Corrole-BODIPY dyads: Synthesis, structure, and electrochemical and photophysical properties

Article abstract of DOI:10.1021/ic502919s

A free-base and its Cu(III) derivative of bichromophoric meso-β linked corrole-BODIPY dyad were synthesized and structurally characterized by single crystal X-ray diffraction (XRD). Both corrole and BODIPY fragments maintained respective ground state elec

Full text of DOI:10.1021/ic502919s

Article
pubs.acs.org/IC
Corrole-BODIPY Dyads: Synthesis, Structure, and Electrochemical
and Photophysical Properties
Biju Basumatary, Adiki Raja Sekhar, R. V. Ramana Reddy, and Jeyaraman Sankar*
Department of Chemistry, Indian Institute of Science Education and Research Bhopal, Indore Bypass Road, Bhopal, India 462066
*
S Supporting Information
ABSTRACT: A free-base and its Cu(III) derivative of
bichromophoric meso-β linked corrole-BODIPY dyad were
synthesized and structurally characterized by single crystal X-
ray diffraction (XRD). Both corrole and BODIPY fragments
maintained respective ground state electronic isolation despite
their connection through a single bond due to a tilted orientation
as observed by XRD. This was further supported by UV−vis and
cyclic voltammetric studies. The Cu(III)-metalated dyad exhibits
temperature-dependent paramagnetic behavior as observed in
1
the variable temperature H NMR due to the presence of a
Cu(II)-corrole-π-cation radical. Importantly, the free-base
exhibits complete fluorescence quenching probably due to
photoinduced electron transfer to a low lying charge separated
state. Interestingly, emission was regained upon addition of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) due to the deprotonation
of corrole. The “turn on” fluorescence behavior and the presence of acidic NH protons were further exploited toward basic anion
sensing utility.
1
. INTRODUCTION
aromaticity, and deformation of the macrocycles from planarity
due to steric hindrance at the core. Thus, the free-base 5,10,15-
Corroles are tetrapyrrolic, 18 π-electron Hu
macrocycles closely related to porphyrin with a direct
pyrrole−pyrrole linkage lacking one methine bridge. The first
synthesis of corrole was carried out by Johnson and Kay in the
late 1960s, motivated because of the similarity of corroles to
that of cobalt chelating corrin in vitamin B . A one-pot
synthesis pioneered by Gross et al. in 1999 followed by
remarkable development in the synthesis of corrole
revolutionized the understanding of this unique tetrapyrrolic
̈
ckel aromatic
tris(pentafluorophenyl)corrole [H (tpfc)] has been identified
3
to be stable under oxidative degradation in light and air due to
the presence of electron withdrawing pentafluorophenyl groups
at the meso-carbon and probably a choice where higher stability
1
15
is desired. Both synthetic and stability constraints limited
1
2
further functionalization of corroles readily. Nevertheless, a few
successful functionalizations of corrole reported in the literature
shows a significant effect on the electronic properties of
2
−6
16
corrole. Functionalization of aryl ring at the meso-position
ligand.
and on the β-pyrrolic position is the most common and
relatively easy way of modification. Therefore, partly inspired
by the lack of literature reports on chromophoric functionaliza-
tion at the meso-carbon of corrole and to understand the
electronic effect of this unique connectivity, we report here the
first successful direct meso-β linked corrole-BODIPY con-
jugates. The structures and electronic and optical properties of
these functionalized bi-chromophoric novel dyads were
investigated, and furthermore their unique fluorescence turn
on behavior was explored toward an anion sensing ability.
Corrole shows a relatively higher fluorescence quantum yield
and molar absorptivity than corresponding porphyrin macro-
7
cycles. Moreover, the noninnocent nature of the corrole
8
ligand in many metallo-complexes is now well documented.
The trianionic nature due to three inner nitrogen protons
imparts many intriguing properties to corroles which are
significantly different from that of porphyrin macrocycles. Its
ability to stabilize relatively higher oxidation states of metal
than those of the corresponding porphyrin complexes
augmented the exploration of many metallocorroles in oxidative
9
10
11
catalysis, group transfer catalysis, and reduction catalysis.
Apart from this, corrole has demonstrated potential applica-
2
. EXPERIMENTAL SECTION
1
2
tions in the field of photochemical sensors, artificial
photosynthesis, and biomedical applications during the
last two decades.
2.1. General Consideration. Pentafluorobenzaldehyde, 2,3-
1
3
14
dichloro-5,6-dicyanobenzoquinone (DDQ), trifluoroacetic acid
TFA), and tetrabutylammonium hexafluorophosphate (TBAPF)
(
Free-base corroles show less stability against light and air
than the porphyrin macrocycles. Lower stability mainly
originates from the electron-rich nature of corrole, reduced
Received: December 9, 2014
©
XXXX American Chemical Society
A
DOI: 10.1021/ic502919s
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
were purchased from Sigma-Aldrich. 4-Bromobenzldehyde, triethyl-
amine (Et N), pyrrole, phosphorus oxychloride (POCl ), and
temperature, the Vilsmeier complex was further stirred for 30 min.
Compound 2 (347 mg, 1 mmol) in 1,2-dichloroethane (80 mL) was
added to the reaction mixture. After the temperature was increased to
70 °C, the reaction mixture was further stirred for 12 h, cooled to
room temperature, and added dropwise into a saturated aqueous
solution of K CO (500 mL) cooled in an ice bath with continuous
3
3
Cu(OAc) ·H O were purchased from Merck and Spectrochem,
2
2
India. Other chemicals were of reagent grade. Hexane, DMF, POCl3,
CH Cl , CHCl , and ethyl acetate were distilled according to standard
2
2
3
procedures prior to use. HPLC-grade solvents were used for
2
3
spectroscopy and electrochemical studies.
2
stirring. After being warmed to room temperature, the reaction
mixture was further stirred for 1 h and extracted with CH Cl . The
.2. Instrumentation. H, C, F, and 11B NMR spectra were
1
13
19
2
2
recorded on Bruker 400, 500, and 700 MHz instruments using CDCl3
as a solvent. Chemical shifts were given in parts per million relative to
organic layers were combined, washed with water, and dried over
anhydrous NaSO , and the solvent was removed under reduced
4
residual CHCl (7.240 ppm). Two-dimensional (2D) NMR experi-
pressure. The crude product was purified by silica gel column
chromatography (100−200 mesh) using ethyl acetate/hexane (20:80,
v/v) as an eluent to give desired product and characterized. Yield =
65% (263 mg, 0.65 mmol), reddish-brown crystalline solid. UV−vis
3
ments were performed on a Bruker 400 or 500 MHz instrument. UV−
vis absorption experiments were performed on an Agilent Cary-100
with 1 cm path length quartz cuvette, and fluorescence emission
spectra were recorded on a Horiba Fluorolog spectrophotometer.
