publishing Company
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48
H.-J. XU ET AL.
chromatography on silica gel with 70% CHCl3-heptane
as eluents followed by recrystallization from CH2Cl2/
methanol to afford pure 9 as bright red crystals (yield:
carried out at the 5-, 10-, and 15-positions, leading to
5,5,10,10,15,15-hexahexyltruxene [4] 3 in 84% yield.
Mono- or full tri-bromination of 3 afforded 4a and 4b
in high yields [4, 12]. The mono-brominated derivative
4a was synthesized according to the reported procedure
for the tris-brominated derivative 4b, only the molar
ratio of the Br2 and truxene is different from the tris-
brominated derivatives. In addition, the progress of
1
10 mg, 92%). H NMR (CDCl3): d, ppm 9.99–9.42 (m,
6 H, H-meso), 8.99 (m, 3H, H-meso), 8.78–8.71 (m, 6H,
H-truxene), 8.20 (d, 3H, J= 5.1 Hz, H-truxene), 3.62 (m,
18H, -CH2-), 3.50 (s, 18H, -CH3), 3.32 (m, 6H, -CH2-),
3.20 (s, 12H, -CH3), 2.77 (m, 24H, -CH2-), 1.50–1.42
(m, 48H, -CH2-), 1.18–1.07 (m, 24H, -(CH2)4CH2CH3),
0.81–0.78 (m, 12H, -(CH2)4CH2CH3). HR-MS (MALDI-
TOF): m/z 2377.2059 [M]+•, 2377.2323 calcd. for
C153H180N12Zn3. UV-vis (CH2Cl2): lmax, nm (e × 10-3
M-1.cm-1) 312.0 (48.7), 410.0 (588.3), 535.9 (33.5),
571.0 (24.9).
1
the reaction was monitored by H NMR. By-products
containing a little of di- or tris-brominated derivatives
were observed. The pure compounds 4a and 4b were
isolated after purification by chromatography column
and recrystallization. Mono- (5a) or tri-carbaldehyde-
hexahexyltruxene (5b) were obtained in 84% and 87%
yields, respectively, by the mono- or tri-lithiation of 3 in
the presence of dry DMF followed by hydrolysis of the
intermediate imidate salt [7].
RESULTS AND DISCUSSION
The syntheses of mono- or triporphyrins containing
the truxene derivatives are outlined in Schemes 3 and 4,
respectively. Based on our previously reported method
[22], mono- (5a) or tricarbaldehydehexahexyltruxene
(5b) and a,c-biladiene dibromide 2 were dissolved
in ethanol and were reacted in the presence of para-
toluenesulfonic acid (PTSA) for two days, redissolved
in CH2Cl2 after removal of the reaction solvent under
vacuum and washed with an excess of sodium hydrogen
carbonate. Final purification by column chromatog-
raphy on silica gel afforded the free base mono- or
trisporphyrins TruP (6) and TruTriP (8) in 29 and 13%
yield, respectively.
Synthesis
3,4-dimethyl-1H-pyrrole-2-carboxylic acid 1 was
readily obtained from the reaction of 3,4-dimethyl-
1H-pyrrole-2-ethylcarboxylate with sodium hydroxide
and then with acetic acid in 75% yield (Scheme 1).
3,3′-dicarboxy-5,5′-diformyl-4,4’-dimethyl-dipyrryl
methane reacts with 1 in the presence of a hydrobromic
acid/acetic acid solution in N2 atmosphere to give a,c-
biladiene dibromide 2 in 77% yield, as described in the
literature [21].
