Programmed metalloporphyrins for self-assembly within light-harvesting stacks: (5,15-dicyano-10,20-bis(3,5-di-tert-butylphenyl)porphyrinato)zinc(II) and its push-pull 15-N,N-dialkylamino-5-cyano congeners obtained by a facile direct amination

Article abstract of DOI:10.1021/jp801510b

The title dicyano compound was synthesized via cyanation and it self-assembles in nonpolar solvents giving red-shifted and broad absorption maxima just as the bacteriochlorophylls which are encountered in the light-harvesting organelles of early photosynthetic bacteria. In the crystal, stacks are formed through a hierarchic combination of π-stacking and a CN-Zn electrostatic interaction. Push-pull 15-N,N-dialkylamino-5-cyano congeners could be obtained in high yields using a solvent- and catalyst-free direct amination of mesobromoporphyrins. Importantly, the fluorescence of the self-assembled species due to the very orderly manner in which the chromophores are arranged is not entirely quenched and has a surprisingly long lifetime of over 1 ns. This lends hope of using the trapped energy in biomimetic hybrid solar cells.

Full text of DOI:10.1021/jp801510b

5512
J. Phys. Chem. B 2008, 112, 5512-5521
Programmed Metalloporphyrins for Self-Assembly within Light-Harvesting Stacks:
(5,15-Dicyano-10,20-bis(3,5-di-tert-butylphenyl)porphyrinato)zinc(II) and Its Push-Pull
15-N,N-Dialkylamino-5-cyano Congeners Obtained by a Facile Direct Amination
Mihaela Carmen Balaban,† Andreas Eichho1fer,† Gernot Buth,‡ Robert Hauschild,|
Je¸drzej Szmytkowski,| Heinz Kalt,| and Teodor Silviu Balaban*,†
Karlsruhe Institute of Technology (KIT), Forschungszentrum Karlsruhe (FZK), Institute for Nanotechnology
(INT), and Institute for Synchrotron Radiation (ISS), Postfach 3640, D-76021 Karlsruhe, Germany, Faculty for
Applied Physics, UniVersita¨t Karlsruhe (UKA), Karlsruhe, Germany, and Center for Functional Nanostructures
(CFN), UniVersita¨t Karlsruhe, Germany
ReceiVed: February 20, 2008
The title dicyano compound was synthesized via cyanation and it self-assembles in nonpolar solvents giving
red-shifted and broad absorption maxima just as the bacteriochlorophylls which are encountered in the light-
harvesting organelles of early photosynthetic bacteria. In the crystal, stacks are formed through a hierarchic
combination of π-stacking and a CNsZn electrostatic interaction. Push-pull 15-N,N-dialkylamino-5-cyano
congeners could be obtained in high yields using a solvent- and catalyst-free direct amination of meso-
bromoporphyrins. Importantly, the fluorescence of the self-assembled species due to the very orderly manner
in which the chromophores are arranged is not entirely quenched and has a surprisingly long lifetime of over
1 ns. This lends hope of using the trapped energy in biomimetic hybrid solar cells.
Introduction
CHART 1: BChls and Their Mimic 1 Together with the
New Compounds 3 and 8 Addressed in This Worka
Early green photosynthetic bacteria have developed a unique
organelle for light-harvesting in deep waters where they can
scavenge photons even at 100 m under the water surface.1
Recently, an oceanic microbe living at a depth of 2390 m, near
a volcanic vent, was identified to harvest infrared radiation and
thus is obligatory photosynthetically active in the absence of
solar radiation.2 The antenna organelles of these green bacteria
are different from the light-harvesting complexes of for instance
purple bacteria, cyanobacteria, algae, or plants where proteins
organize the photoactive pigments such as (bacterio)chloro-
phylls, [(B)Chl]s, and carotenoids into precise orientations
enabling an energy funneling from the peripheral antenna
systems to the core where charge separation occurs within the
reaction center.3 Because of their much simpler genomes, these
bacteria can avoid using a protein scaffold by self-assembling
BChl c, d, and e forming nanostructures called chlorosomes
(from Greek “green sacs”) with a large photon capture cross
section.4 In spite of considerable research efforts, in the absence
of crystallographic proof, the exact chlorosomal structure
remains elusive and several models have been proposed
recently.5 We could replicate the BChl self-assembly with fully
synthetic chromophores, such as porphyrins and chlorins, by
appending to them the same functional groups capable of
forming supramolecular interactions.6 Tamiaki7 and Wu¨rthner8
have used semisynthetic approaches starting from Chl a to
prepare after self-assembly functional antenna systems. In all
these cases, a hydroxy group, a central metal atom within the
tetrapyrrolic core, and a carbonyl group were present in the
constructs. Using the very versatile 3,5-di-tert-butylphenyl group
a In bacteria, BChls are present as mixtures of R8 and R12 homologues
(Me, Et, nPr, iBu) as well as R/S epimers of the 31 stereocenter. Farnesol
is the most frequently encountered fatty alcohol esterifying the 17-
propionic acid residue. Stearol, cetol, geranyl-geraniol, and phytol are
other such hydrophobic alcohols. In BChl c, R7 and R20 are both CH3
groups. In BChl d, R7 is CH3 while R20 is H. In BChl e, R7 is CHdO
while R20 is CH3.
* To whom correspondence should be addressed. Fax: +49-724-782-
8298. E-mail: silviu.balaban@int.fzk.de; silviu.balaban@kit.edu.
† KIT, FZK, INT, and CFN.
as solubilizing moiety, but which at the same time packs well
into the crystal lattice, we could recently crystallize two
assemblies of the zinc porphyrin 1.9 Contrary to all previously
‡ KIT, FZK, ISS.
| KIT, UKA, and CFN.
10.1021/jp801510b CCC: $40.75 © 2008 American Chemical Society
Published on Web 04/11/2008

Programmed Metalloporphyrins for Self-Assembly
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5513
proposed chlorosomal models, no hydrogen bonds exist within
the crystal lattices of 1. Stacks are formed by a strong Zn-OH
ligation and π-π interactions, while the carbonyl group is
involved in a weak electrostatic Zn-OdC< interaction, with
∼3.5 Å Zn to oxygen spacing which is beyond a usual
supramolecular bonding distance. The carbonyl group is there-
fore unavailable for hydrogen bonding.9
added to the top of a chromatography column filled with silica
gel (H ) 30 cm, Φ ) 2.5 cm) which was eluted with a mixture
of dichloromethane/n-hexane (80/20, v/v, degassed previously
by sonication). A first eluted minor fraction of the zinc
monocyano compound 6, which is pink colored, was followed
by the main fraction of the blue-green zinc dicyano compound
3. By NMR, this fraction contains various amounts of the
aromatic ligands which can be completely removed by resus-
pending the green residue (115 mg) in dry dichloromethane (2
mL) and inducing aggregation in 50 mL of dry n-heptane within
a centrifuge vessel. Centrifugation for 5 min (Hettich centrifuge
equipped with a 1624 rotor) at 3950 rpm for 5 min produced a
slightly greenish supernatant, which was discarded, and a well-
formed pellet. Resuspension in a fresh portion of n-heptane (20
mL) and centrifugation gave a colorless supernatant and a pellet
which after drying in vacuum weighed 64.1 mg of pure 3 (68%
yield). Upon doubling of the reaction scale, after boosting of
the catalyst loading to 30 mol % and after the same workup
procedure was followed, 107.1 mg of pure 3 could be obtained
after centrifugation and drying (56% yield).
