Regioisomeric BODIPY Benzodithiophene Dyads and Triads with Tunable Red Emission as Ratiometric Temperature and Viscosity Sensors

Article abstract of DOI:10.1002/chem.201902952

Regioisomeric acceptor-donor (AD) molecular rotors (p-AD, m-AD and m-ADA) were synthesized and characterized, wherein dyads p-AD and m-AD, and triad m-ADA contained 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) and benzodithiophene (BDT) as electron-acceptor and electron-donor, respectively. In all the compounds, the donor and acceptor moieties are electronically decoupled by a phenyl spacer, either through a para coupling or through a meta coupling. The dyad counterparts p-AD and m-AD showed distinct photophysical characteristics in which dyad p-AD showed TICT band at ca. 654 nm characterized by a Stokes shift of ca. 150 nm and prominent solvatochromism. However, meta regioisomeric triad m-ADA showed well-defined aggregation in solution. Notably, because of the temperature-tunable and solvent-viscosity-dependent emission, efficient ratiometric temperature sensing with positive and negative temperature coefficients and viscosity sensing was observed for all compounds. Interestingly, the fluorescence of dyad m-AD (in 10/90 v/v THF/water) revealed a near-white light emission with CIE chromaticity coordinates (x, y) of (0.32, 0.29). Furthermore, the fluorescence emission of p-AD in THF at 0 °C also showed a near-white light emission with chromaticity coordinates (x, y) of (0.34, 0.27). Such multifunctional rotors with readily tunable emission in the red region and prominent temperature- and viscosity-sensing abilities are promising for sensing and bioimaging applications.

Full text of DOI:10.1002/chem.201902952

A Journal of
Accepted Article
Title: Regioisomeric BODIPY-Benzodithiophene Dyads and Triads
with NIR Tunable Emission as Ratiometric Thermometers and
Molecular Viscometers
Authors: Aswathy Plakkal Rajeev, Sushil Sharma, Narendra Pratap
Tripathi, and Sanchita Sengupta
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201902952
Link to VoR: http://dx.doi.org/10.1002/chem.201902952
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Regioisomeric BODIPY-Benzodithiophene Dyads and Triads with
Tunable Red Emission as Ratiometric Temperature and Viscosity
Sensors†
Aswathy P. R.‡, Sushil Sharma‡, Narendra Pratap Tripathi and Sanchita Sengupta*[a]
Abstract: Regioisomeric acceptor-donor (AD) molecular rotors (p-AD, m-AD and m-ADA) were synthesized and characterized where dyads
p-AD and m-AD and triad m-ADA contained BODIPY and Benzodithiophene (BDT) as electron acceptor and donor respectively. In all the
compounds, the donor and acceptor moieties are electronically decoupled by a phenyl spacer, either through a para coupling or through a
meta coupling. The dyad counterparts p-AD and m-AD showed distinct photophysical characteristics where dyad p-AD showed TICT band at
~ 654 nm characterized by a Stokes shift of ~ 150 nm and prominent solvatochromism. However, meta regioisomeric triad m-ADA showed
well-defined aggregation in solution. Notably, owing to temperature tunable and solvent viscosity dependent emission, efficient ratiometric
temperature sensing with positive and negative temperature coefficients and viscosity sensing was observed for all compounds. Interestingly,
fluorescence of dyad m-AD (in 10/90 v/v THF/water) revealed a near white light emission with CIE chromaticity coordinates (x, y) of (0.32,
0.29). Furthermore, fluorescence emission of p-AD in THF at 0 oC also showed a near white light emission with chromaticity coordinates (x, y)
of (0.34, 0.27). Such multifunctional rotors with readily tunable emission in red region and prominent temperature and viscosity sensing
abilities are promising for sensing and bioimaging applications.
properties such as white light emission[13,14] desirable for
Introduction
(opto)electronic applications. The emission of these rotor
molecules can be further tuned through aggregate induced
emission (AIE)[15,16] and mechanochromism.[17] TICT rotors are
promising viscosity probes because they exhibit emission
enhancement due to restricted intramolecular rotation in
solvents of increasing viscosity.[10,11,18–20] Examples of ratiometric
temperature and/or viscosity[8,11] sensors based on TICT rotors
involve fluorophores such as tetraphenylethylenes (TPE),[4]
Design of ratiometric fluorescent molecular probes for
temperature detection is an emerging research area having
witnessed tremendous progress in the last decade.[1,2]
Ratiometric temperature and viscosity molecular probes play a
vital role in accurate (bio)molecular detection, sensing, imaging
and theranostic applications.[3] Such ratiometric molecular
probes have low sensitivity to external factors and thus, high
spatio-temporal resolution and accuracy.[1,2] Moreover,
fluorescent molecular temperature/viscosity sensors are
advantageous for biological applications due to their non-
invasive nature.[3] Among the various design strategies for small
organic molecule based ratiometric temperature and viscosity
probes,[4] donor-acceptor (D-A) rotor molecules[5] that show
twisted intramolecular charge transfer (TICT) are of particular
interest.[4,6–9] In recent years, multifunctional donor-acceptor
indocyanines,[10,21]
diaza-s-indacene
porphyrins,[22]
4,4-difluoro-4-bora-3a,4a-
(BODIPY),[23–27]
arylboranes,[28]
N,N-
dimethylaniline based compound[29] and many others.[25,26,30–35]
Luminescent thermometers based on molecular rotors are being
intensively investigated for temperature sensing of cellular
microenvironments as well as of microfluidic devices.[36] In
recent years, TICT rotors[25] based on the BODIPY dyes[37–40]
have been utilized efficiently for temperature sensing[9] as well
as for imaging of cellular local micro viscosity using fluorescence