Scheme 2 shows the synthetic route to access to the
mono- and tri- carbaldehydehexahexyltruxene (5a and 5b,
respectively). In order to remove the reactive hydrogen
atoms and to improve the solubility, full alkylation was
Standard procedures were then used to metalate
the porphyrin core using zinc acetate. A solution of
porphyrin TruP (6) or TruTriP (8) in CHCl3 was stirred
NH HN
+HN
HN
NH
OHC
CHO
1. NaOH, EtOH
2Br-
CO2C2H5
CO2H
N
H
N
H
2. acetic acid
HBr/acetic acid
NH+
ethanol, N2
1
2
Scheme 1
R1
R1
2
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
15
5
Br2/DCM
1) n-BuLi/THF
2) n-C6H13Br
1) n-BuLi/Et2O
2) DMF
C6H13
C6H13
C6H13
Br
CHO
7
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
10
R1
R1
12
Truxene
4a: R1 = H
4b: R1 = Br
5a: R1 = H
5b: R1 = CHO
3
Scheme 2
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SHAPE-PERSISTENT POLY-PORPHYRINS ASSEMBLED BY A CENTRAL TRUXENE
49
+HN
HN
NH
2Br-
NH+
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
CHO
C6H13
C6H13
2
N
N
N
M
N
ethanol, PTSA, N2
reflux
C6H13
C6H13
C6H13
C6H13
6 (TruP): M = 2H
7 (TruZnP): M = Zn
Scheme 3
N
N
M
N
N
+HN
HN
CHO
NH
2Br-
NH+
C6H13
C6H13
C6H13
C6H13
C6H13
C6H13
2
C6H13
C6H13
OHC
ethanol, PTSA, N2
reflux
C6H13
C6H13
N
N
CHO
C6H13
C6H13
M
N
N
N
N
M
N
N
8 (TruTriP): M = 2H
9 (TruTriZnP): M = Zn
Scheme 4
with 4 equiv. of zinc acetate dihydrate dissolved in a
small volume of absolute methanol at room temperature.
The metalation reaction was monitored by UV-vis
spectroscopy and MALDI-TOF mass spectrometry. The
zinc complexes TruZnP (7) and TruTriZnP (9) were
obtained in nearly quantitative yield.
and 8.13 ppm, respectively, are ascribed to the truxene
protons. In the aliphatic region, quartet signals at 4.11
(12H) ppm are accounting for the methylene protons
connected to the pyrrole ring. The protons of the three
different types of methyl groups linked to the pyrrole
ring display signals located at 3.68, 3.62 and 2.72 ppm,
respectively. The resonances of the b-methylene protons
linked to the truxene appear as multiplets at 3.44 (6H)
and 2.50 (6H) ppm.
1
Compounds 6–9 were characterized by H NMR,
MALDI-TOF mass spectrometry, and UV-vis
spectroscopy (Experimental section and Supporting
1
information, SI). The H NMR spectrum of porphyrin
The methyl protons of the long alkyl chains display
signals at 1.01 ppm as triplets. However, the other alkyl
chain protons are less well-defined with resonances in the
1.29–1.45 ppm range. Compounds TruP (6), TruZnP
TruTriP (8) displays two singlets at d = 10.23 (6H)
and 9.99 (3H) ppm, assigned to the nine meso-protons
(Fig. 1). The chemical shifts for the internal NH-pyrrole
resonances range between -3.00 and -3.19 ppm and
appear as doublets due to the unsymmetrical porphyrinic
structure. Nine aromatic protons placed at 8.89, 8.41
1
(7) and TruTriZnP (9) exhibit similar H NMR spectra
as for TruTriP (8). Their 1H NMR data are given in the
Experimental Section and their spectra are placed in the
Copyright © 2013 World Scientific Publishing Company
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50
H.-J. XU ET AL.
Fig. 1. Representative 1H NMR spectrum of porphyrin TruTriP (8) (300 MHz, 298 K, CDCl3, *CH2Cl2 (solvent)
)
SI. The truxene moiety of TruTriP (8) and TruTriZnP
(9) has a perfect planar C3h symmetry with three
peripheral porphyrin rings. Three para-C atoms (2-, 7-,
12-) exhibit an equilateral triangle with the edge length of
~9.65 Å for TruTriP (8) and TruTriZnP (9), as reported
in the literature [20].
By mass spectrometry, the molecular ions of the free-
base porphyrins and metal complexes 6–9 were observed
in each case as the most intense peak, and the data agree
well with the expected molecular formula. For example,
the HR-MS MALDI-TOF mass spectrum of TruTriP
(8) exhibits the parent-ion peak at m/z = 2191.5042
(calculated for C153H186N12 m/z 2191.4923) (SI).
