Spectroscopic and 1O2 Sensitization Characteristics of a Series of Isomeric Re(bpy)(CO)3Cl Complexes Bearing Pendant BODIPY Chromophores

Article abstract of DOI:10.1021/acs.inorgchem.9b00102

Two new Re(I)bipyridyltricarbonyl chloride complexes, Re(BB3)(CO)₃Cl and Re(BB4)(CO)₃Cl, featuring BODIPY groups appended to the 5,5′- or 6,6′-positions of the bipyridine ligand, respectively, were synthesized as structurally isomeric compliments to a previously reported 4,4′-substituted homologue, Re(BB2)(CO)₃Cl. X-ray crystal structures of the compounds show that the 4,4′-, 5,5′-, and 6,6′-substitution patterns place the BODIPY groups at progressively shorter distances of 9.43, 8.39, and 5.56 ?, respectively, from the complexes' Re centers. The photophysical properties of the isomeric complexes were investigated to ascertain the manner in which the heavy rhenium atom might induce intersystem crossing of the pendant BODIPY moieties positioned at progressively shorter through-space distances. Electronic absorption spectroscopy revealed that the three metal complexes retain the strong visible absorption features characteristic of the bpyBODIPY (BB2-BB4) ligands; however, the fluorescence of the parent borondipyrromethane appended ligands is attenuated by more than an order of magnitude in Re(BB2)(CO)₃Cl and Re(BB3)(CO)₃Cl and by more than two orders of magnitude in Re(BB4)(CO)₃Cl. Furthermore, phosphorescence from Re(BB4)(CO)₃Cl is observed under a nitrogen atmosphere, consistent with highly efficient ISC to the triplet-excited state. Singlet oxygen sensitization studies confirm that all three complexes produce singlet oxygen with quantum yields that increase as the distance of the BODIPY groups to the heavy rhenium center is decreased. The trends observed across the series of rhenium complexes with respect to emission and ¹O₂ sensitization properties can be rationalized in terms of the varied distal separation between the metal center and BODIPY groups in each system.

Full text of DOI:10.1021/acs.inorgchem.9b00102

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
1
Spectroscopic and O Sensitization Characteristics of a Series of
2
Isomeric Re(bpy)(CO) Cl Complexes Bearing Pendant BODIPY
3
Chromophores
Andrea M. Potocny, Justin J. Teesdale, Alize Marangoz, Glenn P. A. Yap, and Joel Rosenthal*
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
*
S Supporting Information
ABSTRACT: Two new Re(I)bipyridyltricarbonyl chloride
complexes, Re(BB3)(CO) Cl and Re(BB4)(CO) Cl, featur-
3
3
ing BODIPY groups appended to the 5,5′- or 6,6′-positions of
the bipyridine ligand, respectively, were synthesized as
structurally isomeric compliments to a previously reported
4
,4′-substituted homologue, Re(BB2)(CO) Cl. X-ray crystal
3
structures of the compounds show that the 4,4′-, 5,5′-, and
6
,6′-substitution patterns place the BODIPY groups at
progressively shorter distances of 9.43, 8.39, and 5.56 Å,
respectively, from the complexes’ Re centers. The photo-
physical properties of the isomeric complexes were inves-
tigated to ascertain the manner in which the heavy rhenium
atom might induce intersystem crossing of the pendant BODIPY moieties positioned at progressively shorter through-space
distances. Electronic absorption spectroscopy revealed that the three metal complexes retain the strong visible absorption
features characteristic of the bpyBODIPY (BB2−BB4) ligands; however, the fluorescence of the parent borondipyrromethane
appended ligands is attenuated by more than an order of magnitude in Re(BB2)(CO) Cl and Re(BB3)(CO) Cl and by more
3
3
than two orders of magnitude in Re(BB4)(CO) Cl. Furthermore, phosphorescence from Re(BB4)(CO) Cl is observed under a
3
3
nitrogen atmosphere, consistent with highly efficient ISC to the triplet-excited state. Singlet oxygen sensitization studies confirm
that all three complexes produce singlet oxygen with quantum yields that increase as the distance of the BODIPY groups to the
heavy rhenium center is decreased. The trends observed across the series of rhenium complexes with respect to emission and
1
O sensitization properties can be rationalized in terms of the varied distal separation between the metal center and BODIPY
2
groups in each system.
INTRODUCTION
effect to promote intersystem crossing (ISC) through
■
1
1
enhanced spin orbit coupling. Accordingly, numerous
halogenated BODIPY derivatives featuring bromine or iodine
substituents directly attached to, or in π-conjugation with the
pyrrolic backbone, have been synthesized and reported to
Boron dipyrromethenes (BODIPYs) are well-known for their
strong absorption of visible light, fluorescence quantum yields
1
,2
that can approach unity, and good photostability. These
characteristics have inspired studies of BODIPY-based
compounds for applications as light harvesters in dye-
1
6−21
efficiently access the triplet state.
Ligands bearing
3
4
BODIPY moieties have also been complexed with heavy
sensitized solar cells, as laser dyes, and as fluorescent probes
for biomolecules, pH, certain cations, anions, or gases, reactive
oxygen or nitrogen species, redox potential, and viscosity,
2
2−24
25−29
30−32
transition metals such as Ru(II),
and Re(I)
Pt(II),
Ir(III),
3
3−35
to deliver systems that exhibit lower
5
−7
fluorescence quantum yields and, in some cases, detectable
phosphorescence from the BODIPY triplet state following
photoexcitation. Along these lines, we reported a 2,2′-
bipyridine ligand functionalized with two BODIPY substitu-
among other stimuli.
