C3-Symmetric Positional Isomers of BODIPY Substituted Triazines: Synthesis and Excited State Properties

Article abstract of DOI:10.1021/acs.jpca.8b02967

A series of meso-O-aryl functionalized BODIPY trimers positioned along the C₃-symmetric axis of triazine ring have newly been synthesized to probe the ground and excited state intramolecular type interactions between the BODIPY entities within the trimer. The developed synthetic strategy resulted in BODIPY trimers in good yields. The electron rich, meso-O-aryl functionalized BODIPYs revealed larger HOMO-LUMO gap and higher Stokes shift and fluorescence lifetimes compared to the traditional BODIPY derivatives having an aryl group attached at the meso position. The optical absorption, steady-state fluorescence, and electrochemical studies revealed weak, if any, intramolecular type interactions among the BODIPY entities within the trimer and the central triazine unit to be both photo- and redox-salient. The possibility of singlet-singlet energy migration among the BODIPY entities was investigated using time-resolved emission and femtosecond transient absorption studies. Excitation of a BODIPY entity in the trimers led to successful formation of ¹BODIPY?, which populated the ³BODIPY? via intersystem crossing. Among the three trimers, although very weak, only trimer 8 revealed excitation transfer to some extent. The present findings suggest that the meso-O-aryl functionalized BODIPYs due to their superior fluorescence properties are better probes to build light energy harvesting supramolecular oligomeric systems and for other applications such as sensing and imaging.

Full text of DOI:10.1021/acs.jpca.8b02967

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
3
C-Symmetric Positional Isomers of BODIPY Substituted
Triazines: Synthesis and Excited State Properties
Ramesh Maragani, Michael Thomas, Rajneesh Misra, and Francis D'Souza
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.8b02967 • Publication Date (Web): 07 May 2018
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C3-symmetric Positional Isomers of BODIPY Substituted Triazines:
Synthesis and Excited State Properties
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Ramesh Maragani,a Michael B. Thomas,b Rajneesh Misra,*,a and Francis D’Souzab,*
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aDepartment of Chemistry, Indian Institute of Technology, Indore 453552, India.
bDepartment of Chemistry, University of North Texas, 1155 Union Circle, #305070, Denton, TX
76203-5017, United States.
(Abstract). A series of meso-O-aryl functionalized BODIPY trimers positioned along the C3-
symmetric axis of triazine ring have newly been synthesized to probe the ground and excited state
intramolecular type interactions between the BODIPY entities within the trimer. The developed
synthetic strategy resulted in BODIPY trimers in good yields. The electron rich, meso-O-aryl
functionalized BODIPYs revealed larger HOMO-LUMO gap, higher Stokes shift and fluorescence
lifetimes compared to the traditional BODIPY derivatives having an aryl group attached at the
meso position. The optical absorption, steady-state fluorescence and electrochemical studies
revealed week, if any, intramolecular type interactions among the BODIPY entities within the
trimer and the central triazine unit to be both photo- and redox-salient. The possibility of singlet-
singlet energy migration among the BODIPY entities was investigated using time-resolved
emission and femtosecond transient absorption studies. Excitation of BODIPY entity in the
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trimers led to successful formation of BODIPY* that populated the BODIPY* via intersystem
crossing. Among the three trimers, although much weak, only trimer 8 revealed excitation transfer
to some extent. The present findings suggest that the meso-O-aryl functionalized BODIPYs due
to their superior fluorescence properties are better probes to build light energy harvesting
supramolecular oligomeric systems and for other applications such as sensing and imaging.
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1.
Introduction
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Over the last two decades research efforts have been directed to unveil the complex
excitation transfer phenomenon occurring in natural photosynthetic light-harvesting complexes.1-
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The covalently linked molecular arrays developed to mimic the natural light-harvesting have
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the potential for building molecular photonic and artificial light-harvesting devices.21-33 Such
molecularly engineered systems provide control over the spatially well-organized molecular
architectures. A fundamental challenge in the photodynamics of such arrays is elucidation of
excitation energy transfer between the fluorophores and the degree of the excitation delocalization.