Spectroscopic and photophysical properties
The absorption spectra of the free base porphyrins
(TruP (6) and TruTriP (8)) exhibit the intense Soret
band (S0–S2 transition) at ~407 nm and the Q-bands (S0–
S1 transitions) at ~502 nm at 298 K (see TruTriP (8) in
Fig. 2).
For the zinc(II) porphyrins (TruZnP (7) and
TruTriZnP (9)), the Soret and Q-bands are observed at
~410 nm and ~540 nm, respectively, at 298 K (Fig. 2). The
absorption maxima in Soret and Q-bands for TruTriP (8)
and TruTriZnP (9), exhibit very little variations relative
to TruP (6) and TruZnP (7), respectively, illustrating
the poor conjugation between the truxene and porphyrin
units. This is consistent with the right angle formed by
these aromatic planes as previously discussed by one of
us [14], and the preliminary X-ray structure obtained for
TruP (6) (below; ~90°; see Fig. S9).
Fig. 2. UV-vis spectra (CH2Cl2) of TruTriP (8) (black) and
TruTriZnP (9) (red)
presented in Table 1 and the increase of the number of
porphyrins (for example, the comparison between TruP
(6) and TruTriP (8)) results in the obvious absorptivities
enhancement, implying the increase in effective light
harvesting ability.
The shape and positions of the Soret and Q-bands
of the porphyrin units (TruP (6) and TruTriP (8)) are
the same. Moreover, the fluorescence maxima of TruP
(6) and TruTriP (8) are also almost the same further
demonstrating that there is a very weak conjugation
between porphyrin and truxene p-systems [14]. Finally,
there is essentially no red-shift of Soret and Q-bands
going from TruZnP (7) to TruTriZnP (9), further
evidencing the poor conjugation between the porphyrin
and truxene p-systems.
The bands associated with the Tru unit are observed
at ~310 nm and due to a truxene-centered p–p* transition
[14]. The UV-visible data of the target porphyrins are
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SHAPE-PERSISTENT POLY-PORPHYRINS ASSEMBLED BY A CENTRAL TRUXENE
51
Fig. 3. Absorption (black), corrected excitation (red) and emission (blue) spectra of TruP (6), TruZnP (7), TruTriP (8) and
TruTriZnP (9) in 2MeTHF at 298 K
Table 1. UV-vis absorption data for all target dyads in 2MeTHF at 298 K
Compound
l
abs, nm (ε × 10-3 M-1.cm-1)a
Truxene
Soret
Q
TruP (6)
308 (54.9)
312 (81.6)
309 (45.3)
404 (242.7) 504 (15.7), 536 (5.2), 578 (4.4), 653 (1.65)
407 (779.2) 502 (66.6), 534 (24.6), 580 (18.7), 650 (7.65)
413 (307.2) 543 (16.5), 578 (10.2),
TruTriP (8)
TruZnP (7)
TruTriZnP (9)
b
—
416 (923.5) 544 (58.3), 578 (31.7)
aThe absorption bands are assigned to the truxene, the porphyrin free-base or zinc(II) porphyrin
(Soret band and Q-bands). bNot observed.
The fluorescence quantum yields (FF) of the porphyrin
of the donor, whereas the overall emission intensity
arising from the free base or zinc(II) porphyrin unit
remains strong. For TruTriP, the truxene fluorescence is
almost absent at 298 K due to S1 energy transfer (Fig. 6).
In addition, the excitation spectra monitored in the
porphyrin fluorescence at 298 K (lmax = 628 nm for TruP
(6) and TriTruP (8), and 580 nm for TruZnP (7) and
TriTruZnP (9)) overlap well the combined absorptions
of the truxene unit and the porphyrin core (Fig. 3),
evidencing the energy transfer from truxene to free base
porphyrins or zinc(II) porphyrins.
