New triazine bridged triads based on BODIPY-porphyrin systems: Extended absorption, efficient energy transfer and upconverted emission

Article abstract of DOI:10.1016/j.dyepig.2021.109137

Two novel triads connecting a BODIPY to ethylenediamine substituted porphyrins via triazine linker have been synthesized and characterized. One of the triads is a linear D-A structure with one BODIPY (D) and one porphyrin (A) bridged by the triazine linke

Full text of DOI:10.1016/j.dyepig.2021.109137

Dyes and Pigments 187 (2021) 109137
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Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
New triazine bridged triads based on BODIPY-porphyrin systems: Extended
absorption, efficient energy transfer and upconverted emission
a
,
*
b
,
c
,
**
b
a
Marcos C. Souza , Carla I.M. Santos
, In ˆe s Mariz , Bruno S. Marques ,
a
a
,
d
c
c
Luana A. Machado , Leandro F. Pedrosa , Jos ´e A.S. Cavaleiro , Maria G.P.M.S. Neves ,
Ricardo F. Mendes , Filipe A.A. Paz , Jos ´e M.G. Martinho , Ermelinda Maç oˆ as
e
e
b
b,***
a
Departamento de Química Org aˆ nica, Instituto de Química. Universidade Federal Fluminense, 24020-141, Niter ´o i, RJ, Brazil
CQE, Centro de Química Estrutural, Instituto Superior T ´e cnico, Av. Rovisco Pais, 1049-001, Lisboa, Portugal
b
c
LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193, Aveiro, Portugal
Departamento de Química, Instituto de Ci ˆe ncias Exatas, Universidade Federal Fluminense, 27213-145, Volta Redonda, RJ, Brazil
d
e
CICECO-Aveiro Institute of Materials, 3 Department of Chemistry, University of Aveiro, 3810-193, Aveiro, Portugal
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two novel triads connecting a BODIPY to ethylenediamine substituted porphyrins via triazine linker have been
synthesized and characterized. One of the triads is a linear D-A structure with one BODIPY (D) and one porphyrin
Porphyrins
BODIPY
(
4
A) bridged by the triazine linker and the other one is a branched A-D structure with the porphyrin core linked
Triazine triads
Energy transfer
to four BODIPY units. The triads show extended absorption in the visible region with contributions from both
porphyrin (Soret band centred at 410–430 nm) and BODIPY units (strong absorption at ≈ 502 nm) in good
agreement with the expected molar ratio. Both triads exhibit linear and nonlinear optical properties featuring an
efficient energy transfer from the BODIPY donor to the porphyrin acceptor. The nonlinear upconverted emission
properties of the triads were studied by two-photon excitation in the Near-infrared (NIR, 710–930 nm). The
maximum two-photon absorption cross-section values for the triads (40–70 GM) are larger than those typically
reported in this wavelength range for porphyrins and BODIPY. Both the green emission of BODIPY (≈514 nm)
and the red emission of porphyrins (650–750 nm) were observed under NIR excitation at 930 nm. The distinct
features of triads, namely i) an extended absorption; ii) an efficient energy transfer and iii) the nonlinear
upconverted emission featuring a large separation between the excitation and emission wavelengths could be
beneficial for application in sensing and imaging procedures.
1
. Introduction
[20–22]. The absorption spectrum of porphyrins has a typical profile
consisting of an intense Soret band at λ = 400–450 nm followed by a
In recent years, a wide range of compounds have been combined to
construct multichromophoric and -conjugated macromolecules with
weaker quartet of bands at λ = 550–650 nm known as the Q bands. The
absorption gap between 450 and 550 nm is a significant limitation for
the use of these macrocycles in photocurrent generation phenomena.
One of the approaches to expand the absorption profile of porphyrin
derivatives is their association to chromophores with strong absorption
in the above region. Green chromophores such as 4,4-difluor-
o-4-bora-3a,4a-diaza-s-indacene, commonly known as BODIPY, with
absorption features complementary to porphyrins, are excellent candi-
dates for a synergistic improvement of the intrinsic optical properties of
the individual units [4,23–29]. BODIPY derivatives have been widely
π
excellent photophysical profiles [1,2]. The presence of conjugated sys-
tems improves the efficiency of electron and energy transfer processes
[
3,4]. Porphyrins, which are tetrapyrrolic ligands with high chemical
stability, unique optical properties and strong redox activity are an
example of these compounds [5–7]. During the last two decades, such
compounds have been extensively studied for use in numerous appli-
cations such as solar cells [8,9], chemosensors [10–13], artificial
photosynthesis [14], catalysis [15–19], and biomedical applications
*
Corresponding author.
*
*
* Corresponding author.
** Corresponding author.
E-mail address: marcoscs@id.uff.br (M.C. Souza).
https://doi.org/10.1016/j.dyepig.2021.109137
Received 27 October 2020; Received in revised form 15 December 2020; Accepted 2 January 2021
Available online 9 January 2021
0
143-7208/© 2021 Elsevier Ltd. All rights reserved.

M.C. Souza et al.
Dyes and Pigments 187 (2021) 109137
used as light harvesting antenna groups due to their excellent photo-
of organoids and in vivo imaging of small animals) or photosensitizers in
scattering media [46–48]. Two-photon induced processes hold also
great interest for applications in micromachining and optical power
limiting [49]. NIR excitation grants the possibility of penetrating deeper
into most materials and thick biological tissue due to reduced absorption
and scattering of NIR radiation. The synthesized triads exhibit linear and
nonlinear optical properties shaped by the efficient energy transfer from
the BODIPY to the porphyrin. We observed both green and red emission
upon excitation at 930 nm featuring a large separation between exci-
tation and emission wavelengths that can be of potential value for
sensing and imaging purposes.
4
chemical stability, high molar extinction coefficient (
ε > 6 × 10
ꢀ
1
ꢀ 1
cm
M at the maximum absorption wavelength) and easily tunable
optical and electrochemical properties [30]. However, the light har-
vesting ability of BODIPY fluorophores is also limited by their typically
narrow absorption spectra. Panchromaticity can be achieved by
coupling the BODIPY fluorophores with porphyrins. In addition, the
preparation of compounds carrying multiple BODIPY units [31–33] can
increase the amount of light that is collected by molecular chromo-
phores avoiding undesired effects associated with aggregation in highly
concentrated films.