In bacterial photosynthetic systems such as R. Sphaeroides, two types of antenna
complexes exist: a core light-harvesting antenna (LH1) and a peripheral light-harvesting antenna
(LH2).34 The reaction center where a series of photoinduced electron transfer events occur resides
within LH1.35-36
The LH2 comprised of two wheel-like structures: B800 with 9
bacteriochlorophyll a (Bchl a) entities and B850 with 18 Bchl a entities revealing different degrees
of intra-dimer distances causing weak (in the case of B800) to strong (in the case of B850)
interactions.37-38
In designing molecular arrays to mimic the natural photosynthetic systems, most of them
have been prepared based on linear arrangement via macrocycle peripheral functionalization.1-20
Circular arrangement of fluorophores in an even manner is crucial for mimicry of natural LH-
arrays in view of their geometry. In the majority of the systems built for this purpose, porphyrins
due to their close structural resemblance to chlorophyll and BChl, have been the primary choice
of chromophores.1-6 However, in recent years, model compounds based on BF2-chelated
dipyrromethene (BODIPY) have attracted wide attention due to their synthetic diversity and well-
defined and tunable spectral and photophysical properties.43-51
In the present study, we have utilized BODIPY as a fluorophore to model natural LH-arrays
and synthesized cyclic trimers using triazine scaffold (see Figure 1). The BODIPYs employed
here are functionalized to have an electron rich oxygen at the meso-position43 (ether type linkage,
see structure of compound 10 in Figure 1) which were found to be a superior probe compared to
BODIPY derivatives functionalized with an alkyl or aryl substituents at the meso-position for their
large Stokes shift, fluorescence quantum yield and lifetime, thus making them an ideal sensitizer
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to build chemosensors for in vivo and in vitro imaging, to build photosynthetic donor-acceptor
model compounds for light energy harvesting, and oligomeric systems to seek novel
applications.21-33 The BODIPY entities in the cyclic timers synthesized in the present study are
positioned around a triazine ring in a symmetric fashion. This arrangement resulted in minimal
perturbation of the ground and excited state properties of BODIPY within the trimers. Time-
resolved emission and femtosecond transient absorption studies revealed that in trimer 8 energy
migration (energy hopping) occurs among the BODIPY entities to some extent.
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Figure 1. Structures of BODIPY carrying C3-symmetric positional isomers of substituted
triazines, 7-9, and control monomers, 10 and 11 investigated in the present study.
2.
Results and Discussion
The BODIPY substituted triazines 79 were synthesized by the cyclotrimerization and
nucleophilic substitution reactions of hydroxyl benzonitrile derivatives (13) (Scheme 1 and
experimental section for details). In BODIPY substituted triazines 79, the BODIPY moiety is
attached at different positions via phenoxy spacer around the 1,3,5-triazine core (Figure 1). The
key intermediates 46 were synthesized by the cyclotrimerization reaction of the corresponding
hydroxyl benzonitrile derivatives 13 under strong acid condition (triflouromethanesulfonic acid)
in chloroform solvent for 16 h at room temperature and Chloro BODIPY, 12 synthesized by
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reported procedure.52 The triazine tri-hydroxy derivatives 4–6 were dissolved in acetone in the
presence of a base (K2CO3) at room temperature, stirred for 10 minutes and treated with BODIPY
12 for 16 h, which resulted tri-BODIPY substituted triazines 79 in 50–58% yield (Scheme 1).
Compound 10 was synthesized by reported procedure.52 All the BODIPY substituted triazines
710 were fully characterized by 1H, 13CNMR, and HR-MS techniques.
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Scheme 1. Synthesis of BODIPY substituted triazines, 7-9.
