The first example of cofacial bis(dipyrrins)

Article abstract of DOI:10.1039/c5nj03347k

Two series of cofacial bis(dipyrrins) were prepared and their photophysical properties as well as their bimolecular fluorescence quenching with C₆₀ were investigated. DFT and TDDFT computations were also performed as a modeling tool to address the nature of the fluorescence state and the possible inter-chromophore interactions. Clearly, there is no evidence for such interactions and the bimolecular quenching of fluorescence, in comparison with mono-dipyrrins, indicates that C₆₀-bis(dipyrrin) contacts occur from the outside of the "mouth" of the cofacial structure.

Full text of DOI:10.1039/c5nj03347k

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The first example of cofacial bis(dipyrrins)†
Jude Deschamps,a Yi Chang,b Adam Langlois,a Nicolas Desbois,b Claude P. Gros*b
Cite this:DOI: 10.1039/c5nj03347k
and Pierre D. Harvey*a
Two series of cofacial bis(dipyrrins) were prepared and their photophysical properties as well as their
bimolecular fluorescence quenching with C60 were investigated. DFT and TDDFT computations were
also performed as a modeling tool to address the nature of the fluorescence state and the possible
inter-chromophore interactions. Clearly, there is no evidence for such interactions and the bimolecular
quenching of fluorescence, in comparison with mono-dipyrrins, indicates that C60-bis(dipyrrin) contacts
occur from the outside of the ‘‘mouth’’ of the cofacial structure.
Received (in Montpellier, France)
26th November 2015,
Accepted 21st March 2016
DOI: 10.1039/c5nj03347k
www.rsc.org/njc
varied as 3.80 (DPB), 4.32 (DPX), 4.94 (DPA), 5.53 (DPO), and 6.32 Å
(DPS),14 and there is an obvious gradual progression of the
Introduction
The generally strongly fluorescent dipyrrins are BODIPY-type photophysical parameters (fluorescence lifetimes and quantum
dyes structurally related to porphyrins (Fig. 1).1–4 They have been yields, non-radiative and radiative rate constants,15 and the rate
investigated due to their ability to form metal complexes,5–7 and of singlet and triplet energy transfer)16 of the cofacially placed
more recently the construction of bis(dipyrrin)s was performed free-base porphyrins with the distance.
and reviewed.8 However, investigation of bis(dipyrrins) placed in
However, for cofacial bis(dipyrrins) systems using the classic
a cofacial fashion was never reported. For instance, upon placing boron-center, the tetrahedral geometry of this atom intuitively
two porphyrin units in a cofacial fashion, it is well known that induces obvious steric hindrance preventing strong p-interactions
inter-ring interactions, as those shown in Fig. 1, lead to expected between the pyrrole groups. We now report the synthesis and
modifications of the optical properties,9,10 and more recently photophysical characterization in order to address this point, and
provided valuable models for the special pairs.11–13
indeed the two isolated dipyrrin units almost act as if they are
In the cofacial bis(porphyrin) compounds listed in Fig. 1, the independent (Fig. 2).
Cmeso–Cmeso distance (meso carbons directly linked to the spacer)
Experimental
Experimental section
Materials. The handling of all air/water sensitive materials
was carried out using standard techniques. DCM was distilled
from CaH2. Unless specified otherwise all other solvents were
used as commercially supplied. Where mixtures of solvents
were used, ratios are reported by volume. Column chromato-
graphy was carried out on silica gel 60 at normal pressure. For
photophysical measurements, all chemicals were of analytical
reagent quality and were used as received. THF was distillated
over Na/benzophenone and 2-MeTHF filtrated over alumina,
Fig. 1 Structures of the cofacial bisporphyrins recently investigated and then distillated under an inert atmosphere using CaH2
(M = 2H, transition metal).
as a drying agent. Dry 2-MeTHF was degassed in a sonic bath
by repeated cycles of vacuum and purging with argon, and
a
´
´
´
Departement de Chimie, Universite de Sherbrooke, Sherbrooke, Quebec, J1K 2R1,
then stored inside a glove box under an almost oxygen-free
argon atmosphere (O2 levels less than 10 ppm). Anhydrous
1,2-dichlorobenzene (Sigma-Aldrich) was used without any
further drying, but was degassed and stored in the same manner
as the 2-MeTHF.
Canada. E-mail: Pierre.Harvey@USherbrooke.ca
b
´
´
ICMUB (UMR CNRS 6302), Universite de Bourgogne Franche-Comte, 21 000 Dijon,
France. E-mail: Claude.Gros@u-bourgogne.fr
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5nj03347k
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BOD-OMe (3). To a stirred solution containing dipyrrin-OMe
(1) (0.1 g, 0.19 mmol) in CH2Cl2 (15 mL), was added triethyl-
amine (100 mL) and the mixture was stirred for 5 min at room
temperature. BF3 (48%, 150 mL) was added and the resulting
solution was stirred for 3 h at rt. The reaction mixture was
washed with saturated NaHCO3 aqueous solution, dried over
MgSO4, and concentrated to a small volume that was loaded
directly on a silica gel column, using CH2Cl2 as the solvent.
The purple fraction was collected to yield the pure compound.
BOD-OMe (3) (purple solid; 85 mg, 74%). 1H NMR (CDCl3,
300 MHz) d 7.78 (dd, 2H), 7.40–7.32 (m, 2H), 7.01 (m, 2H),
6.95–6.93 (d, 1H), 6.91 (d, 1H), 6.70 (d, 2H), 6.65 (d, 2H), 3.79
(s, 6H). 13C NMR (CDCl3, 75 MHz) d 157.7, 135.2, 131.9, 131.1,
127.8, 126.4, 123.6, 121.5, 120.3, 111.0, 55.8. ESI-HRMS:
m/z calcd for C29H18B1F7N2O2Na+: 593.12469; found: 593.12286.
