Donor-acceptor architectures of tetraphenylethene linked aza-BODIPYs: Synthesis, crystal structure, energy transfer and computational studies

Article abstract of DOI:10.1016/j.dyepig.2020.108249

This work is focused on the design and synthesis of donor-acceptor pairs comprising of aza-BODIPY and tetraphenylethene (TPE) fluorogens with large pseudo Stokes shifts (386–424 nm). Femtosecond transient absorption spectroscopy, single crystal X-ray technique and TD-DFT studies were used to get more insight about their spectral behavior. As compared to the tetra-phenylaza-BODIPY, the four aza-BODIPYs exhibited 50–90 nm red shifted absoprtion and emission spectra. When the donor-acceptor architectures were excited at 300 nm, the aza-BODIPY unit emitted strong fluorescence in red region. This suggested an intramolecular energy transfer between donor (TPE) and the aza-BODIPY acceptor; with 81–90% energy transfer efficiencies. Femtosecond transient absorption studies also supported energy transfer process in the molecules, with 3.7 ps time constant associated with it. Aggregation studies in THF/n-heptane solution showed hypsochromic shifts in the aggregated form; X-ray packing structures indicated H-type aggregates (slip angle > 54.7°) in the solid state. Scanning electron microscopy (SEM) measurement revealed 200–700 nm size sphere shaped structures in the aggregated forms.

Full text of DOI:10.1016/j.dyepig.2020.108249

Journal Pre-proof
Donor-acceptor architectures of tetraphenylethene linked aza-BODIPYs: Synthesis,
crystal structure, energy transfer and computational studies
Naresh Balsukuri, Neha Manav, Mohsin Y. Lone, Shigeki Mori, Aritra Das, Pratik Sen,
Iti Gupta
PII:
S0143-7208(19)32844-X
DOI:
https://doi.org/10.1016/j.dyepig.2020.108249
DYPI 108249
Reference:
To appear in: Dyes and Pigments
Received Date: 4 December 2019
Revised Date: 22 January 2020
Accepted Date: 26 January 2020
Please cite this article as: Balsukuri N, Manav N, Lone MY, Mori S, Das A, Sen P, Gupta I, Donor-
acceptor architectures of tetraphenylethene linked aza-BODIPYs: Synthesis, crystal structure,
energy transfer and computational studies, Dyes and Pigments (2020), doi: https://doi.org/10.1016/
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Manuscript Title:
Donor-Acceptor architectures of tetraphenylethene linked aza-BODIPYs:
Synthesis, crystal structure, energy transfer and computational studies
Graphical Abstract

Donor-Acceptor architectures of tetraphenylethene linked aza-BODIPYs:
Synthesis, crystal structure, energy transfer and computational studies
a
a
a
b
c
c
a
Naresh Balsukuri, Neha Manav, Mohsin Y. Lone, Shigeki Mori, Aritra Das, Pratik Sen, and Iti Gupta*
a
Indian Institute of Technology Gandhinagar, Palaj Campus, Gandhinagar, Gujarat- 382355, India
b
Advanced Research Support Center, Ehime University, Matsuyama 790-8577, Japan
c
Department of Chemistry, Indian Institute of Technology Kanpur, Uttar Pradesh-208016, India
*Corresponding author, e-mail: iti@iitgn.ac.in
Abstract
This work is focused on the design and synthesis of donor-acceptor pairs comprising of aza-
BODIPY and tetraphenylethene (TPE) fluorogens with large pseudo Stokes shifts (386 to 424
nm). Femtosecond transient absorption spectroscopy, single crystal X-ray technique and TD-
DFT studies were used to get more insight about their spectral behavior. As compared to the
tetra-phenylaza-BODIPY, the four aza-BODIPYs exhibited 50-90 nm red shifted absoprtion and
emission spectra. When the donor-acceptor architectures were excited at 300 nm, the aza-
BODIPY unit emitted strong fluorescence in red region. This suggested an intramolecular energy
transfer between donor (TPE) and the aza-BODIPY acceptor; with 81-90% energy transfer
efficiencies. Femtosecond transient absorption studies also supported energy transfer process in
the molecules, with 3.7 ps time constant associated with it. Aggregation studies in THF/n-
heptane solution showed hypsochromic shifts in the aggregated form; X-ray packing structures
o
indicated H-type aggregates (slip angle > 54.7 ) in the solid state. Scanning electron microscopy
(SEM) measurement revealed 200-700 nm size sphere shaped structures in the aggregated forms.
Keywords: Aza-BODIPY, Donor-Acceptor System, Aggregation studies, Energy transfer,
pseudo Stokes shifts, TD-DFT, Transient absorption.
1

Introduction
The π-conjugated organic NIR (near infra-red) fluorescent dyes have gained much attention due
to their highly varied applications, ranging from chemosensors, bio-markers, as photosensitizers
in DSSCs (Dye Sensitized Solar Cells) and in OLEDs (Organic Light Emitting Diodes) as
electroluminescent materials [1-4]. As NIR light causes minimal photo-damage and can
penetrate deep into biological tissue, thus NIR dyes can be used as contrast agents in bioimaging
[
5]. Among the numerous fluorescent dyes, the BODIPY (BF -chelated dipyrromethenes) is one
2
of the most extensively studied fluorophore with wide applications in toxic metal sensing,
optoelectronic materials, fluorescent tags, photovoltaics and light harvesting antenna systems [6-
1
0]. The BODIPYs have excellent photochemical stability, large absorption coefficients,
amenable to structural modifications and narrow emission spectra with high quantum yields
11,12]. In aggregated state, BODIPYs show reduced fluorescence and minor Stokes Shifts (10-
5 nm) due to aggregation-caused quenching (ACQ); which limits their applications [13]. On the
other hand, the propeller shape molecules viz. hexaphenylsilole (HPS) and tetraphenylethyene
TPE) exhibit strong fluorescence upon aggregation; due to the restriction of intramolecular
[
2
(
motions (RIM) which reduces the non-radiative decay processes. This phenomenon is known
aggregation induced emission (AIE) and was first reported by Tang and coworkers [14,15]. The
TPE derivatives show good chemical stability and can be easily functionalized; their AIE
property can be exploited to prepare fluorescent probes for biomolecules and explosives [16-18].
Tang and coworkers demonstrated that, a typical ACQ fluorophore when linked to the HPS or
TPE molecules; it can produce novel fluorogens with enhanced fluorescence properties [19]. The
BODIPYs substituted with TPE have exhibited large Stokes shifts and tunable emission
properties [20]. When the meso-carbon of the dipyrromethene core of the BODIPY is exchanged
2