Figure 2 shows the normalized absorption and fluorescence spectra of the investigated
compounds in toluene while the spectral data are summarized in Table 1 both in polar benzonitrile
and nonpolar toluene. The characteristic BODIPY peak in these meso-O-aryl functionalized
compounds was located in the 460 nm range that was about 60 nm blue-shifted compared to meso-
alkyl/aryl substituted BODIPYs.53-54 Compared to monomer spectra, the absorption and
fluorescence spectra of cyclic BODIPY trimers were slightly blue-shifted by up to 5 nm as a
consequence of triazine functionalization. Importantly, there was no significant spectral
broadening compared to the monomer absorption spectrum suggesting lack of intra-chromophore
interactions. Concentration dependence studies revealed linear Beer-Lambert’s plots in the range
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of 0.05 mM to 2 mM suggesting lack of aggregation. The fluorescence emission for the present
series of compounds was located in the 500 nm range that was also blue-shifted by about 20 nm
compared to meso-alkyl/aryl substituted BODIPYs (see Figure 2b).16 Generally, BODIPYs show
a relatively small Stokes shift of ~ 7 x 103 cm-1 (see Table 1). However, the calculated Stokes shift
for the monomer and trimer of the present meso-O-aryl functionalized BODIPYs were in the range
of 2-7 x 104 cm-1 (see Table 1). Such large Stokes shift generally helps in minimizing the inner
filter effect in fluorescence studies.55 The fluorescence intensity of the trimers at the same optical
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Figure 2. (a) Normalized absorption to the visible peak maxima, and (b) fluorescence spectra of
the indicated compounds in toluene. Molar absorption coefficient was found to be 24244, 43853,
11713, and 6367 L mol-1 cm-1 for compounds 7, 8, 9, and 10, respectively. Fluorescence quantum
yields were found to be 0.94, 0.97, 0.70, and 0.31 for compounds 7, 8, 9, and 10, respectively.
Tolyl BODIPY was used as the reference (φf = 0.73). The compounds were excited at the most
intense visible band peak maxima. (c) Differential pulse voltammograms of indicated compounds
in benzonitrile containing 0.1 M (TBA)PF6 (V. vs. Ag/AgCl). The first oxidation process
corresponding to ferrocene used as an internal reference.
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density of the monomer (OD = 1.0) were found to be within +15% of the monomer, 10. Often,
electron and energy transfer events quench the fluorescence intensity of the primary sensitizer in
multi-modular systems.56-59 Lack of such quenching suggest absence of intra-chromophore
interactions between the BODIPY entities in the trimer.
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This lack of intra-chromophore interactions was also revealed by the electrochemical
studies. The differential pulse voltammograms (DPV) corresponding to oxidation of BODIPY
entity in the trimers are shown in Figure 2c. The oxidation process corresponding to the monomer
and trimers were located in the 0.58-0.64 V range vs. Fc/Fc+ range revealed that were 80-140 mV
cathodically shifted compared to the BODIPY control (see Table 1). The easier oxidation could
be attributed to the presence of electron rich oxygen at the meso-position. As expected for systems
Table 1. Absorption and fluorescence peak maxima and fluorescence lifetimes of investigated
compounds in benzonitrile and toluene.
CompoundSolvent
Stokes shift,
cm-1
Eox, V vs.
Fc/Fc+
abs, nm flu, nm
, ns
1.76 x 104
1.69 x 104
1.85 x 104
1.91 x 104
1.82 x 104
6.94 x 103
1.71 x 104
1.67 x 104
1.84 x 104
1.85 x 104
1.78 x 104
6.92 x 103
PhCN
PhCN
PhCN
PhCN
PhCN
PhCN
462
461
460
459
459
501
503
500
503
503
501
519
504
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503
501
518
5.56
2.91
4.17
5.58
5.58
2.86
5.56
5.32
5.61
5.89
5.89
3.36
0.64
0.58
0.62
0.62
0.61
0.72
--
7
8
9
10
11
BODIPY
Toluene 464
Toluene 463
Toluene 462
Toluene 460
Toluene 460
Toluene 500
7
--
8
--
9
10
--
--
11
--
BODIPY
with large HOMO-LUMO gap, (optical HOMO-LUMO gap ~ 3.60 eV for the present BODIPY
series), no reduction within the potential window of -1.80 V of benzonitrile was observed.
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Importantly, no splitting or peak broadening of the oxidation waves, expected for strongly
interacting electroactive entities, was observed thus confirming the lack of intra-chromophore
interactions. The functionalized triazine was also found to be electro-inactive within the potential
range.
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As a consequence of the directly attached electron rich oxygen atom to the BODIPY meso-
position, the fluorescence lifetime of monomer, 10 was found to be higher compared to regular
BODIPY derivatives (Table 1, this also translates into higher quantum yields).16 Interestingly, the
lifetime of trimers were slightly lower, especially for compound 8, in both polar and nonpolar
solvents. In the absence of aggregation and charge transfer effects, this effect could be attributed
to energy migration, among the BODIPY entities within the trimer, as depicted in Scheme 2. In
an effort to gain support for the proposed energy migration mechanism, time-resolved emission of
the monomer and trimers were recorded by varying their concentrations from 0.05 mM to 2 mM.