UV-vis lmax (CH2Cl2)/nm 565 (e/dm3 molꢀ1 cmꢀ1 11 000), 344
(2400). Anal. calcd for C29H18BF7N2O2: C, 61.08; H, 3.18; N,
4.91. Found: C, 61.69; H, 3.60; N, 5.13.
BOD-O (4). The experimental procedure was adapted from
the methodology described in the literature for the preparation
of the phenyl analog. To a stirred solution of BOD-OMe (3)
(0.4 g, 0.72 mmol) in CH2Cl2 (12 mL), BBr3 (0.68 mL, 7.2 mmol)
was added at 0 1C under an argon atmosphere. The reaction
mixture was stirred and allowed to warm up to room tempera-
ture and left for 3 days before quenching with methanol. The
mixture was evaporated and dissolved again with methanol. To
the resulting mixture, conc. HCl (37%) was added and the
reaction mixture was heated under reflux for 3 h. After cooling,
the mixture was neutralized with a saturated NaHCO3 aqueous
solution and extracted with ethylacetate. The organic layer was
dried over MgSO4, evaporated to dryness and purified by column
chromatography using a 1 : 1 mixture of CH2Cl2/heptane as
the solvent. The expected compound (2) was isolated as a dark
purple solid (0.12 g, 32%).
Fig. 2 Structures of the investigated mono- and bis(dipyrrins).
The product above was directly used for the next step reaction.
To a stirred solution containing (2) (30 mg, 0.06 mmol) in 5 mL of
Synthesis
Dipyrrin-OMe (1). To
a
stirred solution containing CHCl3 was added B(OMe)3 (100 mL). The reaction mixture was
2-(2-methoxyphenyl)pyrrole (0.82 g, 4.74 mmol, 1.0 eq.) and heated under reflux for 4 h. After cooling, the mixture was
pentafluorobenzaldehyde (0.42 g, 2.14 mmol, 0.5 eq.) in CH2Cl2 evaporated to dryness and the residue was purified by column
(40 mL), trifluoroacetic acid (60 mL) was added under an argon chromatography on silica gel using hexane–CH2Cl2 (1 : 1) as the
atmosphere and the mixture was stirred for 2 h at room solvent, and recrystallized from CH2Cl2/MeOH to give BOD-O (4)
1
temperature. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, (25 mg, 83%). H NMR (CDCl3, 300 MHz) d 7.80 (dd, 2H), 7.38
0.55 g, 2.42 mmol) was added and the resulting solution was (m, 2H), 7.08 (m, 2H), 6.99 (dd, 2H), 6.94 (s, 4H). 13C NMR
stirred overnight at rt. The reaction mixture was washed with (CDCl3, 75 MHz) d 154.4, 151.7, 134.6, 132.9, 127.8, 126.1,
saturated NaHCO3 aqueous solution, extracted with CH2Cl2, dried 120.7, 119.9, 119.3, 117.1. MALDI-TOF MS: m/z calcd for
over MgSO4, and concentrated to a small volume that was loaded
directly on a short alumina pad. The filtrate was evaporated to
C
C
27H12BF5N2O2: 502.0912; found: 502.791. Anal. calcd for
27H12BF5N2O2: C, 64.57; H, 2.41; N, 5.58. Found: C, 64.32; H,
dryness, taken in the minimum amount of CH2Cl2 and purified 3.34; N, 5.24.
over a silica gel column, using a 8 : 2 : 0.01 mixture of CH2Cl2/
4,6-Bis((Z)-(5-(2-methoxyphenyl)-1H-pyrrol-2-yl)(5-(2-methoxy-
EtOAc/NEt3 as a solvent (golden brown solid; 0.96 g, 77%). phenyl)-2H-pyrrol-2-ylidene)methyl) dibenzo[b,d]furan (5a).
1H NMR (CDCl3, 300 MHz) d 8.03 (dd, 2H), 7.34 (ddd, 2H), 7.02 Dibenzo[b,d]furan-4,6-dicarbaldehyde (173 mg, 0.77 mmol) and
(m, 4H), 6.94 (d, 2H), 6.47 (d, 2H), 3.87 (s, 6H). 13C NMR (CDCl3, 2-(2-methoxyphenyl)-1H-pyrrole (500 mg, 2.9 mmol) were dissolved
75 MHz) d 157.4, 153.9, 140.3, 130.3, 129.1, 126.5, 121.9, 120.9, 119.6, in dry CH2Cl2 (80 mL) under an argon atmosphere. Trifluoroacetic
111.6, 55.9, 26.9. MALDI-TOF MS: m/z calcd for C29H19F5N2O2: acid (TFA, 75 mL) was added, and the solution was stirred for 3 h at
522.1367; found: 522.883. Anal. calcd for C29H19F5N2O2: C, 66.67; room temperature in the dark (until TLC indicated complete
H, 3.67; N, 5.36. Found: C, 66.85; H, 4.38; N, 5.03.
consumption of the aldehyde). 2,3-Dichloro-5,6-dicyanoquinone
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(DDQ, 300 mg, 1.3 mmol) was added, and the mixture was
C
58H42B2F4N4O5: C, 71.63; H, 4.35; N, 5.76. Found: C, 71.25; H,
stirred for an additional 4 h. The reaction mixture was washed 4.16; N, 5.94.