BODIPY fluorophores are also particularly popular in bioimaging
and sensing applications due to their near unitary fluorescence quantum
yield [34–36]. However, like most organic dyes, their small Stokes shift
2. Material and METHODS
(
10–20 nm) limits the signal-to-noise ratio due to reabsorptions of
2.1. Chemicals
emitted photons and cross-talk between the excitation source and the
fluorescent emission. In this sense, molecules containing both porphyrin
and BODIPY units are promising [37–40], and have been the focus of our
recent effort to achieve conjugates able to combine the properties of
photosensitizers (PS) for photodynamic therapy (PDT) with those of
fluorescent markers for biologic studies by exploring efficient intra-
molecular energy transfer processes and enhanced optoelectronic
properties in the blue-green region of the spectrum.
The reagents used for the preparation of porphyrins and BODIPY-
Porphyrin conjugates were purchased from Sigma-Aldrich. All solvents
were used as received from different commercial sources, with the
exception of tetrahydrofuran that was dried using standard procedures.
2.2. Instrumentation
For the construction of the conjugates, we selected 1,3,5-triazine as
the linker between the BODIPY and the porphyrin units due to its suit-
ability to join up to three substituents of interest, according to the well-
known temperature-dependent stepwise substitutions of the three Cl
atoms from the starting cyanuric chloride [41]. In fact, triazine bridged
triads having covalently bonded porphyrin and/or BODIPY constituents
have been considered in the literature as promising agents in fast
transfer systems [42] and in dye sensitized solar cells (DSSC) [43–45]. In
the present study we constructed a fluorescent framework that can
accommodate a combination of three different functional groups, where
the BODIPY block acts as the fluorophore energy donor (D), the
porphyrin core serves as the energy acceptor/photosensitizer (A) and
the third substituent functions as an auxiliary group (Fig. 1).
Melting points are uncorrected and were determined on a Büchi B-
540 device. The mass spectra were recorded using a ESI-MS MALDI-
Micromass Q-TOF2 equipment (potential 40–260 eV). UV–Vis spectra
were recorded on a Shimadzu UV-2501PC spectrophotometer. NMR
spectra were obtained on a Varian Unity Plus/Bruker Advance DPX
1
13
spectrometer (300 or 500 MHz): H: 300.13 MHz or 500.13 MHz; C:
75.47 MHz or 125.76 MHz and 19F: 282.38 MHz or 470.27 MHz 13C
assignments were made based on 2D HSQC and HMBC experiments
(delay for long-range J C/H couplings were optimized for 7 Hz). Samples
were dissolved in the specified solvent, CDCl3 or DMSO‑d . Chemical
6
shifts were expressed in δ (ppm) for the following references: tetrame-
1
13
thylsilane (internal) for H and C and hexafluorobenzene (external) for
1
9
F. Coupling constants (J) were expressed in Hertz (Hz) and the areas of
Two triads were synthesized, a linear D-A structure with one BODIPY
and one porphyrin unit bridged by the triazine linker and one branched
the signals were obtained by electronic integration. Multiplicity of sig-
nals was noted as: s-singlet, d-doublet, t-triplet, q-quartet, quint-quintet,
m-multiplet.
4
A-D structure with the porphyrin core linked to four BODIPY units. The
structure and optical properties of the triads were fully characterized.
The nonlinear upconverted emission properties of the triads where
studied by two-photon excitation in the Near-infrared (NIR, 730–930
nm). NIR excitation is of outmost convenience for the use of fluo-
rophores in high resolution imaging of 3D samples (e.g. in vitro imaging
2.3. Spectroscopic measurements
The linear absorption spectra were recorded on a JASCO V-540
spectrophotometer. The fluorescence spectra were recorded using a
Horiba Jobin Yvon Fluorlog 3–22 Spectrofluorimeter with a xenon lamp
of 450 W. The spectra were recorded in spectroscopic grade dime-
thylsulfoxide (DMSO) using 5 × 5 mm quartz cells. The fluorescence
quantum yields where determined upon excitation of the Soret band at
4
22 nm and the BODIPY at 470 nm using 5,10,15,20-tetraphenylpor-
phyrin (TPP) in toluene (Φ = 0.11) and fluorescein at pH 11 as stan-
dards (Φ = 0.92) [50].
Two-photon absorption (TPA) spectra were measured in 5–10
μ
M
DMSO solutions by two-photon fluorescence (TPF) using fluorescein at
pH 11 and TPP in CCl as standards, to account for collection efficiency
4
and pulse characteristics [51]. A modified setup that follows closely the
one described by Xu and Webb was used to estimate the TPA cross
section in the 710–930 nm region. The two-photon emission (TPE) was
measured within a narrow wavelength bandwidth selected by the
H20Vis Jobin Yvon monochromator placed at the entrance of a
PMC-100-4 photomultiplier tube (Becker and Hickl GmbH). The exci-
tation source was a Ti:sapphire laser (Tsunami BB, Spectra-Physics,
7
10–990 nm, 1.7 W, 100 fs, 82 MHz).
The integrated TPE over the entire emission band was extrapolated
using the overall emission spectra measured in the fluorimeter upon
excitation at half the two-photon excitation wavelength (λTPA/2). The
two photon absorption cross section was calculated from Equation (1):
Fig. 1. General framework highlighting triazine bridged triads.
2

M.C. Souza et al.
Dyes and Pigments 187 (2021) 109137
ꢀ
3
3
σ
= (F
2
/ΦCn) (ΦCn
σ
2
/ F
2
)ref
(1)
= 1.393 g cm , red plate with crystal size of 0.19 × 0.19 × 0.08 mm .
s
Of a total of 43,098 reflections collected, 5406 were independent (Rint
=
where F
2
stands for integrated fluorescence intensity, Φ is the fluor-
0.0437). Final R1 = 0.0600 [I > 2σ(I)] and wR2 = 0.1646 (all data).
◦
escence quantum yield, n refers to the refractive index in solution, C is
the concentration, and s and ref are relative to the sample and the TPA
reference, respectively. The emission intensity dependence of the exci-
tation power was checked. The relative error of the cross sections values
is at most ±20%. The fluorescence decays were measured in 5 mm
quartz cuvettes by the Single-Photon Timing technique under excitation
at 330 nm by collecting the emission at 514 and 700 nm. The excitation
source was the 2nd harmonic of a Coherent Radiation Dye laser 700
series (laser dye DCM, 610–680 nm, 130 mW, 5 ps, 4 MHz). The emis-
sion was collected at the magic angle by a Jobin Yvon HR320 mono-
chromator (Horiba Jovin Ivon Inc.). The instrument response functions
Data completeness to theta = 25.24 , 99.7%. CCDC 1951549.
2.5. Synthesis
All reactions sensitive to air or moisture were carried out under N2.