While population of the triplet
excited-state manifold of traditional BODIPY compounds is
typically negligible, other photosensitizers that support a more
extensive triplet photochemistry have found uses in triplet−
triplet annihilation upconversion,
photodynamic therapy. The ease with which the BODIPY
framework can be synthetically modified
efforts aimed at enabling the use of this popular class of
luminophore for applications that require the photochemical
generation of reactive triplet intermediates.
36
8
,9
10
ents at the 4- and 4′-positions (BB2) and subsequently
photocatalysis, and
11
incorporated this ligand into a rhenium(I)tricarbonyl chloride
37
1
2−14
complex to form Re(BB2)(CO) Cl (Scheme 1). In related
has buoyed
3
work, we synthesized the analogous 5,5′-substituted 2,2′-
bpyBODIPY ligand (BB3), which had been introduced
15
A common strategy for transforming conventional fluo-
Received: January 11, 2019
rophores into triplet photosensitizers exploits the heavy atom
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00102
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Scheme 1. Metallation of bpyBODIPY Ligands to Generate an Isomeric Series of fac-Re(bpy)(CO) Cl Complexes
3
previously in the literature using a slightly different synthetic
syringe or cannula techniques. Reagents and solvents were purchased
from Sigma-Aldrich, Acros, Fisher, Strem, or Cambridge Isotopes
3
8
strategy, as well as the homologous 6,6′-substituted 2,2′-
39
Laboratories. Re(BB2)(CO) Cl and the BB3 and BB4 ligands were
bpyBODIPY ligand (BB4). In this study, BB3 and BB4 have
also been incorporated into rhenium(I)tricarbonyl chloride
complexes (Re(BB3)(CO) Cl and Re(BB4)(CO) Cl, respec-
3
36,37,39
prepared according to published procedures.
Solvents used for
synthesis were of reagent grade or better. Anhydrous solvents were
dried by passage through activated alumina and stored over 4 Å
molecular sieves prior to use. Column chromatography was
performed with 40−63 μm silica gel from Silicycle.
3
3
tively) to complete the set of three homologous complexes
shown in Scheme 1.
For each of the isomeric Re complexes of Scheme 1, the
positions at which the BODIPY units are attached to the
bipyridine scaffolds ultimately dictates the chromophores’
proximities (∼5−10 Å) to the rhenium center. Accordingly, we
anticipated that the photophysics of the pendant BODIPY
units might be perturbed by varying degrees for each Re
complex, and conducted steady state absorption and emission
studies to probe how altering the (Re···BODIPY) separation
influences the photophysical properties of the three isomers.
The absorption profiles of the complexes are dominated by the
strong BODIPY features in the visible region, while the
emission spectra reveal evidence of greater spin−orbit coupling
when the BODIPY groups are positioned closer to the metal
center. Additionally, Re(BB2)(CO) Cl, Re(BB3)(CO) Cl,
Compound Characterization. 1H NMR and 13C NMR spectra
were recorded at 25 °C on a Bruker 400 MHz spectrometer with a
cryogenic QNP probe. Proton spectra are referenced to the residual
proton resonance of the deuterated solvent (CDCl = δ 7.26), and
3
carbon spectra are referenced to the carbon resonances of the solvent
42
(CDCl = δ 77.16). All chemical shifts are reported using the
3
standard δ notation in parts per million, and positive chemical shifts
are to higher frequency from the given reference. High-resolution
mass spectrometry analyses were performed by the University of
Delaware Mass Spectrometry Laboratory in the Department of
Chemistry and Biochemistry.
Rhenium[5,5′-bis(BODIPY)-2,2′-bipyridine](CO) Cl (Re(BB3)-
3
(
CO) Cl). 5,5′-bis(BODIPY)-2,2′-bipyridine (BB3) (84 mg, 0.11
3
mmol) and rhenium(I) pentacarbonylchloride (40 mg, 0.11 mmol)
were combined in 50 mL of 1,2-dichloroethane. The resulting
solution was heated to reflux while being stirred for 20 h under
nitrogen and underwent a change in color from orange to dark red.
The solvent was removed under reduced pressure, and the crude
product was purified on silica by eluting with 5% hexanes in CH Cl
3
3
1
and Re(BB4)(CO) Cl all show the ability to sensitize O ,
3
2
but Re(BB4)(CO) Cl, which has the shortest average Re···
3
1
BODIPY separation, photosensitizes the formation of O with
2
the highest quantum yield across the series. These constructs
are notable as they form a structurally characterized set of
isomeric homologues that add to the small number of BODIPY
compounds tethered to a rhenium heavy atom to induce a
2
2
followed by straight CH Cl . The product was obtained as the second
2
2
band immediately following a band of unreacted ligand. The desired
product was obtained as 41 mg of a dark red powder (yield = 35%).
1
3
3−35
H NMR (400 MHz, CDCl , 25 °C) δ /ppm: 9.04 (d, J = 1.7 Hz,
3
triplet photochemistry
(
and as new examples of rhenium-
1
2H), 8.42 (d, J = 8.2 Hz, 2H), 8.11 (dd, J = 8.2, 1.9 Hz, 2H), 2.57 (s,
I) complexes capable of generating O upon irradiation with
2
11,33,40,41
1
1
1
2H), 2.39−2.28 (m, 8H), 1.47 (s, 6H), 1.46 (s, 6H), 1.02 (dt, J =
wavelengths of light longer than 500 nm.