twice with water and brine, dried over MgSO4, and concentrated
DPA-OMe (8). Compound 6a (90 mg, 0.10 mmol) was dis-
at reduced pressure. The crude product was firstly purified solved in dry CH2Cl2 (20 mL) under an argon atmosphere. The
by alumina and further purification by silica-gel column reaction mixture was treated with triethylamine (250 mL) for
chromatography (dichloromethane/ethyl acetate: 4/1) was used 5 min. Boron trifluoride etherate (200 mL) was added and the
to get the red solid. Compound 5a (red solid; 150 mg, 22% mixture was stirred for another 3 h. The reaction mixture was
yield). 1H NMR (CDCl3, 300 MHz) d 13.47 (s, 2H), 8.11 (dd, 2H), washed with water and brine, dried over Na2SO4, and concen-
7.86 (dd, 4H), 7.58 (dd, 2H), 7.45 (t, 2H), 7.25–7.20 (m, 4H), trated under reduced pressure. The crude product was purified by
6.88 (t, 8H), 6.75 (d, 4H), 6.44 (d, 4H), 3.72 (s, 12H). silica-gel column chromatography (ethyl acetate/dichloromethane:
13C NMR (CDCl3, 75 MHz) d 157.3, 154.6, 152.2, 140.8, 1/5) to get the product. Compound (8) (red brown solid; 40 mg,
1
132.8, 130.5, 129.2, 129.0, 128.2, 124.2, 122.7, 122.2, 120.7, 40%). H NMR (CDCl3, 300 MHz) d 8.89 (s, 1H), 8.55 (s, 1H),
118.4, 111.5, 55.8. HRMS (MALDI-TOF): m/z calcd for 8.12 (d, 2H), 7.65 (d, 4H), 7.59–7.46 (m, 4H), 7.24–7.16 (m, 4H),
C
58H45N4O5: 877.3384; found: 877.3393 [M + H]+. Anal. calcd 6.74 (t, 8H), 6.42 (d, 4H), 6.28 (d, 4H), 3.59 (s, 12H). 13C NMR
for C58H44N4O5: C, 79.43; H, 5.06; N, 6.39. Found: C, 79.44; H, (CDCl3, 75 MHz) d 157.5, 155.5, 140.8, 136.4, 132.7, 132.7,
5.08; N, 6.21. 132.6, 131.5, 131.0, 130.3, 130.0, 129.9, 129.0, 127.2, 124.8,
1,8-Bis((Z)-(5-(2-methoxyphenyl)-1H-pyrrol-2-yl)(5-(2-methoxy- 124.5, 122.0, 122.0, 120.3, 110.8, 55.7. HRMS (ESI): m/z calcd
phenyl)-2H-pyrrol-2-ylidene)methyl) anthracene (6a). Anthracene- for C60H44B2F4N4O4Na+: 1005.33954; found: 1005.33440. Anal.
1,8-dicarbaldehyde (200 mg, 0.85 mmol) and 2-(2-methoxyphenyl)- calcd for C60H44B2F4N4O4: C, 73.34; H, 4.51; N, 5.70. Found: C,
1H-pyrrole (600 mg, 3.5 mmol) were dissolved in dry CH2Cl2 73.84; H, 4.80; N, 5.66.
(100 mL) under an argon atmosphere. Trifluoroacetic acid
DPO-O (9). To a stirred solution containing 5a (50 mg,
(TFA, 90 mL) was added, and the solution was stirred for 3 h 0.057 mmol) in dry CH2Cl2 (5 mL) was added BBr3 (0.2 mL) at
at room temperature in the dark (until TLC indicated complete ꢀ50 1C under an Ar atmosphere. The reaction mixture was
consumption of the aldehyde). 2,3-Dichloro-5,6-dicyanoquinone stirred for 3 days allowed to warm up to room temperature and
(DDQ, 350 mg, 1.5 mmol) was added, and the mixture was quenched with methanol. The mixture was evaporated and
stirred for an additional 12 h. The reaction mixture was washed dissolved again with methanol. To the obtained mixture conc.
twice with water and brine, dried over MgSO4, and concentrated HCl (0.5 mL) was added and heated under reflux for 3 h. The
at reduced pressure. The crude product was firstly purified mixture was cooled and neutralized with saturated NaHCO3
by alumina and further purification by silica-gel column aqueous solution and extracted with ethyl acetate. The organic
chromatography (dichloromethane/ethyl acetate: 3/1) was used layer was dried over MgSO4, evaporated to dryness. The crude
to get the red solid. Compound (6a) (red solid; 200 mg, 27%). product above was dissolved in dry CHCl3 (10 mL) under an
1H NMR (CDCl3, 300 MHz) d 13.21 (s, 2H), 8.47 (s, 1H), 8.29 argon atmosphere. B(OMe)3 (100 mL) was added and the mix-
(s, 1H), 7.84 (dd, 2H), 7.57–7.48 (m, 4H), 7.24 (d, 2H), 7.00–6.87 ture was heated to reflux for 3 h. After cooling, the mixture was
(m, 6H), 6.55 (dd, 8H), 6.36 (d, 4H), 5.95 (d, 4H), 3.38 (s, 12H). evaporated to dryness and the residue was purified by column
13C NMR (CDCl3, 75 MHz) d 157.2, 151.8, 141.7, 136.7, 136.1, chromatography on silica gel using CH2Cl2 as the solvent.
131.8, 131.4, 128.9, 128.7, 128.1, 126.4, 125.9, 124.6, 122.7, Compound DPO-O (9) (green solid; 15 mg, 31%). 1H NMR
120.7, 118.2, 111.5, 55.7. MALDI-TOF MS: m/z calcd for (d6-acetone, 300 MHz) d 8.32 (dd, 2H), 7.87–7.35 (m, 10H),
C
60H47N4O4: 887.3519; found: 887.289 [M + H]+. Anal. calcd 7.25–6.80 (m, 14H), 6.72–6.53 (m, 4H). 13C NMR (150 MHz,
for C60H46N4O4: C, 81.24; H, 5.23; N, 6.32. Found: C, 80.66; H, CDCl3) d 154.3, 153.9, 153.8, 150.6, 150.2, 134.6, 134.6, 132.2,
5.32; N, 6.20. 132.1, 132.0, 130.8, 129.7, 129.3, 126.0, 125.6, 125.0, 123.5, 122.7,
DPO-OMe (7). Compound 5a (100 mg, 0.11 mmol) was 122.5, 120.3, 120.1, 119.8, 119.7, 119.4, 118.8, 116.1, 116.0. HRMS
dissolved in dry CH2Cl2 (10 mL) under an argon atmosphere. (MALDI-TOF): m/z calcd for C54H30B2N4O5: 836.2402; found:
The reaction mixture was treated with triethylamine (100 mL) 836.2464. HRMS (ESI): m/z calcd for C54H30B2N4O5Na+: 859.23114,
for 5 min. Boron trifluoride etherate 48% (200 mL) was added found: 859.22896; calcd for C54H30B2N4O5H+: 837.24920, found:
and the mixture was stirred for another 3 h. The reaction 837.24920. Anal. calcd for C54H30B2N4O5: C, 77.54; H, 3.62; N,
mixture was washed with water and brine, dried over Na2SO4, 6.70. Found: C, 77.15; H, 4.16; N, 6.41.