lifetime imaging (FLIM). However, red emissive BODIPYs[41–43]
and with temperature and viscosity sensing capability are
extremely rare in literature.[44]
TICT
rotors
with
large
Stokes
shift,
pronounced
solvatochromism, aggregate induced emission, temperature and
viscosity sensing have been reported.[9–11] The rational design of
a multichromophoric rotor is a complex task because the
emission of TICT rotors are governed by the subtle equilibrium
between planar and twisted conformations in excited states,
often non-trivial to predict computationally.[12] Owing to the large
Stokes shift of TICT rotors, minimal overlap occurs between
their absorption and emission leading to least reabsorption.[1,2]
As a result, an improved signal-to-noise ratio can be achieved
while utilizing such TICT rotors for fluorescence imaging.[1,2]
Owing to the inherent dual emission due to TICT and local
excited (LE) states, and sensitivity of their emission to solvent
polarity and temperature, such molecules show pronounced
ratiometric temperature sensing behavior.[1,2] Moreover, as a
consequence of multiple emission, such rotors exhibit other
Challenges
in
the
development
of
ratiometric
temperature/viscosity molecular probes for sensing and
especially for bioimaging applications include development of
photostable rotors with red emission for deeper tissue
penetration
and
large
Stokes
shift
for
least
autofluorescence.[3,45,46] Significant efforts have been ongoing in
the development of TICT with large Stokes shift, red emission
and possessing ratiometric temperature/viscosity sensing
capabilities.[22,33,47–51] Kuimova et al. reported porphyrin dimers
(red em. ~ 780 nm, Stokes shift ~ 300 nm), uniquely constructed
for sensing of viscosity by ratiometric or lifetime-based methods,
in solvents of different viscosity at different temperatures.[22]
Mallard et al. reported glycoconjugated porphyrin dimers (red
em. ~ 740-800 nm) for ratiometric temperature sensing.[48] Other
notable examples of rotors for ratiometric sensing with emission
around ~ 630-650 nm are the T-shaped pyridoquinoxaline
(PQCz-T) rotor (red em. 635 nm, Stokes shift ~ 195 nm),[49]
dimalononitrile–dicarbazole-substituted 1,4-butadiene (MCBD)[51]
(red em. 640 nm, Stokes shift ~ 80 nm) and pentamethine
cyanine Ry3 (red em. 650 nm, Stokes shift 250 nm).[33] Very
recently, Tang and co-workers adopted an approach to design
AD’DD’A type rotor based on benzo-bisthiadiazole and
triphenylamine (TPA) and thiophene spacer that showed unique
[a]Aswathy P. R., S. Sharma, N. P. Tripathi and Dr. S. Sengupta
Department of Chemical Sciences,
Indian Institute of Science, Education and Research (IISER) Mohali,
Punjab 140306, India.
*Email: sanchita@iisermohali.ac.in
†Supporting Information for this article is given via a link at the end of the
document.
‡
Contributed equally.
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TICT behavior in aggregated state with emission in near infra
red (NIR) window II (900-1500 nm).[50] In many of these
examples, temperature sensitivity in wide working temperature
range were obtained, as comprehensively reviewed in a recent
literature.[49] Notably, rotors with positive temperature
coefficients in this context are extremely desirable because the
increase in emission intensities with temperature can
significantly supress background interference at high
temperatures and thus are advantageous for sensing/imaging
purpose.[29] Interestingly however, rotors showing positive
temperature coefficients are rather few.[28,29,49,51,52]
regioisomeric meta- and para-dyads and meta-triad, the
corresponding meta- and para-bromo BODIPYs were
synthesized. Regioisomers BDP-p-Br and BDP-m-Br were
synthesized starting from 4-bromobenzaldehyde and 3-
bromobenzaldehyde respectively and pyrrole in the presence of
catalytic amount of HCl in an aqueous medium to yield
corresponding dipyrromethane (DPM) compounds according to
a reported procedure for a different DPM.[53,59,60] Subsequently,
corresponding BODIPY BDP-p-Br and BDP-m-Br were
synthesized in 69 % and 19% respectively (see supporting
information for synthetic procedures and characterization).[53]
Recently, we reported an acceptor-donor-acceptor (p-ADA, Distannylated BDT (D2) was synthesized according to
a
Figure 1) type triad rotor consisting of BODIPY acceptor and
benzodithiophene (BDT) donor that showed efficient TICT with
Stokes shift of ~ 194 nm (red em. upto 700 nm), the highest
known Stokes shift for BODIPY compound and charge carrier
mobility of ~ 4.45  10-4 cm-2/Vs.[53] The D and A units were
connected at the para positions of the two phenyl spacers thus,
it was named p-ADA (Figure 1).[53] Interestingly however,
studies on the effect of regioisomerism in relation to TICT/AIE in
organic rotor molecules are rather few.[49,54–58] To the best of our
knowledge, regioisomeric BODIPY based multichromophoric
TICT rotors with large Stokes shift, tunable emission in red
region and unique temperature and viscosity sensing capabilities
have not been reported.
To bridge this knowledge gap, we present synthesis of
new regioisomeric triad m-ADA and new dyads p-AD and m-AD
(Figure 1). We intend to study the effect of regioisomerism
(para- vs. meta- connectivity of phenyl spacer to D and A) on
TICT, AIE and temperature as well as viscosity sensing
properties of these compounds. Their optical properties such as
UV/Vis absorption, fluorescence, fluorescence solvatochromism,
aggregate induced emission, viscosity and temperature
dependent emission have been investigated in-depth. By
utilizing the TICT and LE emissions, we present ratiometric
temperature sensors based on these BODIPY-BDT
regioisomers that possess either positive or negative
temperature coefficients depending upon the solvent of study.