by a nitrogen atom, the resultant molecule is called an Aza-BODIPY [21-24]. Aza-BODIPYs
absorbs and emits in the NIR (near infra-red) region (650-900 nm) and they also suffer from
ACQ effect; however owing to their strong absorption in biological window they are ideal
candidates for photodynamic therapy, deep tissue imaging and energy transfer systems [25,26].
Fluorescence resonance energy transfer (FRET) process can be used to adjust the optical
and electronic properties of donor-acceptor (D-A) systems including BODIPYs and Aza-
BODIPYs [27-32]. The efficiency of energy transfer in FRET based systems may vary
depending on the D-A distance, their relative orientation and also on the spectral overlap
between the D emission and A absorption [33-39]. Particularly, in BODIPY based D-A designs
[40-42] employing a D with high emission quantum yield leads to leakage of fluorescence from
the D; this drawback may restrict their application in biology. Recent report on dark resonance
energy transfer (DRET) based BODIPYs by Chang and coworkers [43], demonstrated a
complementary approach where a non-fluorescent molecules is used as “dark donor”. The use of
TPE in D-A based molecular designs reduces fluorescence leakage from the D; as a result the
energy transfer efficiency is relatively higher than FRET based systems [44]. Furthermore, the
large pseudo Stokes shifts and emission shifts rule out the fluorescence detection errors and
possibility of self-quenching [44,45]. The AIE active fluorophore like TPE can be linked to aza-
BODIPY to demonstrate energy transfer phenomenon in NIR region; and NIR dyes can be
designed for biological applications and solar energy systems. With this intention, we have
designed four novel AIE fluorogens comprising of TPE and NIR aza-BODIPYs. Herein we
present, the synthesis, X-ray structure, optical, aggregation induced enhanced emission (AIEE)
and electrochemical studies of NIR aza-BODIPYs. The 1,7-positions of the aza-BODIPY core
3

were substituted by 2-thienyl, phenyl and N-butylcarbazole rings; whereas 3,5-phenyl rings were
linked with TPE groups at their para-positions (Chart 1).
Chart 1. TPE substituted aza-BODIPYs 13-16 reported in this work (data recorded in toluene).
All four aza-BODIPYs exhibit considerable red shifs (50-90 nm) in their absoprtion and
emission maxima w.r.t. the parent tetraphenyl aza-BODIPY. The fluorescence studies in THF
(
tetrahydrofuran)/ heptane mixture were investigated by varying the n-heptane content (f ≥ 10-
h
9
0%). The molecular packing diagram obtained from X-ray crystal structure, solid state
absorption, emission and singlet oxygen generation studies were also reported for the selected
aza-BODIPYs. To the best of our knowledge this is the first report, where TPE substituted NIR
aza-BODIPYs are exhibiting considerably large pseudo Stokes shifts due to energy transfer. In
4

addition, DFT and TD-DFT calculations were performed to acquire the spectroscopic insight of
absorption and emission properties of the aza-BODIPYs reported here.
Results and Discussion
Synthesis and Characterization
The synthetic protocol to prepare aza-BODIPYs 13-15 is shown in schemes 1 and 2. The
synthesis of aza-BODIPYs was carried out in four steps; which involve preparation of chalcones,
nitrochalcones, aza-dipyrromethenes and aza-BODIPYs [46].
Scheme 1. Synthetic route for aza-dipyrromethenes 7-9. a) NaOH, Grinding method; b) KOH, CH NO , EtOH,
3
2
4
reflux 12 h; c) NH OAc, EtOH, reflux 24 h.
5

The chalones 1-3 were prepared by aldol condensation reaction between 4-bromoacetophenone
and the corresponding aldehydes (Scheme 1). The compounds 1-3 were formed as yellow
coloured solids in 57-66% yields. Next, 1,4-Michael addition was carried out between chalcones
1
-3 and nitromethane in the presence of base KOH in ethanol; the pure nitrochalcones 4-6 were
obtained as the oily compounds in 50-70% yield after silica gel chromatography.
Scheme 2. Synthetic route for aza-BODIPYs 13-15. d) Pd(PPh ) , toluene/THF, EtOH, K CO ; e) BF .OEt ,
3
4
2
3
3
2
CH Cl / NEt , rt, 24 h.
2
2
3
6

Scheme 3. Synthesis of aza-BODIPY 16.
Then compounds 4-6 were reacted with ammonium acetate in ethanol to form aza-
dipyrromethenes 7-9 as blue colour precipitate; washing with cold ethanol gave pure 7-9 in 16-
2
0% yields (Scheme 1). After preparing the aza-BODIPYs framework next step was to attached
energy donor groups (TPE) and it was achieved by Suzuki-Miyaura coupling of 7-9 with TPE-
B(OH) [47]. The TPE-B(OH) was synthesized as per the reported procedure [47]. Compounds
-9 were refluxed (Scheme 2) with TPE-B(OH) in the prescence of palladium catalyst
2
2
7
2
Pd(PPh ) and K CO base for 24 h. The coupling of TPE-B(OH) with compounds 7-9 was
3
4
2
3
2
checked by TLC (thin layer chromatography) and the observation of new blue green spot on
TLC indicated formation of the desired product. The TPE substituted aza-dipyrromethenes 10-12
were relatively unstable and crude reaction mixture was directly used for the synthesis of aza-
BODIPYs. The aza-BODIPYs 13-15 were formed as dark brown (13) and blue colour (14 and
1
3 2
5) compounds when aza-DPMs 10-12 were reacted with BF OEt for 24 h. The pure aza-
BODIPYs 13-15 were obtained in 38-46% yields after silica gel column chromatography. In
order to make triplet photosensitizer based on aza-BODIPY, the iodo substituents were linked at
the β-pyrrole position of aza-BODIPY.
7