and concentrated under reduced pressure. The crude pro-
DPA-O (10). The process for the synthesis of compound (10)
duct was purified by silica-gel column chromatography (ethyl DPA-O is the same as for compound (9) DPO-O. Compound (10)
acetate/dichloromethane: 1/7) to get the product. Compound (green solid; 10 mg, 10%). 1H NMR (300 MHz, CDCl3) d 8.92 (s,
1
DPO-OMe (7) (red brown solid; 70 mg, 60%). H NMR (CDCl3, 1H), 8.55 (d, 1H), 8.13 (dd, 2H), 7.69–7.41 (m, 8H), 7.31–7.21 (m,
300 MHz) d 8.21 (d, 2H), 7.68 (m, 6H), 7.55 (m, 2H), 7.26 (m, 2H), 7.15–7.05 (m, 2H), 6.49–6.27 (m, 16H). 13C NMR (126 MHz,
4H), 6.92–6.82 (m, 8H), 6.70 (d, 4H), 6.45 (d, 4H), 3.66 (s, 12H). CDCl3) d 153.8, 153.6, 153.3, 153.0, 149.7, 149.3, 149.2, 148.8,
13C NMR (CDCl3, 75 MHz) d 157.5, 155.9, 153.8, 136.7, 135.5, 134.5, 134.5, 134.4, 134.3, 134.2, 131.0, 130.9, 130.8, 130.7,
132.1, 132.0, 130.5, 130.4, 129.2, 124.6, 123.0, 122.3, 122.3, 130.7, 130.6, 130.5, 129.9, 129.8, 129.7, 129.4, 129.2, 128.7,
122.0, 120.1, 119.4, 110.9, 55.7. HRMS (ESI): m/z calcd for 128.4, 128.2, 127.7, 127.5, 127.2, 126.4, 126.1, 124.8, 124.4,
C58H42B2F4N4O5Na+: 995.31563; found: 995.31876. Anal. calcd for 124.3, 124.2, 124.1, 124.0, 123.6, 122.7, 119.5, 119.3, 119.2,
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118.9, 118.8, 118.6, 118.5, 118.3, 118.2, 115.4, 114.8, 114.2. Relative quantum efficiencies were obtained by comparing
HRMS (MALDI-TOF): m/z calcd for C56H32B2N4O4: 846.2610; the areas under the corrected emission spectra of the sample
found: 846.2579.
relative to a known standard, and the following equation was
used to calculate the quantum yield:
DFT calculations
FF sample = (FF standard) ꢁ (Isample/Istandard) ꢁ (Fstandard/Fsample
ꢁ (Zsample2/Zstandard2),
)
All density functional theory (DFT) and time dependent density
functional theory (TD-DFT) calculations were performed using
Gaussian 0917 at the Universite de Sherbrooke with the
´
where FF(standard) is the reported quantum yield of the standard,
I is the integrated emission spectrum, F is the absorptance
(F = 1–10ꢀA, where A is the absorbance) at the excitation wavelength,
and Z is the refractive index of the solvents used. Rhodamine 6G
(FF = 0.94 in methanol)35 and cresyl violet (FF = 0.54 in methanol)36
were used as standards. In all FF determinations, correction for the
solvent refractive index (Z) was applied (in 2-MeTHF, Z = 1.406;
in methanol, Z = 1.328).37
´
´ ´
Mammouth supercomputer supported by Le Reseau Quebecois
De Calculs Hautes Performances. The DFT geometry optimi-
sations as well as TD-DFT calculations18–27 were carried out
using the B3LYP method. A 6-31g* basis set was applied to all
atoms.28–33 All calculations were carried out in a THF solvent
field. The calculated absorption spectra were obtained from
GaussSum 2.1.34
Instrumentation
Results and discussion
Synthesis
UV-Vis spectra were recorded in solutions using a Varian Cary
50 spectrophotometer (1 cm path length quartz cell). NMR
spectra were recorded at room temperature using Bruker Avance
300 and Bruker Avance II 600 instruments with the chemical
shifts reported as d in ppm. Accurate mass measurements (HRMS)
were carried out using a Bruker microTOF-QTM ESI-TOF mass
spectrometer. MALDI-TOF mass spectrometry was carried out
using a Bruker Ultraflex II MALDI- TOF mass spectrometer and
dithranol as the matrix.
The acid-catalyzed condensation of 2-(2-methoxyphenyl)-pyrrole
and pentafluorobenzaldehyde in methylene chloride and sub-
sequent oxidation with DDQ afforded the dipyrrin precursor 1 in
77% yield. Deprotection of the phenol moieties with BBr3
afforded the N2O2 dipyrrin 2 in 32% yield. The boron complex
4 was prepared according to a procedure recently reported in the
literature upon treatment with B(OMe)3 (Scheme 1).38
The synthesis of these bis(dipyrrins) 5a and 6a is outlined
in Scheme 2. They were obtained in one step in 22–27%
yield starting from the dialdehyde linker (e.g. antracene dialde-
hyde or dibenzofuran dialdehyde) and 2-(2-methoxyphenyl)-
pyrrole, where the latter reagent was prepared in only one
step from commercially available pyrrole and bromoanisole.