High resolution mass spectroscopy (HRMS) electrospray ionization
−
4
−1
−1
1
(CH Cl ): λ , nm (ε × 10 , M cm ) 365 (2.05), 499 (7.65). H
2
2
max
3
NMR (400 MHz; CDCl ; 298 K): δ 6.72 (d, J
= 4.5 Hz, 1H,
3
H−H
3
(
ESI) mass spectra and atmospheric pressure chemical ionization
pyrrolic), δ 7.10 (d, J
= 4.5 Hz, 1H, pyrrolic), δ 7.25 (s, 1H,
= 8.5, 2H, Ph), δ 7.71 (d, J
H−H
3
3
(
APCI) mass spectra were recorded on a Bruker micro TOF-Q II mass
pyrrolic), δ 7.44 (d, J
= 8.5 Hz,
H−H
H−H
spectrometer. Matrix assisted laser desorption/ionization (MALDI)
were recorded on a Bruker ultra flextreme mass spectrometer. Cyclic
voltammograms were carried out using a three-electrode system
2H, Ph), δ 8.17 (s, 1H, pyrrolic), δ 8.26 (s, 1H, pyrrolic), δ 9.83 (s, 1H,
CHO). 13C NMR (101 MHz; CDCl ; 298 K): δ 121.79, 126.53,
3
1
1
−
28.51, 131.71, 131.85, 131.97, 132.24, 134.26, 134.60, 136.75, 143.19,
+
19
consisting of a Pt disk as working electrode, Ag/Ag reference
47.96, 149.93, 184.64. F NMR (376 MHz; CDCl , 298 K: δ
3
1
1
11
electrode, and a Pt-wire as counter electrode on a CH-Instrument
potentiostat. Tetrabutylammonium hexafluorophosphate (TBAPF) in
dichloromethane was used as supporting electrolyte (0.1 M). X-ray
diffraction measurements were carried out at 300 K (2 and 3) and 105
K (6), on a Bruker APEX II diffractometer equipped with a graphite
monochromator and Mo Kα (λ = 0.71073 Å) radiation. Data
collections were performed using ϕ and ω scans. The structures were
solved using direct methods followed by full matrix least-squares
144.90 (dd, J
= 55.8 Hz, J
= 27.9 Hz, BF2), B NMR (128
F−B
F−B
1
MHz; CDCl ; 298 K: δ 0.14 (t, J
= 27.9 Hz, 1B, BF ). HR-MS
3
B−F
2
+
(APCI): Calculated for C H BBrF N O [(M + H) ]: 375.0113,
16 11 2 2
observed: m/z 375.0128
BODIPY Dipyrromethane (4). BODIPY 3 (100 mg, 0.2 mmol) and
excess of freshly distilled pyrrole (0.5 mL) were added to it in a round-
bottom flask under Argon atmosphere. A catalytic amount of TFA (0.1
equiv) was added to the reaction mixture and stirred vigorously for 5
min. The reaction mixture was diluted with 100 mL of CH Cl .
2
refinements against F (all data HKLF 4 format) using SHELXTL. All
2
2
refinements were carried out using SHELXL 97, PLATON 99, and
WinGX systemVer-1.6414. Disordered solvent molecules were
removed by using SQUEEZE command in PLATON. All computa-
tional investigations were performed with the Gaussian 09 program.
For computational studies, initial geometry for compound 6 was taken
from single crystal data and fully optimized without any symmetry
restriction. The calculations were performed by the density functional
theory (DFT) method with restricted B3LYP (Becke’s three-
parameter hybrid exchange functional and Lee−Yang−Parr correlation
function) level, employing basis sets 6-31G(d) for H, B, C, N, F, and
Br and LANL2DZ for Cu.
Triethylamine (0.2 equiv) was added and stirred for another 5 min.
The solvent and excess pyrrole were evaporated under reduced
pressure, and the crude mixture was purified by silica gel column
chromatography (100−200 mesh) using ethyl acetate/hexane (15:75,
v/v) as an eluent to give the desired product, and characterized. Yield
1
=
2
76% (99 mg, 0.20 mmol), red powder. H NMR (400 MHz; CDCl ;
3
3
98 K): δ 5.35 (s, 1H), δ 5.95 (br s, 2H), δ 6.12 (dd, J
= 5.7 Hz,
= 1.4 Hz, 1H), δ
H−H
4
3
4
JH−H = 2.8 Hz, 2H), δ 6.53 (dd, J
= 3.9 Hz, J
= 1.4 Hz, 2H), δ 6.71 (s, 1H), δ 6.86
= 8.4 Hz, 2H), δ 7.63 (d,
H−H
JH−H = 8.4 Hz, 2H), δ 7.73 (s, 1H), δ 7.89 (s, 1H), δ 7.93 (s, 2H).