Materials and Methods
General Remarks. Solvents were dried by prolonged heating
under reflux (dichloromethane over calcium hydride, toluene
and tetrahydrofuran over sodium benzophenone ketyl, n-heptane
over sodium metal), freshly distilled under nitrogen, and freed
from any residual oxygen by passing a flow of dry nitrogen for
3 min. The solvents for chromatography were sonicated shortly
before use. All reactions flasks and chromatography columns
were protected from ambient light with aluminum foil. Reactions
were performed under nitrogen or argon atmosphere. NMR
spectra were recorded at 300 MHz (for 1H) with a Bruker DPX
300 Avance spectrometer. Chemical shifts are given in ppm
relative to the signal of CHCl3, which was taken as δ ) 7.26
Rf (CH2Cl2 stabilized with amylenes) ) 0.46. Mp: ∼330 °C
without decomposition. Compound 3 sublimes neatly with green
1
vapors at 2.5 × 10-2 mbar. H NMR (300 MHz, dioxane-d8;
1
(for H) and 77.00 (for 13C). Coupling constants are given in
δ, ppm): 9.90 (4H, d, J ) 4.8, 3,7,13,17-H4), 9.26 (4H, d,
J ) 4.5, 2,8,12,18-H4), 8.24 (4H, d, J ) 1.5, 2′6′-H2), 8.12
(2H, t, J < 2, 4′-H), 1.75 (s, 36H, t-Bu). In CDCl3 broad signals
due to aggregation are encountered. 13C NMR (75 MHz,
dioxane-d8; δ, ppm): 153.3, 152.2, 150.6, 135.8, 132.5, 131.2,
130.1, 129.6, 36.2 (CMe3), 32.4 (CMe3). Even after 40 000 scans
the CN signal and the porphyrin quaternary carbons are not
visible. UV-vis (CH2Cl2) [λmax, (log ꢀmax)]: 423 (5.56), 561
(4.06), 602 (4.55). FT-IR (KBr) [ν (cm-1)]: 2962 (s), 2904
(m), 2869 (m), 2207 (vs), 1592 (m), 1536 (w), 1502 (w), 1420
(w), 1394 (w), 1362 (m), 1294 (mw), 1247 (mw), 1210
(m), 1077 (m), 1071 (m), 1004 (ms), 932 (ms), 902 (mw) 881
(m), 817 (m), 793 (vs), 716 (m), 709 (m). MS (MALDI-ToF)
(m/z): 797.8, [M]+ with correct isotopic pattern; calcd for
C50H5064ZnN6) 798.4. HR-FAB-MS (m/z): 800.3540; calcd for
C50H5066ZnN6) 800.3357; calcd for [M + 2H]+ C50H5264ZnN6
) 800.3545.
5,15-Dicarboxy-10,20-bis(3,5-di-tert-butylphenyl)porphy-
rin (4). Porphyrin 3 (133 mg, 0.16 mmol) was stirred vigorously
with 15 mL of H2SO4 (95-97%) and 3 drops water at 100 °C
for 2 h and with exposure to air. The reaction was then cooled
at room temperature and left to stir overnight. The solution was
poured cautiously into ice-water. An attempt to extract the
diacid with dichloromethane failed due to its low solubility. The
green aqueous layer was neutralized with solid sodium dicar-
bonate, and then the aqueous solution was concentrated on rotary
evaporation with caution. Intense foaming could be controlled
by using a 250 mL flask and carefully controlling the bath
temperature and vacuum level. The remaining solid was
extracted with acetone in which the inorganic salts are insoluble.
Final evaporation of the acetone extract left a raspberry colored
powder (109 mg, 88% yield).
Hz. UV/vis spectra were measured with a Varian Cary 500
equipped with a Peltier variable-temperature unit. Fluorescence
spectra were measured on a Varian Eclipse fluorometer.
MALDI-ToF spectra were obtained on a Voyager Instrument
from Applied Biosystems with 1,8,9-anthracenetriol as matrix.
HR-FAB-MS results were recorded using 3-nitrobenzyl alcohol
(NBA) as a matrix on a Finnigan MAT 90 machine. Column
chromatography was performed with Merck silica gel 0.040-
0.063 nm. Retention factors are given for silica gel TLC plates
(Macherey Nagel silica gel Polygram SIL 6/UV254) using
dichloromethane (0.2% ethyl alcohol as stabilizing agent) unless
otherwise specified.
(5,15-Dicyano-10,20-bis(3,5-di-tert-butylphenyl)porphyri-
nato)zinc(II) (3). 5,15-Dibromo-10,20-bis(3,5-di-tert-butylphe-
nyl)porphyrin free base (2)10 (100 mg, 0.12 mmol), tris-
(dibenzylideneacetone)dipalladium (Pd2(dba)3, 32.5 mg, 0.035
mmol, 30 mol %), 1,1′-bis(diphenylphophino)ferrocene (dppf,
39 mg, 0.07 mmol, 60 mol %), activated zinc powder (7 mg,
0.1 mmol, 90 mol %), and anhydrous zinc cyanide (22 mg, 0.19
mmol, 160 mol %) were weighed under nitrogen atmosphere
and filled as solids in an overnight oven-dried (at 111 °C) 50
mL flask. Anhydrous N,N-dimethylacetamide (DMA, 12 mL)
purchased from Aldrich was degassed by passing a flow of argon
for 3 min and was cannulated to the reaction flask which was
then heated under magnetic stirring at 120 °C for 4 h under a
dry nitrogen atmosphere. After the temperature reached 120 °C,
usually a deep pine tree green color was observed. With smaller
catalyst loading (e.g., 4 mol %), the induction period is longer
and the reaction can be triggered by adding fresh Pd2(dba)3,
dppf, and activated zinc (without zinc cyanide) with up to 30
mol % loading. The reaction mixture was then left to stir
overnight at room temperature. The mixture was transferred into
a separatory funnel, followed by rinsing of the reaction vessel
and stirrer with a total of 100 mL of dichloromethane giving a
dichroic greenish-violet organic layer, which was washed once
with 50 mL of 12% aqueous NH4OH. After subsequently
washing of the sample with brine (50 mL) and distilled water
until neutral pH, the organic layer was concentrated on a rotary
evaporator without using a drying agent (e.g., MgSO4 or
Na2SO4) to avoid losses by adsorption of aggregates. The
greenish blue residue was taken up in dichloromethane and
UV-vis (acetone) [λmax, (log ꢀmax)]: 411 (4.92), 508 (3.77),
584 (3.26). MS (MALDI-ToF) (m/z): 772.87. HR-FAB-MS-
(m/z): 772.3995; calcd for C50H52N4O4 [M - 2H]+ ) 772.3989.
(5-Cyano-10,20-bis(3,5-di-tert-butylphenyl)porphyrinato)-
zinc(II) (6). In a similar fashion, 5-bromo-10,20-bis(3,5-di-tert-
butylphenyl)porphyrin free base (5)10 (100 mg, 0.13 mmol) was
admixed with 35 mg of Pd2(dba)3 (0.04 mmol, 30 mol %), 43
mg of dppf (0.08 mmol, 60 mol %), 8 mg of activated Zn (90
mol %), 12 mg of Zn(CN)2 (0.1 mmol, 80 mol %), and 12 mL

5514 J. Phys. Chem. B, Vol. 112, No. 17, 2008
Balaban et al.
1
of anhydrous DMA. After stirring of the sample overnight at
120 °C and after workup as described above, isolation by
column chromatography furnished 40 mg (40% yield) of pink-
violet pure compound. Reproducible yields were obtained over
several runs, but should the starting material have contained
some dibromo compound (2) the final color of the reaction
mixture was green.