Chemicals of analytical grade were purchased from commercial sources
(Sigma-Aldrich or Merck) and were used as received. Solvents were
distilled and dried, according to known methodologies. Analytical TLC
was carried out on precoated sheets with silica gel (0.2 mm thick,
Merck), the eluents were prepared volume-by-volume (v/v) and visu-
alization was performed by revelation under ultraviolet radiation (254/
366 nm) and/or iodine. Column chromatography was carried out with
silica gel 70–230 mesh (Aldrich). 5,10,15,20-tetrakis(pentafluoro-
phenyl)porphyrin (TPPF20), tert-butyl-(2-aminoethyl)carbamate 1, and
4,4-difluoro-1,3,5,7-tetramethyl-8-(4-hydroxyphenyl)-4-bora-3a,4a-
diaza-s-indacene (BODIPY-OH) were synthesized as described in the
literature [61–63].
(
35–80 ps FWHM) was estimated using a scattering dispersion of
colloidal silica in water. Decay curves were stored in 1024 channels with
2
4.4–48.8 ps per channel and an accumulation of 20k counts in the peak
channel. The fluorescence decays were analyzed by a nonlinear least-
squares reconvolution method using the TRFA DP software by SSTC
(
Scientific Software Technologies Centre, Belarusian State University,
Minsk, Belarus).
Synthesis of TPPF19NHCH CH NHBoc (2). A 50 mL round-bottom
2
2
flask containing a magnetic stirring bar was charged with 75 mg of
2
.4. Single-crystal X-ray diffraction studies
TPPF20 (0.0767 mmol), 12 mg of mono-protected ethylenediamine 1
(
0.0767 mmol), 5.5 mg of potassium carbonate (0.0384 mmol) and 20
◦
Single crystals of compound 7 were manually harvested from the
mL of toluene. The mixture was heated at 120 C for 24 h under nitrogen
atmosphere. The flask was allowed to cool to room temperature and the
solvent was removed under reduced pressure. Then, the residue was
diluted with chloroform and the organic phase was washed with water
(3 × 20 mL). After separation of the organic phase, the solvent was
removed under vacuum and the residue was purified by silica gel pre-
parative TLC using chloroform as the eluent. From the first fraction was
recovered the starting porphyrin (6%) and compound 2 was obtained
from the second fraction in 68% yield, after recrystallization in a
mixture of chloroform and methanol. Compound 2 was characterized by
crystallization vials and immersed in highly viscous FOMBLIN Y per-
fluoropolyether vacuum oil (LVAC 140/13, Sigma-Aldrich) to avoid
degradation caused by the evaporation of the solvent [52]. Crystals were
mounted on either Hampton Research CryoLoops or MiTeGen Micro-
Loops, typically with the help of a Stemi 2000 stereomicroscope
equipped with Carl Zeiss lenses.
Crystal data were collected at 150(2) K on a Bruker X8 Kappa APEX II
CCD area-detector diffractometer (Mo K graphite-monochromated ra-
α
diation, λ = 0.71073 Å) controlled by the APEX3 software package [53]
and equipped with an Oxford Cryosystems Series 700 cryostream
monitored remotely using the software interface Cryopad [54].
Diffraction images were processed using the software package SAINT+
1
19
H and
Information).
Synthesis of TPPF16(NHCH
sphere (N ), a round-bottom flask, equipped with a magnetic stirring bar
F NMR, UV–Vis and MS (Figs. S1–S4, Supplementary
2
2 4
CH NH-Boc) (3). Under inert atmo-
[
55], and data were corrected for absorption by the multiscan
2
semi-empirical method implemented in SADABS 2016/2 [56].
All structures were solved using the algorithm implemented in
SHELXT-2014/5 [57], which allowed the immediate location of almost
all of the heaviest atoms composing the molecular unit of compound 7.
The remaining missing and misplaced non-hydrogen atoms were located
from difference Fourier maps calculated from successive full-matrix
least-squares refinement cycles on F2 using SHELXL from the 2018/3
release [58]. All structural refinements were performed using the
graphical interface ShelXle [59].
was charged with 75 mg of TPPF20 (0.0767 mmol), 49 mg of tert-butyl
(2-aminoethyl) carbamate 1 (0.11 mmol), 21 mg of potassium carbonate
(0.015 mmol) and 20 mL of toluene. The mixture was refluxed for 72 h
until consumption of the starting porphyrin TPPF20. After cooling to
room temperature, the mixture was diluted with chloroform and washed
with water (3 × 20 mL). The organic phase was concentrated under
vacuum and the residue was purified by silica gel preparative TLC using
as eluent a mixture of chloroform-methanol (9:1) Compound 3 was
isolated in 63% and characterized by 1H and F NMR and UV–Vis
(Figs. S5–S8, Supplementary Information).
19
Hydrogen atoms bound to carbon were placed at their idealized
positions using appropriate HFIX instructions in SHELXL: 43 (aromatic
carbon atoms) and 137 (for the methyl groups). These hydrogen atoms
were included in subsequent refinement cycles with isotropic thermal
displacements parameters (Uiso) fixed at 1.2 (for the former family of
hydrogen atoms) or 1.5 × Ueq (solely for those associated with the
methyl group) of the parent carbon atoms.
Synthesis
of
NH
TPPF19NHCH
2
CH
2
NH
2
(4)
and
TPPF16(NHCH
2
CH
2
2 4
) (5). In a two necked round-bottom flask
containing porphyrin 2 or 3 (15 mg) and dichloromethane (1 mL) it was
added TFA (1 mL, 1.5 equiv.). The resulting mixture was maintained
under stirring for 2 h at room temperature. At this time, TLC showed the
complete consumption of the starting material. Then, the reaction was
neutralized with an aqueous solution of NaOH 1 M until pH to ~12.
Finally, the organic product was extracted with chloroform, the organic
The last difference Fourier map synthesis showed for 7, the highest
peak (0.813 eÅ 3) and the deepest hole (ꢀ 0.392 eÅ ) located at 1.54
and 0.58 Å from Cl1, respectively. Structural drawings have been
created using the software package Crystal Impact Diamond [60].
Crystallographic data (including structure factors) for the crystal
structure of 7 has been deposited with the Cambridge Crystallographic
Data Centre. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB2 2EZ, U.K. FAX:
ꢀ
ꢀ 3
2 4
phase separated, dried over anhydrous Na SO and the solvent removed
under reduced pressure. The products (4 and 5) were fully characterized
by NMR, mass and UV–Vis techniques (Figs. S9–S16, Supplementary
Information).
Synthesis of 4,6-dichloro-N,N-diphenyl-1,3,5-triazin-2-amine
(6) [64]. Under a nitrogen atmosphere 1.84 g (0.01 mol) of cyanuric
chloride and 2.12 g (0.02 mol) of sodium carbonate were dissolved in 20
(
+44) 1223 336033. E-mail: deposit@ccdc.cam.ac.uk.