13
1
0.8, 7.6 Hz, 12H). C{ H} NMR (101 MHz, CDCl , 25 °C) δ
3
96.02, 156.88, 155.35, 154.99, 152.40, 139.30, 138.18, 136.22,
EXPERIMENTAL SECTION
General Materials and Methods. Reactions requiring an inert
atmosphere were carried out under positive pressure of N using flasks
■
134.44, 133.80, 130.64, 130.50, 129.89, 123.13, 29.50, 16.92, 16.89,
+
14.41, 14.29, 13.45, 13.01, 12.63, 12.51. HR-ESI-MS: [M + H] m/z:
2
calcd for C H B ClN F O Re, 1067.3391; found, 1067.3428.
47 51
2
6
4
3
fitted with Suba-Seal rubber septa and standard Schlenk techniques.
Air and moisture sensitive reagents were transferred using standard
Rhenium[6,6′-bis(BODIPY)-2,2′-bipyridine](CO) Cl (Re(BB4)-
3
(CO) Cl). 6,6′-bis(BODIPY)-2,2′-bipyridine (BB4)(84 mg, 0.11
3
B
DOI: 10.1021/acs.inorgchem.9b00102
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
mmol) and rhenium(I) pentacarbonylchloride (40 mg, 0.11 mmol)
were combined in 50 mL of 1,2-dichloroethane. The resulting
solution was heated to reflux while being stirred for 20 h under
nitrogen and underwent a change in color from orange to dark red.
The solvent was removed under reduced pressure, and the crude
product was purified on silica by eluting 25% hexanes in CH Cl
were excited at λ = 500 nm, and emission was monitored from λ
=
em
ex
515−950 nm using a step size of 1 nm and an integration time of 0.25
s. Emission spectra were also measured following exposure of each of
the samples to air. All reported spectra are the average of five
individual acquisitions. Excitation spectra recorded for Re(BB4)-
(CO) Cl were monitored at either 575 or 828 nm while varying the
2
2
3
followed by 1% methanol in CH Cl . The desired product was
excitation wavelength from 420−650 nm.
2
2
1
obtained as 75 mg of a dark red powder (yield = 64%). H NMR (400
A solution of [Ru(bpy) ](PF ) in air-saturated acetonitrile was
3
6 2
MHz, CDCl , 25 °C) δ 8.46 (d, J = 8.2 Hz, 2H), 8.20 (t, J = 7.9 Hz,
prepared at a concentration of ∼120 μM such that its absorbance at λ
3
−1 −1
2
H), 7.72 (d, J = 7.7 Hz, 2H), 2.52 (s, 12H), 2.28 (q, J = 7.5 Hz, 4H),
.20 (q, J = 7.6 Hz, 4H), 1.65 (s, 6H), 1.07 (s, 6H), 0.95 (t, J = 7.5
= 500 nm fell between 2.60 × 10 and 3.15 × 10 for use as a
45
2
luminescence reference (Φref = 0.018). Emission quantum yields
were calculated using the expression below:
1
3
1
Hz, 6H), 0.87 (t, J = 7.5 Hz, 6H). C{ H} NMR (101 MHz, CDCl ,
3
2
1
1
5 °C) δ /ppm: 192.04, 187.33, 158.68, 158.09, 158.04, 155.29,
2
39.29, 138.93, 135.70, 134.50, 133.86, 133.13, 131.18, 129.94,
ij I yz ij A yz ij n y
s
ref
s z
j
zj
zj
z
Φ = Φ j zj
zj
zj
z
z
28.16, 124.26, 17.26, 17.12, 14.65, 14.25, 13.53, 13.09, 12.83, 11.36.
s
ref j
zj
I
A
n
+
k ref {k s {k ref {
HR-ESI-MS: [M + H] m/z: calcd for C H B ClN F O Re,
1
4
7
51
2
6
4
3
067.3391; found, 1067.3435.
where Φ and Φ are the emission quantum yield of the sample and
the reference, respectively, I and I are the integrated emission
intensities of the sample and reference, respectively, A and A are the
measured absorbances of the sample and reference at the excitation
s
ref
X-ray Structural Analysis. Crystals of Re(BB3)(CO) Cl and
3
s
ref
Re(BB4)(CO) Cl were obtained via slow evaporation of saturated
3
s
ref
solutions of the respective complexes in 1,2-dichloroethane. Crystals
were mounted on a glass fiber using viscous oil and cooled to the data
collection temperature. Data were collected on a Bruker-AXS APEX
CCD diffractometer with graphite-monochromated Mo Kα radiation
wavelength, respectively, and η and η are the refractive indices of
s
ref
the solvents used for the sample and reference, respectively.
Determination of Singlet Oxygen Sensitization Quantum
Yields. O production was quantified by monitoring the fluorescence
1
(λ = 0.71073 Å). Unit cell parameters were obtained from 60 data
2
1
frames, 0.3° ω, from three different sections of the Ewald sphere. The
systematic absences in the diffraction data are uniquely consistent
with Pmn2 /n for Re(BB4)(CO) Cl with the Flack parameter refined
to virtually nil, indicating the true hand of the data had been
determined. For Re(BB3)(CO) Cl, no symmetry higher than triclinic
was observed, and solution in the centrosymmetric space group
option yielded chemically reasonable and computationally stable
results of refinement. The data sets were treated with SADABS
absorption corrections based on redundant multiscan data (Sheldrick,
G., Bruker-AXS Inc., Madison, WI, 2001). For Re(BB4)(CO) Cl, the
compound molecule was located on a mirror plane. Molecules of 1,2-
dichloroethane solvent were located cocrystallized in a 1:1 ratio with
quenching of the O2 trapping agent, 1,3-diphenylisobenzofuran
4
6,47
(DPBF).