Furthermore, chromaticity analysis for some of these molecules
revealed near white light emission that justify their
multifunctionality for possible applications in (opto)electronics.
previously reported procedure starting from thiophene-3-
carboxylic acid.[61] Furthermore, with the intention to synthesize
dyad compounds for which monostannylated BDT was required,
some optimization reactions were performed and mixture of
monostannylated and distannnylated BDT (D1+D2) was
obtained (see supporting information). The isolation of
monostyannylated BDT was challenging owing to its negligible
difference in Rf values from the distannylated or the starting
compound. The synthesis of m-ADA was achieved by Stille
coupling of BDP-m-Br and D2 in dry toluene with catalytic
amounts
of
tris(dibenzylidene-acetone)
dipalladium(0)
(Pd2(dba)3) and tri(o-tolyl)phosphine (P(o-tol)3) under an inert
atmosphere and m-ADA was isolated in 54 % yield (supporting
infromation for details). Syntheses of dyads m-AD and p-AD
were achieved under similar Stille coupling conditions using
D1+D2 as donor and BDP-m-Br and BDP-p-Br as acceptors
respectively in low yields of 14 % and 11 %.
Scheme 1. Synthesis of dyads m-AD and p-AD and triad m-ADA.
Figure 1. Chemical structures of regioisomeric ADA triads m-ADA, p-ADA
and dyads p-AD and m-AD synthesized and investigated in this work.
The synthesized compounds were purified by column
chromatography as well as preparative thin layer
chromatography (TLC) using petroleum ether/ethyl acetate or
dichloromethane (DCM) mixtures (v/v) as eluents. All precursors
Results and Discussion
1
Synthesis
and m-ADA, p-AD and m-AD were characterized with H NMR,
13C NMR and high resolution mass spectrometry (HRMS)
(details provided in supporting information).
The synthesis of dyads p-AD and m-AD and triad m-ADA were
performed as outlined in Scheme 1. In order to achieve the
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Absorption and Emission
(THF). Subsequent to steady state emission measurements,
fluorescence quantum yields were determined for p-AD, m-AD
and m-ADA in CHCl3 by relative method using Rhodamine B as
the reference compound in ethanol (Table S1) and low quantum
yields of ~ 0.031, 0.021 and 0.035 were obtained for p-AD, m-
AD and m-ADA respectively. TICT compounds are usually
known to have low fluorescence quantum yields owing to the
forbidden nature of the transition from the v = 0 vibrational level
of the TICT state to the ground state.[4]
UV/Vis and fluorescence spectra were recorded for all
compounds in chloroform (CHCl3) at a concentration of 10-5 M.
Figure 2a shows the comparative UV/Vis absorption spectra of
triad m-ADA and dyads m-AD and p-AD in CHCl3 (c ~ 10-5 M)
and the absorption profiles of model acceptors BDP-m-Br and
BDP-p-Br and donor alkylated BDT, are provided in Figure S1,
supporting information. The absorption spectra of m-ADA, m-
AD and p-AD were compared with their corresponding monomer
units, i.e., donor and acceptor units. All compounds have three
major peaks corresponding to BODIPY (~ 504 nm), BDT (~ 312-
322 nm), and broad peak around ~ 360-380 nm corresponding
to extended conjugation of phenyl unit with BDT (Figure 2a).
Peaks around ~ 504 nm correspond to BODIPY S0-S1 transition
and peaks around 312-322 nm correspond to BDT π-π*
transition. Notably, no electronic communication was observed
between BDT and BODIPY since their corresponding peak
positions were maintained in the dyads p-AD and m-AD and
triad m-ADA. This observation is in agreement with density
functional theory (DFT) calculations (vide infra) revealing
compartmentalization of HOMO and LUMO on the donor and
acceptor respectively. Owing to the weaker coupling of m-phenyl
with the BDT unit in m-ADA and m-AD compared to the
coupling in p-phenyl with BDT (in p-AD), peak splitting to a
greater extent was observed in meta compounds (Figure 2a)
than in para compound p-AD. Similar peak splitting occurring in
meta-regioisomer and its absence in para-regioisomer has been
reported earlier.[53] The fluorescence emission spectra of p-AD in
CHCl3 were measured upon excitation at two wavelengths 377
nm (donor) and 504 nm (acceptor) in Figure 2b. Excitation at
377 nm led to emission at 410 nm and 430 nm from donor, 516
nm from acceptor and a broad but intense band around ~ 654
nm corresponding to a TICT state and a Stokes shift of ~ 150
nm (Figure 2b).
Quantum Chemical DFT Calculations
In order to gain insights about the frontier molecular orbitals
(FMOs) as well as torsion angles of m-ADA, m-AD and p-AD,
DFT calculations were performed using the Gaussian 09
package (see reference S1, supporting information) by the
B3LYP/6-31G(d,p) method. The torsional angles in m-ADA
between the BODIPY and phenyl spacer (Φ1, Φ4) and between
phenyl spacer and BDT (Φ2, Φ3) were ~ 55.4o, 54.2o and ~ 22.5o,
22.5o respectively (Table S2, S3) confirming the non-planarity of
this molecule. The calculated HOMO and LUMO energy levels
of m-ADA were -5.39 eV and -2.87 eV respectively, with the
LUMO and LUMO+1 being energetically degenerate (Figure 3).