In this regard, diiodo- substituted aza-BODIPY 16 was synthesized by treating aza-BODIPY 15
with N-iodosuccinimide (Scheme 3) for 10 h to get product in 62% yield. All the compounds
1
13
were characterized by H, C NMR, ESI- or MALDI-MS spectrometry (Supplementary
1
9
11
Information). The F and B NMR also confirmed the formation of aza-BODIPYs 13-16. In
aza-BODIPY 13-16, the coupling of two fluorine atoms with the adjacent boron atom (I = 3/2)
1
9
caused F signal split into quartet. Simultaneously boron atom showed up as triplet attributed to
1
9
coupling with fluorine atom (I = 1/2). For aza-BODIPYs 13-16 the F signals were appeared
1
1
arround -130.36 to -131.40 ppm; and the B signals were observed between 0.92 to 1.16 ppm
Supplementary Information).
(
X-ray Crystal Studies
The single crystal suitable for X-ray diffractions was developed by slow evaporation of the
solution of aza-BODIPY 13 (CCDC 1814178) in chloroform/n-heptane solvent mixture. The
crystal structure of aza-BODIPY 13 appeared as green block crystal with triclinic system having
P-1(#2) space group. The X-ray structure and packing diagram with intramolecular interactions
are shown in figures 1 and 2, respectively. X-ray crystal structure parameters are provided in the
supplementary information (Tables S1, S2). The four phenyl rings of two TPE units are
arranged in propeller shape conformation due to steric bulkiness. Noticeably, the aza-dipyrrin
core is perpendicular to the two TPE units (Figure 2a). The dihedral angles between aza-dipyrrin
plane and one of the lower phenyl rings linked to TPE unit were quite small [C7-C8-C53-C54,
o
o
2
.2(6) and N3-C8-C53-C58, 1.1(7) ]; which made the lower phenyl ring almost coplanar to the
aza-dipyrrin core. The torsion angles [C1-C2-C9-C10, N1-C1-C9-C14] between the other lower
o
o
phenyl ring and aza-dipyrrin plane are 29.6(6) and 30.1(7) , respectively.
8

Figure 1. X-ray structure of aza-BODIPY 13, dotted colorized lines stand for weak intramolecular interactions.
Also, the torsion angles [C12-C13-C15-C20, C57-C56-C59-C60] between the lower phenyl rings
o
o
and the phenyl ring of TPE are 26.6(6) and 29.8(6) , respectively. These smaller dihedral angles
indicated enhanced electronic communication between the aza-dipyrrin core and the TPE units in
compound 13. The packing diagram of 13 obtained from X-ray crystal structure, revealed slipped
stack arrangements of aza-dipyrrin planes of two adjacent molecules (Figure 2a) with significant
π−π interactions. The solid state packing diagram of 13 and the orientation of the transition
dipoles relative to the stacking axis can be helpful to explain the type of aggregates [48]. The
center to center distances between these aza-dipyrrin cores are 4.55 and 6.08 Å and the ground
state dipole moment (0.88 D) of 13, was aligned with Nmeso→ B vector which has antiparallel
alignment in the columnar stacks (Figure 2b). The two aza-dipyrrin units of neighboring
9

molecules were parallel to each other and the interplanar separation of the mean planes of the
adjacent molecules was of 3.94 Å.
Figure 2. (a) Packing diagram of 13, displaying center to center distances and slip angle; (b) Distances of the mean
planes of adjacent molecules, H atoms and TPE units are omitted for clarity.
Thus, aza-BODIPY 13 displayed intermolecular slipped π−π stacking in the crystal state with
o
slip angle of 77.04 (Figure 2a). According to Kasha’s molecular exciton model [49], the H-type
aggregates are non-fluorescent due to forbidden nature of the lowest excited state in the
aggregates. Exceptionally, emission from H-type aggregates is observed when forbidden nature
of the excited state is reduced due to minor deviations of the direction of the transition dipoles
between adjacent neighbours [50]. When the molecules possess inclined geometry in the
10

aggregated state, the transition dipoles would not be cancelled out, therefore fluorescence is not
completely quenched from H-type aggregates. This could be the probable reason of the observed
Figure 3. Molecular packing diagram of aza-BODIPY 13 with inter-molecular interactions.
fluorescence from aggregated state of aza-BODIPY 13; which exhibited H-type aggregates with
o
slip angle
θ
> 54.7 in the packing diagram (Figure 2a). Also, hypsochromic shift in the
fluorescence emission during aggregation studies indicated H-type aggregates in solution.
Furthermore, intermolecular H-bonding (B-F-----H) interactions (Table S3) as evident in crystal
packing diagram, are likely to favor the parallel slipped π−π stacking of molecules in solid state
(
Figure 3).
1
1