Measurements were carried out on an automated
1
3
Photon Technology International (PTI) QuantaMaster 40 fluor-
ometer equipped with a 75-W Xenon arc lamp, an LPS-220B lamp
power supply, and a Hammamatsu R2658 photomultiplier tube using
quartz cuvettes (6Q) of 1.0 cm path length. Each cuvette contained
2.0 mL of N,N-dimethylformamide solution that was 10.0 μM in
Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, Re(BB4)(CO) Cl, or [Ru-
3
3
3
3
48
(bpy) ](PF ) (used as a reference, Φ = 0.57) and 1.0 μM in
3
6
2
Δ
3
DPBF. An additional cuvette containing only N,N-dimethlyforma-
mide and 1.0 μM DPBF was used as a control. Consumption of DPBF
was monitored by observing the change in its integrated emission
intensity following irradiation with light from an Intralux 9000 light
source (Volpi) fitted with a 10 nm (fwhm) bandpass filter centered at
500 nm (Thor Laboratories, FB500−10). During the studies of
Re(BB2)(CO) Cl and Re(BB3)(CO) Cl, the cuvettes were irradiated
the Re(BB4)(CO) Cl complex with the solvent disordered at a mirror
plane. For Re(BB3)(CO) Cl, the pyrrole(C)−ethyl(C) bonds were
3
3
restrained to be similar with one ethyl group found disordered in two
positions having a refined site occupancy of 60/40 with the
disordered chemically equivalent atoms constrained with equal
atomic displacement parameters and treated with three-dimensional
rigid-bond displacement restraints. The structures were solved using
direct methods and refined with full-matrix, least-squares procedures
3
3
1
for 10 s intervals for a total of 40 s. Due to its greater O generating
efficiency, Re(BB4)(CO) Cl was studied separately and irradiated for
5 s intervals for a total of 20 s. DPBF emission spectra were obtained
by exciting at λex = 405 nm and scanning from λem = 400−600 nm
using a step size of 1 nm and an integration time of 0.25 s.
2
3
2
43
on F . All nonhydrogen atoms were refined with anisotropic
displacement parameters. All other hydrogen atoms were treated as
idealized contributions. Structure factors are contained in the
SHELXTL program library. CIFs are deposited under CCDC
depositary numbers 1879091 and 1879092.
49−52
As has been reported previously,
calibration curves of the
integrated emission intensity versus the concentration of unreacted
DPBF remaining in solution were generated to correct for absorption
of the photosensitizers and standard between 400−600 nm. Emission
44
Steady-State Spectroscopy Experiments. UV−visible absorb-
ance spectra were collected at room temperature on a StellarNet
Silver-NoVA CCD array UV−vis spectrometer using quartz cuvettes
spectra were collected from 10.0 μM solutions of Re(BB2)(CO)
3
Cl,
con-
Re(BB3)(CO) Cl, Re(BB4)(CO) Cl, and [Ru(bpy) ](PF )
3 3 3 6 2
taining DPBF concentrations of 0, 0.25, 0.50, 0.75, 1.00, 1.25, or 1.50
μM. Linear regression lines fit to the calibration data from each
solution enabled the integrated emission intensity values obtained
sensitization experiments to be converted into the
corresponding concentrations of unreacted DPBF. A final plot of the
concentration of unreacted DPBF versus irradiation time formed a
straight line of slope m, which allowed for calculation of the O
quantum yields via the following expression:
(
6Q) with a 1.0 cm path length from Firefly Scientific. Absorption
spectra of BB2, BB4, Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, and
3
3
1
Re(BB4)(CO) Cl were collected in N,N-dimethylformamide at
concentrations of 1.0, 2.0, 3.0, 4.0, and 5.0 μM. Absorption spectra
of BB3 were collected in 10% CH Cl in N,N-dimethylformamide at
from the
O
2
3
2
2
1
concentrations of 1.0, 2.0, 3.0, 4.0, and 5.0 μM.
2
Emission and excitation spectra were recorded on an automated
Photon Technology International (PTI) QuantaMaster 40 fluor-
ometer equipped with a 75-W Xenon arc lamp, an LPS-220B lamp
power supply and a Hamamatsu R2658 photomultiplier tube. All
samples were prepared in screw cap quartz cuvettes of 1.0 cm path
length from Starna Cells, Inc. Solutions of BB2 (7.0 μM), BB4(6.0
μM), Re(BB2)(CO) Cl (8.0 μM), Re(BB3)(CO) Cl (9.0 μM) and
Re(BB4)(CO) Cl (10.0 μM) in N,N-dimethylformamide, and a
solution of BB3(6.0 μM) in 10% CH Cl in N,N-dimethylformamide
were prepared in a nitrogen-filled glovebox such that the absorbance
i m yiε
y
j
s zj ref z
j
zj
z
Φ = Φ j
zj
z
s
refj
zj
zj
z
z
j
m
n
k
ref {k s {
1
where Φ and Φ are the O sensitization quantum yields for the
s
ref
2
sample and [Ru(bpy) ](PF ) reference, respectively, m and m are
3
3
3
6
2
s
ref
the slopes of the concentration of unreacted DPBF vs irradiation time
plots for the sample and reference, respectively, and ε and ε are the
extinction coefficients at the wavelength of irradiation (500 nm) for
the sample and reference, respectively. All reported O quantum
3
2
2
s
ref
−
1
1
values of all samples at λ = 500 nm ranged between 2.60 × 10 and
3
2
.15 × 10− (1 cm path length). Samples for emission experiments
1
yields were obtained from an average of three trials.