The 1H NMR spectra of the C2 symmetric derivative exhibit
Photophysical studies
Absorption spectra were measured on a Varian Cary 300 Bio
UV-vis spectrometer at 298 K and on a Hewlett-Packard 8452A
diode array spectrometer with a 0.1 second integration time at
77 K. Steady state fluorescence and excitation spectra were
acquired on either a Fluorolog SPEX 1680 equipped with double
monochromators for both excitation and emission arms or on
an Edinburgh Instruments FLS980 phosphorimeter equipped
with single monochromators. All fluorescence spectra were
corrected for instrument response. Fluorescence lifetime
measurements were made using a GL3300 Nitrogen laser
equipped with a high resolution (full width at half-maximum
(FWHM) = 1.4 ns) GL302 dye laser from PTI or on the FLS908
phosphorimeter using a 378 nm picosecond pulsed diode laser
(FWHM = 78 ps) as an excitation source. Data collection on
the FLS980 system is done by time correlated single photon
counting (TCSPC).
Quantum yield measurements
Measurements were performed in distillated 2-methyl-tetra-
hydrofuran (2-MeTHF), and spectrophotometric grade methanol
(Aldrich) was used for reference. Quartz cuvettes of 3 mL with a
path length of 1 cm equipped with a septum were used, and
all solutions were Ar-degassed prior to measurements. Three
different measurements (i.e., different solutions) were performed
for each quantum yield. The sample concentrations were
chosen to obtain an absorbance of about 0.05. The fluorescence
quantum yield (FF) measurements were performed with the
slit width of 0.5–1.5 nm for both excitation and emission.
Scheme 1
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the monodipyrrin derivative as the reference compound. The
UV-visible data are reported in the experimental section.
Fig. 4 The absorption (black), excitation (blue) and fluorescence (red)
spectra of BOD-O (4) (top), DPA-O (10) (middle), and DPO-O (9) (bottom)
in 2MeTHF at 298 and 77 K. The excitation and monitoring wavelengths are
placed in the frames.
Scheme 2
the characteristic pattern of two meso-substituted dipyrrin units
cofacially linked by an aromatic bridge (see Experimental section).
Peak assignments were made on the basis of chemical shifts,
multiplicity, integrations, and spectral intercomparisons with
Table 1 Spectral absorption and emission data of the bis(dipyrrins)
Absorption (nm)
[e (ꢁ103 Mꢀ1 cmꢀ1)]
Emission (nm)
Compound
298 K
77 K
298 K
77 K
BOD-OMe (3)
284 [21.2]
343 [11.9]
357 [11.3]
570 [47.3]
348
366
555(sh)
588
622
665(sh)
543
621
665(sh)
DPO-OMe (7)
DPA-OMe (8)
280 [58.7]
370 [24.0]
536 [88.9]
398
569
611
668(sh)
612
667
253 [74.5]
280 [40.2]
359 [15.2]
534 [62.9]
376
540
557
584
611
665(sh)
610
664
BOD-O (4)
DPO-O (9)
314 [35.6]
447 [57.5]
607(sh) [16.1]
648 [48.2]
328
448
600
658
679
740(sh)
672
736
303 [62.8]
440 [19.1]
593 [42.5]
625 [71.5]
306
340
442
595
630
667
727(sh)
658
723
DPA-O (10)
254 [99.2]
306 [68.4]
467 [14.0]
593 [52.6]
622 [55.4]
310
482
592
626
672
728(sh)
657
718
Fig. 3 The absorption (black), excitation (blue) and fluorescence (red)
spectra of BOD-OMe (3) (top), DPA-OMe (8) (middle), and DPO-OMe (7)
(bottom) in 2MeTHF at 298 K and 77 K. The excitation and monitoring
wavelengths are placed in the frames.
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Steady state properties
decrease by 2 to 3fold on going from the mono(dipyrrin) to the
bis(dipyrrins) species. This effect is associated with lower FF
values in the bis(dipyrrins) species (excluding the uncertainty
for the comparison between the data for BOD-OMe (3) and
DPA-OMe (8)). The knr value for DPO-OMe (7) appears to be
unexplainably larger (associated with a lower FF value).
Noteworthy, the fact that tF is independent of whether the
DPA- and DPO-spacers are used indicates that the two dipyrrin
units are not interacting, a behaviour that can easily be
addressed as well as demonstrated for the PACMAN systems
(see Fig. 1).15 This behaviour is corroborated using the
optimized geometry of the cofacial compounds below. More-
over, the tF value (9.45 ns) for BOD-O (4) is similar to that
The absorption, excitation and fluorescence spectra of BOD-OMe
(3), DPO-OMe (7), DPA-OMe (8), BOD-O (4), DPO-O (9), and DPA-O
(10) in 2MeTHF are presented in Fig. 3 and 4, and the data are
given in Table 1. The assignment for fluorescence is based on the
close proximity of the fluorescence and the lowest energy absorp-
tion bands, and their lifetimes (below). The excitation spectra
superpose well the absorption indicating that the fluorescence
arises from the absorbing species (i.e. no impurities, or other
emitting species). The band shapes of both the absorption and
fluorescence spectra are reminiscent of those for BODIPY.39–41
The fluorescence lifetimes, tF, quantum yields, FF, the radia-
tive kF (FF/tF) and non-radiative, knr ((1 ꢀ FF)/tF), at 298 K are
compared in Table 2. Two trends are obvious. The rigidification
of the skeleton, for example, on going from BOD-OMe (3) to
BOD-O (4), increases tF by nearly 2-fold. This effect is consistent
with the decrease in non-radiative processes associated with the
flexibility of the skeleton by removing low-frequency vibration
enhancing relaxation (i.e. internal conversion). The kF values
Table 3 Selected structural parameters
Table 2 Photophysical parameters (tF, FF, kF and knr) at 298 K (in 2MeTHF)
Compound
lex
FF
kF (106 sꢀ1
)
knr (106 sꢀ1
)
Bꢃ ꢃ ꢃB Cmesoꢃ ꢃ ꢃCmeso Xꢃ ꢃ ꢃX Shortest
(Å) (Å) (Å) dist. (Å)
y1 = y2 (1) y3 (1)
26.7
BOD-OMe (3)
DPA-OMe (8)
DPO-OMe (7)
BOD-O (4)
DPA-O (10)
DPO-O (9)
500
500
500
550
550
550
0.55
0.48
0.15
0.49
0.20
0.18
91
50
50
52
15
12
75
54
280
54
62
56
DPO-OMe (7) 6.96 5.62
DPA-OMe (8) 5.31 5.17
5.12 5.27 (meso-b) 62.3
3.06 5.05 (meso-a) 70.6
6.49 4.98 (meso-b) 59.6
4.45 4.49 (meso-a) 68.4
5.63
28.9
16.9
DPO-O (9)
7.43 5.65
DPA-O (10) 6.12 5.26
298 K
Compound tF (ns)
77 K
tF (ns)
Monitoring
wavelength (nm)
w2
w2
BOD-OMe (3) 6.04 ꢂ 0.05 1.085 5.94 ꢂ 0.05 1.107
DPA-OMe (8) 9.63 ꢂ 0.05 1.089 9.56 ꢂ 0.06 1.046
DPO-OMe (7) 3.03 ꢂ 0.05 1.004 9.68 ꢂ 0.10 1.044
625
610
610
680
665
665
BOD-O (4)
DPA-O (10)
DPO-O (9)
9.45 ꢂ 0.05 1.118 10.47 ꢂ 0.05 1.074
12.9 ꢂ 0.05 1.050 17.6 ꢂ 0.05 1.033
14.7 ꢂ 0.10 1.024 18.2 ꢂ 0.05 1.027
Fig. 6 Representations of the frontier MOs for BOD-OMe (3) (top) and
DPO-OMe (7) (bottom) as representative examples (see ESI,† for the other
compounds). The energies are in a.u.