Crystal data for 7: C34 28BClF
O, M = 620.88, monoclinic, space
group P2
17) Å, β = 105.547(2) , V = 2961.0(6) Å ,
◦
H
N
2 6
mL of dry acetone at 0 C. To this mixture a solution of 1.42 g (0.0084
1
/n, Z = 4, a = 10.8010(12) Å, b = 18.800(2) Å, c = 15.1359
mol) of diphenylamine in 10 mL of dry acetone was added dropwise. The
◦
3
ꢀ 1
◦
(
μ
(Mo-K
α
) = 0.182 mm , D
c
reaction was kept at 0 C and stirred for about 4 h, monitored by TLC.
3

M.C. Souza et al.
Dyes and Pigments 187 (2021) 109137
Upon reaction completion the white precipitate was filtered off and the
solvent evaporated under reduced pressure. The remained solid was
washed with acetone three times, then purified by column chromatog-
raphy using CH
2
Cl
2
:hexane (2:1) as the eluent. A pale yellow solid (m.p.
◦
1
78–179 C) was obtained in 64% yield (see characterization in
Figs. S17–S18, Supplementary Information).
Synthesis of compound 7. Under nitrogen atmosphere 41.0 mg
(
0.12 mmol) of BODIPY-OH and 38.06 mg (0.12 mmol) of 1,3,5-triazin
6
were dissolved in 9 mL of dry acetone, then 19 mg (0.18 mmol) of
◦
sodium carbonate was added. The mixture was stirred at 35 C for 27 h,
and the evolution of the reaction was monitored by TLC. Upon reaction
completion the white precipitate was filtered off and the solvent evap-
orated under reduced pressure. The crude product was purified by col-
umn chromatography using CH
2
Cl
2
:hexane as the eluent (gradient 2:1 to
◦
4
:1). A bright orange-red solid (dec. > 230 C) was obtained in 56%
yield and fully characterized by NMR, mass and UV–Vis (see charac-
terization in Figs. S19–S25, Supplementary Information).
Synthesis of compound 8. Under nitrogen atmosphere 5.5 mg
(
0.0054 mmol) of 4 and 3.06 mg (0.0049 mmol) of 7 were dissolved in 7
mL of THF. Two drops of DIPEA were added and the resulting mixture
was refluxed for 5 h. Upon reaction completion (monitored by TLC) the
white precipitate was filtered off and the solvent was evaporated under
reduced pressure. The residue was dissolved in CH
phase washed twice with water and dried with anhydrous Na
following standard procedures. The crude material was purified by
preparative TLC using CH Cl as eluent, to give a reddish brown solid in
1% yield from the second upper fraction. The full structural charac-
2
Cl
2
, the organic
SO
2 4
2
2
8
terization is presented in Figs. S26–S31, Supplementary Information.
Synthesis of compound 9. Under nitrogen atmosphere 5.75 mg
(
0.0051 mmol) of 5 and 11.5 mg (0.0185 mmol) of 7 were dissolved in 6
mL of THF. 20 mg of K CO were added and the resulting mixture was
2
3
refluxed for 28 h. Upon reaction completion (monitored by TLC) the
white precipitate was filtered off and the solvent was evaporated under
reduced pressure. The residue was dissolved in CH
2
Cl
2
, the organic
SO
Scheme
1. Synthesis
of
the
) K
Cl , rt, 2 h.
ethylenediamine
substituted
) K
fluo-
CO
rophenylporphyrins 4 and 5. a
1
2
3
CO , toluene, reflux, 24 h; a
2
2
3
,
phase washed twice with water and dried with anhydrous Na
2
4
toluene, reflux, 72 h b) TFA, CH
2
2
following standard procedures. The crude material was purified by
preparative TLC using THF:hexane (3:2) as the eluent, to give a reddish
brown solid in 40% yield. The full structural characterization is pre-
sented in Figs. S32–S40, Supplementary Information.
TPPF20 was controlled by the number of equiv. of the amino derivative 1
and the reactions were performed in toluene at reflux in the presence of
potassium carbonate [62,66]. The mono-N-Boc protected ethylene
diamine porphyrin 2 was isolated in 68%, after 24 h of reaction of
TPPF20 with 1 equiv. of amine 1, while the tetra-substituted porphyrin 3
was obtained in 63% after 72 h of reaction using 4 equiv. of the amine.
The evolution of both reactions was controlled by TLC and were stopped
after verifying the almost consumption of the starting porphyrin
accompanied by the formation of the desired main product. The struc-
tures of porphyrins 2 and 3 were confirmed by NMR, UV–vis and
high-resolution mass spectrometry, see the Supplementary Information,
3
. Results and DISCUSSION
8
-(4-hydroxyphenyl)-1,3,5,7-tetramethyl BODIPY, herein abbrevi-
ated as BODIPY-OH, was elected as the fluorophore in our study due to
its high fluorescence and appropriate nucleophilicity with respect to
cyanuric chloride [32]. BODIPY-OH was synthesized in 21% yield by
the mechanochemical method described by Laramine et al. [65]
exploring the green chemistry concept. Diphenylamine was chosen as
the auxiliary group, which is expected to modify the chemical and
physical properties of the triad with particular emphasis on the solubi-
lity. Ultimately two porphyrins derived from the well-studied 5,10,15,
1
Figs. S1–S8. The analysis of H NMR spectra (Figs. S1 and S5) show in
both cases the expected absorption at lower field of the eight β-pyrrolic
protons of the porphyrin core. In the aliphatic region the resonances of
the methylene protons from one alkyl or four alkyl chains appear with
the expected pattern - two triplets at δ ~3.8 ppm and δ ~3.6 ppm. The
resonances of the Boc protons appear as a singlet at δ ~1.45 ppm and of
the two nitrogen internal protons at δ ~ ꢀ 2.9 ppm as a broad singlet.
The analysis of 19F NMR spectra confirms also the expected loss of one
p-fluorine atom in the case of porphyrin 2 and of four p-fluorine atoms in
porphyrin 3 (Figs. S2 and S6). In the 13C NMR spectra the carbon res-
onances of the Boc methyl groups are located at δ ~29.7 ppm and those
due to the remaining methylene carbons are found at δ ~47.1 ppm and δ
2
0-tetrakis(pentafluorophenyl)porphyrin (TPPF20) [61] containing one
or four ethylenediamine branches, respectively 4 and 5, were synthe-
sized (Scheme 1) to complete the triads 8 and 9 presented in Scheme 2.
Two-carbon chains separate the porphyrin core from the terminal amino
group to minimize the steric interference towards the nucleophilic
substitution reaction over the chlorine atom of the triazine intermediate
7
.