H−H
H−H
3
4
6
.64 (dd, J
= 3.4 Hz, J
H−H
H−H
3
3
(
d, J
= 4.0 Hz, 1H), δ 7.38 (d, J
H−H
3
19
F
2
.3. Synthesis and Characterization of Compounds. 4-
1
NMR (376 MHz; CDCl ; 298 K): δ −144.66 (dd, J
= 57.6 Hz,
3
F−B
Bromophenyl BODIPY (2). BODIPY 2 was prepared in a one-pot,
two-step synthetic procedure according to the modified literature
1
11
J
= 28.7 Hz, 2F), B NMR (128 MHz; CDCl ; 298 K: δ 0.14 (t,
F−B
3
1J
= 28.8, 1B). C NMR (101 MHz; CDCl ; 298 K): δ 106.97,
13
17
method. DDQ (227 mg, 1 mmol) in CHCl (10 mL) was added to 1
B−F
3
3
1
1
08.58, 117.78, 118.74, 125.64, 128.25, 128.94, 129.06, 131.24, 131.82,
(
301 mg, 1 mmol) in 50 mL of CHCl , cooled in an ice bath under N
3
2
31.89, 132.50, 134.58, 134.73, 135.84, 144.28, 144.45, 145.50; HR-
atmosphere. The reaction mixture was stirred for 10 min. Triethyl-
+
MS (APCI): Calculated for C H BBrF N [(M − H) ]: 489.0696,
observed: m/z 489.0653
amine (2 mL) and immediately followed by BF ·Et O (2 mL) were
24 17
2
4
3
2
added to the reaction mixture. The reaction mixture was further stirred
for another 3 h at room temperature and washed with 0.2 M NaOH
solution (100 mL) and water (100 mL). The organic layer was
collected and dried over anhydrous Na SO , and the solvent was
Corrole-BODIPY Dyad (6). 3 (100 mg, 0.26 mmol) and 5 (163 mg,
0.52 mmol) were dissolved in MeOH (40 mL). A solution of HCl (aq)
(36%), 1 mL in 20 mL of H
mixture, and stirred for 2 h. The pink color reaction mixture was
extracted with CHCl , and the organic layer was washed twice with
O, dried over anhydrous Na SO , filtered, and diluted to 100 mL
with CHCl . DDQ (340 mg, 1.5 mmol) was added, and a dark green
O, was added slowly to the reaction
2
4
2
removed under reduced pressure. The crude product was purified by
using silica gel column chromatography (100−200 mesh) using ethyl
acetate/hexane (10:90, v/v) as an eluent to give the desired product
and characterized. Yield = 72% (250 mg, 0.72 mmol), reddish
3
H
2
2
4
3
−
4
−1
−1
crystalline solid. UV−vis (CH Cl ): λ , nm (ε × 10 , M cm );
mixture was stirred for another 2 h. The reaction mixture was
concentrated and subjected to filtration column through neutral
2
2
max
1
3
52 (1.88), 503 (6.3). H NMR (400 MHz; CDCl ; 298 K): δ 6.54 (d,
3
3
3
J H−H = 4.1 Hz, 2H, pyrrolic), δ 6.89 (d, J
= 4.2 Hz, 2H, pyrrolic),
alumina using CH
and evaporated to dryness. Silica gel column chromatography (100−
00 mesh) using ethyl acetate/hexane (10:90, v/v) as an eluent and
2 2
Cl ; all fractions containing product were collected
H−H
3
3
δ 7.43 (d, J
= 8.5, 2H, Ph), δ 7.67 (d, J
= 8.5, 2H, Ph), δ
H−H
H−H
13
2
7
1
.94 (s, 2H, pyrrolic). C NMR (101 MHz; CDCl ; 298 K): δ 118.76,
3
1
9
25.42, 131.29, 131.81, 132.57, 134.64, 144.54, 145.73. F NMR (376
repeated precipitation with hexane/methanol gave the desired product.
1
1
MHz; CDCl ; 298 K): δ −145.08 (dd, J
2
= 57.5 Hz, J
= 28.7,
Yield = 14% (36 mg, 0.037 mmol), purple colored powder. UV−vis
3
F−B
F−B
11
1
−4
−1
−1
F, BF ), B NMR (128 MHz; CDCl ; 298 K): δ 0.25 (t, J = 28.7
Hz, 1B, BF ). HR-MS (APCI): Calculated for C H BBrF N [(M) ]:
(CH Cl ): λmax, nm (ε × 10 , M cm ) 411 (22.33), 487 (10.48),
2 2
1
2
3
B−F
+
553 (6.4), 616 (3.57). H NMR (400 MHz; CDCl ; 298 K): δ 6.70 (d,
3
H−H = 2.9 Hz, 1H, bodipy pyrrolic), δ 7.08 (d, JH−H = 4.0 Hz, 1H,
2
15 10
2
2
3
3
346.0086, observed: 346.0098
J
β-Formyl BODIPY (3). Freshly distilled POCl (10 mL) was slowly
added to DMF (10 mL), cooled in an ice bath under argon
atmosphere, and stirred for 10 min. After being warmed to room
bodipy pyrrolic), δ 7.63 (s, 1H, bodipy pyrrolic), δ 7.77−7.66 (m, 4H,
3
3
Ph), δ 8.11 (s, 1H, bodipy pyrrolic), δ 8.55 (d, J
= 3.9 Hz, 2H,
H−H
3
corrole pyrrolic), δ 8.72 (s, 1H, bodipy pyrrolic), δ 8.73 (d, J
= 5.11
H−H
B
DOI: 10.1021/ic502919s
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Scheme 1. Synthesis of Precursors 2, 3, and 4
a
(
i) DDQ/CH Cl , Et N/BF ·OEt , RT, 2 h, yield = 72%. (ii) DMF/POCl at 0 °C, C H Cl , 70 °C, yield = 65%. (iii) pyrrole/TFA, RT, 4-5 min,
2 2 3 3 2 3 2 4 2
yield = 76%.
a
Scheme 2. Synthesis of Compounds 6 and 7
a
(
iv) TFA/CH Cl , yield = 16% or BF .OEt /CHCl , DDQ, yield = 24% or MeOH/H O, CHCl /DDQ, yield = 14%. (v) Cu(OAc) ·H O, CHCl /
2
2
3
2
3
2
3
2
2
3
MeOH, yield = 82%.
3
Hz, 2H, corrole pyrrolic), δ 8.91 (d, J
= 4.7 Hz, 2H, corrole
form/hexane (50:50, v/v) as eluent gave a trace of 8 (confirmed by
mass spectroscopy, Figure S27, Supporting Information) along with
H−H
3
19
pyrrolic), δ 9.08 (d, J
= 4.2 Hz, 2H, corrole pyrrolic). F NMR
H−H
(
376 MHz; CDCl ; 298 K: δ −160.76 (m, 4F, meta Ph-F), δ −152.17
major product 9. Yield = 35%, (54 mg, 0.035 mmol), purple colored
3
3
1
1
(
t, J
= 20.9 Hz, 2F, para Ph-F), δ −144.87 (dd, J
= 56.6 Hz,
powder. H NMR (500 MHz; CDCl ; 298 K): δ −2.69 (s, 2H, NH), δ
F−F
F−B
3
1
3
J
= 28.1 Hz, 2F, BF ), δ −136.96 (d, J = 18.8 Hz, 4F, ortho Ph-
6.73 (m, 3H, bodipy pyrrolic), δ 7.12 (m, 3H, bodipy pyrrolic), δ 7.65 (s,
2H, bodipy pyrrolic), δ 7.68 (s, 1H, bodipy pyrrolic), δ 7.72−7.74 (m,
F−B
2
F−B
1
1
1
F). B NMR (128 MHz; CDCl ; 298 K): δ 0.26 (t, J
= 28.2 Hz,
3
B−F
1
B, BF ). HR-MS (APCI): Calculated for C H BBrF N [(M +
12H, corrole pyrrolic), δ 8.17 (s, 3H, bodipy pyrrolic), δ 8.70 (s, 3H,
2
46 21
12
6
+
3
H) ]: 975.0915, observed: m/z 975.0912
bodipy pyrrolic) δ 8.78 (d, J
= 4.7 Hz, 2H, corrole pyrrolic), δ 9.15
H−H
Cu(III)-corrole BODIPY Dyad (7). Compound 6 (50 mg, 0.048
(m, 6H, corrole pyrrolic). 19F NMR (376 MHz; CDCl ; 298 K): δ
−161.84 (m, 2F, meta Ph-F), δ −152.29 (t, J
3
3
mmol) was dissolved in 15 mL of CHCl /MeOH (1:2) at room
temperature, and Cu(OAc) ·H O (95 mg, 0.48 mmol) was added to it.