Fig. 5 Optimized geometries (DFT, B3LYP) for DPA-OMe (8) (top) and
DPA-O (10) (bottom).
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reported for similar (and only example) mono-o-chelated bis- from the non-nil dihedral angle y3. The fact that y3(DPA) o y3(DPO)
(dipyrrin) (9.6 ns).42
is predictable due to the intrinsic geometry of the spacer.
Concurrently, both y1 and y2 are similar for both spacers
(DPA B 701; DPO B 611). The slight difference is related to the
b-hydrogen atoms with the spacer. The computed Cmesoꢃ ꢃ ꢃCmeso
separations (B5.2 Å for DPA and B5.6 Å for DPO) compare
favourably to those experimentally measured by X-ray crystallo-
graphy (respectively 4.94 Å and 5.53 Å in the PACMAN series;
Fig. 1).14 Based on the shortest calculated atom-atom separations
between the two chromophores, the computed geometry does not
result from any contact as the evaluated separations exceed the
sum of van der Waals. The closest distance is 3.06 Å for FꢃꢃꢃF in
DPA-OMe (8) (the van der Waals radius for F is 1.35 Å).43 Other
distances of 4.13 Å (F–N) in DPA-O (10) and 5.39 Å (F–Ca) in DPO-
OMe (7) are also noted but clearly these values are larger than the
sum of the van der Waals radii. It is concluded that the geometry
is not limited by inter-atomic contacts inside the ‘‘sandwich’’ area
of the cofacial dimers, but rather by steric limitations between the
b-protons and the spacer. There is no computational evidence for
through space inter-dipyrrin interactions.
Optimized geometry
In the absence of X-ray structures, the geometry of the cofacial
dimers has been addressed using DFT computations. Representative
examples of optimized geometries are provided in Fig. 5 (see ESI,†
for DPO-OMe (7) and DPO-O (9)). The geometry of bis(dipyrrin)s is
best described as slipped dimers as defined by the dihedral angles y1
and y2 made by the average planes of the dipyrrin and spacer units
(Table 3). These dimers further exhibit an ‘‘open mouth’’ geometry
Table 4 Percent distribution of the molecular orbitals over selected
molecular fragments of DPO-OMe (7)
Fragment
Hꢀ4
Hꢀ3
2.1
48.9
49.0
Hꢀ2
5.5
47.3
47.2
Hꢀ1
3.6
47.9
48.5
HOMO
DPO
Dipyrrin 1
Dipyrrin 2
2.9
48.6
48.4
2.1
49.2
48.7
Fragment
LUMO
L+1
L+2
L+3
L+4
Excited state description
DPO
Dipyrrin 1
Dipyrrin 2
10.5
44.8
44.7
8.7
45.6
45.7
89.7
5.2
5.2
5.2
47.4
47.4
2.6
48.6
48.7
The nature of the excited states was addressed by DFT and TDDFT.
In all cases the atomic contributions are p-orbitals arising from
Table 5 Computed oscillator strengths (F), positions of the first electronic transitions and major contributions of BOD-OMe (3), DPA-OMe (8),
DPO-OMe (7), BOD-O (4), DPA-O (10) and DPO-O (9)a
Compound
lb (nm)
F
Major contributions (%)
BOD-OMe (3)
1
2
3
533.5 (588)
452.8
421.0
0.6403
0.0826
0.0845
HOMO - LUMO (96)
Hꢀ1 - LUMO (99)
Hꢀ2 - LUMO (95)
DPA-OMe (8)
1
2
3
4
574.3 (569)
572.4
537.7
0.0129
0.0078
0.0180
0.2454
Hꢀ1 - L+1 (20), HOMO - LUMO (74)
Hꢀ1 - LUMO (31), HOMO - L+1 (63)
Hꢀ2 - LUMO (18), Hꢀ1 - L+1 (71), HOMO - LUMO (10)
Hꢀ2 - L+1 (47), Hꢀ1 - LUMO (44)
532.8
DPO-OMe (7)
1
2
3
4
565.0 (584)
565.0
534.9
0.0005
0.0022
0.0213
1.1673
Hꢀ1 - LUMO (43), HOMO - LUMO (11), HOMO - L+1 (37)
Hꢀ1 - LUMO (11), Hꢀ1 - L+1 (37), HOMO - LUMO (43)
Hꢀ1 - LUMO (45), HOMO - L+1 (53)
525.0
Hꢀ1 - L+1 (53), HOMO - LUMO (45)
BOD-O (4)
1
2
3
580.8 (658)
480.9
415.8
0.3993
0.1187
0.0594
HOMO - LUMO (99)
Hꢀ1 - LUMO (98)
Hꢀ2 - LUMO (98)
DPA-O (10)
1
2
3
4
608.8 (630)
605.8
572.3
0.0137
0.0038
0.0057
0.4780
Hꢀ1 - L+1 (20), HOMO - LUMO (79)
Hꢀ1 - LUMO (48), HOMO - L+1 (51)
Hꢀ1 - L+1 (79), HOMO - LUMO (19)
561.6
Hꢀ2 - L+1 (16), Hꢀ1 - LUMO (46), HOMO - L+1 (37)
DPO-O (9)
1
2
3
4
597.5 (626)
597.2
571.3
0.0001
0.0018
0.0066
0.7257
Hꢀ1 - L+1 (38), HOMO - LUMO (62)
Hꢀ1 - LUMO (55), HOMO - L+1 (45)
Hꢀ1 - L+1 (62), HOMO - LUMO (37)
Hꢀ1 - LUMO (45), HOMO - L+1 (54)
559.1
a
b
For the 75 first electronic transitions, see ESI. H = HOMO, L = LUMO. The values in parentheses are those experimentally measured at 77 K
from Table 1.