Absorption and Emission Properties
The experimentally determined photophysical properties of aza-BODIPYs 13-16 are presented in
table 1. A comparison of absorption spectra of compounds 13-16 is shown in figure 4a, an
-
1
-1
intense band between 600-820 nm (ε = 42,600-79,500 M cm ) corresponds to S
and another band around 500-600 nm with medium intensity (ε = 14,500-45,500 M cm ) was
assigned to S transitions originating from the aza-BODIPY core [21]. Additional absorption
o 1
→S transitions
-
1
-1
→S
o 2
-1
-1
band at ~340 nm of moderate intensity (ε = 46,500-89,000 M cm ) was assigned to the two TPE
units attached on aza-BODIPYs [44]. The major absorption band of compounds 13-16 was
moved to NIR region, depending upon the nature of substituents present at 1,7- and 3,5-positions
of the aza-BODIPY chromophore. Change of the substituents from phenyl (13) to thiophene (14)
at 1,7-positions of aza-BODIPY, led to bathochromic shift of 17 nm in the absorption maximum
of 14. The bathochromic shift in 14 can be ascribed to the smaller ring size of thiophene, making
the electron rich heterocycle more coplanar with the aza-BODIPY plane.
Table 1. Absorption and emission data of compounds 13-16 in various solvents.
a
Compound
Solvent
λab (nm), ε (log E)
λem
nm)
734
734
736
Φf
Stokes shift -
1
(
(nm)
(cm )
520
1
1
1
3
4
5
Toluene
Chloroform
Tetrahydrofuran
707 (4.82), 510 (4.16), 348 (4.67)
703 (4.69), 509 (4.09), 345 (4.56)
705 (4.63), 505 (4.00), 345 (4.51)
0.42 27
0.38 31
0.19 31
600
597
Toluene
Chloroform
Tetrahydrofuran
724 (4.90), 537 (4.29), 344 (4.78)
722 (4.87), 545 (4.23), 342 (4.72)
721 (4.80), 543 (4.19), 342 (4.68)
753
754
753
0.18 29
0.15 32
0.10 32
531
587
589
Toluene
Chloroform
Tetrahydrofuran
738 (4.92), 604 (4.66), 344 (4.95)
738 (4.87), 614 (4.69), 343 (4.97)
741 (4.82), 621 (4.65), 342 (4.91)
770
782
790
0.09 32
0.07 44
0.05 49
563
762
837
1
6
Toluene
Chloroform
685 (4.93), 520 (4.31), 325 (4.78)
680 (4.84), 519 (4.23), 329 (4.69)
728
729
727
0.02 43
0.02 49
0.01 52
862
988
1047
Tetrahydrofuran 679 (4.71), 512 (4.13), 329 (4.57)
a: 1,7-dinaphthyl-3,5-dithienyl azaBODIPY (Φ = 0.14, CHCl ) used as standard, λ = 650 nm.
f
3
ex
12

1
00000
(a)
(b)
1
1
3
4
13
1
.0
14
15
16
TPAB
8
6
4
2
0000
0000
0000
0000
0
15
1
6
0.8
TPAB
0
0
0
.6
.4
.2
0.0
4
00
500
600
700
800
700
750
800
850
Wavelength (nm)
Wavelength (nm)
Figure 4. Comparison of (a) absorption and (b) emission spectra of aza-BODIPYs TPAB, 13-16, recorded in
toluene (λex = 650 nm).
Furthermore, due to the presence of strong electron donor aromatic ring like N-butylcarbazole,
compound 15 showed significantly red shifted (31 nm) absorption in solution. Remarkably, the
absorption of 13 was 57 nm red shifted as compared to parent tetraphenyl aza-BODIPY (TPAB,
λ
abs = 650) [51]. It may be due to the extended π conjugation of TPE units present on the 3,5-
phenyls in the compound. Noticeably, as compared to TPAB the major absorption bands of 14
and 15 were red shifted by 74 and 88 nm, respectively. The major absorption band of 2,6-diiodo
substituted aza-BODIPY 16 was 35 nm red shifted as compared to the TPAB. The fluorescence
spectra of aza-BODIPYs 13-16 are shown in figure 4(b) and emission quantum yields are given
in table 1. All aza-BODIPYs 13-16 displayed intense emission maxima in the range of 734-770
nm due to S →S transitions (Figure 4b). The emission maxima of 13-16 were substantially red
1
0
shifted (Table 1) as compared to the parent TPAB emission (λem = 672 in CHCl ) [51]. As
3
compared to TPAB, the aza-BODIPYs 13 and 16 with phenyl rings on the 1,7-positions
exhibited 62 and 56 nm bathochromically shifted emission maxima in solution.
13

1
0
0
0
0
.0 (a)
1.0 (b)
13 (λ = 738 nm)
1
1
3 (λ = 766 nm)
4 (λ = 770 nm)
14
(
λ = 764 nm
)
1
5
(
λ = 805 nm
)
.8
.6
.4
.2
0.8
0.6
0.4
0.2
15 (λ = 775 nm)
4
00
500
600
700
800
900
750
800
850
900
Wavelength (nm)
Wavelength (nm)
Figure 5. (a) Normalized absorption and (b) emission spectra of aza-BODIPYs 13 (λ = 680 nm), 14 and 15 (λ =
ex
ex
7
00 nm) on drop-cast film.
Whereas, the emission bands of aza-BODIPYs 14 and 15 having thiophene and N-butylcarbazole
rings at 1,7-positions displayed red shifts of 81 and 98 nm, respectively as compared to TPAB.
This could be attributed to the presence of electron donor substituents like thiophene and N-
butylcarbazole on the aza-BODIPY core. However, the most red shifted emission maximum (790
nm) was observed for aza-BODIPY 15 in THF; making it a potential candidate for biological
applications. The calculated emission quantum yields of aza-BODIPYs 13-15 were found to be
in the range of 0.02 to 0.42 in toluene (Table 1); however owing to heavy atom effect lower
quantum yield was observed for aza-BODIPY 16. The solvent dependency of the absorption and
emission of the compounds 13-16 was also studied (Table 1). However, only slight red shifts (2-
8
nm) in the absorption and emission maxima were observed for 13, 14 and 16; this is a typical
behavior observed for the BODIPY chromophores reported in literature. Nonetheless, the
emission of aza-BODIPY 15 showed decent red shifts (10-20 nm) in polar solvents. Stokes shifts
-1
were found to be in the range of 520 to 1047 cm for aza-BODIPYs 13-16. The observed
14