=20.9 Hz, 1F, para
3
F−F
1
1
Ph-F), δ −144.54 (dd, J
= 56.6 Hz, J
= 27.8 Hz, 6F, BF ), δ
F−B 2
2
2
F−B
11
The mixture was stirred for 1 h, solvent was removed under reduced
pressure, and the product was purified by silica gel column
chromatography (100−200 mesh) with CH Cl /hexane (2:1, v/v) as
−136.78 (m, 2F, ortho Ph-F). B NMR (128 MHz; CDCl ; 298 K): δ
0.68 (t, J = 28.2 Hz, 3B, BF ). MS (MALDI-TOF, without matrix):
3
1
B−F
2
+
2
2
Calculated for C H B Br F N [(M + H) ]: 1509.0935, observed:
71 40
3
3 11 10
an eluent and characterized. Yield = 82%, (44 mg, 0.041 mmol), purple
m/z 1509.336.
−
4
−1
−1
colored powder. UV−vis (CH Cl ): λ , nm (ε × 10 , M cm )
4
2
2
max
1
00 (27.11), 484 (12.13), 551 (14.00). H NMR (700 MHz; CDCl ;
3
3
3. RESULTS AND DISCUSSION
2
98 K): δ 6.65 (d, J
= 3.2 Hz, 1H, bodipy pyrrolic), δ 7.02 (d,
H−H
3
JH−H = 3.8 Hz, 1H, bodipy pyrrolic), δ 7.15 (s, 1H, bodipy pyrrolic), δ
3.1. Synthesis. Preparation of precursors 2, 3, and 4 were
carried out (Scheme 1) according to reported procedure.
Synthesis of 2 involved two steps in a one-pot reaction;
oxidation of corresponding dipyrromethane 1 with DDQ
3
7
.24 (br, 2H, corrole pyrrolic), δ 7.44 (d, J
= 3.6 Hz, 2H, corrole
H−H
3
3
pyrrolic), δ 7.50 (d, J
= 8.20 Hz, 2H, ph), δ 7.51 (d, J
= 4.21
H−H
H−H
3
Hz, 2H, corrole pyrrolic), δ 7.69 (d, J
= 8.2 Hz, 2H, ph), δ 7.90 (d,
H−H
3
JH−H = 2.5 Hz, 2H, corrole pyrrolic), δ 8.07 (s, 1H, bodipy pyrrolic), δ
followed by subsequent addition of triethylamine immediately
8
−
.23 (s, 1H, bodipy pyrrolic). 19F NMR (376 MHz; CDCl ; 298 K): δ
17
3
3
followed by BF ·OEt gave a good yield. A drop in yield was
3 2
160.73 (m, 4F, meta Ph-F), δ −152.16 (t, J = 20.9 Hz, 2F, para
F−F
1
1
observed with delayed addition of BF ·OEt , after the addition
3 2
Ph-F), δ −144.88 (dd, J
= 56.6 Hz, J
= 28.3 Hz, 2F, BF ), δ
F−B
F−B
2
3
11
of triethylamine. Mono β-formylated BODIPY 3 was prepared
according to a modified Vilsmeier−Haack procedure reported
−
136.97 (d, J
= 17.8 Hz, 4F, ortho Ph-F). B NMR (128 MHz;
F−B
1
CDCl ; 298 K): δ 0.26 (t, J = 28.4 Hz, 1B, BF ). HR-MS (APCI):
3
B−F
2
+
18
Calculated for C H BBrCuF N [(M + H) ]: 1034.9976, observed:
m/z 1034.9928.
for BODIPY formylation. Compound 4 was prepared by
46
18
12
6
condensation of corresponding aldehyde and excess of pyrrole
in the presence of TFA. Meso-β directly linked corrole-
BODIPY dyad 6 and copper metalated dyad 7 were prepared
for the first time (Scheme 2). We started with a Brønsted acid
Corrole-BODIPY Triads (8) and Porphyrin-BODIPY Pentad (9). 4
(
50 mg, 0.1 mmol) and pentafluorobenzaldehyde (9.8 mg, 0.05 mmol)
were dissolved in 10 mL of dry CH Cl under argon. A solution of
2
2
BF ·OEt (0.3 equiv) was added to the reaction mixture and stirred for
3
2
(
TFA) catalyzed reaction of the respective aryl-aldehyde (3)
1
h. Then, 22.7 mg (0.1 mmol) of DDQ dissolved in 1 mL of toluene
and pentafluorophenyl-dipyrromethane (5) in CH Cl , and
was added, and the mixture was further stirred for 30 min. The solvent
was removed under reduced pressure and subjected to column
chromatography through silica gel (100−200 mesh) using chloro-
2
2
subsequent oxidation by DDQ gave 6 in low yield (16%).
Multiple column chromatographic separations were performed
C
DOI: 10.1021/ic502919s
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Scheme 3. Synthesis of Compounds 8 and 9
a
(
vi) BF ·OEt /CHCl , DDQ, 8 (trace amount), 9 (yield = 35%).
3 2 3
1
1
Figure 1. H− H COSY spectra of compound 6 (left) and 7 (right) in CDCl at 25 °C.