Synthesis of the ethylenediamine substituted fluorophenylporphyr-
ins 4 and 5.
The synthetic access to the amine porphyrins 4 and 5 involved, in
~
40.9 ppm. The deprotection of the amino group or groups in por-
phyrins 2 and 3 in the presence of trifluoroacetic acid (TFA) afforded
after the work up the desired porphyrins 4 and 5 in 82 and 87% yields,
respectively. The structures of porphyrins 4 and 5 were unambiguously
confirmed by 1D, and 2D NMR studies and their molecular formulas by
both cases, the reaction of 5,10,15,20-tetrakis(pentafluorophenyl)
porphyrin (TPPF20) with the mono-protected Boc-ethylenediamine 1,
prepared according to literature [62], followed by the amine depro-
tection step (Scheme 1).
+
HRMS. In these spectra, the ion peak [M+H] at 1015.12537 for 4 and
The nucleophilic substitution of one or four para fluorine atoms in
4

M.C. Souza et al.
Dyes and Pigments 187 (2021) 109137
Scheme 2. Synthetic pathway for BODIPY-Porphyrin conjugates 8 and 9.
at 1135.31798 for 5 confirmed the successful cleavage of the N-Boc
compound 6.
group(s) (Figs. S12 and S16). Moreover, in the aliphatic region of their
Moreover, the presence of a resonance for two β-pyrrolic protons (δ
= 5.98 ppm), two sets of resonances for two methyl groups each (δ =
2.56 ppm and 1.27 ppm) and three sets of resonance for a total of
1
H NMR spectra is patent the absence of the signal due to the Boc protons
and the presence of two triplets corresponding to the methylene hy-
drogens at δ ~3.7 and 3.1 ppm; a broad singlet due to the resonance of
NH of the ethylenediamine fragment(s) appear(s) for porphyrin 4 at δ ~
1
fourteen aryl protons (δ = 7.16–7.39 ppm) in the H NMR spectrum, and
1
9
the characteristic signal at δ = ꢀ 142.35 in the F NMR spectrum cor-
5
ppm and for porphyrin 5 at δ ~ 6.5 ppm). The resonances of the
2
responding to the BF group, corroborate the expected structure (see the
β-pyrrolic protons as a singlet at δ 9.2 ppm for 5 and as the two doublets
at δ 8.9 and 8.8 ppm for porphyrin 4 is on line with the substitution
degree (Figs. S9 and S13).
complete NMR assignments in the Supplementary Information).
Single-crystal X-ray diffraction was also used to fully elucidate the
structural features of compound 7 and to unequivocally prove the suc-
cess of the nucleophilic substitution of 6 with BODIPY-OH. Compound 7
crystallizes in the centrosymmetric P21/n space group, with the asym-
metric unit being composed of a whole molecular unit as depicted in
Fig. 2. The close packing of individual molecular units is mainly driven
by the need to effectively fill the available space. A number of weak
supramolecular contacts are present within the crystal structure, mainly
C–H⋯F hydrogen bonding interactions [between two methyl groups
3
.1. Synthesis of triazine-BODIPY-porphyrin triads
The initial step for the synthesis of the triads 8 and 9 exhibited in
Scheme 2 involves the reaction of cyanuric chloride with diphenylamine
◦
in the presence of sodium carbonate at 0 C in dry acetone [64]. Spe-
cifically, the choice of this amine as the first group to be connected to the
triazine ring is due to its poor nucleophilicity, affording the known in-
termediate 6 without scrambling with poly-substituted products. Com-
pound 6 was conveniently collected in 64% yield after column
chromatography and its mono-substitution pattern confirmed by MS. In
the subsequent step, it was performed the replacement of a chlorine
atom of 6 using the equivalent amount of BODIPY-OH in the presence of
(
positions 1 and 7 of the BODIPY system) and F1, and one o-phenyl
carbon with F2]: dC⋅⋅⋅F distances found in the 3.187(4)-3.361(4) Å range
with the corresponding <(CHF) interaction angles in the range 117-
◦
1
45 . Despite the presence of several aromatic rings, no structurally-
relevant
π
⋅⋅⋅π
contacts were found.
The last step to achieve triads 8 and 9 consisted in the nucleophilic
substitution of the mono- and tetra-amine porphyrins 4 and 5, respec-
tively, over intermediate 7.
◦
sodium carbonate in dry acetone at 35 C. In the course of the reaction, a
new fluorescent orange spot related with the product was observed in
The protocol that has been used by many authors when amino-
porphyrins are the nucleophile over cyanuric chloride derivatives
makes use of DIPEA as the HCl acceptor [42,43,45,68].
2 2
the TLC plates (CH Cl :hexane 2:1) after 2 h, along with a considerable
amount of 6. In our case, the consumption of 6 only took place after 27 h
of stirring, and this is significantly longer than the 2 h period reported in
the literature for similar conditions (see references [31,32,64,67] as
examples). The crude reaction mixture was passed through a chroma-
tography column affording the new compound 7 in 56% yield. In the
HRMS-ESI spectrum, the ion peak of 7 [M+H]+ at 621.21247 un-
equivocally confirms that one BODIPY unity was introduced into
In our experiments, DIPEA reacted as expected in the synthesis of the
mono-adduct 8, but represented a persistent contaminant in the syn-
2 3
thesis of the tetra-adduct 9 and so due to that it was replaced by K CO .
In both cases a 10% excess of 4 and 5 was added and the reactions were
followed by TLC until the stage when a minimum amount of BODIPY-
5

M.C. Souza et al.
Dyes and Pigments 187 (2021) 109137
observed in the 19F NMR spectra (Fig. 3). In compound 8 the two fluo-
rine atoms of BF
2
give rise to a resonance centred at δ = ꢀ 142.77 ppm,
which is consistent with the integral ratio of the fluoroporphyrin system:
six ortho-F, six meta-F, three para-F, two ortho’-F and two meta’-F. On the
other hand, the symmetric compound 9 has a simpler spectrum showing
a resonance centred at δ = ꢀ 142.66 ppm for a total of eight fluorine
2
atoms of four BF groups, in agreement with the integral ratio of eight
ortho-F and eight meta-F of the fluoroporphyrin moiety.
The UV–Vis absorption and fluorescence emission spectra of the
triazine-BODIPY-porphyrin triads are compared with those of the indi-
vidual units in Fig. 4. Table 1 presents a summary of the optical prop-
erties of compounds 4, 5, 7, 8 and 9.
The ethylenediamine substituted fluorophenylporphyrins 4 and 5
show typical absorption spectra of meso-tetraarylporphyrins with the
intense Soret band centred in the 410–430 nm region and being
accompanied by less intense Q-bands between 550 and 650 nm (Fig. 4).