bathochromic shifts in the emission bands of aza-BODIPYs, reveal larger dipoles of the CT
excited states in polar solvents [52,53]. Particularly, the Stokes shifts of di-iodo aza-BODIPY 16
in polar solvents were much larger than the rest of the BODIPYs 13-15 (Table 1); this can be
explained by DFT studies. The HOMO-2 to LUMO transition in compound 16 is attributed to the
ICT (intramolecular charge transfer) process (Figure 11); the electron density is spreaded on the
donor TPE units in the HOMO-2 and the LUMO is clealry localized on the boron-dipyrrin core.
Also, the larges dipole moment difference in the ground state and the excited CT state leads to
electron phonon coupling with the solvent phonons; and the prescene of two iodine atoms on the
2
,6-position of the BODIPY core results in the enhanced internal conversion and higher Stokes
shifts in 16 [51,54]. The quantum yields of aza-BODIPYs 13-16 were slightly higher in non-
polar solvent than those of in polar solvents (Table 1). In polar solvents the large dipole moment
difference between ground state and charge-transfer excited state enable internal conversion in
the molecules, resulting in the lower quantum yields [53].
To illustrate the potential of AIEE property arising from aza-BODIPYs 13-15 for solid
state applications, the photophysical properties were examined on solid films. The thin films
were prepared on glass plate by drop cast method and film based absorption and emission spectra
of 13-15 are shown in figure 5. In solid state, aza-BODIPYs 13-15 exhibited significantly red
shifted and comparatively broad absorption bands with 31, 40 and 67 nm shifts, , with respect to
their corresponding absorption in solution state (Figure 5a). Typically, fluorophores display
aggregation caused quenching (ACQ) of emission in solid state. In contrast, the aza-BODIPYs
1
3-15 exhibited moderately intense fluorescence in solid films at 766, 770 and 775 nm,
respectively (Figure 5b). Furthermore, the solid state emission bands of 13-15 were broad and
bathochromically shifted by 32, 17 and 5 nm, with respect to their corresponding emission in
15

solution phase. In general, BODIPYs with bulky substituents directly attached to the core
reported to show solid state emission; since the steric crowding of large groups minimizes
interchromophoric interactions in solid state. The amorphous films of aza-BODIPYs 13-15
prepared by drop-cast method, exhibited broad and red shifted absorption and emission due to
the formation of parallel inclined π−π stacks of the molecules in solid state.
Heavy-Atom Effect and Singlet Oxygen Generation
In order to test the ability of aza-BODIPY 16 to produce singlet oxygen due to heavy atom
effect; singlet oxygen generation study with 1,3-diphenylisobenzofuran (DPBF) was carried out
[27]. The study was undertaken by monitoring the reaction of known singlet oxygen scavenger
(DPBF) in solution with the singlet oxygen generated by our compound 16 (Figure 6). Due to the
presence of two iodo groups on the aza-BODIPY core, compound 16 should have significant
triplet population upon excitation at 633 nm, and can act as triplet sensitizer.
A
0.5
16
MB
B
0
sec
0
0
0
0
.6
.4
.2
.0
0
0
0
0
.4
.3
.2
.1
5
0 sec
0.0
800
4
00
500
600
700
0
10
20
30
40
50
Irradiation time, s
Wavelength (nm)
-
5
-5
Figure 6. (a) Absorption spectra of DPBF (2 x 10 M) upon irradiation in the presence of aza-BODIPY 16 (1 x 10
2
M) from 0 to 50 sec (recorded at 10 sec interval) in DCM. Monochromatic light (633 nm at 0.5 mW/cm ) was used;
b) Straight line plot of change in the absorbance of DPBF at 414 nm with the irradiation time in presence of aza-
BODIPY 16 against methylene blue (MB, 3.1 x 10 M) as the standard in DCM.
(
-6
16

The significant inter-system crossing (ISC) would lead to the energy transfer from
photosensitizer (16) to triplet ground state of oxygen molecules; which in turn get excited to
singlet state and react with DPBF. The disappearance of DPBF absorption band at 414 nm was
monitored over a time period of 50 seconds. Monochromatic light source (633 nm at 0.5
2
mW/cm ) was used to excite the aza-BODIPY 16; as the time progressed the intensity of the
DPBF absorption band at 414 nm gradually decreased (Figure 6a). The triplet photosensitization
ability of compound 16 was compared with the reference photosensitizer (methylene blue,
∆
Φ =
0
.57 in DCM) under similar conditions (Figure S5, SI) [27]. The singlet oxygen quantum yield,
calculated for aza-BODIPY 16 was 0.68 in DCM; confirming that it has substantial spin-orbit
coupling due to heavy atom effect. Moreover, the presence of iodo groups directly on the aza-
BODIPY core in 16 can considerably reduce the loss of excited state energy by non-radiative
,
decay processes.
Energy Transfer Studies
Recent reports on BODIPYs based D-A systems demonstrated efficient FRET and TBET
(through bond energy transfer) based molecular architectures. In the BODIPY based D-A
systems, where TPE is used as donor the energy transfer process was named DRET (dark
resonance energy transfer) [43]. DRET is similar to FRET phenomenon, though the donor
chromophore is named “dark donor” as it is non-fluorescent in nature. According to the literature
reports, the efficiency of energy transfer is reasonably high in DRET based system, because the
loss of energy due to non-radiative decay is eliminated in the molecules [43]. Lately, TPE
connected BODIPYs have been described with decent energy transfer from TPE donor to the
BODIPY unit. TPE and its derivatives have been extensively studied for AIE properties, owing
to facile synthetic modifications and high emission intensity in aggregated state. Atilgan and
17

coworkers [44] have reported D-A systems of BODIPY where, TPE is employed as “dark energy
donor”. The intramolecular energy transfer in aza-BODIPYs 13-16 was evaluated by comparing
the fluorescence emission of compounds excited at the donor and acceptor units in toluene
(Table 2, Figure 7). The emission spectra of 13-16 in toluene displayed moderate emission
intensity when excited at λ = 650 nm; the aza-BODIPY core absorbs strongly here. On the other
hand, the fluorescence signals of 13-16 were very unusual when excited at λ = 330 nm; the TPE
units absorb intensely here.
1.0x105
.0x105
3
2
2
1
1
5
Excitation
(a)
(b)
Excitation
.5x105
13 (330 nm)
3 (650 nm)
8.0x104
14 (330 nm)
1
14 (650 nm)
.0x105
.5x105
.0x104
6
4
.0x104
.0x105
2.0x104
.0
.0x104
0
.0
0
7
00
750
800
850
700
750
800
850
Wavelength (nm)
Wavelength (nm)
8
.0x104
.0x104
.0x104
.0x104
(c)
Excitation
1
1
5 (330 nm)
5 (650 nm)
6
4
2
0
.0
7
00
750
800
850
Wavelength (nm)
-6
Figure 7. Comparative emission spectra of compounds: (A) 13, (B) 14 and (C) 15 in toluene (2 x 10 M), excited at
donor (black) and acceptor (red). (D) Comparative emission maxima of compounds 13, 14 and 15 in toluene excited
at donor and acceptor.
Table 2. Energy transfer data of aza-BODIPYs 13-16 in toluene.
Compound
TPAB
Excitation (λex)
650 nm
Emission (λem)
672 nm
Acceptor (Φ )
ETE (%)
f
[25]
__
__
b
1
3
650 nm
734 nm
0.42
90%
18