3
to separate overlapping bands and repeated precipitation with
hexane/methanol provided pure compound. Similar separation
difficulties were also observed with BF ·OEt in CHCl , but
opted for precursor 4 which reacted with pentafluorobenzalde-
21
hyde in the presence of BF ·OEt in CHCl (Scheme 3). We
3
2
3
observed only a trace amount of expected compound 8
(confirmed by mass spectroscopy, Figure S27) along with A3B
type porphyrin 9 as the major product. This unexpected
3
2
3
with relatively improved yield (24%). Gryko and co-workers
reported an excellent method for the synthesis of meso-
arylcorrole using H O/MeOH (1:1, v/v) mixture as a solvent
formation of A B porphyrin was possibly an outcome of the
2
3
in the presence of HCl (aq) and the subsequent oxidation of
bilane intermediate by DDQ/para-chloranil in CHCl3. In our
case, solubility of β-formyl BODIPY (3) in MeOH was rather
scrambling of dipyrromethane 4. The scrambling of dipyrro-
methane in acid was observed as a function of steric and
electronic factor at the meso-carbon. Sterically hindered and
electron-deficient substituent at the meso-carbon of dipyrro-
methane shows less scrambling than dipyrromethane bearing a
19
poor, but MeOH/H O (2:1, v/v) as solvent gave 6 in low yield
2
(
14%) with relatively easy separation protocol. We chose to
22
follow a methanol−water procedure to avoid tedious separation
less hindered and electron-rich group.
3.2. NMR Spectra. H, F, and B NMR spectral
characterization of 6 confirmed the proposed structure. H
1
19
11
despite lower yields. Treatment of 6 with Cu(OAc) ·H O in
2
2
1
CHCl /MeOH (1:2) afforded copper metalated compound 7
3
20
in good yield (82%).
NMR spectrum exhibits characteristic sharp peaks in
accordance with 17 protons in the region δ 6.6−9.1 ppm
(Figure S17). Out of these signals four doublets (δ 8.55, δ 8.73,
Furthermore, with an idea to attach a large number of
BODIPY units directly to the meso-position of corrole, we
D
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Article
1
Figure 2. (a) H NMR variable temperature experiment of 7 in toluene-d . (b) Plot of chemical shift deviation at different temperatures with respect
8
to 25 °C in toluene-d . Positive and negative deviation corresponds to the deviation toward downfield and upfield regions with respect to 25 °C
8
respectively.
Figure 3. Single crystal structures of compounds 2 and 3 (ORTEP representations). Thermal ellipsoids at 50% probability.
δ 8.9, δ 9.08) were assigned for eight corrole β-pyrrolic protons,
with a characteristic coupling constant of 4−5 Hz. Signal for the
meso-aryl ring protons of BODIPY, two doublets with roof
effect were observed at δ 7.69 and δ 7.72 ppm. Two doublets (δ
resonances of H and H . BODIPY β-pyrrolic protons H −H
h
i
a
c
and H −H give relatively weak correlation due to long-range
d
e
coupling in both compounds, whereas proton H shows a
b
1
strong correlation between both H and H . H NMR signals of
a
c
6
.70, δ 7.08) with characteristic coupling constant of 3−4 Hz
corrole β-pyrrolic protons in compound 7 appear relatively
broad at room temperature. To understand the effect of
temperature on the chemical shift and nature of the signal, a
and three singlet (δ 7.63, δ 8.11, δ 8.72) were assigned to five
1
9
BODIPY pyrrolic protons. F NMR shows three signals (δ
136.96, δ −152.17 and δ −160.76) corresponding to two
1
−
variable temperature H NMR experiment was conducted for
C F (Figure S18), and a doublet of doublets at δ −144.87 ppm
compound 7 in toluene-d (Figure 2). All corrole β-pyrrolic
6
5
8
1
1
was assigned to BF . B NMR exhibited a slightly deshielded
signals experience considerable shift in either direction along
with simultaneous broadening at a higher temperature. This
broadening of peaks can be ascribed to Cu(II)-corrole π-radical
2
triplet signal (δ 0.26, J = 28.4 Hz) with reference to 2 and 3
(Figures S3 and S8). In compound 7, all proton resonances
20a,23
appeared slightly shielded compared to that of compound 6.
Correlation and assignment of pyrrolic protons for 6 and 7
cations.
Saddling geometry of Cu (III)-corrole imparts
unique properties of temperature-dependent magnetic behav-
iors. Saddling induces a part of the electron density from
HOMO of the corrole moiety to flow into the empty
1
1
were carried out on the basis of H− H COSY (Figure 1),
NOESY (Figures S32 and S36), ROESY (Figure S34), and
HSQC (Figure S33). Black connecting lines represent a
correlation among BODIPY β-pyrrolic protons, whereas red
lines indicate a correlation between β-pyrrolic protons of
2
4
2
2
d
orbital of the Cu(III) metal center. Theoretical
x −y
calculation by Abhik Ghosh et al. suggested that ground state
Cu(III)-corrole complex is only slightly lower in energy than
1
1
25
corrole. The H− H correlation spectrum reveals the resonance
of bipyrrolic β-protons of 6, namely, H and H flanked by the
Cu(II)-corrole π-radical cationic state. This small energy
barrier can be thermally accessible. Thus, a temperature-
f
g
E
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Article
Figure 4. Single crystal X-ray structures of compound 6 (ORTEP representations). (a) (side view) and (b) (view toward corrole plane). Thermal
ellipsoids at 50% probability.
8
dependent equilibrium between diamagnetic d Cu(III)-corrole
Table 1. Crystal Data and Structures Refinement for
Complexes
9
and paramagnetic d Cu(II)-corrole π-cation radical was
strongly reflected in this copper complex. The BODIPY β-
2
3
6
pyrrolic protons H and H directly connected to the meso-
e
d
CCDC
formula
1023162
1023022
1022285
carbon of corrole are the most affected by the paramagnetism
of corrole core due to close proximity and further resonate at
the shielded region, while the remaining protons of BODIPY
do not indicate considerable temperature-dependent behavior
other than the appearance of doublet splitting pattern as the
temperature increases. In the case of 6 temperature has a very
weak effect except broadening of the signal with a decrease in
temperature (Figure S37).