In an event of energy migration the lifetime of the sensitizer would be lowered, and by increasing
the concentration this effect should be more pronounced. Figure 3 shows the normalized decay
profiles of the investigated compounds in toluene. For the monomer 10 and 11, the lifetimes were
virtually same in the entire concentration range (also indicates lack of inner filter effects in the
concentration range, supported from the earlier discussed steady-state emission results). This was
also the case for trimers 7 and 9 where the lifetime were the same within the experimental error.
However, for trimer, 8 the decay was biexponential at lower concentrations with short and long
lived components. The lifetime of the long-lived component became smaller with increasing the
concentration, and at 2.0 mM only the short-lived component persisted revealing a
monoexponential decay profile. These observations suggest occurrence of energy migration in 8,
however, in case of 7 and 9, the concentration range was perhaps too narrow to observe this trend.
Changing the solvent to polar benzonitrile also revealed such a trend in the case of 8 (see Figure
S1) but not for 7 and 9. It seems subtle structural changes in the trimers cause noticeable changes
in their excitation transfer properties.
Further, computational studies were performed to visualize the geometry and to ascertain
absence of intramolecular type association between the BODIPY entities in the trimers.60 The
optimized structures are shown in Figure 4. The dihedral angle in para substituted BODIPY
triazine 7 between triazine core and BODIPY through oxygen atom was 123, whereas triazine 8
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Scheme 2. Cartoon showing energy hopping among the BODIPY entities in the cyclic BODIPY
trimer.
Figure 3. Normalized fluorescence decay profiles (log count vs. time) of the indicated compounds
at the indicated concentrations (in mM) in toluene. The samples were excited using 374 nm
nanoLED and monitored at their respective emission maxima.
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and 9 shows dihedral angle of 123 and 118 between triazine and BODIPY core as shown in
Figure 4. Importantly, although flexible, the BODIPY entities were almost perpendicular with
respect to the triazine core with the dihedral angles varying between 77-82o. In the optimized
structures, the frontier HOMO and LUMO were found to be on the BODIPY entity without any
contributions on the triazine entity (see Figure S19 for frontier orbitals). These observations
suggest triazine being a redox-silent entity in these trimers and acts only as a scaffold. At the
excitation wavelengths of 460-465 nm where the trimers are excited, the central triazine also acts
as a photo-silent entity. It is also important to point out that in none of the trimers steric hindrance
or intramolecular type association between the BODIPY entities within a given trimer was
observed.
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Figure 4. B3LYP/6-31G** optimized structures of the investigated BODIPY trimers 7-9.
In an effort to seek evidence for energy migration in trimer 8, femtosecond transient
spectral studies were performed in both toluene and benzonitrile. As shown in Figure 5a,
immediately after excitation, monomer 10, revealed a positive peak at 884 nm due to transitions
involving from the 1BODIPY* state and two negative peaks, the first one at 468 nm due to ground
state bleaching and the second one at 515 nm as a shoulder due to stimulated emission. With time,
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the 468 nm peak revealed a small red shift of 5-6 nm perhaps due to solvation/vibrational
relaxation. The decay and recovery of the positive and negative peaks which lasted over 3 ns was
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accompanied by a new peak at 574 nm that was assigned to BODIPY* state. This was pretty
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much the case for timers 7 and 9 as shown in Figures 5b and d, respectively. In the case of trimer
7, positive peaks at 614 and 1075 nm and negative peaks at 465 and 502 nm (due to ground state
bleach and stimulated emission) were observed. The 3BODIPY* state developed at the later delay
time was at 602 nm. In the case of trimer 9, a positive peak at 1054 and negative peaks at 462 and
504 nm (due to ground state bleach and stimulated emission) were observed. The 3BODIPY* state
developed at the later delay time was at 608 nm. In contrast, for trimer 8 where energy migration
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Figure 5. Femtosecond transient absorption spectra at the indicated delay times of (a) monomer
10, (b) trimer 7, (c) trimer 8, and (d) trimer 9 in benzonitrile. The solution concentration was held
at 0.1 mM. The samples were excited at the corresponding visible peak maxima.