FMO composition analysis for triad m-ADA indicated major
contribution of BDT in the HOMO (91 % contribution, table S4,
figure S3) and BODIPY contributing towards the LUMO level
(91 %, table S4) with no overlap between the HOMO and LUMO
(Figure 3). Therefore, no electronic communication was present
between BDT and BODIPY as confirmed by experimental
absorption spectrum of m-ADA. Similar localization of HOMO
and LUMO of m-AD and p-AD on BDT and BODIPY
respectively led to electronic decoupling of BDT and BODIPY
units (Figure 3, Table S4). However, coupling of phenyl ring with
BDT was present in all cases and this coupling is different in m-
ADA/m-AD and p-ADA/p-AD that is reflected in their optical
properties. The DFT calculated HOMO and LUMO levels of p-
AD were at -5.33 eV and -2.83 eV respectively. Torsion angles
between the BODIPY and phenyl (Φ1) and between phenyl and
BDT (Φ 2) in m-AD were 55.2o and 23.3o respectively while in p-
AD they were 52.3o and 22.9o respectively (Figure 3, Table S2).
Figure 2. a) Comparison of absorption spectra for m-AD, p-AD, m-ADA in
CHCl3 (c ~ 10-5 M), b) Normalized emission spectra for m-AD, p-AD, m-ADA
in CHCl3 (c ~ 10-5 M) upon exciting at 362 nm, 377 nm, and 380 nm
respectively.
In comparison to p-ADA that showed Stokes shift of 194
nm,[53] the Stokes shift of p-AD was ~ 150 nm which was 44 nm
less than that of p-ADA. Fluorescence emission spectra of m-
ADA were measured upon three different wavelengths, 322 nm,
380 nm (donor) and 504 nm (acceptor) respectively. Upon
excitation at 322 nm and 380 nm, donor emission at 430 nm,
acceptor emission at 517 nm and a broad but less intense band
around ~ 662 nm were observed. The band around ~ 662 nm
can be attributed to aggregation and this band was observed in
the emission spectra of other solvents also (vide infra). In case
of m-AD, three distinct emission peaks were observed. Upon
excitation at 362 nm (donor) and 504 nm (acceptor), a donor
emission peak at 430 nm and acceptor emission at 522 nm were
observed. A broad but a weaker TICT type band was observed
around ~ 645 nm that was also observed in tetrahydrofuran
Figure 3. DFT calculated (B3LYP/6-31G(d,p)) geometry optimized structures
of p-AD, m-AD and m-ADA with indication of torsion angles and
HOMO/LUMO levels.
In order to obtain insights about the redox properties and FMO
energies for dyads and triad, cyclic voltammetry (CV)
measurements were performed for p-AD, m-AD and m-ADA as
well as for the model subunits BDP-p-Br, BDP-m-Br and alkyl
BDT in dry DCM (Figure S3, Table S5 and S6). From CV studies,
the HOMO levels were calculated to be -5.88 eV, -5.91 eV and -
5.61 eV for p-AD, m-AD and m-ADA respectively.
Corresponding LUMO values of -4.12 eV, -4.20 eV and -4.22 eV
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were obtained for p-AD, m-AD and m-ADA respectively. Thus,
CV studies substantiate the theoretically calculated FMO energy
levels as well as the fact that BDT and BODIPY serve as
electron donor and acceptor respectively in all dyads and triad.
photon counting (TCSPC) measurement technique (Figure S5).
The fluorescence lifetime of p-AD, m-AD and m-ADA were
measured upon an excitation with 375 nm laser diode. Decay
profiles at emission wavelengths of 430 nm (corresponding to
donor decay) and TICT emission at 647 nm were studied for p-
AD. The decay corresponding to charge transfer emission is
known to exhibit a longer rise time characteristic of charge
transfer and a slower component characteristic of the lifetime of
the equilibrated molecules.[7,58] Moreover, in the decay profile of
the CT band, a growth component can be observed (Fig. S5c)
which is well known for fluorescence decay of TICT
molecules.[58] Accordingly, a slower decay was observed for p-
AD emission at 647 nm with a lifetime of ∼ 3 ns than the decay
at 430 nm (lifetime of ~ 0.42 ns) as shown in Figure S5c. The
average lifetimes of the decay corresponding to 430 nm and 647
nm were ~ 0.7 ns and ~ 1.83 ns respectively. The decay at 430
nm is in close agreement with the neat donor decay with lifetime
of ~ 0.8 ns reported earlier.[53]
Fluorescence Solvatochromism
Since compound p-AD showed the formation of a TICT
state, fluorescence solvatochromism was investigated in order
to assess the polarity of the ground and first excited state.[4,6]
Emission maximum of p-AD in toluene was observed at ~ 530
nm which was due to local excited (LE) emission since toluene
being a non-polar solvent can not stabilize the TICT state with
high dipole moment. A bathochromically shifted band at ~ 654
nm was observed in CHCl3 arising due to TICT state (Figure 4a).