C H BBr
C H BBr
F N O
C H BBr
15 10
16 10
46 20
F N
12
F2N2
346.96
triclinic
2
2
6
Mw (g mol−1)
crystal system
space group
a (Å)
374.97
monoclinic
P21/c
975.39
monoclinic
P21/c
P1
̅
7.7681(4)
7.9220(4)
11.8432(6)
85.630(3)
80.383(3)
74.105(3)
690.74(6)
2
11.5531(8)
8.7314(7)
15.0856(10)
90
15.9618(19)
8.356(1)
34.582(4)
90
b (Å)
c (Å)
3.3. X-ray Diffraction Studies. Structures of 2, 3, and 6
α (deg)
are shown in Figures 3 and 4 respectively. Crystals suitable for
X-ray diffraction analysis for 3 and 6 were grown by vapor
diffusion of n-hexane into a dichloromethane solution at 4 °C,
whereas for 2 was obtained by slow evaporation of dichloro-
methane solution at room temperature. The relevant crystallo-
graphic data for these compounds are summarized in Table 1.
Compound 6 was crystallized in monoclinic P21/c space group.
β (deg)
97.196(3)
90
113.041(5)
90
γ (deg)
V (Å3)
1509.77(19)
4
4244.5(9)
4
Z
Dcalcd (g cm−3
)
1.668
1.650
1.526
μ (mm−1)
F(000)
2.990
2.748
1.063
344.0
744.0
1944.0
105
Twenty-three atoms in C N corrole core have nonplanar
1
9
4
temperature (K)
296
296
conformation with a small saddle distortion and are quite
similar to that previously observed for free-base corrole
λ [Å]
0.71073
28.690
0.71073
27.980
0.71073
28.010
θ-range for data
collection (deg)
2
6
macrocycles. The mean plane deviation of its 23 atoms is
.102 Å, with a maximum of 0.232 Å for N5. The interior N
0
measured reflection
3568
3635
10240
atoms in the corrole core are forced to deviate out from
coplanarity, two above and two below, due to steric repulsion
from three inner hydrogens. The bond distance (1.473 Å)
between the meso-carbon of corrole connected to BODIPY β-
pyrrolic carbon is slightly shorter than two pentafluorophenyl
meso-substituents (1.482 and 1.497 Å). Mean planes of
BODIPY units are rotated by about 45.41° with respect to
the corrole core mean plane, while the two pentafluorophenyl
rings are tilted by about 72.46° and 53.99°, respectively.
The DFT optimized geometry (Figures 5 and S38),
calculated bond distances and angles, matches fairly well with
the experimental values and is summarized in Table 2. In
compounds 6 and 7 HOMO is mainly localized on the
electron-rich corrole moiety. Notably, in 6, LUMO is localized
on the BODIPY fragment, whereas in the case of 7, LUMO+1
is localized on the BODIPY fragment.
Tmin, Tmax
0.307, 0.460
0.0475(2401)
0.1736(3469)
0.307, 0.463
0.0445(2479)
0.1615(3624)
1.092
0.655, 0.775
0.0858(5561)
0.2419(1015)
1.041
R
R
[I > 2σ(I)]
[I > 2σ(I)]
1
w
goodness-of-fit [GOF] 1.183
3.5. Redox Properties. Electrochemical properties of
compounds 2, 3, 6, 7 and 5,10,15-tris(pentafluorophenyl)-
corrole [H (tpfc)] were investigated in CH Cl , and potentials
3
2
2
+
with reference to Ag/Ag electrode are given in Table 3. Cyclic
voltammograms of 2 and 3 (Figures S39 and S40) consist of
two irreversible oxidation peaks, and two reversible reduction
peaks. Because of the presence of the formyl group in 3, it is
relatively difficult to oxidize and easier to reduce than 2. The
difference in the first oxidation potential and reduction
potential between 2 and 3 varied by ∼21 mV and ∼25 mV
respectively. A thorough investigation on redox behavior of
F
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Article
reverse scan at E = 0.12 V were assigned to a corrole fragment.
p
A BODIPY centered oxidation process at potential 1.57 V was
observed in DPV. The redox properties of 5,10,15-tris-
(
pentafluorophenyl)corrole [H (tpfc)] as reference compound
3
in CH Cl were also investigated (Figure S41). A similar
2
2
voltammogram was observed which indicates the involvement
of similar redox active species. The cyclic voltammogram of
compound 7 is displayed in Figure 7 and consists of three
reduction and two oxidation processes. Copper metal-centered
facile one electron reversible reduction peak at E1/2 = 0.02 V
was observed, which is similar to that observed for the Cu(III)-
16a,f−i
corrole complex.
Ligand centered irreversible reduction
and reversible oxidation were observed at E = −1.29 V and
p
E
= 0.90 V respectively, whereas irreversible oxidation and
1/2
reduction peaks at E = 1.59 V and E = −0.92 V were assigned
p
p
to the BODIPY unit. Redox properties of BODIPY fragments
in 6 and 7 do not show any considerable difference to that
observed in reference compounds 2. This electronic isolation
between the chromophoric fragments remains intact despite the
shortest bond connection and probably due to the tilted
orientation of the BODIPY unit with respect to the corrole
fragment as observed in solid state structural studies.
3
.6. UV−vis Absorption Study. Ground state absorption
spectra of compounds 2, 3, 6, 7, and H (tpfc) in CH Cl have
Figure 5. DFT optimized geometry and frontier orbital plots of
compounds 6 (left) and 7 (right).