In addition, the excitation spectra monitored at these
porphyrin-centered fluorescence bands exhibits the
bands arising from both the truxene and porphyrin units
also providing further evidence for energy transfer. The
singlet–singlet energy transfers from S1(Tru) to S1(P) and
S1(ZnP), or to S2(P) to S2(ZnP) are possible [14], but this
cannot be discriminated in this work. The photophysical
data for the truxene, free base and zinc(II) porphyrin
chromophores are listed in Table 3.
moiety excited in the Q-bands (lexc = 500 for (6) and
(8) and 540 nm for (7) and (9)) are 0.13, 0.020, 0.067
and 0.019 for TruP (6), TruZnP (7), TruTriP (8) and
TruTriZnP (9) at 298 K, respectively. At 77 K, a strong
phosphorescence arising from the zinc(II) porphyrin
(TruZnP (7) and TruTriZnP (9)) (lexc = 540 nm) is
observed at ~800 nm (Fig. 4). The 0–0 phosphorescence
peaks are observed at 789 nm and 812 nm for TruZnP
(7) and TriTruZnP (9), respectively.
The truxene moiety exhibits a fluorescence band in
the 350–400 nm window with some vibronic structure at
298 K, and a strong phosphorescence band at 77 K with
a detailed vibronic structure starting at 457 nm [13, 23].
Based on the position of the 0–0 peaks in the absorption
andemissionspectraoftruxeneandporphyrins, theenergy
diagram and the roles of energy donor and acceptor are
defined (Fig. 5). Selective excitation at 310 nm, where the
truxene absorbs the most, results in a large decrease of
the fluorescence intensity and phosphorescence intensity
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52
H.-J. XU ET AL.
Table 2. Emission data for all dyads in 2MeTHF
c
Compound
Chromoa
lem, nmb
lem, nm
FF
298 K
77 K
298 K
(lexc = 540 nm)
(lexc = 504 nm)
d
TruP (6)
Tr
P
366, 380
360, 427
628, 663, 697
622, 655, 690
0.127
d
TruZnP (7)
TruTriP (8)
TruTriZnP (9)
Tr
P
366
580, 637
d
404, 424, 432
575, 633, 706, 789
0.020
d
Tr
P
360, 378, 435, 501, 543
622, 654, 691, 730
629, 661, 697
0.067
d
Tr
P
366, 426
581, 637
404, 429, 492
583, 642, 726, 812
0.019
aTr: chromophore = truxene; P: chromophore = porphyrin free-base; ZnP: chromophore =
zinc(II) porphyrin. bThe emission spectra were recorded using lexc = 310 nm under the inert
atmosphere at 298 K. cFluorescence quantum yields (FF) of the samples in 2MeTHF were
measured by using tetraphenylporphyrin free-base (lexc = 500 nm) and tetraphenylporphyrin
zinc (lexc = 540 nm) (H2TPP, FF = 0.10 in THF, ZnTPP, FF = 0.033 in THF) as standards.
dToo weak to be accurately measured.
Fig. 4. Absorption (black), corrected excitation (red) and emission (blue) spectra of TruP (6), TruTriP (8), TruZnP (7) and
TruTriZnP (9) in 2MeTHF at 77 K
Fig. 5. State diagram representing the different fragments involved in the energy transfer processes in this work. Phos.:
phosphorescence; SSET: singlet–singlet energy transfer; TTET: triplet–triplet energy transfer; TSET: triplet–singlet energy
transfer; ISC: intersystem crossing
Copyright © 2013 World Scientific Publishing Company
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SHAPE-PERSISTENT POLY-PORPHYRINS ASSEMBLED BY A CENTRAL TRUXENE
53
porphyrin or zinc(II) porphyrins), respectively. In all
cases, the truxene unit is considered as comparison
species. At 298 K, the singlet kET values are 5.0, 1.4,
1.0, and 1.3 (ns)-1 for TruP (6), TruTriP (8), TruZnP
(7) and TruTriZnP (9) respectively. At 77 K, the rates
are 5.9, 1.3, 2.3, and 0.86 (ns)-1 for TruP (6), TruTriP
(8), TruZnP (7) and TruTriZnP (9) respectively.