The maximum molar extinction coefficient of the Soret band is 17.97 ×
4
ꢀ 1
ꢀ 1
4
ꢀ 1 ꢀ 1
cm for 5 (Table 1). The
1
0
M
cm for 4 and 12.37 × 10
M
BODIPY derivative 7 shows a narrow absorption band centred at 502 nm
4
ꢀ 1
ꢀ 1
with molar absorptivity of 7.99 × 10 M cm in good agreement with
the UV–Vis absorption spectrum of the BODIPY fluorophore.
The absorption spectra of triads 8 and 9 show features of both
porphyrin and BODIPY units in good agreement with the expected molar
ratio. The molar extinction coefficient of the Soret band in the triads (15-
4
ꢀ 1
ꢀ 1
1
6 × 10 M cm ) are similar to those of the isolated porphyrins, thus
confirming the presence of one porphyrin unit per triad. The molar
extinction coefficients of the green absorption of BODIPY in triad 8 (5.9
4
ꢀ 1 ꢀ 1
×
10 M cm ) is of the same order of magnitude of derivative 7,
Fig. 2. Schematic representation of the molecular unit present in the crystal
structure of 7. Non-hydrogen atoms are represented as thermal ellipsoids drawn
at the 50% probability level and hydrogen atoms as small spheres with arbi-
trary radii.
thought it appears to be 30% lower. This band is nearly four times
stronger in triad 9 than in triad 8, thus confirming the tetra-substitution
of the porphyrin core in triad 9.
3
.2. Steady-state emission
OH was detected. Mono-adduct 8 was isolated in 81% yield by prepar-
2 2
ative TLC (CH Cl ) after 5 h in refluxing THF, while completing the total
Two emission bands are observed at 660 and 706 nm for compound 4
of four substitutions on tetra-adduct 9 required a longer time (28 h). The
and 660 and 714 nm for compound 5. The emission quantum yield Φ of
the mono-substituted fluorophenylporphyrin 4 in DMSO (Φ = 3.8%) is
slightly lower than that of the tetra-substituted derivative 5 (Φ = 7.4%).
The quantum yield of the fluorophenylporphyrin is 2–3 times lower than
that of the TPP standard used to estimate the quantum yield. This is a
well-known effect of having the hydrogen atoms in the meso-aryl moi-
eties replaced by fluorine, which leads to a larger distortion from
planarity. In addition, the halogen atoms promote a spin-orbit coupling
that decreases the radiative deactivation pathways while increasing the
non-radiative pathways [69]. For compound 7, the emission of BODIPY
is centred at 516 nm with a Stokes shift of only 14 nm and Φ = 65%,
considerably higher than that of the porphyrin derivatives.
latter product was isolated from preparative TLC (THF:hexane 3:2) in
4
0% yield. In addition to the losses inherent to the TLC purifications of
multi substitution reactions, the lower yield of 9 compared to 8 could be
attributed to BODIPYs partially substituted porphyrins, whose isolation
was not the focus of this work. The structural identities of 8 and 9 were
confirmed by ESI- HRMS, 1H, C and F NMR spectroscopy and the
13
19
respective spectra are depicted in Supplementary Information
(
Figs. S26–S40). For each compound, the major peak in the ESI-HRMS
spectrum appeared as half the calculated molecular weight in agree-
2
+
ment with the double-charged [M+2H] species.
In the 1H NMR spectrum of 8, the porphyrin core shows a set of
resonances for the eight asymmetric β-pyrrolic protons around δ = 9
ppm and one resonance for two inner NH protons at δ = ꢀ 2.92 ppm. The
resonances due to the ethylenediamine fragment appear as a set of sig-
nals between δ = 3.90–3.65 ppm for the four ethylene protons and as
two broad peaks for the two NH protons at δ = 5.16 and 4.72 ppm. The
BODIPY moiety of 8 showed the same pattern of signals as its precursor 7
and similar chemical shifts. One difference, however, is that the two
methyl groups at positions 1 and 7 of the BODIPY system in compound 8
are split into two resonances at δ = 1.25 and 1.23 ppm, denoting slightly
different environments. The tetra-substitution pattern in compound 9 is
confirmed, for example, by the 1:4 ratio in the number of inner NH
hydrogens of the porphyrin centre (2H, δ = ꢀ 2.89 ppm) and the pe-
ripheral BODIPY β-pyrrolic protons (8H, = 5.90 ppm). Highlighted as-
signments are: a resonance at δ = 2.52 ppm for twenty four methyl
protons (positions 3 and 5 of the BODIPY system), two sets of resonance
at δ = 1.24 and δ = 1.20 ppm for twelve methyl protons each (positions 1
and 7) and a set of resonance in the range δ = 7.09–7.35 ppm relative to
fifty six aryl protons.
The photoluminescence spectra of the triads upon excitation at 470
nm feature a narrow band in the green (≈515 nm) and a doublet in the
red (660/708 nm and 660/718 nm for 8 and 9) ascribed to the BODIPY
and the porphyrin units, respectively. The peak positions of the emission
bands in the triads are nearly coincident with those observed in the
isolated components evidencing the lack of conjugation between the
porphyrin and the BODIPY units. Emission in the green is exclusively
due to direct excitation of the BODIPY. This is supported by the fact that
the excitation spectra collected in the green (514 nm) shows only the
typical BODIPY absorption band at 502 nm (dashed blue trace in Fig. 4).
On the other hand, emission in the red is observed from either direct
excitation of the porphyrin and excitation of the BODIPY followed by an
efficient energy transfer from the BODIPY donor (D) to the porphyrin
acceptor (A). The energy transfer process is supported by the fact that
the excitation spectra of the triads recorded upon collection of the
emission in the red (660 or 700 nm) overlaps perfectly with the ab-
sorption spectra (dashed green and red lines in Fig. 4). Furthermore, the
good spectra overlap of both emission and excitation spectra of the tri-
ads with the spectra of the individual units excludes the formation of
The most distinct feature in the characterization of triads 8 and 9 is
6

M.C. Souza et al.
Dyes and Pigments 187 (2021) 109137
1
9
Fig. 3. F NMR spectra of triads 8 and 9 with integral ratio and fluorine assignment. Optical properties of BODIPY-porphyrin triads.
emissive charge transfer complexes in both the ground and electronic
excited state. This is further supported by the fact that the lifetime of the
porphyrin moiety does not changes in the hybrid compared to the free
porphyrin, which excludes a charge transfer dynamic quenching. The
emission quantum yield of 8 is of the same magnitude order of the
quantum yield of porphyrin 4 (Φ = 4.1% and 4.7% upon excitation at
Fig. 5 shows that at equimolar concentrations the red emission of the
porphyrin upon excitation of the BODIPY unit at 470 nm is significantly
stronger in the triads (8 and 9) than in the isolated porphyrins (4 and 5).