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among the BODIPY entities was predicted, the transient spectra revealed distinct changes (Figure
5b). The two negative peaks were located at 466 and 500 nm, in agreement with the absorption
and emission peak maxima of the trimer. However, the intensity of the near-IR peak in the 1045
nm range involving optical transitions (singlet-singlet) from the 1BODIPY* state was rather weak.
This was also the case for the peak corresponding to 3BODIPY* expected in the 600-620 nm range
(see Figure S20 for nanosecond transient spectral data). This could be rationalized by lesser
availability of 1BODIPY* due to rapid energy hopping. It may be pointed out here that the energy
migration in trimer 8 is not as efficient as that reported for porphyrin cyclic oligomers in
literature.1-6 Changing solving to nonpolar toluene also revealed such spectral features (see Figure
S2 in SI).
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3.
Summary
The results of the present study bring out several interesting observations. The meso-O-
aryl functionalized BODIPYs revealed larger HOMO-LUMO gap and Stokes shift compared to
BODIPY derivatives having an aryl group attached at the meso position. This was also the case
for singlet excited state lifetimes, which were found to be almost double as compared to the
traditional BODIPYs because of directly attached electron rich oxygen atom at the meso position.
The BODIPY entities in the trimer positioned symmetrically around a triazine ring revealed
minimal ground state interactions between them as shown by spectral, electrochemical and
computational studies. Concentration dependent time-resolved studies revealed presence of short
and long-lived components for trimer 8 in which the long-lived component disappeared at higher
concentration supporting the occurrence of excited state energy migration at higher concentration
lowering the lifetime, and such an effect is not due to any charge transfer type interactions between
BODIPY and the substituents. The performed femtosecond transient absorption studies also
supported such a mechanism, although the overall energy migration efficiency is lower than that
reported for cyclic porphyrin systems. The present study brings out the importance of molecular
structure controlled energy migration in BODIPY oligomers. Further studies along this line are in
progress in our laboratories.
4.
Experimental Section
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Chemicals. All the reagents were from Aldrich Chemicals (Milwaukee, WI) while the bulk
solvents utilized in the syntheses were from Fischer Chemicals. Tetra-n-butylammonium
perchlorate, (n-Bu4N)ClO4, used in electrochemical studies was from Fluka Chemicals.
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General Procedure for the preparation of Triazines 4-6
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Synthesis of 2,4,6-tri(4’-hydroxyphenyl)-1,3,5-triazine (4). The 4-hydroxybenzonitrile
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(0.5g, 4.23 mmol) was dissolved in chloroform (50 ml), then reaction mass cooled to 0 C, added tri
flouro methane sulfonic acid (1.22g, 8.13 mmol) drop wise under argon atmosphere, and stirred for 24
h at rt, after completion of the reaction, the residue was neutralized with an aqueous NaHCO3, water,
methanol, acetone, and hexane in this order, and then dried under vacuum to afford 4 as an off-white
solid (0.8 g, 53%); 1H NMR (400 MHz, Acetone-d6): δ = 9.14 (s, 3H), 8.68 (d, J = 8.00 Hz, 6H), 7.06
(d, J = 8.00 Hz, 6H), ppm; 13C NMR (100 MHz, Acetone-d6): δ = 206.31, 171.59, 162.53, 131.72,
128.75, 116.34 ppm; HRMS (ESI): calcd. for C21H15N3O3 358.1186 [M+H]; found 358.1182.
Synthesis of 2,4,6-tri(4’-hydroxyphenyl)-1,3,5-triazinetris(1-methoxyphenol) (5). The
4-hydroxy-3-methoxybenzonitrile (0.5g, 3.35 mmol) was dissolved in chloroform (50 ml), then
o
reaction mass cooled to 0 C, added tri flouro methane sulfonic acid (1.22g, 8.13 mmol) drop wise
under argon atmosphere, and stirred for 24 h at rt, after completion of the reaction, the residue was
neutralized with an aqueous NaHCO3, water, methanol, acetone, and hexane in this order, and then
dried under vacuum to afford 5 as an off-white solid (0.65 g, 43%); 1H NMR (400 MHz, Acetone): δ
= 8.38 (dd, J = 4.00 Hz, 3H), 8.31 (d, J = 4.00 Hz, 3H), 7.13 (d, J = 8.00 Hz, 3H), 4.06 (s, 9H) ppm;
13C NMR (100 MHz, Acetone-d6): δ = 206.24, 154.58, 148.86, 126.36, 116.59, 113.52, 56.55 ppm;
HRMS (ESI): calcd. for C21H15N3O3 448.1503 [M+H]; found 448.1504.