c
3
30 nm
650 nm
30 nm
650 nm
30 nm
650 nm
30 nm
734 nm
753 nm
754 nm
771 nm
771 nm
727 nm
731 nm
0.38
b
1
1
1
4
5
6
0.18c
81%
88%
67%
3
0.14
b
0.08c
3
0.07
b
0.02
c
3
0.014
b: Quantum yield of acceptor excited at acceptor; c: Quantum yield of acceptor excited at donor.
Intense fluorescence was observed from the aza-BODIPYs 13-16 in between 730 to 771 nm
(
Figure 7). This behavior can be explained by energy transfer phenomenon, where TPE units are
donor and the aza-BODIPY unit is acting as acceptor. The compounds 13-16 were excited at two
wavelengths, corresponding to the absorption of the donor (TPE units) and the acceptor (aza-
BODIPY unit); then the fluorescence quantum yields were calculated (Table 2). The “energy
transfer efficiency” (ETE, %) was calculated [52], which suggest the total energy transfer from
donor to acceptor including non-radiative decays of the molecules in question [55]. The % ETE
values are given in the table 2, suggesting effective intra-molecular energy transfer from TPE
units to the aza-BODIPY unit in molecules 13-16. Except diiodo aza-BODIPY (16), rest of the
compounds exhibited ETE between 81-90%, which could be due to lack of fluorescence-leakage
in these molecules. The pseudo-Stokes shifts [44] were calculated for compounds 13-16 by
observing the difference between λem (aza-BODIPY core) and λabs (TPE); and the values were
found to be in the range of 386 to 424 nm. The D-A systems 13-16 with high values of
intramolecular energy transfer and huge pseudo-Stokes shifts are very attractive for prospective
biological applications. The previous report on TPE substituted aza-BODIPY dimers [15]
described the novel synthesis and AIE property of such molecules; though the energy transfer
studies were not explored. To the best of our knowledge, molecules 13-16 are the first such TPE-
aza-BODIPY conjugates which exhibit very large pseudo-Stokes shifts (386 to 424 nm). The
19

crystal structure of 13 showed that, the propeller shape TPE moieties and aza-dipyrrin core are
not in the same plane; therefore possibility of energy transfer through space is not very favorable.
Femtosecond Transient Absorption Spectroscopy
In order to determine the exact time constants of the energy transfer process, we have performed
femtosecond broadband transient absorption (TA) spectroscopy [27]. The molecule 13 consists
of two different chromophores; one is the TPE and the other being the aza-BODIPY. The TPE
chromophore absorbs around 350 nm and is non-fluorescent in nature. Whereas, the aza-
BODIPY unit absorb around 650 nm; and emits at around 720 nm. The TA study was carried out
in toluene and compound 13 was excited at 400 nm, to exclusively excite the molecules at the
TPE chromophore.
Figure 8. (a) TA spectra at some representative delay times along with the fitted spectra (green solid lines); (b)
Kinetics at two representative wavelengths 560 nm and 720 nm along with the fitting (black solid lines); (c) Decay
associated spectra (DAS) obtained from global analysis of the TA response of 13 in toluene after exciting at 400 nm.
20

The TA signal of 13 consists of a broad excited state absorption (ESA) band around 500-600 nm
and a stimulated emission (SE) band centered at 720 nm (Figure 8a). The observation of the
emission from aza-BODIPY unit in TA signal for 13 confirms the energy transfer process in the
excited state. The TA responses of 13 were globally analyzed and successfully fitted with four
different time constants; the fitted kinetics at two representative wavelengths is displayed in
figure 8b. The observed time constants are 3.7 ps (ꢀꢁ), 40 ps (ꢀꢂ), 120 ps (ꢀꢃ) and 3300 ps (ꢀꢄ).
Decay associated spectra (DAS, Figure 8c) for 13 show that the spectra associated with ꢀꢁ is
quite similar but of opposite sign compared to that of the spectra associated with ꢀꢄ. Which
suggest that, ꢀꢁ and ꢀꢄ represent the formation and decay of a same state in the excited states
manifold. Moreover, the spectra associated with ꢀꢂ are same but negative of the DAS of ꢀꢃ. Thus
ꢀꢂ and ꢀꢃ represent the formation and decay of another exited state species. It is clear from figure
8
c that, though four different time constants are obtained from the global analysis, all DAS are
quite similar. DAS spectra suggest that the two excited state species are structurally similar. The
species associated with ꢀꢁ and ꢀꢄ is strongly emissive in nature, whereas the other one is weakly
emissive. We suggest that, following is the singlet singlet energy transfer process; the excited
state species undergo some kind of structural change to form the other species. A plausible
pattern to describe the excited state phenomena of 13 is shown in scheme 4. We assign the 3.7 ps
time constant as the time constant associated with energy transfer, which is also the formation
time constant of the species [D-A*]. The lifetime of this species is 3300 ps. Following the energy
transfer process [D-A*] structurally changes to [D-A*']. The formation time constant of this [D-
A*'] is 40 ps and has a decay constant of 120 ps. The other molecules under investigation having
21

the similar donor-acceptor moieties are expected to exhibit similar dynamics. The experimental
data for 15 is provided in the figure S52 of the supporting information.
Scheme 4. Schematic of the excited state processes in aza-BODIPY 13.
Aggregation Studies
1
1
1
4000
2000
0000
0
.40
Aza-BODIPY 13
Quantum Yield
1
0% Heptane
(a)
20% Heptane
0
.35 (b)
30% Heptane
40% Heptane
50% Heptane
60% Heptane
70% Heptane
80% Heptane
90% Heptane
0.30
0.25
8000
6000
4000
2000
0.20
0.15
0.10
0.05
0.00
1
0
20
30
40
50
60
70
80
90
7
00
750
Wavelength (nm)
800
Heptane fraction (%)
Figure 9. (a) Emission spectra of aza-BODIPY 13 in THF-heptane mixture with different heptane fractions. (b) Bar
diagram of quantum yield of aza-BODIPY 13 with increasing heptane fractions.
22