Experimentally, there is no major change in rate constants
for all the cases at the two temperatures, consistent with
an energy transfer process but more relevant to this work,
with an absence of dyad conformation. There is, indeed,
no possibility of significantly changing the dihedral angle
between the porphyrin and truxene planes as the methyl
groups in the porphyrin b-positions are forcing it to be
perpendicular (~90˚). Attempts to accurately confirm this
angle by X-ray crystallography were made but suitable
crystals were not obtained. TruP (6) crystals exhibited
too much disorder for publication purposes. Nonetheless,
the dihedral angle was indeed found to be ~90° (see
Fig. S9 in the SI). These rates are considered somewhat
efficient but medium when compared to other closely
spaced multi-porphyrinic assemblies [15].
Importantly, the kET values are very similar to those
reported for TetraTruP, TetraTruZnP, DodecaTruP
and DodecaTruZnP (Chart 1; 0.74 < kET < 5.9 (ns)-1),
which do not have b-methyl groups. The similarity
between the rates indicates that indeed the mean angle
between the truxene and porphyrin planes is also ~90° for
the latter as previously anticipated.
Fig. 6. Top: corrected emission spectrum of TruTriP (8) at
298 K in 2MeTHF (lex = 310 nm). Bottom: expansion of the
350–600 nm region. The sharp peaks at ~440, ~480 and ~545
nm are artifacts
The dipole–dipole interactions (Förster) [24–26] and
exchange (Dexter) [26, 27] mechanisms are assumed to
be operative in the singlet–singlet energy transfers for
short donor–acceptor–separations as those investigated
here [26]. The donor and acceptor are expected to be
very poorly conjugated owing to the quasi right angle
All fluorescence lifetimes and kET data are listed in
Table 3. The singlet–singlet energy transfer rate (kET)
o
.
is extracted by kET = (1/tF)-(1/tF ),[15] where tF and
o
tF are the fluorescence lifetimes of the donor (truxene)
in the presence and absence of the acceptor (free base
Table 3. Fluorescence lifetimes and singlet–singlet energy transfer rates of the porphyrin systems in 2MeTHF
at 298 K and 77 K
Compound
Chromoa
tF, nsb lem, nmb
kET, ns-1
tF, nsb lem, nmb
kET, ns-1
298 K
77 K
298 K
77 K
Et4Me4PhP
Tr
P
1.70 0.10
1.94 0.10c
Tr
56.26 0.15 380 (310)
64.64 0.52
380 (310)
TruP (6)
P
15.51 0.43 620 (500)
0.25 0.05 380 (310)
24.97 0.47
0.17 0.04
620 (500)
380 (310)
Tr
5.0
1.4
1.0
1.3
5.9
1.3
2.3
0.86
TruTriP (8)
TruZnP (7)
TruTriZnP (9)
P
17.20 0.22 620 (500)
0.75 0.03 380 (310)
22.47 0.46
0.82 0.15
620 (500)
380 (310)
Tr
P
1.41 0.04 580 (568)
0.97 0.18 380 (310)
1.17 0.06
0.43 0.13
580 (568)
380 (310)
Tr
P
1.16 0.17 580 (568)
0.81 0.11 380 (310)
1.32 0.08
1.15 0.05
580 (568)
380 (310)
Tr
aTr: chromophore = truxene; P: chromophore = porphyrin free-base; ZnP: chromophore = zinc(II) porphyrin.
bThe lifetime was recorded at the shown emission wavelength and the samples were under an inert atmosphere.
The values in parentheses indicate at which wavelength the samples were excited. cFrom Ref. 17.
Copyright © 2013 World Scientific Publishing Company
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54
H.-J. XU ET AL.
formed by the b-pyrrole-alkyl-substituted porphyrin
ring and truxene, which is totally consistent with
the very small red-shift of the absorption (Soret and
Q-bands), and emission bands going from TruP (6) to
TruTriP (8), or from TruZnP (7) to TruTriZnP (9).
This fact greatly diminishes the efficiency in singlet–
singlet energy transfer operating according to the Dexter
exchange mechanism, in which the spatial orbital overlap
is necessary. Therefore it is most likely that the Dexter
mechanism cannot contribute to the overall rate. The
rates are found to be slower again due to the poor orbital
conjugation.