Synthesis of 2,4,6-tri(3’-hydroxyphenyl)-1,3,5-triazine (6). The 3-hydroxybenzonitrile
o
(0.5g, 4.23 mmol) was dissolved in chloroform (50 ml), then reaction mass cooled to 0 C, added tri
flouro methane sulfonic acid (1.22g, 8.13 mmol) drop wise under argon atmosphere, and stirred for 24
h at rt, after completion of the reaction, the residue was neutralized with an aqueous NaHCO3, water,
methanol, acetone, and hexane in this order, and then dried under vacuum to afford 6 as an off-white
solid (0.6 g, 39%); 1H NMR (400 MHz, Acetone-d6): δ = 8.87 (t, 3H), 8.29 (d, J = 8.00 Hz, 6H), 7.50
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(m, 3H), 7.17 (d, J = 8.00 Hz, 3H) ppm; C NMR (100 MHz, Acetone-d6): δ = 206.31, 171.59,
162.53, 131.72, 128.75, 116.34 ppm; HRMS (ESI-TOF):: calcd. for C21H15N3O3 358.1186 [M+H];
found 358.1185.
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Synthesis and Characterization of Triazine-BODIPYs 7-9. Triazines 4 or 5 or 6 (100
mg, 0.44 mmol) was dissolved in dry Acetone (7 mL) and K2CO3 (121 mg, 0.88 mmol) were added
after 30 minutes 8-chloro BODIPY added and stirred at ∼25 °C for 16 h. Upon completion of the
reaction, the mixture was evaporated to dryness and the crude product was dissolved in EtOAc,
chromatographed on silica compound eluted using EtOAc/hexane (80:20) to get 7 or 8 or 9 as a
yellow solid.
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2,4,6-tris[4’-(4’’,4’’-difluoro-4’’-bora-3a’’,4a’’-diaza-s-indacenato-8’’-yl)oxyphenyl]-1,3,5-
triazine (7). Yellow crystalline solid. Yield: 47% (0.125 g). 1H NMR (CDCl3, 400 MHz, ppm): δ
8.94 (d, J = 12.00 Hz, 6H), 7.81 (s, 6H), 7.50 (d, J = 8 Hz, 6H), 6.87 (d, J = 4.00 Hz, 6H), 6.46 (d,
J = 4.00 Hz, 6H). 13C NMR (DMSO-d6, 100 MHz, ppm): 172.98, 171.00, 170.21, 133.83, 130.95,
130.93, 126.01, 124.83, 116.72, 115.90, 112.91, 110.68, 110.31 ppm. HRMS (ESI-TOF): calcd.
for C48H30B3F6N9O3 950.2569 [M+Na]; found 950.2569.
2,4,6-tris[4’-(4’’,4’’-difluoro-4’’-bora-3a’’,4a’’-diaza-s-indacenato-8’’-yl)oxyphenyl]-1,3,5-
1
triazinetris(1-methoxyphenol) (8). Yellow crystalline solid. Yield: 48% (112 mg). H NMR
(CDCl3, 400 MHz, ppm): δ 8.57 (d, J = 8.00 Hz, 3H), 7.78 (s, 6H), 7.53 (d, J = 8.00 Hz, 3H), 6.86
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(d, J = 4.00 Hz, 6H), 6.43 (d, J = 4.00 Hz, 6H), 3.97 (s, 9H). C NMR (DMSO-d6, 100 MHz,
ppm): 170.44, 151.79, 148.18, 130.93, 127.24, 126.29, 124.82, 115.90, 113.15, 110.79, 110.31,
55.36 ppm. HRMS (ESI-TOF): calcd. for C51H36B3F6N9O6 1040.2887 [M+Na]; found 1040.2890.