The ground state of p-AD has a smaller dipole moment than the
excited state, because UV-VIS absorption spectra of molecule in
solvents of different polarities did not show any change in the
absorption maxima (Figure S4). Notably, the TICT band was
non-existent in high polarity solvents acetonitrile (ACN) and
methanol (MeOH) owing to the limited solubility of the compound
in these solvents and in DCM due to rapid non-radiative
deactivation. These observations are consistent with similar
TICT rotors that show negligible emission in ACN and MeOH.[49]
Formation of TICT state in p-AD was justified from DFT
calculated torsion angles that indicate the non-planarity of the
compound. This results in twisting around the BODIPY-phenyl
single bond in the excited state leading to pronounced charge
separation of D and A units accompanied by an increase in the
dipole moment in the excited state. The fluorescence
solvatochromism was further probed in binary solvent mixtures
of hexane/THF (v/v) by sequentially increasing the percentage of
THF from 10% to pure THF and thus achieving a wide range of
solvent polarity. Interestingly, when the solvent polarity was
decreased from 100 % THF sequentially 99/1 (v/v) hexane/THF,
the band at 647 nm shifted to ~ 523 nm (through a broad band
at ~ 580 nm corresponding to an intermittent LE state) (Figure
4b, e). Thus, a pronounced hypsochromic shift upon decreasing
solvent polarity substantiates the formation of TICT state in p-
AD. Interestingly, the meta counterpart m-ADA of previously
published p-ADA compound[53] does not show any TICT in
various solvents. Solvent polarity dependent absorption
measurements did not show significant variations as shown in
Fig The absence of any TICT in m-ADA could be attributed to a
weaker electronic coupling between the D and A units through
the meta positions of the phenyl.[54,58] Solvent polarity dependent
fluorescence spectra of m-ADA showed apart from the BODIPY
emission at 530 nm, a weaker peak at ~ 662 nm in THF, toluene
and DCM and a peak at ~ 675 nm in MeOH (Figure 4c). The
peak position of this band remained unchanged in polar and
non-polar solvents indicating that this peak did not result from
TICT but presumably due to the aggregate formation at low
concentrations (c ~ 10-5 M). The m-phenylene connection in m-
ADA restricts the free rotation of phenyl ring and so the -
stacking is probably favored for this compound. Furthermore, in
m-ADA the comparable terminal twist angles (Φ 1 and Φ 4) of
55.4o and 54.2o to p-ADA and smaller Φ2 and Φ3 of 22.5o, 22.5o
than p-ADA (32.2o, 32.2o) renders the molecule more planar
than p-ADA. Thus, m-ADA being less twisted than p-ADA has
presumably a higher propensity to form intermolecular π-stacks.
Similar solvatochromism studies were performed for m-AD and
a weaker but a broad band around ~ 643 nm was observed only
in CHCl3 and THF which could be attributed to a very weakly
emissive TICT-type transition (Figure 4d).
Figure 4. Normalized fluorescence emission spectra of p-AD a) upon
excitation at 377 nm in solvents of different polarity; b) in different hexane/THF
(v/v) solvent mixtures; c) Fluorescence emission spectra of m-ADA and d) m-
AD upon excitation at 380 nm and 362 nm respectively in different solvents; e)
Fluorescence solvatochromism of p-AD in hexane/THF mixtures (from left to
right: 99/1, 90/10, 80/20, 70/30, 60/40, 50/50 v/v hexane/THF) under handheld
UV lamp of wavelength 365 nm.
Similarly, the fluorescence lifetimes of m-ADA at emission
wavelengths of 432 nm (donor emission) and 662 nm
(aggregate emission) were ~ 0.88 ns and ~ 5 ns respectively.
Such distinct decay profiles for m-ADA and m-AD indicate the
presence of two or more distinct species/conformer in the
solution and the fact that all the decays could be fitted with bi-
exponential or tri-exponential functions substantiates the
presence of multiple conformers or species in the solution. The
fluorescence decays as well as fitted plots with fitting
parameters and lifetimes are presented in Figure S5 and Table
S7.
Aggregation Induced Emission (AIE)
Fluorescence lifetime measurements were performed for p-
AD, m-AD and m-ADA in CHCl3 using time correlated single
Aggregate induced emission (AIE) property was investigated in
dyads p-AD and m-AD and triad m-ADA by measuring
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fluorescence in binary mixture of water and THF. In case of p-
AD, starting from pure THF when the water fraction was
increased to 10 vol% up to 50 vol%, the fluorescence intensity
decreased drastically (Figure 5a). This can be attributed to the
increasing polarity of the solvent mixture upon increasing the
water content that favours TICT and therefore non-radiative
deactivation. However, upon further increasing the water fraction
from 60 vol% onwards i.e., 60/40, 70/30, 80/20 and 90/10 v/v
water/THF, fluorescence intensity increased up to two to three
times than the intensity at 10/90 v/v water/THF. The increased
fluorescence intensity can be attributed to the formation of well-
defined nanoaggregates, without any agglomeration or
precipitation, which restrict the intramolecular rotation. For m-
ADA the fluorescence intensity increased for 90/10 and 80/20
(v/v) in THF/water than from pure THF. Further it uniformly
decreased upon increasing percentage of water. A new band
around ~ 670 nm was observed due to aggregation upon
increasing the water percentage (Figure 5b). In case of m-AD, it
followed the similar behaviour as in p-AD. It decreased first and
upon increasing the water content from 30% onwards, the
intensity started increasing (Figure 5c) (corresponding UV/Vis of
p-AD and m-AD in Figure S6).
The solutions of m-ADA in THF/water (v/v) showed sharp
colour change from orange to pink as the solvent is changed
from 100% THF to 90/10 (v/v) water/THF (Figure 5d, inset) due
to aggregate formation. Upon changing the solution composition
from 60/40 (v/v) THF/water to 50/50, the aggregation is evident
from appearance of a bathochromically shifted new band at 550
nm in absorption spectra (Figure 5d) and a colour change from
orange (monomer) to pink (aggregates). Interestingly triad m-
ADA has no propensity for TICT owing to much weaker
electronic coupling of meta phenylene spacer and probably a
higher barrier for TICT and planar states.