C
DOI: 10.1021/acs.inorgchem.9b00102
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
37
Figure 1. Solid state structures of Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, and Re(BB4)(CO) Cl. Thermal ellipsoids are shown at the 30%
3
3
3
probability level (C, black; O, red; N, blue; Cl, dark green; Re, purple; B, brown; F, light green). Hydrogen atoms and cocrystallized solvent
molecules were omitted for clarity.
RESULTS
Table 2. Dihedral Angles and Average Re···BODIPY Distal
■
Separation of Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, and
3
3
BB2, BB3, and BB4 were synthesized according to published
3
9
Re(BB4)(CO) Cl
3
procedures, and metalation of these derivatives was achieved
by heating solutions of the ligands with Re(CO) Cl in 1,2-
bpy−BODPIY dihedral
average Re···BODIPY
5
a
angles (°)
separation (Å)
dichloroethane at reflux for 20 h (Scheme 1). As reported
37
previously for Re(BB2)(CO) Cl, both Re(BB3)(CO) Cl
Re(BB2)
CO) Cl
73.8, 83.2
9.43
3
3
(
3
and Re(BB4)(CO) Cl can be obtained in four steps from
3
Re(BB3)
CO) Cl
83.9, 87.5
8.39
commercially available materials, as illustrated by Scheme S1 in
the Supporting Information.
(
3
b
b
Re(BB4)
79.5
5.56
Slow evaporation of concentrated solutions of Re(BB3)-
(CO) Cl
3
(
CO) Cl, and Re(BB4)(CO) Cl dissolved in 1,2-dichloro-
a
3
3
Dihedral angles measured between planes defined by each BODIPY
b
ethane yielded single crystals of both complexes. Figure 1
displays the solid-state structures obtained for Re(BB3)-
unit and the directly appended pyridine ring. Due to the presence of
a mirror plane, the dihedral angles and Re···BODIPY distances
measured for each BODIPY unit in Re(BB4)(CO) Cl are identical.
(
CO) Cl and Re(BB4)(CO) Cl along with a reproduction of
3
3
3
37
the structure obtained previously for Re(BB2)(CO) Cl.
3
Crystallographic parameters for Re(BB3)(CO) Cl and Re-
3
directly appended for each Re complex. For all three
(
BB4)(CO) Cl are summarized in Table 1, and fully labeled
3
complexes, the bpy−BODIPY dihedral angles range from
thermal ellipsoid plots for both complexes are available in the
Supporting Information (Figure S1 and S2). The crystal
structures of all three isomeric Re complexes reveal that the
BODIPY units are nearly orthogonal to the bipyridine
backbone. Table 2 summarizes the dihedral angles between
each BODIPY moiety and the pyridine ring to which it is
1
∼
(
74°−88°. Additionally, the H NMR spectra of Re(BB3)-
CO) Cl and Re(BB4)(CO) Cl show a desymmetrization of
3
3
the aliphatic resonances attributable to the BODIPY alkyl
1
substituents that is not seen in the H NMR spectra of the free
BB3 or BB4 ligands (Supporting Information Figure S3 and
1
S4). Similar observations from the H NMR spectra of BB2
and Re(BB2)(CO) Cl were attributed to a lack of free rotation
3
Table 1. Crystallographic Parameters for Re(BB3)(CO) Cl
3
about the indacene−bipyridine bonds, which places opposite
sides of each BODIPY group in distinct environments as a
result of being in closer proximity to either the axial chloride or
the axial carbonyl ligand on each complex’s Re center (Figure
and Re(BB4)(CO) Cl
3
Re(BB3)(CO) Cl
Re(BB4)(CO) Cl
3
3
37
empirical formula
C H B C F N O Re
C H B C F N O Re
4
9
54
2
13
4
6
3
49 54
2
13
4
6
3
1). Given that dimethyl substitution at the 1- and 7-positions
of the BODIPY framework has been reported to hinder
formula weight
crystal system
space group
1165.15
triclinic
P1
̅
1165.15
orthorhombic
Pmn2(1)
53−55
rotation of meso substituents,
the rigid, nearly perpendic-
ular orientation of the BODIPY groups with respect to the
a
b
c
α
13.383(3) Å
14.555(3) Å
14.563(3) Å
88.305(4)°
81.330(4)°
62.646(4)°
2488.1(9) Å
2
27.663(4) Å
11.5474(15) Å
8.0100(10) Å
90°
90°
90°
2558.7(6) Å3
2
2
,2′-bipyridine backbone observed in all three rhenium
complexes is not surprising.