This gives a clear evidence of efficient energy transfer from the BODIPY
to the porphyrins in the triads. The energy transfer in the triads can be
estimated as E = 1ꢀ τ where τDA and τDrefers to the average lifetime of
DA
τ
D
4
22 and 470 nm, respectively). The emission quantum yield of 9 is about
the donor emission measured in the presence (8 and 9) and absence (7)
of the acceptor, respectively. An energy transfer efficiency of nearly 90%
is estimated for both triads (92% for 8 and 88% for 9).
twice higher than that of porphyrin 5 (Φ = 11.3% and 15.9% upon
excitation at 422 and 470 nm, respectively). A strong quenching of the
BODIPY emission is observed in both triads while the porphyrin emis-
sion efficiency remains similar to that observed in the individual por-
phyrins 4 and 5.
The transfer of excitation energy from an electronic excited molecule
(
donor) to another (acceptor) is possible if the emission spectrum of the
donor partially overlaps with the absorption spectrum of the acceptor.
For allowed donor and acceptor transitions, the dipole-dipole term of the
expansion of the coulombic interaction is predominant for large donor to
acceptor separations. In the limit of very weak coupling, the rate of
energy transfer by a dipole-dipole interaction between an energy donor
and an acceptor separated by a distance R is given by the F o¨ rster
Equation (2) [70]:
3
.3. Fluorescence decay curves
The fluorescence decays were measured in DMSO and fitted with a
sum of exponentials after convolution with the experimental response
∑
aiτ
i
ai
function. The average lifetimes
τ
= ∑
where ai are the pre-
exponential factors and
τ
i the lifetimes from the fluorescence decay
1 R
0
6
D
k
ET = τ
( )
R
(2)
curves fitting, are collected in Table 1. The average lifetimes are 6.7 and
5
.7 ns, respectively for 4 and 5. The BODIPY derivative 7 has a slightly
where
τ
D is the lifetime of the donor in the absence of acceptor and R
0
is
lower emission lifetime of 4 ns The energy transfer in the triads results in
a faster decay of the green emission of BODIPY (<1 ns) as compared with
emission of derivative 7 (4 ns). As shown in Fig. S41 of the Supple-
mentary Information, a very clear fast decay component is observed in
the green emission decay of 8 and 9. The corresponding rise time
component in the red emission of the triads is not so evident. Both hy-
brids show multiexponential red emission decays with average lifetimes
in the same range of the individual porphyrins (5–7 ns).
the F o¨ rster radius at which the rate of energy transfer and decay of the
excited donor are equally probable. The F o¨ rster radius can be calculated
from spectroscopic data of the donor and acceptor molecules, following
Equation (3) [71],:
7

M.C. Souza et al.
Dyes and Pigments 187 (2021) 109137
Fig. 5. Fluorescence emission of equimolar concentration solutions of the
BODIPY (7) and porphyrins compared with the corresponding triads in panel A
(
4, 7 and 8) and panel B (5, 7 and 9) upon excitation at 470 nm. The intensity
of 4, 5, 8 and 9 are read on the left axes while that of derivative 7 is shown on
the right axes.
∞
∫
κ2
R = 8.79 × 10ꢀ 5(Φ
D
n4
)
F
(λ)
ε
A
(λ)λ dλ
(3)
6
4
o
D
0
where ΦD is the emission quantum yield of the donor, n is the refractive
index of the medium, and the integral on the right side of Equation (3) is
Fig. 4. Normalized UV–Vis absorption, emission and excitation spectra of
compounds 4, 5, 7, 8 and 9 in DMSO. The absorption spectra in straight black
trace, the emission spectra upon excitation at 422 nm and 470 nm in straight
blue and green traces, respectively, and the excitation spectra recorded upon
collection of emission at 514 nm, 660 nm and 700 nm are shown as dashed
traces in blue, green and red, respectively.
the overlap integral of the normalized donor emission spectra, F
D
(λ),
A(λ), and κ is an orienta-
tion factor between the transition dipole moments of the donor and the
acceptor molecules. A R
2
with the absorption spectra of the acceptor,
ε
0
= 43 Å is estimated for the BODIPY-Porphyrin
2
donor-acceptor pair in DMSO assuming an orientation factor κ = 2/3
corresponding to freely rotating donor and acceptor dyes. The efficiency
of F o¨ rster Resonance Energy Transfer (FRET) is given by,
Table 1
k
+
1
ET
E
FRET
=
=
(4)
Summary of absorption and emission properties of the synthesized compounds:
maximum absorption and emission wavelengths (λabs and λem, nm), photo-
luminescence emission quantum yields (Φ, %) and average lifetimes (
1
R
6
k
ET
1 + ( )
τ
D
R
0
τ
, ns).
which allows the estimation of the average distance between D and A.
For an efficiency of 90%, estimated before from the decay times of the
donor, a value of R ≈ 30 Å was calculated, which is larger than the
distance estimated based on the molecular structure of the conjugates
(≈20 Å depending on the conformation). The discrepancy in the R
values is due to the fact that the donor and acceptor separation is not
fixed, but varies with the conformation and Brownian motion of the
triads, in which case a distribution of distance has to be considered and
the separation R, when evaluated from equation (4), is an apparent
separation. Furthermore, the donor and acceptor molecules are not
point dipoles and the rotation of the donor and acceptor is not
compound
absorption
Emission
4
ꢀ 1
ꢀ 1
λ
max/nm (
ε
/10 M cm )
λmax/nm
Φ
τ
(
%)
(ns)
a
c
4
5
413 (17.97), 508 (1.84), 582 (0.60),
32 (0.31)
421 (12.37), 514 (1.67), 547 (0.46),
86 (0.42)
660, 706
660, 714
3.8
6.7
6
a
c
7.4
5.7
5
b
d
7
8
502 (7.99)
516
65.0
4.0
a
d
413 (16.3), 503 (5.9), 578 (0.48)
514, 660,
4.1
0.3
b
c
7
08
4.7
4.1
a
b
d
9
426 (15.22), 502 (21.20), 548 (0.67),
514, 658,
718
11.3
15.9
0.5
c
5
88 (0.54)
7.2
completely free, which introduces uncertainty in the F o¨ rster radius, R
.
0
a
Electronic structure calculation of the lowest energy electronic
transition of triad 8 and the isodensity surfaces of the frontier molecular
orbitals involved (Fig. S42) confirm that the mechanism behind the
quenching of the emission of BODIPY is a FRET process. The lowest
energy transitions involve orbitals localized in the porphyrin fragment
Excitation at 422 nm.
b
c
c
Excitation at 470 nm. .