Interestingly, chromaticity analysis for the emission of m-AD in
all the water/THF solvent mixtures showed emission colours
distributed in the blue, green and purple region (Figure 5e). A
chromaticity analysis using Internationale de l’Eclairage (CIE)
coordinates revealed a near-white light emission component
with chromaticity coordinates of x = 0.32, y = 0.29 (Figure 5e) for
10/90 (v/v) THF/water solution. Pure white light emissive
singular compounds (with x, y coordinates of 0.33, 0.33) are
rare[13,14,62–64] and easily tunable emission in blue, green and
purple region are rare to obtain in a single compound. This is
achievable in the present case through molecular design and
only by changing solvent compositions.
Temperature Induced Tunable Fluorescence
The emission of molecular rotors are highly sensitive to
solvent temperature and show different emission properties
upon varying the temperature. Temperature sensing in rotor
molecules originates from two decay channels corresponding to
the twisted and planar molecular conformations in the excited
state.1 In order to obtain insights about the temperature
sensitivity of fluorescence of these dyads and triads,
temperature-dependent fluorescence were performed for p-AD,
p-ADA, m-AD and m-ADA in various solvents. Emission
measurements were performed in the temperature range of -20
o
oC upto 85 C (depending upon the boiling point of the chosen
solvent) in quartz cuvettes in a cuvette holder thermally
equilibrated using a Peltier thermostat.
In p-AD, upon increasing the temperature from -20 oC to 55 oC
in CHCl3, a drastic increase in the fluorescence intensity was
observed and the TICT band was blue shifted from ~ 670 nm to
~ 636 nm (Figure 6a). This observation is attributed to the fact
that heating helps in crossing the activation barrier from TICT to
the LE state and the LE state gets more populated thereby
causing the hypsochromic shift in the emission band and
increased intensity. For p-AD, temperature dependent
fluorescence measurements in THF (Figure 6b) also showed
increase in the TICT band intensity and a concomitant blue shift.
The effect of temperature on p-AD in toluene however was
different as evident from Figure 6c.
Figure 6. Temperature dependent fluorescence emission spectra of dyad p-
AD in a) CHCl3; b) THF and c) Toluene; d) CIE chromaticity plot of
temperature dependent emission of p-AD in THF showing a near-white light
emission at 0oC (CIE coordinates of 0.33, 0.27), 5 oC (0.34, 0.27) and 10 oC
(0.35, 0.27).
Figure 5. Aggregate induced emission in a) p-AD, b) m-ADA, and c) m-AD in
different mixture of water/THF. d) Absorption of m-ADA in water/THF mixture.
e) CIE Chromaticity plot of m-AD in water/THF showing a single component
near-white light emission (0.32, 0.29) at 10/90 (v/v) THF/water mixture. Inset in
(d): Solutions of m-ADA in THF/water (v/v) left to right: 90/10, 80/20, 70/30,
60/40, 50/50, 30/70, 20/80, 10/90 showing AIE effect.
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In toluene, the TICT band was absent and the spectra showed
two bands at 530 nm corresponding to the BODIPY emission
and ~ 580 nm corresponding to an intermittent LE state at low
temperatures. Upon increasing the temperature, the emission
intensity decreased due to enhanced molecular motions and
possibility of non-radiative decay channels. Interestingly, there
was no change in absorption spectra with the variation of
temperature ruling out any possibility of degradation of the
compounds (Figure S7). Temperature dependent fluorescence
of p-AD in 2-methyl THF (2MeTHF) was also performed in the
o
o
Figure 8. Fluorescence emission spectra of dyad a) p-AD and b) p-ADA in
temperature range of -15 C to 75 C as shown in Figures S8,
S9. Figure 6d shows the CIE chromaticity plot for temperature
dependent emission of p-AD in THF. Interestingly, at
2MeTHF at cryogenic temperature (-196 oC), at 25 oC and 75 oC.
Cryogenic temperature (-196 oC) fluorescence was measured for
p-AD and p-ADA in 2MeTHF in a cryogenic setup and the
emission centred at ~ 519 nm and ~ 523 nm were observed for
p-AD and p-ADA respectively (Figure 8), whereas any emission
from the TICT state were completely absent for both compounds.
o
o
o
temperatures of 0 C, 5 C and 10 C, near white light emission
with CIE coordinates (x, y) of (0.33, 0.27), (0.34, 0.27) and (0.35,
0.27) respectively were obtained (Figure 6d). Temperature
dependent emission spectra were measured for p-ADA in DCM,
THF, toluene and 2MeTHF in the temperature range from -15 oC
upto the boiling point of the respective solvents (Figure 7a-d).
The TICT band of p-ADA initially observed at ~ 700 nm in DCM
was blue shifted upto 678 nm accompanied by an increase in
the emission intensity upon increasing temperature upto 35 oC.
In THF, the TICT band initially located at ~ 660-670 nm was
blue-shifted upto ~ 645 nm upon increasing the temperature to
50 oC. Temperature dependent emission measurements for p-
AD and p-ADA were performed in 2MeTHF because this solvent
forms a glassy matrix at cryogenic temperature (-196 oC) and
thus, can reveal the complete confinement of LE state at -196
oC.[65] Temperature dependent emission for compounds p-ADA
and p-AD in 2MeTHF showed two different trends in the lower
Ratiometric Temperature Sensing Analysis
Ratiometric analysis of temperature dependent emission is
possible when one emission band changes with temperature
while the other band remains unchanged or changes negligibly
with temperature.[1] The plot of ratio of intensities with
temperature can be fitted with linear,[28,49,66] exponential,[51] or
polynomial functions[52] to obtain temperature coefficients and
sensitivities of such molecular thermometers. Since temperature
dependent emission of p-AD and p-ADA in different solvents
revealed pronounced temperature dependent intensity changes
in the TICT band (I2) while the LE/BDT emission band remained
relatively unchanged (I1), ratiometric analyses of these data
were performed. The ratiometric analyses (i.e., plot of I1/I1+I2 vs.