The solid-state structures of Re(BB2)(CO) Cl, Re(BB3)-
3
(
CO) Cl, and Re(BB4)(CO) Cl also allow for the comparison
3
3
β
of the proximity of the BODIPY luminophores to each of the
complexes rhenium centers to be assessed across the series of
isomeric compounds. The average through-space distance from
the boron atoms of the two appended BODIPY groups to the
central rhenium atom was determined to be 9.43 Å for
Re(BB2)(CO) Cl, 8.39 Å for Re(BB3)(CO) Cl, and 5.56 Å
γ
3
volume
Z
temperature
200(2) K
1.555 g/cm
1.725−28.302°
200(2) K
1.512 g/cm
1.91−28.49°
2.592 mm (Mo Kα)
−
3
−3
Dcalcd
3
3
2θ range
for Re(BB4)(CO) Cl, as summarized in Table 2. Accordingly,
3
−
1
−1
μ
2.666 mm (Mo Kα)
the BODIPY substituents are positioned increasingly closer to
the heavy metal as their points of attachment are changed from
the 4,4′- to the 5,5′- to the 6,6′-positions on the 2,2′-bipyridine
ligand.
reflections
unique
R(int)
27,813
12,249
0.0482
0.0588
29,089
6513
0.0716
0.0390
0.0772
R1
The steady state photophysical properties of Re(BB2)-
(CO) Cl, Re(BB3)(CO) Cl and Re(BB4)(CO) Cl were
wR2
0.1297
3
3
3
D
DOI: 10.1021/acs.inorgchem.9b00102
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
1
studied and compared to those of the corresponding
bpyBODIPY ligands in DMF. As shown in Figure 2 and
summarized in Table 3, the absorption spectrum of each of the
three rhenium complexes is dominated by a strong feature in
the visible region with a maximum extinction coefficient of
cm− at 542 nm for Re(BB4)(CO) Cl. These absorption
3
bands are attributed to the BODIPY-based S
transition, and similar features with maximum extinction
coefficients of 86,000 M cm at 526 nm, 95,100 M cm
→ S (π → π*)
1
0
38
−
1
−1
−1
−1
at 528 nm, and 100,800 M cm−1 at 526 nm are evident in the
absorption profiles of the BB2, BB3, and BB4 ligands,
respectively (Figure 2). While metalation weakly attenuates
the characteristic BODIPY absorption features and shifts them
toward slightly longer wavelengths, overall, the electronic
spectra of the metal complexes are very similar to those of the
corresponding bpyBODIPY ligands. This observation is
−1
−
1
−1
7
9,400 M cm at 530 nm for Re(BB2)(CO) Cl, 52,700
3
−
1
−1
−1
M
cm at 533 nm for Re(BB3)(CO) Cl and 88,500 M
3
notable, as Re(bpy)(CO) Cl complexes lacking additional
3
pendant chromophores usually absorb poorly at wavelengths
5
6,57
longer that ∼450 nm.
We note that the BODIPY based
adsorption observed for Re(BB3)(CO) Cl is reduced as
3
compared to the other two isomeric complexes considered.
This difference in absorptivity is presumably due to the
differences in coupling between the BODIPY units when
attached to the 5,5′-positions of the bipyridine ligand.
The room temperature emission spectra of BB2, BB3, BB4,
Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, and Re(BB4)(CO) Cl
3
3
3
were collected from DMF solutions under a nitrogen
atmosphere with λexc = 500 nm and are overlaid with the
corresponding absorption spectra in Figure 2. In contrast to
the minor differences observed in the absorption spectra
between each free bpyBODPY ligand (i.e., BB2−BB4) and the
corresponding Re complexes, incorporation of the BODIPY-
based bpy ligands onto the rhenium tricarbonylchloride center
results in dramatic changes to the indacenes’ emission
properties. For example, BB2 shows a single, intense emission
feature centered at λ = 550 nm, and similar features appear in
fl
the emission spectra of BB3 and BB4 centered at λ = 548 nm.
fl
Table 3 lists the emission quantum yields determined for the
bpyBODIPY ligands; BB2 and BB4 display Φ values close to
fl
−1
unity, and the quantum yield for BB3 (Φ = 3.5 × 10 ) is also
relatively large. Accordingly, the three ligands exhibit the small
fl
Stokes shifts and high fluorescence quantum yields that are
2
,12
hallmarks of traditional BODIPY photophysics. By contrast,
each of the isomeric rhenium complexes emit at slightly longer
wavelengths (λ = 561 nm for Re(BB2)(CO) Cl; λ = 553 nm
fl
3
fl
for Re(BB3)(CO) Cl; and λ = 564 nm for Re(BB4)(CO) Cl)
3
fl
3
and with considerably diminished intensity. The fluorescence
quantum yields of Re(BB2)(CO) Cl and Re(BB3)(CO) Cl
3
3
−
2
−2
are Φ = 1.4 × 10 and Φ = 1.3 × 10 , respectively,
fl
fl
representing ∼10−100 fold reductions in magnitude relative to
the corresponding free bpyBODIPY ligands (i.e., BB2 and
BB3). The fluorescence from Re(BB4)(CO) Cl, in which the
3
BODIPY groups are held closest to the rhenium center is
−
3
attenuated even further with Φ = 1.3 × 10 . The decrease in
fl
fluorescence quantum yield observed when BB2 and BB3 are
complexed to rhenium, which places the BODIPY fluoro-
phores ∼9.5 and 8.4 Å, respectively, from the metal center,
coupled with the even lower Φ obtained when the BODIPY
fl
units are brought within ∼5.5 Å of the rhenium atom in
Re(BB4)(CO) Cl, suggest that enhanced ISC to the triplet
3
excited state is being promoted by the heavy rhenium atom.