2,4,6-tris[3’-(3’’,3’’-difluoro-4’’-bora-3a’’,4a’’-diaza-s-indacenato-8’’-yl)oxyphenyl]-1,3,5-
triazine (9). Yellow crystalline solid. Yield: 60% (0.160 g). 1H NMR (CDCl3, 400 MHz, ppm): δ
8.81 (d, J = 12.00 Hz, 3H), 8.63 (s, 3H), 7.80 (m, 9H), 6.84 (d, 6H), 6.43 (s, 6H). 13C NMR (CDCl3,
100 MHz, ppm): 180.54, 170.89, 156.11, 141.23, 138.24, 131.09, 127.66, 126.33, 124.68, 120.48
ppm. HRMS (ESI-TOF): calcd. for C48H30B3F6N9O3 950.2569 [M+Na]; found 950.2566.
Spectral measurements
1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were recorded on a Bruker Avance
(III) instrument by using CDCl3, Acetone-d6 and DMSO-d6 as solvents. 1H NMR chemical shifts
are reported in parts per million (ppm) relative to the solvent residual peak. Multiplicities are given
as s (singlet), d (doublet), t (triplet), q (quartet), dd (doublet of doublets), and m (multiplet), and
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the coupling constants, J, are given in Hz. C NMR chemical shifts are reported relative to the
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solvent residual peak and HRMS was recorded on a Bruker-Daltonics micrOTOF-Q II mass
spectrometer.
The UV-visible spectral measurements were carried out with a Shimadzu Model 2550
double monochromator UV-visible spectrophotometer. The fluorescence emission was monitored
by using a Horiba Yvon Nanolog coupled with time-correlated single photon counting with
nanoLED excitation sources. A right angle detection method was used. Differential pulse
voltammograms were recorded on an EG&G PARSTAT electrochemical analyzer using a three
electrode system. A platinum button electrode was used as the working electrode. A platinum wire
served as the counter electrode and an Ag/AgCl electrode was used as the reference electrode.
Ferrocene/ferrocenium redox couple was used as an internal standard. All the solutions were
purged prior to electrochemical and spectral measurements using nitrogen gas.
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Femtosecond pump-probe transient spectroscopy. Femtosecond transient absorption
spectroscopy experiments were performed using an Ultrafast Femtosecond Laser Source (Libra)
by Coherent incorporating diode-pumped, mode locked Ti:Sapphire laser (Vitesse) and diode-
pumped intra cavity doubled Nd:YLF laser (Evolution) to generate a compressed laser output of
1.45 W. For optical detection, a Helios transient absorption spectrometer coupled with
femtosecond harmonics generator both provided by Ultrafast Systems LLC was used. The source
for the pump and probe pulses were derived from the fundamental output of Libra (Compressed
output 1.45^^W, pulse width 100 fs) at a repetition rate of 1 kHz. 95 % of the fundamental output
of the laser was introduced into a TOPAS-Prime-OPA system with 290—2600 nm tuning range
from Altos Photonics Inc., (Bozeman, MT), while the rest of the output was used for generation
of white light continuum. In the present study, the second harmonic 400^^nm excitation pump was
used in all the experiments. Kinetic traces at appropriate wavelengths were assembled from the
time-resolved spectral data. Data analysis was performed using Surface Xplorer software supplied
by Ultrafast Systems. All measurements were conducted in degassed solutions at 298 K.
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at:
DOI: xxxx
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1H, 13C and HRMS spectra of the conjugates, frontier orbital diagrams, nanosecond transient
spectral data on the trimers and Cartesian coordinates of the optimized structures. (PDF)
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AUTHOR INFORMATION
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Corresponding Authors
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* E-mail: rajneeshmisra@iiti.ac.in, Francis.Dsouza@unt.edu
ORCID
F. D’Souza: 0000-0003-3815-8949
Author Contributions
The manuscript was written through contributions of all authors.
Notes
The authors declare no competing financial interest.
Acknowledgment
R. M. thanks the Department of Science and Technology (Project No. EMR/2014/001257), New
Delhi, and INSA (Project No. SP/YSP/139/2017/2293), New Delhi, for the financial support. We
are grateful to the Sophisticated Instrumentation Centre (SIC), IIT Indore and the National Science
Foundation (Grant No. 1401188).
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