Rf (CH2Cl2) ) 0.38. H NMR (300 MHz, CDCl3; δ, ppm):
9.59 (d, 2H, J ) 4.5), 9.48 (d, 2H, J ) 4.8), 8.96 (d, 2H, J )
4.8), 8.80 (d, 2H, J ) 4.5), 7.98 (d, 4H, J ) 1.8), 7.82 (d, 2H,
J ) 1.5), 3.99 (m, 4H, morpholinyl-H), 3.84 (m, 4H, morpholi-
nyl-H), 1.56 (s, 36H, C(CH3)3). The chemical shifts are
depending on the concentration. 13C NMR (75 MHz, CDCl3/
dioxane-d8, ) 1/1, v/v; δ, ppm): 151.61, 150.29 (3′,5′-C),
149.50, 148.34 140.76, 134,79, 134.05, 131.20 (2′,6′-C), 129.27,
129.00, 123.75, 120.67 (4′-C), 83.28 (CN), 68.11, 57.81, 34.42
(C(CH3)3), 31.01 (C(CH3)3). UV-vis (CH2Cl2) [λmax (log ꢀmax)]:
316 (4.09), 424 (5.18), 557 (4.08), 592 (3.97). FT-IR (KBr)
[ν (cm-1)]: 3446 (b, residual water), 2957 (s), 2899 (ms), 2863
(ms), 2208 (ms), 1593 (m), 1552 (w), 1524 (w), 1501 (w), 1476
(w), 1444 (m), 1393 (w), 1348 (w), 1362 (ms), 1287 (w), 1260
(w), 1248 (w), 1213 (w), 1202 (w), 1172 (w), 1117 (w), 1095
(w), 1073 (w), 1059 (w), 971 (w), 930 (w), 915 (w), 900 (w),
882 (w), 861 (w), 820 (w), 798 (ms), 785 (ms), 729 (m), 715
(m). MS (MALDI-ToF) (m/z): 857.81; calcd for C53H59N6O64-
Zn [M]+ ) 858.40. HR-FAB-MS (m/z): 859.4041; calcd for
C53H59N6O64Zn [M]+ ) 859.4042.
1
Rf (CH2Cl2) ) 0.64. H NMR (300 MHz, CDCl3; δ, ppm):
10.36 (1H, s, 15-H), 9. 63 (2H, d, J ) 4.8, 3,7-H2), 9. 41 (2H,
d, J ) 4.5, 13,17-H2), 9.18 (2H, d, J ) 4.5, 2,8-H2), 9.10 (2H,
d, J ) 4.5, 12,18-H2), 8.08 (d, 4H, J ) 1.8, 2′6′-H2), 7.85 (t,
2H, J ) 1.8, 4′-H), 1.58 (s, 36H, t-Bu). 13C NMR (75 MHz, 72
h, dioxane-d8; δ, ppm): 152.8, 152.6, 151.7, 150.3, 142.8, 135.8,
133.8, 131.5, 131.2, 125.0, 123.5, 122.5, 86.3 (CN), 55.2
(impurity, CH2Cl2), 38.2, 36.2, 35.4, 32.5. UV-vis (CH2Cl2)
[λmax (log ꢀmax)]: 418 (5.57), 511 (3.50), 548 (4.19), 583 (4.02).
MS (MALDI-ToF) (m/z): 774.04, [M]+ with correct isotopic
pattern. HR-FAB-MS (m/z): found, 775.3589; calcd for
C49H53N564Zn, 775.3592.
(5-Bromo-15-cyano-10,20-bis(3,5-di-tert-butylphenyl)por-
phyrinato)zinc (7). 5-Cyano-10,20-bis(3,5-di-tert-butylphenyl)-
porphyrinato zinc (6) (280 mg, 0.36 mmol) was dissolved in
dichloromethane (150 mL), and the solution was cooled to
0 °C. Pyridine (1.0 mL) was added via a syringe. Then
N-bromosuccinimide (96 mg, 0.54 mmol) was added in portions.
The color of the reaction turns from pink to blue-violet. The
reaction was stirred for 30 min at 0 °C and then quenched (after
TLC control) with 15 mL of acetone. After the evaporation of
the solvent, the violet-greenish residue was taken up in
dichloromethane and deposited onto silica gel by vacuum
evaporation. Column chromatography on silica gel (H ) 20 cm,
Φ ) 6 cm) eluted with 98:2 dicloromethane/methanol (v/v) gave
the main fraction of the green-violet compound 7 (215.5 mg,
70% yield).
(5-Piperazino-15-cyano-10,20-bis(3,5-di-tert-butylphenyl)-
porphyrinato)zinc (8b). In a flask equipped with a reflux
condenser porphyrin, 7 (50 mg, 0.058 mmole) was stirred with
a solution of 3.0 g of piperazine in 15 mL of chloroform. The
general procedure was modified by adding a solvent because
piperazine is a solid amine. The reaction mixture was heated at
50 °C under argon overnight. After the solvent was evaporated,
the mixture of unreacted piperazine and the crude compound
8b was extracted in the minimal quantity (ca. 10 mL) of
dichloromethane. The column chromatography on silica gel (H
) 20 cm, Φ ) 6 cm) was eluted first with dichloromethane
and afterward with a mixture of dichloromethane/methanol (95:
5, v/v) to afford the porphyrin 8b (25.8 mg, 52% yield).
Rf (CH2Cl2/MeOH, 90/10, v/v) ) 0.60. 1H NMR (300 MHz,
dioxane-d8; δ, ppm): 9.96 (broad s), 9.75 (broad s), 9.16
(broad s), 9.08 (broad s), 8.2 (broad s), 8.08 (broad s), 5.55
(broad s, NH-piperazinyl), 4.77 (broad s, 4H, piperazinyl-H),
4.05 (broad s, 4H, piperazinyl-H), 1.45 (s, 36H, C(CH3)3). UV-
vis (CH2Cl2) [λmax (log ꢀmax)]: 322 (3.84), 428 (4.6), 574 (3.89),
619 (3.88). FT-IR (KBr) [ν (cm-1)]: 3295 (w), 2950 (s), 2935
(ms), 2849 (ms), 2207 (ms), 1592 (m), 1550 (w), 1444 (m),
1362 (ms), 1285 (m), 1247 (m), 1214 (m), 1069 (m), 1060 (m),
1001 (ms), 881 (w), 929 (w), 860 (w), 819 (m), 796 (s), 784
(m), 729 (w), 714 (w). MS (MALDI-ToF) (m/z): 856.86; calcd
for C53H58N764Zn [M - H]+ ) 856.40. HR-FAB-MS (m/z):
858.4204; calcd for C53H60N764Zn [M + H]+ ) 858.4201.