In addition, the relative orientation of the transition
moments (k2) of the two chromophores has an important
impact on energy transfer rates according to the Förster
theory [24, 25, 28]. k2 takes a maximum value (4) when
the dipole transition moments are parallel and is minimal
(0) when perpendicular. The quasi right angle between
truxene and porphyrin plane will result in a very low k2
values, hence leading to slower energy transfer rates, as
observed.
between the porphyrin and truxene planes. Evidence
for efficient singlet energy transfers from the truxene to
the porphyrin core has been provided and their rates are
considered efficient but average when compared to other
closely-spaced multiporphyrin assemblies. This effect
is expectedly due to a poor orbital overlap between the
truxene and porphyrin p-systems, which is predictable
from the right angle (X-ray) between the porphyrin and
truxene planes for TruP (6). The close similarity in the
kET values reported for the parent dyad systems (0.74 <
kET < 5.9 (ns)-1),8 which do not exhibit b-methyl groups
(Chart 1), and those investigated in this work (0.86 < kET
< 5.9 (ns)-1), which have b-methyl substituents, clearly
demonstrates that the former series must also exhibit a
right angle between the aromatic planes. This conclusion
was also proposed and interpreted by hexyl-hexyl
interactions between the truxene moieties as corroborated
by computer modeling. This work experimentally
supports this previous conclusion.
Supporting information
Furthermore, because truxene and porphyrin
chromophores are actually nonpolar compounds, the
dipole transition moments are very small. The Förster rate
constant is further found to be slower [24, 25, 28]. Finally,
the integral overlap (J) between the emission spectrum of
donor (truxene) and the absorption spectrum of acceptor
(porphyrin) will play an important role to the singlet
energy transfer assigned to a Förster mechanism. From
Figs 3 and 4, the spectral overlap between the emission
spectrum of truxene and the absorption spectrum of
porphyrin is very weak, again leading to the slower rates.
In conclusion, even though that the Förster mechanism
is assumed to be operative in these molecular systems,
the energy transfer rates are found to be relatively slower
due to the above-mentioned several structural parameters
which are being performed to reduce the singlet–singlet
energy transfer rate constants.
The phosphorescence lifetimes (tP) of TruZnP (7)
and TriTruZnP (9) is 67 ms and 65 ms at 726 nm at
77 K, respectively, which are typical for triplet lifetimes
for this kind of zinc(II) porphyrins. The phosphorescence
of the truxene unit (lmax ~580 nm) was never observed
indicating efficient triplet–triplet energy transfer (truxene
⇒ porphyrins) [14].
1
MALDI-TOF mass spectra of compound 6–9, H
NMR spectra (300 MHz, 298 K, CDCl3) of compound
6–9 (Figs S1–S8), and ortep view of TruP (6) (Fig. S9)
are given in the supplementary material. This material is
available free of charge via the Internet at http://www.
worldscinet.com/jpp/jpp.shtml.
Acknowledgements
This research was supported by the Natural Sciences
and Engineering Research Council of Canada (NSERC),
le “Fonds Québécois de la Recherche sur la Nature et
les Technologies (FQRNT),” the “Centre d’Etudes des
Matériaux Optiques et Photoniques de l’Université de
Sherbrooke (CEMOPUS),” and the “Agence Nationale
de la Recherche (ANR)” for Research “Chair of
Excellence” grant. The “Centre National de la Recherche
Scientifique” (ICMUB, UMR CNRS 6302) is gratefully
thanked for financial support. Hai-Jun Xu also gratefully
acknowledges the “Région Bourgogne” and CNRS
for a post-doctoral fellowship. Support was provided
by the CNRS, the “Université de Bourgogne” and the
“Conseil Régional de Bourgogne” through the 3MIM
integrated project (“Marquage de Molécules par les
Métaux pour l’Imagerie Médicale”). Antony Lapprand
is acknowledged for the registration of few fluorescence
lifetimes.
CONCLUSION
A series of rigid shape-persistent porphyrins substi-
tuted by a central truxene (zinc(II) porphyrin: TruZnP
(7) and TruTriZnP (9) and porphyrin free base: TruP
(6) and TruTriP (8)), have been designed in order
to investigate the singlet energy transfer rates in a
structurally addressable manner. The energy donor
(truxene) and acceptor (porphyrin) were intentionally
tailored in a way to minimize the p-orbital conjugation
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