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various chromophore segments providing evidence for low-energy
p - p* electronic transitions (Fig. 6 and ESI,†). A clear plane of
symmetry between the right and left sides is also observed. An
examination of the HOMO - LUMO transition also suggests the
presence of a minor charge transfer contribution (from the OC6H4
groups to the central dipyrrin unit). Interestingly in all cofacial
chromophores, an equivalent contribution of the p-systems is com-
puted (Table 4). The absence of or very modest atomic contributions
arising from the spacer for the frontier MOs (Hꢀ1, HOMO, LUMO,
L+1) suggests that in all cases p-conjugation is either minimal or
negligible. This conclusion is corroborated by the absence of signi-
ficant (red) shifts of absorption and fluorescence bands (Fig. 3 and 4).
Moreover, the absence of band broadening in the absorption
and fluorescence spectra indicates the absence of MO coupling
between the two dipyrrins. These observations indicate again
that the two chromophores act independently, which is fully
consistent with the conclusion drawn with the comparison of
the photophysical parameters. In brief, the atomic contribu-
tions calculated for both chromophores are due to symmetry,
and not due to coupling or conjugation.
In order to support the current pp* assignment and the absence
of inter-dipyrrin interactions, the positions of the electronic transi-
tions has been calculated (TDDFT; Table 5 and Fig. 7). The
calculated lowest energy electronic transitions (i.e. 0–0 peaks) are
placed in the 533–565 and 580–609 nm range for the BOD-OMe,
DPA-OMe and DPO-OMe, and the BOD-OMe, DPA-O and DPO-O
series, respectively. This red-shift on going from the first to the
second series is consistent with the experimental observations
(Table 1). The comparison between the calculated and experimental
positions is good for the bis(dipyrrin) compounds with discrepan-
cies of only 5–28 nm. Note that the selected data are those measured
at 77 K because the spectra are more resolved allowing a better
evaluation of the positions of the 0–0 components. However, this
comparison is worse for the mono-pyrrin species (55–78 nm differ-
ence) but yet not shocking. By (arbitrarily) assigning a thickness of
500 cmꢀ1 to each calculated transition (in blue, Fig. 7), theoretical
spectra are thus generated (in black). Despite the fact that this
approach does not take into account the contribution of vibrational
progression, the comparison between the generated spectra (Fig. 7)
with the experimental ones (Fig. 3, 298 K, left) is good.
Fig. 7 Graphs reporting the computed oscillator strength (F) as a function
of the calculated positions of the first 75 electronic transitions for BOD-OMe
(3) (top), DPA-OMe (8) (middle) and DPO-OMe (7) (bottom). See the ESI,†
for BOD-O (4), DPA-O (10) and DPO-O (9).
The three first computed lowest energy transitions for the two
mono-pyrrins BOD-OMe (3) and BOD-O (4) exhibit almost of C60, the fluorescence intensity of the dipyrrin species in
pure contributions of the Hꢀ2 - LUMO, Hꢀ1 - LUMO and 1,2-dichlorobenzene undergoes a decrease (see BOD-OMe in
HOMO - LUMO (Table 5). Conversely, the bis(dipyrrin)s exhibit Fig. 9 as a representative example). The Stern–Volmer (left)
mixed contributions. This computational observation is a natural plots report these intensity decreases (F0F/FF = F0/F; i.e. relative
predictable consequence of the symmetry in the atomic contribu- intensity in the absence over in the presence of quencher) with
tions (as illustrated in Table 4), and of the close proximity in MO [C60], but not for the lifetime (t0/t; i.e. relative dipyrrin fluores-
energy between the HOMO and the Hꢀ1, and between the LUMO cence lifetime in the absence over in the presence of a quencher)
and the L+1 (for instance see Fig. 6). Thus, the computed data indicating the presence of static quenching:
Â
Ã
confirm that the nature of the mono- and bis(dipyrrins) fluorescent
states is pp*. A little charge transfer character from the flanking
aromatic OC6H4 groups to the central dipyrrin unit is also computed.
1pyrrinꢄ þ C60
$
1pyrrinꢄ ꢃ ꢃ ꢃ C60
!
½pyrrinꢅþ ꢃ ꢃ ꢃ C60ꢅꢀꢆ
fluorescent
static complex
not emissive
(1)
Quenching analysis
Fluorescence quenching experiments using C60 as an electron
Although it is meant to analyze dynamic quenching
acceptor were carried out on all compounds. Upon the addition (diffusional), for comparison purposes the quenching constant
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Table 6 Stern–Volmer and modified Stern–Volmer analyses for BODIPY
dyads
Knowing that there is not enough space inside the ‘‘mouth’’
of the bis(dipyrrins) based on the optimized geometries, these
static complexes must be formed via outside contacts.