1
Rf (CH2Cl2) ) 0.61. H NMR (300 MHz, dioxane-d8; δ,
ppm): 9.88 (d, 4H, J ) 4.5), 9.77 (d, 4H, J ) 4.5), 9.18 (d,
4H, J ) 4.5), 9.05 (d, 4H, J ) 4.5), 8.21 (s, 4H), 8.08 (s, 2H),
1.72 (s, 36H, C(CH3)3). 13C NMR (75 MHz, dioxane-d8; δ,
ppm): 156.81, 153.47, 153.3, 152.05, 150.35, 150.25 (3′,5′-
C), 131.20 (2′,6′-C), 126.17 (4′-C), 36.19 (C(CH3)3), 32.47
(C(CH3)3). UV-vis (CH2Cl2) [λmax (log ꢀmax)]: 322 (4.11), 425
(5.44), 560 (4.03), 595 (3.92). FT-IR (KBr) [ν (cm-1)]: 2957
(vs), 2907 (ms), 2863 (ms),2202 (ms), 1592 (ms), 1555 (w),
1521(w), 1521 (w), 1505 (w), 1477 (w), 1425 (w), 1393 (w),
1363 (ms), 1344 (w), 1322 (w), 1292 (w), 1246 (w), 1212 (w),
1074 (w), 1071 (w), 1059 (w), 1004 (ms), 950 (w), 937 (w),
898 (w), 882 (w), 816 (w), 807 (w), 793 (s), 766 (w), 737 (w),
729 (w), 715 (w), 698 (w), 668 (w). MS (MALDI-ToF) (m/z):
851.90; calcd for C49H50N579Br64Zn [M]+ ) 851.25. HR-FAB-
MS (m/z): 853.2528; calcd for [M]+ C49H50N581Br64Zn [M]+
) 853.2521, C49H50N579Br66Zn [M]+ ) 853.2510.
(5-Piperidino-15-cyano-10,20-bis(3,5-di-tert-butylphenyl)-
porphyrinato)zinc (8c). Following the general solventless
procedure described above, compound 8c was obtained as a
blue-green powder (36.7 mg, 73% yield).
1
Rf (CH2Cl2) ) 0.70. H NMR (300 MHz, CDCl3; δ, ppm):
General Procedure for the Catalyst-Free Amination of
meso-Bromoporphyrins. A solution of the above porphyrin 7
(50 mg) in excess amine (5 mL) was deaerated with nitrogen
and then heated at 50 °C overnight. After evaporation of the
amine, the residue was deposited onto silica gel by vacuum
evaporation. Column chromatography on silica gel (H ) 20 cm,
Φ ) 6 cm), eluted with dichloromethane, affords the title push-
pull N,N-dialkylaminocyanoporphyrins (8).
9.58 (d, 2H, J ) 4.5), 9.18 (d, 2H, J ) 4.8), 8.95 (d, 2H, J )
4.8), 8.81 (d, 2H, J ) 4.8), 8.02 (d, 4H, J < 2), 7.83 (t, 2H, J
< 2), 4.33 (m, 4H, 2′′,6′′-piperidyl-H), 2.21 (m, 4H, 3′′,5′′-
piperidyl-H), 2.09 (m, 2H, 4′′-piperidyl-H), 1.55 (s, 36H,
C(CH3)3). 13C NMR (75 MHz, CDCl3; δ, ppm): 152.37, 151.34,
150.61 (3′, 5′-C), 149.71, 148.82, 140.92, 138.46, 134.79,
131.47, 130.02 (2′, 6′-C), 129.52, 129.09, 124.62, 121.19 (4′-
C), 82.92 (CN), 59.87, 35.05 C(CH3)3), 31.73, 31.58 (C(CH3)3),
27.97, 25.03, 22.64, 14.12. See Supporting Information for
H-C-correlated NMR spectra. UV-vis (CH2Cl2) [λmax (log
ꢀmax)]: 314 (4.18), 423 (5.22), 557 (4.06), 621 (3.95). FT-IR
(KBr) [ν (cm-1)]: 2949 (s), 2663 (w), 2210 (m), 1592 (m),
1554 (w), 1521 (w), 1442 (m), 1363 (m), 1283 (w), 1247 (m),
(5-Morpholino-15-cyano-10,20-bis(3,5-di-tert-butylphenyl)-
porphyrinato)zinc (8a). Using the above procedure, 50 mg of
7 (0.058 mmol) was heated overnight in 5 mL of morpholine
at 50 °C. After chromatography, porphyrin 8a was obtained as
a blue-green powder (40 mg, 90% yield).

Programmed Metalloporphyrins for Self-Assembly
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5515
1
1213 (w), 1151 (w), 1118 (w), 1076 (w), 1001 (w), 930 (w),
915 (w), 881 (w), 865 (w), 825 (w), 791 (m), 729 (w), 714 (w).
MS (MALDI-ToF) (m/z): 855.57; calcd for C54H59N664Zn
[M - H]+ ) 855.41. HR-FAB-MS (m/z): 856.4168; calcd for
C54H60N664Zn [M]+ ) 856.4171.
Rf (CH2Cl2) ) 0.88. H NMR (300 MHz, CDCl3, at 30 °C;
δ, ppm): 9.58 (d, 4H, J ) 4.5), 8.85 (d, 4H, J ) 4.8), 8.067 (s,
4H), 7.84 (d, 2H, J ) 1.2), 4.37 (m, 8H, morpholinyl-H), 4.30
(m, 8H, morpholinyl-H), 1.56 (s, 36H, C(CH3)3), -2.68 (s, 2H,
NH). 13C NMR (75 MHz, CDCl3, at 30 °C; δ, ppm): 148.79
(3′,5′-C), 141.17 (1′-C), 129.68 (2′,6′-C), 129.03, 123.99, 121.65
(4′-C), 121.03, 69.04, 57.75, 35.07 (C(CH3)3), 31.71 (C(CH3)3).
UV-vis (CH2Cl2) [λmax (log ꢀmax)]: 304 (4.13), 422 (5.35), 519
(4.05), 558 (3.85), 657 (3.53). FT-IR (KBr) [ν (cm-1)]: 3446
(b), 3317 (m), 3960 (s), 2899 (ms), 2848 (ms), 1591 (m), 1505
(w), 1473 (m), 1446 (m), 1392 (w), 1363 (m), 1301 (w), 1262
(m), 1244 (m), 1196 (w), 1172 (w), 1151 (w), 1113 (s), 1066
(w), 1033 (m), 976 (w), 946 (w), 915 (w), 898 (w), 880 (w),
794 (s), 729 (m), 715 (m). MS (MALDI-ToF) (m/z): 857.09;
calcd for C56H69N6O2 [M + H]+ ) 857.54. HR-FAB-MS
(m/z): 857.5479; calcd for C56H69N6O2 [M + H]+ ) 857.5482.
(5-Pyrrolidino-15-cyano-10,20-bis(3,5-di-tert-butylphenyl)-
porphyrinato)zinc (8d). Following the above solventless
procedure, 50 mg of porphyrin 7 (0.058 mmol) heated in 5 mL
of pyrrolidine affords after chromatography the porphyrin 8d
as a blue-green powder C (35.4 mg, 72% yield).
1
Rf (CH2Cl2) ) 0.44. H NMR (300 MHz, dioxane-d8, at
30 °C; δ, ppm): 9.43 (d, 2H, J ) 4.5), 9.35 (d, 2H, J ) 4.2),
8.9 (d, 2H, J ) 4.5), 8.66 (d, 2H, J ) 4.5), 8.13 (d, 4H, J )
1.5), 8.01(d, 2H, J < 2), 4.66 (t, 4H, pyrrolidinyl-H, J ) 6.3),
2.60 (q, pyrrolidinyl-H, J < 2) 1.71 (s, 36H, C(CH3)3). 13C NMR
(75 MHz, dioxane-d8, at 30 °C; δ, ppm): 154.54, 150.43,
150.24, 150.19 (3′,5′-C), 148.46, 142.95, 135.72, 130.705,
130.37 (2′,6′-C), 129.24, 126.02, 122.25 (4′-C), 83.209 (CN),
61.23, 36.15 (C(CH3)3), 32.48 (C(CH3)3), 28.18. See Supporting
Information for H-C-correlated NMR spectra. UV-vis
(CH2Cl2) [λmax (log ꢀmax)]: 302 (4.18), 433 (5.23), 446 (5.07),
628.5 (4.27). FT-IR (KBr) [ν (cm-1)]: 2962 (s), 2870 (ms),
2197 (ms), 1592 (m), 1554 (m), 1521 (m), 1450 (ms), 1392
(w), 1362 (m), 1326 (m), 1247 (w), 1074 (w), 1006 (w), 880
(w), 826 (w), 789 (w), 728 (w). MS (MALDI-ToF) (m/z):
843.90; calcd for C53H59N664Zn [M + H]+ ) 843.41. HR-FAB-
MS (m/z): 842.4019; calcd for C53H58N664Zn [M]+ ) 842.4014.