Aggregation induced enhanced emission (AIEE) behavior of compounds 13-15 was investigated
by promoting aggregate formation through alteration of solvent composition. The systematic
change in the composition of THF/n-heptane solvent system from 100% THF to 90% n-heptane
resulted in the formation of solid aggregates as indicated by the noticeable variations in the
emission spectra. Compounds 13 and 14 showed AIEE phenomenon whereas, 15 exhibited
aggregation caused quenching (ACQ) behavior. In pure THF, compounds 13 and 14 exhibited
f f
weak emissions at 735 nm (Φ = 0.14) and 754 nm (Φ = 0.10), respectively. However, upon
increasing the volume fraction of n-heptane (f = 10% to 90%) in THF, the emission intensities
h
were risen gradually (Figure 9). Moreover, the emission bands of 13 and 14 became narrower
with significant blue shifts (~12 nm) in higher heptane fractions (f = 90%). At a f of 90%,
h
h
significant (3-fold) increase in the emission intensity of aza-BODIPY 13 was observed and the
calculated quantum yield was 0.44 (Table 3). The enhancement in the fluorescence intensity and
quantum yields is due to AIEE behaviors of 13 and 14 (Table 3). The formation of AIEE active
agglomerates of compounds 13 and 14 at f of 90% was confirmed by DLS. The size distribution
h
pattern obtained from DLS showed that nano-aggregates formed are in range of 50-900 nm,
having maximum probability distribution around 200-700 nm (Figure S6, SI). At f of 90%, the
h
morphology of aggregates was further explored by SEM (Scanning Electron Microscopy) and
the nano-aggregates were found to have spherical morphology. The size of nano-aggregates
obtained from SEM analysis, was between 270-750 nm; which was in accordance with the DLS
data (Figure S6, SI).
Electrochemical studies
23

The electrochemical properties of all aza-BODIPYs 13-15 were studied by cyclic voltammetry
technique (Figure S3, SI). The cyclic voltammograms of aza-BODIPYs 13-15 were recorded in
dry dichloromethane under N atmosphere and the data is presented in table 3. The aza-
2
BODIPYs 13-15 exhibited single irreversible oxidation wave in between 0.97 to 1.26 V. Aza-
BODIPYs 13 and 14 showed two reversible reductions in between -0.32 to -1.13 V; whereas
compound 15 showed three irreversible reduction waves in between -0.14 to -1.61 V. The
oxidation potential of aza-BODIPY 13 was a bit anodically shifted as to those of the aza-
BODIPY 14 and 15; indicating that compound 13 was harder to oxidize.
Table 3. Electrochemical redox data (V vs SCE) of compounds 13-15 in DCM, recorded at 50 mV/s scan speed and
energy levels calculated from optical CV data.
Compound
E
1/2(ox)/V
E
1/2(red)/V
HOMO
[eV]
-5.68
-5.64
-5.39
LUMO
[eV]
-4.04
-4.10
-4.28
∆E [eV]
(from CV)
1.64
Optical ∆E
[eV]
I
I
II
III
1
1
1
3
4
5
1.26
1.22
0.97
-0.38
-0.32
-0.14
-1.13
-1.07
-1.03
-
-
2.65
2.65
2.63
1.54
1.11
-1.61
The redox potential data was also used to determine the HOMO-LUMO energies (Table 3) of
aza-BODIPYs 13-15; the frontier orbital energies are presented in figure S4 (SI). In the aza-
BODIPYs 13-15, by varying the substituents at 1,7-positons from phenyl to thiophene and then
to N-butylcarbazole the HOMO-LUMO gaps were decreased from 1.64 to 1.11 eV. Furthermore,
the absorption data of compounds 13-15 was used to calculate the optical band gap (Table 3)
[21]. As compared to compounds 14 and 15, higher optical band gap was observed for the phenyl
substituted compound (13). Among all the compounds, the lowest optical (2.63 eV) and
electrochemical (1.11 eV) band gaps were obtained for compound 15; which showed the most
red shifted absorption and emission spectra as compared to the other aza-BODIPYs. The electron
24

rich N-butylcarbazole ring has considerable effect on the electronic property of the aza-BODIPY
5.
1
Computational Studies
Ground state geometrical parameters
Density functional theory (DFT) and time dependent density functional theory (TD-DFT) were
used to characterize the properties of ground and the low excited electronic states of the aza-
BODIPY molecules 13-16. The important structural parameters viz. bond lengths, valence angles
and dihedral angles obtained as a result of geometry optimization are summarized in tables S4,
S5 and S6 respectively (SI). Comparison of these structural parameters for the molecule 13 with
the X-ray crystallographic structure is of immense importance. The X-ray crystallographic
structure with C symmetry cannot be visualized perfectly; therefore, average values of the
2
structural parameters were used for the correlation. The numbering schemes for bond lengths,
valance angles and for the dihedrals angles are provided in figures S7 and S8, respectively (SI).
It is evident from tables S4 and S5 that the maximum deviation in bond lengths and valance
angles for 13 with respect to the X-ray crystallographic structure is 0.02Å and 1.54° respectively.
The maximum deviation in bond lengths is 0.005Å and the bond angles agree within the 0.46°;
suggesting the sturdiness of the level of approximation used for the DFT study. Furthermore, the
substituents at the aza-BODIPY skeleton have negligible effect on the optimized structures of
compounds 13-16.
Absorption studies
Theoretical simulations also provide the complementary spectroscopic understanding of the
absorption properties of the molecules. To accomplish this, singlet-singlet electronic transitions
25