The rhenium mediated ISC to the triplet provides a competing
process to the facile radiative relaxation of the BODIPY singlet
excited states in each of the isomeric Re complexes.
Additional evidence of triplet state population in the
isomeric rhenium complexes of Scheme 1 is garnered by
observation of a second red-shifted feature in the emission
Figure 2. Electronic absorption spectra (solid lines) recorded under
air and normalized emission spectra (dashed lines) recorded under a
nitrogen atmosphere for (a) BB2 (black) and Re(BB2)(CO) Cl
3
(
green) in DMF, (b) BB3(black) in DMF containing 10% CH Cl
2 2
and Re(BB3)(CO) Cl(orange) in DMF, and (c) BB4 (black) and
3
Re(BB4)(CO) Cl (purple) in DMF. All spectra were recorded at 298
3
K. Inset: Emission spectra of BB4 and Re(BB4)(CO) Cl rescaled to
3
illustrate the weak emission features of the Re complex.
spectrum recorded for Re(BB4)(CO) Cl. This long-wave-
3
E
DOI: 10.1021/acs.inorgchem.9b00102
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Table 3. Photophysical Data Recorded for BB2, BB3, BB4, Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, and Re(BB4)(CO) Cl
3
3
3
λmax abs/nm(ε × 10 M cm−1
3
−1
)
λ /nm(Φ )
λ /nm(Φ )
ΦΔ (%)
fl
fl
ph
ph
BB2
526 (86.0)
530 (79.4)
528 (95.1)
533 (52.7)
526 (100.8)
542 (88.5)
550 (∼1)
561 (1.4 × 10−
2
1
2
)
)
)
Re(BB2)(CO) Cl
BB3
Re(BB3)(CO) Cl
BB4
1.7
3
−
548 (3.5 × 10
553 (1.3 × 10−
548 (∼1)
1.6
3
Re(BB4)(CO) Cl
564 (1.3 × 10−3)
829 (1.6 × 10−4
)
18.6
3
a
Spectroscopic data for BB3 was recorded in 10% CH Cl in DMF. All other data was recorded in DMF.
2
2
length band is centered at λ = 828 nm and has a quantum
photosensitizer compared to the other two complexes of
Scheme 1 is consistent with the spectroscopic observations/
rationale presented above (vide supra); although the rhenium
center promotes ISC due to enhanced spin−orbit coupling in
all three Re complexes, the especially close proximities of the
BODIPY moieties (∼5.5 Å) to the heavy metal center as a
result of their attachment at the 6,6′-positions of the bipyridyl
backbone leads to much more effective population or longer
em
yield of Φ = 1.6 × 10− (see Figure 2c, inset and Table 3).
4
ph
This long-wavelength signal was conspicuously absent when
the emission spectrum of Re(BB4)(CO) Cl was measured
3
under air, while the shorter-wavelength features observed for
Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, and Re(BB4)(CO) Cl
3
3
3
at around 555−565 nm remain unperturbed (see Figure S5).
3
Molecular oxygen in its triplet ground state ( O ) can engage
2
in energy transfer with chromophores and metal complexes in
lifetime of the triplet excited state for Re(BB4)(CO)
such, of the three isomeric Re complexes considered in this
work, Re(BB4)(CO) Cl represents a platform that is most
sensitization and related triplet photochemical
Cl. As
3
triplet excited states to return the excited light-absorber to its
1
ground state and is converted to singlet oxygen ( O ) in the
3
2
1
process. Consequently, emission quenching as a result of
exposure to air is often diagnostic of phosphorescence.
Rhenium bipyridyltricarbonyl chloride complexes are often
suitable for O
2
applications using visible excitation sources.
We emphasize that metal···BODIPY distance may not the
3
1
phosphorescent; however, emission from the MLCT state is
only factor that ultimately controls O
2
sensitization efficiency.
5
7,58
normally observed between 600−650 nm.
The spectral
3
While the series of isomeric rhenium complexes we present
demonstrates how distal separation between the BODIPY light
position of the emission feature of Re(BB4)(CO) Cl (∼830
nm) is more consistent with emission from the BODIPY
π−π* manifold, which has been previously observed at
wavelengths longer than 700 nm. Excitation spectra recorded
absorber and metal center can be correlated with intersystem
3
1
crossing efficiency and O
quantum yield, many other factors
2
15
ultimately control the efficacy of these two processes. In
particular, the strength of spin−orbit coupling, which promotes
ISC to the triplet excited state is largely dependent on identity
for Re(BB4)(CO) Cl (Figure S6) while monitoring either the
3
fluorescence (575 nm) or phosphorescence (828 nm)
emission are consistent with the BODIPY absorbance profile
31
of the metal center. Similarly, the manner by which the
visible light absorber is attached to the metal center is also
critical, as a rhenium bipyridine complex in which a BODIPY
(
Figure 2), demonstrating that both the singlet and triplet
emission of Re(BB4)(CO) Cl arise from the BODIPY
3
light absorber is tethered to a Re(bpy)(CO) Cl core via an
moieties.