5-Morpholino-10,20-bis(3,5-di-tert-butylphenyl)porphy-
rin (10). Similar to the general procedure, 5-bromo-10,20-bis-
(3,5-di-tert-butylphenyl)porphyrin (5, 50 mg, 0.065 mmol) was
heated in 5 mL of morpholine. Column chromatography
afforded the main product 10 as a red powder (31 mg, 61%
yield).
1
Rf (CH2Cl2) ) 0.79. H NMR (300 MHz, CDCl3; δ, ppm):
10.11 (s), 9.73 (d, 2H, J ) 4.8), 9.28 (d, 2H, J ) 4.8), 9.025
(d, 2H, J ) 4.5), 8.99 (d, 2H, J ) 4.8), 8.12 (d, 4H, J ) 1.8),
7.85 (d, 2H, J ) 1.8), 4.40 (dm, 8H, morpholinyl-H), 1.56 (s,
36H, C(CH3)3), -2.90 (s, 2H, NH). 13C NMR (75 MHz, CDCl3;
δ, ppm): 148.90 (3′,5′-C), 140.72 (1′-C), 131.69, 130.88,
129.97, 129.67 (2′,6′-C), 128.59, 121.07 (4′-C), 120.20, 57.83,
35.07 (C(CH3)3), 31.76 (C(CH3)3). See Supporting Information
for H-C-correlated NMR spectra. UV-vis (CH2Cl2) [λmax
(log ꢀmax)]: 302 (4.08), 415 (5.41), 511 (4.11), 547 (3.73), 584
(3.65), 642 (3.65). FT-IR (KBr) [ν (cm-1)]: 3313 (b), 2989
(s), 2899 (ms), 2856 (ms), 1591 (ms), 1474 (m), 1392 (w), 1362
(ms), 1298 (w), 1263 (m), 1245 (ms), 1153 (w), 1115 (m), 1063
(w), 1032 (w), 984 (w), 963 (m), 916 (m), 898 (w), 879 (w),
842 (w), 816 (w), 794 (ms), 716 (m), 690 (w). MS (MALDI-
ToF) (m/z): 771.86; calcd for C52H61N5O [M]+ ) 771.49.
HR-FAB-MS (m/z): 772.4961; calcd for C52H62N5O [M + H]+
) 772.4954.
(5-N,N-Diethylamino-15-cyano-10,20-bis(3,5-di-tert-bu-
tylphenyl)porphyrinato)zinc (8e). The reaction was performed
in a flask equipped with a reflux condenser under an argon-
filled balloon. To the porphyrin 7 (50 mg, 0.058 mmol) was
added 10 mL of diethylamine, and then the reaction mixture
was heated overnight at 50 °C. After evaporation of the amine,
column chromatography as described above afforded 39.6 mg
of compound 8e as a blue-green powder (81% yield).
1
Rf (CH2Cl2) ) 0.90. H NMR (300 MHz, dioxane-d8, at
30 °C; δ, ppm): 9.69 (d, 4H, J ) 2.7), 9.67 (d, 2H, J ) 3),
9.09 (d, 2H, J ) 4.8), 8.93 (d, 2H, J ) 4.8), 8.21 (d, 4H, J )
1.8), 8.06 (d, 2H, J ) 1.8), 4.55 (m, CH2), 1.71 (s, 36H,
C(CH3)3), 1.51 (t, CH2, J ) 7.0). 13C NMR (75 MHz, dioxane-
d8, at 30 °C; δ, ppm): 153.35, 153.35, 151.68, 151.16, 150.36,
150.16 (3′,5′-C), 142.98, 135.68, 132.58, 131.87, 130.90 (2′,
6′-C), 125.29, 123.50, 122.26 (4′-C), 85.11 (CN), 56.32, 36.18
(C(CH3)3), 32.49, 32.47 (C(CH3)3), 16.09. See Supporting
Information for H-C-correlated NMR spectra. UV-vis
(CH2Cl2) [λmax (log ꢀmax)]: 318 (4.15), 423 (5.30), 557 (4.07),
592 (3.95). FT-IR (KBr) [ν (cm-1)]: 2964 (s), 2892 (ms), 2871
(ms), 2210 (ms), 1591 (ms), 1552 (m), 1522 (w), 1501 (w),
1443 (m), 1392 (w), 1362 (ms), 1283 (m), 1283 (m), 1221 (m),
1119 (w), 1070 (m), 1002 (m), 929 (w), 899 (w), 881 (w), 824
(m), 792 (ms), 765 (w), 728 (m), 713 (ms). MS (MALDI-ToF)
(m/z): 845.87; calcd for C53H61N664Zn [M + H]+ ) 845.42.
HR-FAB-MS (m/z): 844.4178; calcd for C53H60N664Zn [M]+
) 844.4171.
5-Morpholino-15-cyano-10,20-bis(3,5-di-tert-butylphenyl)-
porphyrin Free Base (11a). Porphyrin 8a (15 mg, 0.017
mmole) was stirred with a mixture of deoxygenated trifluoro-
acetic acid (3 mL), sulfuric acid (3 mL), and 3 drops distilled
water for 2 h at room temperature. The reaction mixture was
poured very slowly and carefully on crushed ice (20 g) under
stirring. The resulting mixture was extracted with dichlo-
romethane (2 × 20 mL), washed with sodium carbonate (2 ×
30 mL), and then with distilled water until neutral pH. After
evaporation of the solvent, the violet residue was taken up in
dichloromethane and deposited onto silica gel by vacuum
evaporation. After column chromatography on silica gel (H )
15 cm, Φ ) 2 cm), eluted with dichloromethane/methanol )
98/2, v/v, the violet free base porphyrin was obtained in 97%
yield (13.4 mg).
5,15-Dimorpholino-10,20-bis(3,5-di-tert-butylphenyl)por-
phyrin (9). 5,15-Dibromo-10,20-bis(3,5-di-tert-butylphenyl)-
porphyrin (2, 50 mg, 0.06 mmol) was dissolved in 5 mL of
morpholine, the solution was stirred for 72 h at 50 °C. Column
chromatography on silica gel (H ) 20 cm, Φ ) 6 cm) after
elution first with dichloromethane which afforded a minor
fraction of 5-bromo-15-morpholino-10,20-bis(3,5-di-tert-bu-
tylphenyl)porphyrin (8.6 mg) and then with dichloromethane/
methanol ) 90/10 (v/v) provided the compound 9 which was
obtained as a cognac colored powder (25 mg, 49% yield).