3
2
2
been shown for the comparison in Figure 8. BODIPY 2 and 3
display the typical narrow absorption features with λ
max
28
positioned at 503 and 498 nm, respectively. This absorption
band is assigned to the S -S transition. An additional weaker
free-base corrole was carried out by Kadish and co-workers in
nonaqueous solvent (PhCN and pyridine) and confirmed the
involvement of a mixture of in situ generated electroactive
species in the redox process due to facile gain and loss of
0
1
and broad absorption band at ∼300−400 nm was also observed
which corresponds to the S -S transition. Absorption
0
2
2
7
protons. Electroreduction of compound 6 (Figure 6) revealed
maximum of 3 was observed to be ∼5 nm blue-shifted to
that of 2. The observed hypsochromic shift can be rationalized
due to stabilization of HOMO by the presence of an electron-
withdrawing formyl group in the β-position of 3. Compound 6
exhibits a typical corrole Soret type band around 411 nm and
broad Q type bands from 500 to 750 nm. An additional
absorption band corresponding to BODIPY chromophoric
fragment was also observed at 488 nm. Similarly, compound 7
exhibits a Soret type band at 398 nm, Q type bands around
two irreversible peaks at E = −0.94 V (BODIPY centered) and
p
E = −1.37 (corrole centered). The initial electrooxidation of
p
neutral species (Bod-Cor)H was irreversible and located at E
3
p
•
=
0.66 V. This led to the formation of species [(Bod-Cor)H ]
and [(Bod-Cor)H ] . This mixture of electroactive species
2
+
4
further involved two reversible oxidations at E1/2 = 0.74 V and
E1/2 = 0.91 V respectively. A reversible oxidation peak at E1/2
=
1.23 V and an ill-defined oxidation peak observed during
Table 2. Selected X-ray and DFT Calculated Bond Distances [Å] and Angles [deg] for Compound 6 and 7
crystal structure
DFT (B3LYP) optimized geometry
6
6[6-31G (d)]
7 [631G (d) +LANL2DZ]
Br(1)−C(91)
N(11)−B(1)
N(12)−B(1)
B(1)−F(1)
1.886(7)
1.554(9)
1.540(6)
1.357(8)
1.393 (7)
1.407(6)
1.473(6)
1.907
1.566
1.571
1.379
1.382
1.418
1.479
1.905
1.566
1.571
1.378
1.382
1.429
1.469
1.924
1.918
1.919
1.923
112.0
110.1
110.4
117.8
117.7
112.6
112.6
B(2)−F(2)
C(15)−C(27)
C(44)−C(46)
Cu−N(14)
Cu−N(15)
Cu−N(16)
Cu−N(17)
F(1)−B(1)−F(2)
F(1)−B(1)−N(11)
F(1)−B(1)−N(12)
C(35)−C(44)−C(46)
C(45)−C(44)−C(46)
N(7)−C(27)−C(15)
N(6)−C(15)−C(27)
111.7(5)
111.5(5)
110.8(5)
117.5(4)
115.7(4)
116.1(4)
114.0(4)
111.8
110.0
109.1
118.0
116.3
116.1
115.6
G
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Table 3. Half-Wave or Peak Potential (V vs Ag/Ag ) for 2, 3, 6, 7, and H (tpfc) in CH Cl Containing 0.1 M TBAP
Article
+
3
2
2
oxidation
reduction
II
compounds
VI
V
IV
III
II
I
I
III
b
b
a
a
a
b
b
a
b
2
2.05
2.26
0.66
1.59
0.78
1.59
1.80
0.12
0.90
0.22
−0.80
−0.55
−0.83
0.02
−1.68
−1.19
−1.28
−0.83
b
3
b
b
b
b
6
1.57
1.23
0.91
1.04
0.74
0.90
b
b
7
−1.18
a
tpfc
−1.24
a
b
Oxidation or reduction peak potential. DPV oxidation or reduction potential.
Figure 6. Cyclic voltammogram (solid line) at scan rate: 0.1 V/s and differential pulse voltammogram (dotted line) of 6 in CH Cl .
2
2
Figure 7. Cyclic voltammogram (solid line) at scan rate: 0.1 V/s and differential pulse voltammogram (dotted line) of 7 in CH Cl .
2
2
of 250−750 nm. Furthermore, absorption spectra support weak
ground state electronic communication between chromophores
as indicated in cyclic voltammetric studies.
3
.7. Protonation and Deprotonation Study. The
protonation study of 6 was carried out with the addition of
trifluoroacetic acid (TFA) in CH Cl solution. Addition of TFA
2
2
produced a color change from purple to brown as shown in
Figure 11. UV−vis absorption spectrum shows diminishing
bands at 411 and 489 nm with concomitant enhancement of
bands at 474, 556, and 645 nm. The Job’s plot analysis revealed
+
a 1:1 stoichiometry of complexation between H and 6 (Figure
2
S46). With the help of a straight line from [TFA] /A -A versus
[
0
n
2
TFA] , the equilibrium constant log K was calculated to be
5
4
.08 (Figure 9). Notably, protonation by TFA induced band at
89 nm, corresponding to BODIPY absorption to be blue-
Figure 8. UV−vis absorption spectra of compound H (tpfc) (black),
3
shifted by 15 nm. Monoprotonation of inner nitrogen increases
positive charge density on corrole moiety. Thus, probably the
corrole moiety stabilized the HOMO of BODIPY due to
increase in electronegativity, which then resulted in a blue-
shifted band.
6
(blue), 7 (red), 3 (pink), and 2 (green) in CH Cl .
2 2
5
00−750 nm, and an additional absorption band contribution
from the BODIPY fragment at 484 nm. Both choromophoric
fragments in dyads 6 and 7 exhibit an overall additive
absorption spectrum spanning throughout the UV−vis region
Solid state structure and DFT optimized geometry in 6
revealed that the NH-protons suffered severe steric hindrance
H
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Article
changes from brown to green color (Figure 11), while the rest
of tested anions do not produce any effect even with 50 fold
Figure 9. UV−vis absorption change of 6 (∼10−6 M) with the
addition of TFA in CH Cl solution.
2
2
which causes interior protonated nitrogen to deviate out of the
mean pyrrole ring plane. This out-of-plane deviation to
minimize steric repulsion accounts for the observed higher
Figure 11. Colorimetric change of 6, above under daylight and below
−
under UV light. Picture was taken with 10 equiv additions of DBU, F ,
−
−
−4
29
AcO , H PO , and TFA in CH Cl solution of 6 (∼10 M).
acidity for corrole macrocycles. To understand the deproto-
nation behavior, a non-nucleophilic base 1,8-
diazabicyclo[5.4.0]undec-7-ene (DBU) was added to 6 in
CH Cl . This resulted in an immediate color change of the
2
4
2
2
−
addition. Upon higher addition of F (Figures S43 and S44 for
−
−
AcO and H PO respectively), an intense band emerged at
2
4
2
2
4
26 nm, 507 nm, 621 nm with simultaneous disappearance of
solution from purple to green (Figure 11). UV−vis absorption
changes were monitored with the addition of increasing
amount of DBU in CH Cl solution (Figure 10). An
bands at 411 and 560 nm as shown in Figure 12. The Job’s plot
2
2
−6
Figure 12. UV−vis absorption change of 6 (∼10 M) with the
addition of TBAF in CH Cl solution.
2
2
Figure 10. UV−vis absorption change of 6 (∼10−6 M) with the
addition of DBU in CH Cl solution.