3
59
alkyne linker shows Φ = 0.88. Accordingly molecular
To confirm that the air-sensitivity of the long-wavelength
Δ
topology is an important consideration in the construction of
new complexes that support triplet photochemistry.
emission feature of Re(BB4)(CO) Cl is a result of energy
3
transfer to molecular oxygen and to investigate whether
Re(BB2)(CO) Cl and Re(BB3)(CO) Cl also support triplet
3
3
1
CONCLUSIONS AND FUTURE DIRECTIONS
photochemistries and photosensitize the formation of O , the
2
■
three isomeric complexes of Scheme 1 were probed using the
singlet oxygen trapping agent 1,3-diphenylisobenzofuran
Typical BODIPY chromophores strongly absorb visible light
and are characterized by high photostability and large
fluorescence quantum yields that can approach unity. Although
these traits make them effective as fluorophores, standard
BODIPYs are generally not suitable for applications requiring
triplet photosensitizers due to their negligible rates of
4
6,47
(
DPBF)
in DMF and irradiated with λex = 500 nm.
Re(BB2)(CO) Cl, Re(BB3)(CO) Cl and Re(BB4)(CO) Cl
3
3
3
1
all demonstrate an ability to produce O , indicating that even
2
the 4,4′- and 5,5′-substituted isomers undergo at least some
ISC to the triplet excited state upon irradiation with 500 nm
light. Furthermore, because most traditional fac-Re(bpy)-
intersystem crossing. Conversely, fac-Re(bpy)(CO) Cl com-
3
plexes readily populate the triplet excited state due to the
presence of the heavy metal atom, but their utility as triplet
photosensitizers is often limited by poor light absorption
within the visible region (especially beyond 450 nm). In this
(
CO) Cl derivatives fail to absorb light longer than ∼450
3
nm, Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, and Re(BB4)-
3
3
(
CO) Cl join a small group of such complexes capable of
3
1
sensitizing the formation of O upon irradiation with light of λ
≥
study, we detailed an isomeric set of fac-Re(bpy)(CO) Cl
2
3
1
1,33,40,41
500 nm.
Ru(bpy) ] was used as an actinometer (Φ = 0.57 in
homologues with pendant BODIPY chromophores attached at
either the 4,4′-, 5,5′-, or 6,6′-positions of the 2,2-bipyridine
ligand. We anticipated these complexes would combine the
favorable photophysical properties of the two individual classes
2
+
[
3
Δ
4
8
1
DMF) to determine the O quantum yields (Φ ) for each
2
Δ
of the isomeric Re complexes. The values of Φ for
Δ
Re(BB2)(CO) Cl and Re(BB3)(CO) Cl (λ = 500 nm)
of light absorbers (i.e., BODIPY and fac-Re(bpy)(CO) Cl) to
3
3
ex
3
were found to be low and essentially equivalent (Φ = 1.7 and
deliver robust and efficient triplet photosensitizers that strongly
absorb visible light.
Δ
1
.6%, respectively). Re(BB4)(CO) Cl displayed a significantly
3
1
higher quantum yield for O production of Φ = 18.6%. The
enhanced efficiency with which Re(BB4)(CO) Cl acts as a O
Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, and Re(BB4)-
2
Δ
3
3
1
(CO) Cl form a series of three isomeric complexes in which
3
2
3
F
DOI: 10.1021/acs.inorgchem.9b00102
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
the position of the BODIPY units is attached to the 2,2-
bipyridine ligand at progressively shorter distances from the
Crystallographic and spectroscopic data (PDF)
Accession Codes
CCDC 1879091−1879092 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
rhenium center. The Re(BB2)(CO) Cl complex (4,4′-
3
BODIPY substituents), Re(BB3)(CO) Cl complex (5,5′-
3
BODIPY substituents), and Re(BB4)(CO) Cl complex (6,6′-
3
BODIPY substituents) position the light absorbing indacene
moieties at ∼9.4, 8.4, and 5.5 Å from the heavy Re center,
respectively. Electronic absorption spectra recorded for each of
these rhenium complexes show that the characteristic BODIPY
absorption features are preserved and confirm that all three
compounds are strong visible light absorbers. The high
emission quantum yields typical of traditional BODIPY
derivatives are not observed for any of the three rhenium
complexes, however, and the decrease in fluorescence intensity
becomes more extreme as the BODIPY moiety is placed closer
AUTHOR INFORMATION
■
Corresponding Author
E-mail: joelr@udel.edu; Tel.: 302-831-0716.
*
ORCID
to the rhenium center. Re(BB4)(CO) Cl, which has the
Joel Rosenthal: 0000-0002-6814-6503
Notes
The authors declare no competing financial interest.
3
shortest average through-space Re···BODIPY separation (∼5.5
Å) of the three complexes studied, also exhibits weak
phosphorescence. The quenched BODIPY fluorescence
observed across the series of rhenium complexes and the
ACKNOWLEDGMENTS
phosphorescence of Re(BB4)(CO) Cl are consistent with the
■
3
heavy rhenium atom enhancing the rate of ISC to deliver a
triplet excited state using visible light excitation.
This work was supported by NSF CAREER Award
CHE1352120 and NIH P20GM104316. NMR and other
data were acquired at UD using instrumentation obtained with
assistance from the NSF and NIH (NSF-MIR 0421224, NSF-
MIR 1048367, NSF-CRIF MU CHE-0840401, and CHE-
0541775, NIH P20 RR017716).
The ability of Re(BB2)(CO) Cl, Re(BB3)(CO) Cl, and
3
3
Re(BB4)(CO) Cl to undergo ISC following excitation with
3
light of λ = 450−550 nm makes these systems potentially
suitable for triplet photochemistry applications. To this end,
evaluation of these complexes’ abilities to photosensitize the
1
formation of O upon irradiation with light of λ = 500 nm
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