1
Rf (CH2Cl2) ) 0.58. H NMR (300 MHz, CDCl3; δ, ppm):
9.47 (d, 2H, J ) 4.8), 9.42 (d, 2H, J ) 4.8), 8.92 (d, 2H, J )
4.8), 8.71 (d, 2H, J ) 4.8), 8.01 (d, 4H, J ) 1.5), 7.84 (d, 2H,
J ) 1.5), 4.39 (s, 4H, morpholinyl-H), 4.38 (s, 4H, morpholinyl-
H), 1.55 (s, 36H, C(CH3)3), -1.90 (s, 2H, NH). 13C NMR (75
MHz, CDCl3; δ, ppm): 149.20 (3′,5′-C), 139.97 (1′-C), 135.35,
129.68 (2′,6′-C), 123.99, 121.65 (4′-C), 96. 47, 83.14 (CN),
68.79, 58.43, 35.07 (C(CH3)3), 31.71 (C(CH3)3). UV-vis
(CH2Cl2) [λmax (log ꢀmax)]: 426 (5.26), 524 (3.97), 561 (3.91),

5516 J. Phys. Chem. B, Vol. 112, No. 17, 2008
Balaban et al.
SCHEME 1: Reagents and Reaction Conditions: (a)
NBS, Pyridine, CH2Cl2, RT, 30 min 74%; (a′) NBS,
Pyridine, CHCl3, 0 °C, 10 min 70%; (a′′) NBS, Pyridine,
CH2Cl2, 0 °C, 30 min 70%; (b) Zn(CN)2, Zn, Pd2(dba)3,
DPPF, DMA, 120 °C, 4 h. 68%; (b′) Zn(CN)2, Zn,
Pd2(dba)3, DPPF, DMA, 120 °C, 12 h. 40%; (c) Alk2NH
Excess, 50 °C, 18 h, 49-90%. (d) TFA, H2SO4, H2O,
100 °C, 2 h, 88%
561 (4.02), 651 (3.61). FT-IR (KBr) [ν (cm-1)]: 3460 (b,
residual water), 3309 (ms), 2961 (s), 2906 (ms), 2663 (ms),
2213 (m), 1593 (m), 1480 (w), 1363 (m), 1248 (m), 865 (w),
796 (ms), 716 (m). MS (MALDI-ToF) (m/z): 797.02; calcd for
C53H61N6O [M + H]+ ) 797.49. HR-FAB-MS (m/z): 797.4915;
calcd for C53H61N6O [M + H]+ ) 797.4906.
Time-Resolved Fluorescence. For the time-resolved fluo-
rescence measurements, we used a Ti:sapphire laser delivering
sub-150 fs pulses at a repetition rate of 76 MHz. To excite the
compound to second harmonic generation, we employed a BBO
crystal. Predominantely one species only could be excited, either
the monomers or the aggregates, by tuning the excitation
wavelength to 419 and 447 nm, respectively. The spectral width
of the pulses was about 5 nm so that excitation conditions are
comparable to that of the stationary measurements. A closed
loop circulation system consisting of an optics cuvette and a
peristaltic pump avoided photodegradation of the compounds.
The fluorescence signal was detected by a streak camera
(Hamamatsu C5680) coupled to a spectrometer (Jobin-Yvon
HR460, 100 lines/mm grating). Fluorescence spectra were
measured in the wavelength range between 580 and 755 nm,
with a spectral resolution of 2 nm and a temporal resolution of
8 ps. A polarizer was used for compensating the effect of
molecular rotation on the fluorescence decay. The obtained
fluorescence spectra were analyzed globally in terms of decay-
associated emission spectra (DAES) by fitting the following
function to the data:
n
A (λ)e-t/τ
i
I(λ,t) )
∑
i
i)1
Here I(λ,t) is the fluorescence intensity, Ai(λ) are the spectral
weights, and τi are the global decay times.
Crystallographic Details. The data collection of 3‚2CHCl3
was carried out at the ANKA synchrotron source (Forschungs-
zentrum Karlsruhe) on a Bruker AXS D8 diffractometer
equipped with a SMART APEX CCD (λ ) 1.0 Å) while 3‚
2C4D8O2 was measured on a STOE IPDS (image plate diffrac-
tion system) with λ ) 0.8 Å at 150 K. The structure solutions
and full-matrix least-square refinements based on F2 were
performed with SHELX-97 program package.11 Molecular
diagrams were prepared using the program Diamond.12 3‚
2CHCl3: dark green needles, 0.1 × 0.04 × 0.04 mm; Mr )
1039.1; monoclinic, space group P21/n (No. 14); a ) 6.060(4),
b ) 17.733(10), c ) 23.562(13) Å; â ) 96.869(13)°; V ) 2514-
(3) Å3; Z ) 2; Dc ) 1.373 g cm-3; µ(λ ) 1 Å) ) 2.219 mm-1
giving a final R1 value of 0.0788 for 356 parameters and 2878
unique reflections with I g 2σ(I) and wR2 of 0.2272 for all
8836 reflections (Rint ) 0.1692). A disorder has been modeled
for two symmetry-equivalent tert-butyl groups (C17-C20).
However most of the residual peaks namely Q1-Q6 with a
maximum of 1.007 e/Å3 are still located in this area but could
not be refined in a suitable model. 3‚2C4D8O2: violet plates,
0.26 × 0.12 × 0.04 mm; Mr ) 1064.6; triclinic, space group
P1h (No. 2); a ) 7.6081(15), b ) 10.239(2), c ) 19.051(4) Å;
R ) 77.14(3), â ) 88.95(3), γ ) 72.70(3)°; V ) 1379.6(5) Å3;
Z ) 1; Dc ) 1.311 g cm-3; µ(λ ) 1 Å) ) 0.703 mm-1 giving
a final R1 value of 0.0572 for 488 parameters and 3245 unique
reflections with I g 2σ(I) and wR2 of 0.1510 for all 6848
reflections (Rint ) 0.1079). CCDC-632909 and CCDC-653012
contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge at www.ccdc.
cam.ac.uk/conts/retrieving.html [or from the Cambridge Crystal-
lographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
U.K.; fax, (internat.) +44-1223/336-033; e-mail, deposit@ccdc.
cam.ac.uk].
Results and Discussion
Syntheses. By discovering that hydrogen bonding is not a
prerequiste for inducing chlorosomal-type organization of
pigments, we could generalize this self-assembly algorithm.13
Reasoning that π-π interactions combined with weak electro-
static and dispersive forces can also be engineered with other

Programmed Metalloporphyrins for Self-Assembly
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5517
SCHEME 2: Both Reactions Carried Out in Excess
Morpholine at 50 °C: (a) 72 h, Isolated Yield ) 49%;
(b) 12 h, Isolated Yield ) 61%
tion, bromination, and amination gives higher yields than the
alternate one involving bromination of the 5-(dialkylamino)-
poprhyrin. Probably protonation occurs so that the NBS bro-
mination of the 5-dialkylammonium salt is much slower than
the meso-bromination of the 5-cyanoporphyrin. We could thus
prove that even meso-brominated free base porphyrins can be
successfully aminated. The presence of nickel salts as catalysts,
as recently stated,22 is not needed in this direct and facile
reaction.