2
2
referred to 1:2 stoichiometric interactions of 6 with all
interacting basic anions (Figure S47). The visible color and
−
−
−
enhancement of bands at 422 nm, 504 nm, 617 nm with
simultaneous diminishing bands at 411 and 550 nm was
observed. The Job’s plot referred to a 1:1 stoichiometric
interaction between 6 and DBU (Figure S45). With the help of
UV−vis absorption changes with F , AcO , and H PO were
2
4
similar to that as observed in the case of DBU addition. These
results support the fact that the spectral and colorimetric
changes were possibly due to deprotonation of NH moiety of
corrole by basic anions. Furthermore, the initial band at 488 nm
which corresponds to absorption of BODIPY fragment was red-
shifted by 18 nm to a new band at 507 nm with the addition of
2
2
sa traight line plot from [DBU] /A -A versus [DBU] , the
n
0
equilibrium constant log β was found to be 10.27. Encouraged
from the DBU deprotonation study, we opted to explore easy
deprotonation behavior of 6 toward the sensing ability of
−
1
−
F . An H NMR study with F addition shows the appearance
of new peaks in the shielded region as shown in Figure 13.
These shielded proton signals strongly support increased
electron density due to deprotonation of the inner NH proton
of corrole.
−
−
various anions. We studied various common anions (F , Cl ,
−
−
−
−
−
−
Br , I , CH COO , H PO , HSO , and NO ) in the form
3
2
4
4
3
of TBA salt in a CH Cl solution. Addition of highly basic
2
2
−
−
−
anions F , AcO , and H PO shows immediate colorimetric
2
4
I
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Article
32
mostly restores the fluorescence. We observed a fine balance
between complete fluorescence quenching of neutral corrole
molecule probably due to electron transfer and afterward
gradual enhancement of emission solely due to the inherent
higher emission behavior of corrole-anion. Experimental
−
equilibrium constant (log β) for F was found to be 10.88,
−
−
which is close to that of DBU, whereas AcO and H PO have
2
4
a relatively small log β value of 8.99 and 8.27 respectively
−
(
Figures S37 and S38). A higher equilibrium constant of F can
be attributed to higher electronegativity (4.1) and smaller size
1.3 Å). Moreover the size of fluoride ion is compatible with
the smaller corrole core size of 2.5 Å.
.9. Conclusion. We have presented here the synthesis of
(
3
the bichromophoric meso-β linked free-base corrole-BODIPY
dyad 6 and its Cu (III)-metalated derivative 7. Single crystal X-
ray structure of 6 revealed a saddle distortion of corrole core
with two nitrogen atoms alternatively in an up and down
conformation. Distortion of corrole core relieved the steric
hindrance experienced by the three inner nitrogen protons and
resulted in a higher acidic nature. A tilted orientation by about
1
Figure 13. Partial H NMR of 6 with an addition of TBAF in CDCl .
3
Black arrows (corrole) and purple arrows (BODIPY) represent a shift
of corresponding protons.
Furthermore, quite interestingly, we observed a pink color
fluorescence “turn on” phenomenon with the addition of basic
−
−
−
1
anions (DBU, F , AcO , and H PO ) as shown in Figure 11.
45.41° was observed between corrole and BODIPY. H NMR
2
4
−
Addition of F in CH Cl solution shows a gradual enhance-
ment of an emission band at 665 nm, which is a typical corrole
based emission (Figure 14). Compound 6 was initially
of compound 7 exhibits strong temperature-dependent
resonance with broadening and shifting of signals due to
paramagnetic contribution of Cu(II)-corrole π-radical cation.
CV and UV−vis studies suggested weak ground state electronic
communication between the chromophoric fragments.
2
2
Similar to the case of a singly connected porphyrin-BODIPY
dyads, emission was quenched due to a possible electron
transfer process. However, it is demonstrated here that the
fluorescence can be revoked by the addition of either a neutral
−
base or suitable anion like F . Thus, the current study displays
the uniqueness of the corrole-BODIPY dyad which is unlike
that of the porphyrin BODIPY unit.
ASSOCIATED CONTENT
Supporting Information
■
*
S
NMR spectra, mass spectra, UV−vis spectra, CV plot, TCSPC,
and DFT optimized geometry. This material is available free of
charge via the Internet at http://pubs.acs.org.
Figure 14. Fluorescence enhancement of 6 (∼10−5 M) with the
addition of TBAF in CH Cl solution. Excitation at λ = 411 nm.
2
2
AUTHOR INFORMATION
Corresponding Author
E-mail: sankar@iiserb.ac.in. Fax: +91-755-6692392. Tel: +91-
55-6692-334.
■
nonfluorescent, probably because of excited state electron
transfer between the corrole and BODIPY unit, which is one of
the fundamental quenching phenomena for chromophoric
donor−acceptor (D−A) types of molecules as commonly
*
7
Notes
The authors declare no competing financial interest.
1
3,30
observed for many similar systems.
A negative free energy
change (ΔG = −0.78 eV) suggests a thermodynamically
CS
ACKNOWLEDGMENTS
■
feasible photoinduced electron transfer (PET) event from
singlet excited state of corrole to the BODIPY, yielding a
charge separated state (Figure S42). It was observed that
deprotonation of NH enhanced the emission quantum yield of
corrole but does not have much effect in the fluorescence
B.B. acknowledges UGC for JRF fellowship. A.R.S and R.R.
thank IISER, Bhopal for a fellowship. J.S. thanks DST, New
Delhi, MHRD, New Delhi, and IISERB for funding.
31
REFERENCES
lifetime of corrole anion. Fluorescence lifetime of deproto-
■
(
1) Johnson, A. W.; Kay, I. T. J. Chem. Soc. 1965, 1620−1629.
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2
2
(
close to that measured for neutral H (tpfc) (4.9 ns) and
3
1
(
H (tpfc) anion (5.2 ns) in CH Cl (Figure S48). Thus, the turn
3
2
2
on fluorescence in this particular case is not because of
modulation or quenching of any electron transfer process
present, but merely an outcome of higher fluorescence
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different from reported fluorescence “turn on” phenomenon
where quenching of the electron or energy transfer process
Boschi, T.; Smith, K. M. Chem. Commun. 1999, 1307−1308.
(
(
(
4) Gryko, D. T. Chem. Commun. 2000, 2243−2244.
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̈
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DOI: 10.1021/ic502919s
Inorg. Chem. XXXX, XXX, XXX−XXX