T) revealed unique temperature sensing behaviour for p-AD and
p-ADA in different solvents. The ratio of intensities with
temperature plot for p-AD in THF, CHCl3 and 2MeTHF were
fitted with exponential functions with goodness of fit R2 values of
0.98-0.99 (Figures S12, S13, S8 and S9 and Table S8). The
internal sensitivities were obtained by differentiation of the fitted
plots of intensity ratio vs temperature[51] and internal sensitivities
o
o
temperature range (25 C to -15 C) (Figure S10) and in higher
temperature range (25 oC to 75 oC) (Figure 7d, Figure S11).
When temperature was reduced from 25 C to -15 oC for p-ADA,
o
the TICT band initially at 628 nm was bathochromically shifted
upto 648 nm with a concomitant decrease in the emission
o
o
intensity. When temperature was increased from 25 C to 75 C,
the TICT band initially at 628 nm was blue shifted upto 613 nm
with concomitant decrease in the emission intensity (Figure 7d).
Compound p-AD in 2MeTHF showed similar behaviour in the
o
temperature range 25 C to -15 oC where the TICT band at 631
o
in the range of 0.6 % C-1 to -0.8 % oC-1 was obtained in various
nm at 25 oC was red-shifted to 646 nm and decreasing
intensities upon decreasing temperature. Upon increasing
temperatures for p-AD in 2MeTHF (Figures S8, S9) however,
solvents as tabulated in Table S8. The plot of intensity ratio for
p-AD in 2MeTHF with temperature was fitted in two stages, a
lower temperature range (-15 oC to 25 oC) and a higher
temperature range (25 oC to 75 oC). In the lower temperature
o
the TICT band at 631 nm at 25 C was blue shifted upto 613 nm
with decreasing intensities.
range,
a
negative temperature coefficient with internal
o
o
sensitivities of -0.05 % C-1 to -0.67% C-1 was obtained (Figure
S9, Table S8). In the higher temperature range however, a
positive temperature coefficient with internal sensitivities from
0.07 % C-1 to 0.6 % C-1 was obtained (Figure S8, Table S8).
The plot of intensity ratio of p-AD in toluene with respect to
temperature could be fitted with a linear function with goodness
of fit (R2) of 0.97 (Figure S14) with positive temperature
coefficient, slope of 0.0017 and thus a temperature sensitivity of
o
o
0.17 % C-1. The fitted equation obtained (for p-AD in toluene)
o
was:
(퐼431/퐼431 + 퐼532) = 0.0017 푇 + 0.1953
where, I431 and I532 are the fluorescence intensities at emission
wavelengths of 431 nm and 532 nm, respectively.
For p-ADA rotor, the ratio of emission intensities, I1/I1+I2 was
plotted with respect to temperature in different solvents and the
data in toluene, THF, DCM and CHCl3 were fitted with
exponential functions[51] with goodness of fit (R2) 0.99-0.93
(Figures S15-S18). The plots for the internal sensitivities are
Figure 7. Temperature dependent fluorescence emission spectra of triad p-
ADA in a) DCM; b) THF; c) Toluene and d) 2MeTHF.
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10.1002/chem.201902952
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presented in Table S5 and sensitivities in the range of 0.16 %
oC-1 to -1.88 % oC-1 were obtained. Notably, the temperature
coefficients in THF, CHCl3, DCM were negative while that in
toluene was positive. For p-ADA in DCM, the plot of intensity
ratio with temperature was fitted with a linear function with
goodness of fit (R2) was 0.93, a negative temperature coefficient
conformations of these molecules in the excited state. The
fluorescence intensity increased with increasing the viscosity of
solvent using a glycerol/methanol mixture for all compounds.
The solvent viscosity was varied from 100/0 to 10/90
MeOH/glycerol (0.6 cP to 454 cP) v/v.[49] Restricting the rotation
of molecular rotors in a viscous medium enhances fluorescence
intensity. Since glycerol is more polar than MeOH, upon
increasing the percentage of glycerol, there will be an overall
increase in the solvent polarity. However, the viscosity of
glycerol overrules the polarity increment effect and the viscosity
restricts the intramolecular rotation thereby leading to LE
emission exclusively. The restriction of the intramolecular
rotation in the viscous medium hinders the TICT deactivation
pathway and thus the TICT band or the band in the longer
wavelength region was completely absent in case of the dyads
p-AD and m-AD (Figure 9a, b) and in triads p-ADA and m-ADA
(Figure 9c, d). In case of p-AD, the peaks (at ~ 425 nm and ~
520 nm) did not shift upon increasing the percentage of glycerol
but the intensities of the peaks as well as the spectral shape
changed upon increasing percentage of glycerol.
o
and sensitivity of -0.43 % C-1. The fitted equation (for p-ADA in
DCM) obtained was:
(퐼431/퐼431 + 퐼677) = - 0.0043 푇 + 0.5937
where, I431 and I667 are the fluorescence intensities at emission
wavelengths of 431 nm and 677 nm, respectively. The
temperature dependent emission of p-ADA in 2MeTHF in the
decreasing temperature range (25oC to -15oC) was fitted with
exponential function (Figure S10, Table S8) while the data in the
increasing temperature range (25oC to 75oC) was fitted with
linear plot with R2 value of 0.93 (Figure S11), positive
temperature coefficient and slope of 0.0016, thus a sensitivity of
0.16 % oC-1.
Temperature dependent fluorescence study
Similar effect was observed in case of m-AD where upon
increasing the percentage of glycerol, the fluorescence intensity
drastically increased almost five times from initial value (Figure
9b).
was performed also for m-ADA in CHCl3 and Toluene.