Photophysical Properties. An interesting feature of the title
compound 3 is it is aggregate-induced emission.23 Figure 1
shows the absorption and in the inset the fluorescence spectrum
of 3 as a mull in perfluoropolyalkyl ether oil. Usually, with
randomly oriented chromophores self-quenching or concentra-
tion quenching prevent observing steady-state fluorescence from
strongly absorbing solids. Due to the very orderly manner in
which the chromophores 3 are arranged (vide infra), after
excitation at 430-500 nm a weak but measurable emission is
observed. The fluorescence excitation spectra monitored at a
wavelength where only the aggregates emit (700 nm) show that
the Soret band is severely broadened, in contrast to the sharp
band encountered for the monomers (at 413 nm) when the
excitation was monitored at 645 nm (Figure 1).
recognition groups than the ones present in BChls (i.e., as in
the Zn porphyrin 1), we synthesized the title compound 3 in
over 65% yield by a palladium-catalyzed cyanation (see Scheme
1) starting from the known 5,15-dibromo-10,20-bis(3,5-di-tert-
butylphenyl)porphyrin (2)10 and zinc cyanide.14a The mono-5-
cyanoporphyrin 6 was obtained as a byproduct (10% yield), and
it could be prepared in an independent way by the same coupling
procedure14a from the corresponding 5-bromoporphyrin 5 ob-
tained by the method of Arnold.15 While this work was in
progress, other direct cyanations of bromoporphyrins14b and
haloindoles14c have been reported with similar yields. This
reaction has an induction phase which depends on the catalyst
loading and on the concentration of the zinc cyanide. For larger
scale reactions, as an excess of cyanide ions poisons the
catalyst,16 we recommend addition of a fresh portion of catalyst
after 90 min and when complete metalation of the free base
porphyrin has occurred. It has been reported for other cyanation
reactions that water can act as a cocatalyst;17 however, in the
present case, moisture as well as oxygen inhibit the reaction.
With the monocyanoporphyrin 6 in hand we could optimize
a catalyst-free, solventless amination of the novel 5-cyano-15-
bromo zinc porphyrin 7. We were inspired by the recent
amination of brominated perylene diimides discovered by
Imahori and co-workers.18 The high yields and facile workup
recommend this reaction for obtaining push-pull porphyrins
having the donor (D ) NR2) and acceptor (A ) CN) groups
directly linked to the meso positions. There are ample examples
of porphyrins where the D/A groups are linked via phenyl or
ethinyl spacer groups onto the porphyrin macrocycle.19 The
herein described procedure affords for the first time such push-
pull porphyrins with direct, linkerless attachment of the D/A
groups. An analogy to 4-N,N-dialkylaminobenzonitriles, dual
fluorescent and well-studied compounds, both experimentally
and theoretically,20 is obvious. Previously synthesized meso-
N,N-dialkylaminoporphyrins employed either Pd-catalyzed Buch-
wald-Hartwig conditions21 or required the presence of Ni(II)
salts.22 The direct amination of meso-bromoporphyrins works
well also for the compounds 2 and 5 yielding the free bases 9
and 10, respectively (Scheme 2), but from the former reaction
it was not possible to isolate the intermediate 5-(dialkylamino)-
15-bromoporphyrin as this reacts more rapidly to the 5,15-bis-
(dialkylamino)porphyrin (9). For obtaining meso-substituted
push-pull porphyrins, the sequence monobromination, cyana-
Time-resolved measurements followed by fitting of the decay-
associated emission spectra (DAES) reveal two components
which can be assigned to the monomers and to the aggregates
by excitation at 419 nm (Figure 2). When exciting at 447 nm,
where the monomers do not absorb, the longer-lived, monomer
fluorescence is absent, the spectrum equals the DAES, and the
fitting algorithm finds only one (1.2 ns) component (Figure 2).
Thus, the fluorescence is relatively strongly, but not entirely,
quenched in the aggregates and the lifetime is somewhat
shortened. However, a 1 ns lifetime is exceptionally long for
aggregated chromophores and should allow efficient trapping
of the radiant energy. The unquenched fluorescence and
therefore long lifetime is a strong indication of a very orderly
manner in which the porphyrins are arranged. With unspecific
aggregation, fluorescence in the solid state is usually very
strongly quenched and the lifetimes are in the ps to sub-ps time
range. These optical properties are typical of natural self-
organized antenna systems, like the chlorosomes, and make
aggregates of 3 a promising candidate for light-harvesting in
artificial devices. We reported recently very similar absorption
and fluorescence properties for an isomer of the BChl c mimic
1 having the (rac)-hydroxyethyl group in the 3-position and
the acetyl group in the 13-position of the macrocycle.24
The push-pull zinc porphyrins 8a,b self-assemble similarly
to 3 in thoroughly dried nonpolar solvents (Figure 3). In contrast
8c-e do not show any evidence of self-association in the same
solvents.
It is evident from the above spectra that a similar self-
assembly algorithm must operate for the dicyanozinc porphyrin
3 and the push-pull Zn-morpholino-cyano compound 8a.
However, in the latter, self-assembly could additionally occur
via coordination of the zinc atom by the morpholinyl oxygen
from one side of the macrocycle combined with a cyano-zinc
interaction on the other side. The piperazino-substituted com-
pound 8b also self-assembles but gives a split and less broadened
Soret band (Figure 4). In control experiments performed with
the same solvents, the piperidino (8c), pyrrolidino (8d), and
diethylamino (8e) analogs did not give self-assemblies as judged
by the nonscattering solutions in n-heptane and by the mono-
meric type of absorption and fluorescence spectra (Figure 4 and
see also Supporting Information).

5518 J. Phys. Chem. B, Vol. 112, No. 17, 2008
Balaban et al.
Figure 1. (A) Absorption spectra of 3. Blue trace: crystals rubbed between two quartz plates with perfluoropolyalkyl ether oil (ABCR, Karslruhe,
Germany), which were measured in an integrating sphere on a Cary 500 UV-vis-NIR spectrometer. Inset: fluorescence measured on a Cary
Eclipse fluorimeter at right angle from a similar sample with 500 nm excitation. The asterisks denote spikes which do not come from the sample.
Magenta trace: same sample but measured as normal absorption after addition of one drop of dry dichloromethane. Green trace: same sample after
dissolution in dichloromethane, path length 1 mm. (B) Green trace: absorption spectrum of 3 suspended in n-heptane after short sonication with
a path length of 1 cm. The baseline is shifted due to strong scattering from the aggregates. Red trace: after addition of one drop of methanol the
aggregates are disrupted and the monomeric 3‚MeOH adduct is formed, path length 1 mm. (C) Stationary fluorescence spectra of aggregates
suspended in n-heptane with different excitation wavelengths: red trace, 420 nm excitation; black trace, 430 nm excitation; green trace, 450 nm
excitation. (D) Fluorescence excitation spectra monitored at 700 (green trace) and 645 nm (red trace).
Figure 2. DAES from aggregates of 3 in dry n-heptane. Excitation wavelength was 419 nm (A, at left) and 447 nm (B, at right). For color-coded
images of the fluorescence and the goodness of fit, see the Supporting Information.