3
of the molecules 13-16 were calculated in CHCl exclusively, owing to the insignificant effect of
solvent polarities on the absorption profiles of these aza-BODIPYs. The type of transitions for
the molecules 13-16 can be examined, prior to comparison of the lowest energy transitions
(λmax). For this purpose, topologies of the calculated molecular orbitals responsible for the
transitions were figured out. The calculated frontier molecular orbitals (FMOs) and the
absorption maxima along with their oscillatory strengths (f) are presented in figure S9 (SI) and
table 4, respectively. It is apparent from the results that the most intense singlet-singlet
transitions (S
HOMO LUMO (89.78%, 92.48% and 54.08% for 13, 14 and 16 respectively) and HOMO-
LUMO (8.0%, 5.78% and 42.32% for 13, 14 and 16 respectively), with the dominance of the
former. Notably, for 13 the HOMO LUMO transition is a π→π* transition of the aza-BODIPY
core; whereas, the HOMO-2 LUMO corresponds to a charge transfer from TPE chromophore
to the aza–BODIPY unit. Similarly, for the molecule 14, HOMO LUMO and HOMO-
LUMO transitions are the dominant π→π* transitions occurring in the central core and the
0 1
ꢀS ) of the molecule 13, 14 and 16 are due to the electron promotion from
ꢀ
2
ꢀ
ꢀ
ꢀ
ꢀ
2
1
ꢀ
,7 substituted thiophene arms respectively. In contrast, HOMOꢀLUMO transition of the
molecule 16 occurs via a charge transfer from TPE to the iodinated aza–BODIPY unit. On the
other hand, HOMO-2 LUMO transition in 16, is a mixture of intramolecular charge transfer
from the TPE to the aza-BODIPY unit and π→π* transition of the central core. In addition, the
ꢀ
most intense singlet-singlet transition (S0
transition (98.0%).
ꢀS1) of the molecule 15 is sheer HOMOꢀLUMO
26

Table 4: Calculated electronic excitation energies and corresponding oscillator strengths of the low-lying singlet
excited state for the molecules 13-16 in CHCl .
3
Theoretical
Experimental
Molecule
Electronic
Contribution
transition
Energy
(nm)
Oscillatory Absorption in
CI coefficients
strength (f)
CHCl (nm)
3
HOMO-2ꢀLUMO
HOMOꢀLUMO
0.20
0.67
1
1
1
1
3
4
5
6
S
0
S
0
S
0
S
0
ꢀS
ꢀS
ꢀS
ꢀS
1
1
1
1
671
703
702
584
1.12
703
722
738
680
HOMO-2ꢀLUMO
HOMOꢀLUMO
0.17
0.68
1.04
1.20
0.85
HOMOꢀLUMO
0.70
HOMO-2ꢀLUMO
HOMOꢀLUMO
0.46
0.52
*
CI coefficients are in absolute values.
This transition is an amalgam of π→π* and charge transfer characteristics. The former occurs in
the central core region whilst, the later takes place from the 1,7 substituted N-butylcarbazole
arms to the central core of aza–BODIPY 15. The substantial electronic communication between
the substituents and the aza-BODIPY core is reflected with the delocalization of FMOs over the
entire molecular framework. Analogous topologies of the LUMO of all the compounds (13-16)
indicated strong electron accepting nature of the aza-BODIPY core. Despite the fact, that the
numbers of absorption bands were seen in the calculated spectra of aza-BODIPYs 13-16, but for
the sake of brevity λmax was chosen for the comparison. The calculated and observed λmax values
are accommodated in table 5. A significantly good agreement between the calculated and
observed λmax was seen for all the molecules except 16. In particular, the maximum deviation of
9
6 nm was shown by 16, followed by 15, 13 and 14 which have displayed the difference of 36
nm, 32 nm and 19 nm respectively. It is imperative to mention that, all the molecules were
consistently underestimated, because of the limitations of vertical approximation.
27

In addition, the energies of FMOs and the HOMO-LUMO energy gap (∆E) were calculated by
employing DFT and TD-DFT approaches are summarized in table S7 (SI). The DFT calculation
based energy level of HOMO and LUMO was found to be increased by 0.18 eV and 0.19 eV
respectively, as we moved from aza-BODIPY 13-15. In contrast, the experimental HOMO
energy level was increased by 0.29 eV whereas; the LUMO energy level was decreased by 0.24
eV. Consequently, the calculated HOMO-LUMO energy gap was underestimated by 0.62 eV,
0
.54 eV and 0.08 eV for the molecule 13, 14 and 15 respectively. Similarly, the TD-DFT based
HOMO-LUMO energy gap for aza-BODIPY 13, 14 and 15 were severely overestimated by 1.21
eV, 1.22 eV and 1.69 eV respectively from the electrochemical measurements. However, the
TD-DFT based HOMO-LUMO energy gap was found to be in line with the optical
measurements with an overestimation of 0.20 eV, 0.11 eV and 0.17 eV for 13, 14 and 15
respectively. No such comparison was made for the molecule 16 owing to the unavailability of
the experimental data.
Excited state geometrical parameters
In general, the electronic excitation of the molecules may result in the equilibrium geometries
very different from those in their ground states. Therefore, the essential structural parameters viz.
bond lengths, valence angles and dihedral angles of the S of 13-16 were computed and
1
compared with their corresponding S geometrical parameters as presented in tables S4, S5 and
0
S6 respectively (SI). It is evident from the tables S4 and S5 that, the valance angles and bond
lengths of S geometries are not significantly different from S geometries for all the molecules.
1
0
The utmost deviation in bond length and valance angle was found to be 0.027Å and 1.69° for 15
and 13 respectively. On the contrary, significant differences between S and S dihedral angles
were notified for all the molecules under investigation as evident from table S6. To be more
0
1
28