Emission collected in the red (700 nm).
Emission collected in the green (514 nm).
d
(
from H or H-1 to L and L+1), orbitals delocalized between this fragment
and the triazine bridge (from H-3 to L or L+1), or orbitals localized in the
8

M.C. Souza et al.
Dyes and Pigments 187 (2021) 109137
BODIPY fragment (from H-2 to L+2). The lack of orbital overlap be-
tween the porphyrin and the BODIPY fragments supports the assignment
of the energy transfer process to a through space dipole – dipole inter-
action (FRET) rather than through-bond Dexter process.
the porphyrin monomers (40–70 GM). In Fig. 4, we showed that the
emission spectra upon one-photon excitation depends on the excitation
wavelength. The same is true for two-photon induced emission. Never-
theless, the emission should have no memory on the nature of the
excitation process, meaning that as long as we introduce the same
amount of energy in the system the order of the photon interaction
should not affect the nature of final excited state nor its dynamics. Thus,
the nonlinear emission spectrum induced at a given excitation wave-
length should be equal to the linear emission spectrum at half that
wavelength. Indeed, Fig. 6(F) shows that the upconverted emission upon
two-photon excitation at 930 nm is equal to the down-converted emis-
sion spectrum collected upon one-photon excitation at half that wave-
length (470 nm). Both the green emission of BODIPY and the red
emission of porphyrins are observed under 930 nm excitation. The
separation of ≈400 nm between NIR excitation and emission in the
green can be useful for application in sensing and imaging. On the other
hand, excitation in the NIR and emission in the far red might also be of
value for applications requiring non-invasive introduction of a certain
amounted of energy within scattering media. The inserts in Fig. 6 show
that for all the compounds the fluorescence emission upon NIR excita-
3
.4. Nonlinear NIR excitation
The nonlinear emission properties of the triads where studied in the
7
10–930 nm region. Typical nonlinear two-photon absorption (TPA)
cross section of BODIPY and porphyrins in this spectral region have been
reported to be below 20 GM [72–74]. The triazine core has also been
explored as an electron acceptor unit in the design of small molecules
and polymers for upconverted emission based on TPA [75,76]. Triazines
have been used in the construction of NIR antenna polymers for
two-photon activated volumetric data storage based on energy transfer
to photochromic acceptor units [77,78].
The TPA and the upconverted emission spectra of 4, 7 and 8 are
shown in Fig. 6. The corresponding results for 5 and 9 are shown in
Fig. S43 and follow the same trend as for 4 and 8.
The TPA cross-section was determined by two-photon induced
fluorescence using well-known TPA cross-section standards as explained
in detail in the experimental section. The maximum TPA of porphyrin is
observed at shorter wavelengths (40–70 GM at 730 nm). Both 4 and 5
show TPA cross-sections that are at least twice higher than those re-
ported for the isolated porphyrin units in the 710–930 nm region. In line
with earlier reports, the BODIPY derivative 7 has only a modest TPA
cross-section in this wavelength range with the highest value observed
at longer wavelengths (5 GM at 930 nm). Noteworthy, the peak of the
TPA band of 7 should lay just outside our observation window at 1030
nm (2 × 516 nm). In the triads the TPA spectra are dominated by the
contribution of the porphyrin unit, with maximum TPA cross-sections
laying in the same range as those observed for the porphyrins mono-
mer and also peaking at shorter wavelengths. The high TPA cross-
sections observed for the triads at 730 nm (36 and 65 GM for triad 8
and 9, respectively) are in good agreement with the values observed for
2
tion depends quadratically on the excitation power (logF vs log P with a
slope of 2), thus confirming that the upconverted emission is due to a
two-photon excitation process.
4. Conclusion
We synthesized two triads connecting a BODIPY derivative 7 to
mono- and tetra-ethylenediamine substituted porphyrins 4 and 5 via a
triazine linker. Triad 8 has a linear D-A structure with one BODIPY and
one porphyrin unit bridged by the triazine and triad 9 is a branched A-D
4
structure with the porphyrin core linked to four BODIPY units. The
structure and optical properties of the triads were evaluated by different
1
19 13
techniques namely NMR ( H, F, C), HRMS, UV–vis and fluorescence.
The nonlinear upconverted emission properties of the triads were
studied by two-photon excitation in the NIR (710–930 nm). The
Fig. 6. Nonlinear two-photon absorption and emission spectra of 4, 7 and 8: TPA (red symbols) and one-photon absorption (OPA, black line) spectra in panels A, C
and E on the left and two-photon emission excited at 930 nm (red symbols) and one-photon emission excited at 470 nm (black line) in B, D and F on the right. The
inserts show the log-log plot of the NIR excited fluorescence intensity as a function of the excitation power showing a quadratic dependence (linear fit with a slope
of ≈2).
9

M.C. Souza et al.
Dyes and Pigments 187 (2021) 109137
synthesized triads exhibit linear and nonlinear optical properties shaped
by the efficient energy transfer from the BODIPY to the porphyrin. We
observed both green and red emission upon excitation in the NIR at 930
nm featuring a large separation between excitation and emission
wavelengths that can be of value for application in sensing and imaging.
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Jos ´e A. S. Cavaleiro; Maria da Graça P. M. S. Neves: Supervision,
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Carla I. M. Santos; In ˆe s Mariz: Methodology.
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Paz FAA, Tom ´e JPC, Nakagaki S. Copper–porphyrin–metal–organic frameworks as
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The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Acknowledgements
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Authors are grateful to University of Aveiro, Instituto Superior
T ´e cnico (IST, Portugal) and FCT/MCTES (Fundaç a˜ o para a Ci ˆe ncia e
Tecnologia/Minist ´e rio da Ci ˆe ncia e Tecnologia, Portugal) for funding,
the research units: QOPNA (FCT UID/QUI/00062/2019), LAQV-
REQUIMTE (UIDB/50006/2020), CICECO-Aveiro Institute of Materials
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Ref.UID/CTM/50011/2020), (UID/CTM/50011/2019) and (UID/
NAN/50024/2019); the projects: (UID/EMS/00481/2013), (PTDC/FIS-
NAN/4662/2014), (PTDC/NAN-MAT/29317/2017), (PTDC/QUI-QFI/
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020) through national funds and, where applicable, co-financed by
European Union, FEDER, QREN, and COMPETE within the PT2020
Partnership Agreement, and to the Portuguese NMR Network. RFM ac-
knowledges FCT for his Junior Research Position (CEECIND/00553/
1
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