This conclusion is clear when comparing the kQ values
between BOD-OMe (3) and DPA-OMe (8), and between BOD-O
(4) and DPA-O (10) as they are nearly the same. The ‘‘outside’’
interactions are also evidenced by the electronic density map
(Fig. 8) where the electron poor areas will interact more favorably
with the electron rich C60.
It is unclear whether the larger kQ values for the BOD-OMe,
DPA-OMe and DPO-OMe series are larger because the binding
constants between bis(dipyrrin)s and C60 are larger. However,
because of the flexibility of MeOC6H4 groups, these latter aro-
matics can adapt through rotations to favour pp-interactions. This
is not the case for the other series BOD-O, DPA-O and DPO-O. This
possibility does not exclude the possibility that the excited state
driving forces for electron transfers are simply larger due to the
presence of the electron donating OMe groups for the former
series. So, no firm conclusion can be provided at this time on
this trend.
KSV (103 Mꢀ1
)
R2
Stern– Modified kQ
Stern– Modified
Volmer S–Va
Compound
Volmer S–Va (108 Mꢀ1 sꢀ1
)
BOD-OMe (3) 1.37
DPA-OMe (8) 1.53
DPO-OMe (7) 1.26
1.14
1.59
1.42
0.89
1.38
1.01
1.89
1.65
4.69
0.94
1.07
0.69
0.990
0.995
0.992
0.983
0.992
0.991
0.968
0.998
0.983
0.978
0.977
0.955
BOD-O (4)
DPA-O (10)
DPO-O (9)
0.85
1.48
0.90
a
Intercept of the regression curves was forced to 1 (i.e. assuming that f = 1).
extracted from the Stern–Volmer analysis, KSV (from the rela-
tionship (F0/F) = KSVꢃ[C60] + 1) was determined (Table 6). For
static quenching, the relationship is modified and becomes
F0/(F0 ꢀ F) = 1/( fꢃKSVꢃ[C60]) + 1/f, where f (normally extracted
from the intercept) is the fraction of chromophores that are
accessible (so f = 1).44 The KSV constants are given by kqꢃt0 where
kq is the bimolecular quenching rate constant. The kq values
were obtained from the KSV data measured from the modified
Stern–Volmer approach. The extracted values are considered
fast indicating that the process is definitely diffusion controlled
and that these values for the BOD-OMe, DPA-OMe and DPO-OMe
series are larger than those for their corresponding BOD-O,
DPA-O and DPO-O.44
Conclusions
Bis(dipyrrins) were easily prepared using standard procedures
and proved to be useful to demonstrate the absence of inter-
dipyrrin interactions. Indeed, spectroscopy and computer
modeling allow for this evidence. The calculated inter-atomic
distances are systematically larger than the sum of van der Waals
radii and unambiguously demonstrate that, in fact, the rigidity
of the DPO- and DPA-spacers induces this effect. This conclusion
is perfectly in line with the reported absence of triplet–triplet
energy transfers in the cofacial hetero-bis(porphyrin) systems
held by DPA and DPO.14 Indeed, the triplet–triplet energy
transfer is dominated by the Dexter mechanism45 (double electron
exchange) and the absence of significant orbital contacts makes
this process very difficult, not to say inexistent. The bimolecular
quenching of cofacial bis(dipyrrins) with the strong electron
acceptor C60 indicates that diffusion-controlled quenching
occurs from outside contacts between the pairs. Knowing that
one of the cofacial dipyrrin units can act as a shield protecting
Fig. 8 Electronic density map (DFT) for DPA-OMe (8) and DPA-O (10). The
red and blue areas are respectively the electron rich and poor segments of
the molecule. See the ESI,† for the two DPO-bis(dipyrrins).
Fig. 9 Left: Evolution of fluorescence spectra of BOD-OMe (3) in 1,2-dichlorobenzene upon the addition of C60. Middle: Stern–Volmer analysis of
fluorescence quenching of BOD-OMe by C60 in 1,2-dichlorobenzene. Right: Modified Stern–Volmer analysis. See the ESI,† for the five other compounds.
The C60 concentration was increased from 0 to 18.5 equivalents.
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Materials Science, Engineering, Biology and Medicine, ed.
K. M. Kadish, K. M. Smith and R. Guilard, World Scientific
Publishing, Singapore, 2011, pp. 1–179.
one face of the excited other dipyrrin, the similarity in the
bimolecular quenching rate constants between the mono- and
bis(dipyrrins) (except for one unexplained case) suggests that
nonetheless the static complex must be close to both chromo-
phores at the same time. The conclusion drawn from this study
indicates that there is no real advantage to use the cofacial
structure for the design of photonic devices such as photo-
cells, but does not exclude that in the solid state (i.e. bulk
heterojunction-type cell) this situation would be different due
to the formation of aggregates.
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Acknowledgements
This research was supported by the Natural Sciences and
Engineering Research Council of Canada (NSERC), le ‘‘Fonds
´ ´
Quebecois de la Recherche sur la Nature et les Technologies
´
(FQRNT)’’ the ‘‘Centre d’Etudes des Materiaux Optiques et
´
Photoniques de l’Universite de Sherbrooke (CEMOPUS)’’. The
‘‘Centre National de la Recherche Scientifique’’ (ICMUB, UMR
CNRS 6302) is gratefully thanked for financial support.
Yi Chang also gratefully acknowledges the ‘‘Region Bourgogne’’
´
and CNRS for a post-doctoral fellowship. Support was provided
´
by the CNRS, the ‘‘Universite de Bourgogne’’ and the ‘‘Conseil
´
Regional de Bourgogne’’ through the 3MIM integrated project
´
´
(‘‘Marquage de Molecules par les Metaux pour l’Imagerie
´
´ ´
Medicale’’). We are also thankful to the ‘‘Consulat General de
`
´
France a Quebec’’ for a ‘‘Samuel de Champlain’’ grant.
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