Energy transfer between the singlet excited-state of zinc
porphyrins and their free bases is well documented both in
covalent-25 and hydrogen-bonded constructs.26 We have inves-
tigated by time-resolved fluorescence mixtures in n-heptane of
the zinc porphyrin 8a with 5% and 10% addition of its
corresponding free base 11a. Although the aggregate and free
base fluorescence were clearly evident with lifetimes of ∼1.0
and 7.0 ns, respectively, we could not distinguish in the DAES
an energy transfer component (data not shown). This indicates
that the self-assembly of the zinc porphyrins occurs without
intermixing of their free bases which must thus be separated
by a distance larger than the Fo¨rster radius.26f
Single-Crystal X-ray Diffraction Structures. Needlelike
crystals (inset in Figure 5) could be grown reproducibly either
by slow evaporation of CDCl3 from a capped 5 mm diameter
NMR tube or by slowly diffusing cyclohexane into a concen-
trated chloroform solution of 3 through an intermediate layer
of a cyclohexane-chloroform mixture. Because these microc-
rystals diffracted only weakly X-rays, synchrotron radiation had
to be used to obtain data sets which finally led to solving the
crystal structure by direct methods.11 The molecular structure
is shown in Figure 5A and shows the expected geometry. Figure
5B shows that the tetrapyrroles stack as a flight of stairs with
a slightly off-setted geometry, as expected from the strong π-π

Programmed Metalloporphyrins for Self-Assembly
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5519
Figure 3. (A) Absorption spectra of 8a in n-heptane (green trace) and after addition of a methanol aliquot and 3-fold dilution with n-heptane. A
few drops of dichloromethane were added to ensure a homogeneous solution (red trace). (B) Corresponding stationary fluorescence spectra of the
aggregated species with excitation at 420 (green trace) and 450 nm (blue trace) and of the Zn-methanol adduct excited at 430 nm (red trace).
Figure 4. Comparison of the absorption (A, C) and fluorescence spectra (B, D) of 8b (A, B) and 8e (C, D). (A) Absorption spectra of 8b after
sonication in n-heptane (green trace) and after addition of an aliquot of methanol and 2-fold dilution with n-heptane (red trace). Note the split and
broadened Soret band. (B) Corresponding fluorescence of 8b in n-heptane (green trace, excitation wavelength at 425 nm; blue trace, excitation
wavelength at 435 nm) and after addition of methanol (red trace, excitation wavelength at 433 nm). (C) Absorption spectra of 8e in n-heptane
(green trace) and after addition of an aliquot of methanol (magenta trace). (D) Corresponding fluorescence spectra of 8e in n-heptane (green trace,
excitation wavelength at 420 nm) and after addition of an aliquot of methanol (magenta trace excitation wavelength at 430 nm).
interaction between electron deficient porphyrins.27 Figure 5C
represents the packing of stacks along the short crystallographic
axis. Well-ordered chloroform solvent molecules are included
in the crystal lattice, and a C-D(H)-N-C interaction with the
C-N distance of 3.17 Å is encountered. There are no short
contacts between the stacks which define a rhombus with 14.67
and 16.47 Å Zn-Zn spacings. Within a stack Zn-Zn distances
are 6.011 Å.
The stacking interaction with a 3.399 Å spacing between the
porphyrin planes is interesting as a similar stacking was
encountered in the natural BChls as put into evidence with solid
state 13C NMR on uniformly 13C-labeled BChl5a,28 and in the
two crystal modifications of 1.9 The dotted lines in Figure 4B
(3.111 Å long) are only a guide to the eye and should not be
interpreted as “bonds” but as a newly put into evidence
electrostatic interaction between the electron-deficient zinc atom
and the electron-rich nitrogens within the pseudohalogen groups.
The strongly interacting chromophores lead to a considerable
broadening of the Soret and Q bands as shown in Figures 1
and 2, and the aggregate maxima are red-shifted as in J-
aggregates. Usually J-aggregates have sharp absorption maxima.29
In our case the inhomogeneous broadening of the absorption
can be assigned to size heterogeneity in the length of the stacks.
For light-harvesting purposes this is quite beneficial as the whole

5520 J. Phys. Chem. B, Vol. 112, No. 17, 2008
Balaban et al.
Figure 6. Cocrystal of 3 with dioxane. The stacks pack solely by
hydrophobic interactions. Zn-Zn spacings within a stack are 7.608 Å,
while the closest Zn-Zn spacing within adjacent stacks is 10.239 Å.
anes, a much more loosely stacked structure with partial loss
of the π-π interactions is encountered. The Zn-Zn spacings
within a stack are 7.608 Å, only slightly longer than those in
Figure 5 due to a larger overlap of the porphyrin rings. Both
crystal structures have in common the hydrophobic packing of
stacks. Presently it is difficult to predict a priori the outcome
of crystallization when multiple weak interactions act in
conjunction.30 Modeling of polymorphs is an active research
area, and the present structures may serve as an example where
the π-π stacking combined with weak electrostatic interactions
can be outmaneuvered by a more strongly bis-coordinating
solvent, like dioxane.
Figure 5. Crystal structure and packing of 3. (A) Monomeric unit.
Inset: microcrystals under the light microscope. The yellow scale bar
is 50 µm long. (B) Stacks formed by π-π stacking and a CN-Zn
electrostatic interaction (dotted red line 3.111 Å). (C) View of stacks
along the short crystallographic a-axis showing inclusion of chloroform
as well-ordered crystallization solvent. A weak Cl3C-H-NtC interac-
tion is encountered. One of the tert-butyl groups in each 3,5-di-tert-
butylphenyl substituents is disordered over two preferential positions.
In (A) and (B) the tert-butyl groups have been replaced by the ipso-
carbon atom for clarity. Color coding: C, gray; N, blue; Zn, cyan; Cl,
green. H atoms have been omitted for clarity.
Conclusions
The crystal structures of 3 prove that the structural constrains
to obtain a chlorosomal type self-assembly can be relaxed and
that other functional groups than the ones encountered in BChl-
(s) (i.e., 1-hydroxyethyl, carbonyl, and a central metal atom)
can be used. Shorter and high yielding synthetic routes can then
furnish functional light-harvesting assemblies.13 The advantage
of our compounds is that they are fully synthetic and culturing
of bacteria or isolation of Chl a from algae can be avoided.
Our strategy, which starts from preformed porphyrins and then
uses substitution reactions, is optimal if larger quantities of
material are desired. The mono- and dicyano compounds
described herein are interesting building blocks amenable to
further chemistries as for example they can be easily hydrolyzed
to the corresponding carboxylic acids (e.g., 4). The push-pull
dialkylamino-cyano-porphyrins could be used in field-effect
transistors or may have interesting second order nonlinear
harmonic generation properties which are currently under
investigation. Progress along these lines of research will be
reported in due course.
spectral range up to over 700 nm is covered. In dichloromethane
the self-assemblies completely dissociate and only a monomeric-
type spectrum is obtained (green trace in Figure 1A). This
implies that the CN-Zn electrostatic interaction at low
concentrations does not survive a solvent of even medium
polarity. In more concentrated CDCl3 solutions aggregation
shifts are encountered in the 1H NMR spectra (data not
shown).
It is remarkable that the combination of π-stacking with this
for the first time visualized electrostatic interaction wins over
a putative direct coordination of the zinc atom by the nitrogen
lone pair. Self-assembly into stacks is thermodynamically driven,
and the packing of stacks involves multiple hydrophobic
interactions.
From dioxane a different crystal packing is encountered which
includes two solvent molecules/unit cell. While one spectator
dioxane molecule has just a space filling role, a second one is
ligating the zinc atoms in two stacked porphyrins with an
interplanar distance of 7.115 Å (Figure 6). The Zn-O ligations
are 2.596 Å long and much stronger than the previously
described Zn-NtC interactions. Due to the intercalated diox-
Acknowledgment. This work was partially supported by the
DFG Center for Functional Nanostructures at the University of
Karlsruhe through Project C3.5 (T.S.B. and H.K.).
Supporting Information Available: Additional spectro-
scopic data (24 figures). This material is available free of charge
via the Internet at http://pubs.acs.org.

Programmed Metalloporphyrins for Self-Assembly
J. Phys. Chem. B, Vol. 112, No. 17, 2008 5521
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