Interestingly, while the emission in toluene was centred around
525 nm that increased upon reducing the temperature, the
emission in CHCl3 was characterized by a weak broad band at ~
660 nm that indicated aggregation as discussed above. This
aggregation band upon reduction in temperature increased in
intensity owing to reduced intramolecular rotation and reduced
conformational freedom at lower temperatures. The temperature
dependent emission for m-ADA in CHCl3 showed good
ratiometric behavior where the ratio of intensities with
temperature plot could be fitted with a linear function with a
goodness of fit (R2) value of 0.98 (Figure S19). The fitted
equation obtained (for m-ADA in CHCl3) was:
(퐼435/퐼435+ 퐼662) = -0.0041 푇 + 0.6562
where, I435 and I662 are the fluorescence intensities at emission
wavelengths of 435 nm and 662 nm respectively. The fitted plot
had a negative temperature coefficient and slope of -0.0041,
o
thus a temperature sensitivity of 0.41 % C-1. It however showed
a positive temperature coefficient in toluene (Figure S20) with
sensitivity of 0.21
%
oC-1. Compared to m-ADA, dyad
counterpart m-AD was not much temperature responsive in
variety of solvents when compared to p-AD or m-ADA.
Thus, to summarize from ratiometric temperature analyses,
all three rotors p-AD, p-ADA, and m-ADA showed positive
temperature coefficient in toluene. In these cases, the emission
that was monitored was predominantly the LE/BODIPY emission.
Moreover, p-ADA and p-AD showed positive temperature
coefficient in 2MeTHF in the temperature range 25 C to 75 C.
The small sensitivities obtained for p-AD, p-ADA and m-ADA
indicate that a range of temperature can be measured precisely
using these molecules. Rotors p-AD and p-ADA also showed
negative temperature coefficients in THF, CHCl3 and in the lower
temperature range in 2MeTHF. The summary of temperature
behaviour of emission of these rotors in various solvents have
been tabulated in Table S9. Such studies indicate the utility of
these dyad and triad rotors as molecular thermometers for
sensing temperature of microenvironments upto the
macroscopic scale in the physiological temperature range.
Figure 9. Viscosity dependent fluorescence emission spectra of dyads a) p-
AD; b) m-AD; and triads c) p-ADA; d) m-ADA.
While in p-ADA, only the emission from the LE state and the
BDT was observed (Figure 9c) and in m-ADA the peaks were
slightly blue shifted with spectral broadening at 90/10 (v/v) of
glycerol/MeOH mixture (Figure 9d). For all compounds, solvent
polarity had no effect and the blue shift observed was primarily
attributed to the rigidification of the molecular geometry. In order
to quantitatively assess the viscosity sensitivity of the four
molecules, viscosity dependent emission was expressed using
o
o
Förster Hoffmann theory[10,11]
:
log (I/I0) = C + x log η
where, I0 and I are the emission intensities at the initial viscosity
i.e., in 100 % MeOH and in glycerol/ MeOH mixtures with
increasing glycerol percentage respectively.
Viscosity Induced Fluorescence Enhancement
The viscosity of the medium is given by η, C is the experimental
temperature and concentration dependent constant and x is the
viscosity sensitivity of the molecular rotors. According to this
analysis (Figure S21, Table S10), p-AD, m-AD, p-ADA and m-
Viscosity dependent emission were measured for dyads p-
AD and m-AD and triad m-ADA in a series of mixtures of MeOH
and glycerol (v/v) to understand the viscosity effect on
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ADA showed viscosity sensitivity (x) of 0.09, 0.23, 0.12, 0.31
respectively. Typical values of slopes (x) for rotors vary from 0.1
to 1.4 for different rotors.[24,67] Thus, among four compounds, m-
ADA is most sensitive to viscosity and shows the best viscosity
sensing (x = 0.31) characteristics.
Acknowledgements
The financial support from Department of Science and
Technology (DST), New Delhi (DST/TM/SERI/FR/162) and
IISER Mohali is gratefully acknowledged. The authors thank
central (NMR and MS) and departmental facilities of IISERM.
A.P.R. thanks DST-INSPIRE for BS-MS fellowship, S.S. and
N.P.T. thank IISERM for Ph.D. fellowships. We thank Dr.
Kamalakannan Kailasam, Institute of Nano Science and
Technology, Mohali for providing access to temperature
dependent UV/Vis facility. We extend our sincere gratitude to Mr.
Mayank Saraswat for insightful discussion and help regarding
the FMO composition analysis. We thank Dr. Sugumar
Venkataramani and Dr. Arijit K. De for fruitful discussion
regarding some of the photophysical studies.
Furthermore, temperature effect on a particular viscosity was
also investigated for all compounds. A solution of glycerol/
MeOH mixture 70/30 (v/v) (104 cP) of p-AD, p-ADA, m-AD and
m-ADA were subjected to temperature dependent fluorescence
measurement. The fluorescence intensity decreased on
increasing the temperature (Figure S22). It is reported that at
higher temperatures, viscosity and polarity tend to decrease
dramatically.[11,68] Reduction in fluorescence intensities upon
increasing the temperature is indicative of reduced viscosity and
thus higher molecular motion and thus rapid non-radiative
deactivation. Furthermore, no (hypsochromic) shifts in peak
positions were observed upon increasing the temperature
indicating that solvent polarity did not change significantly. The
decrease in fluorescence intensity was thus predominantly due
to viscosity effect at variable temperatures.
Keywords: BODIPY • Regioisomer • Ratiometric temperature
sensor
• Twisted intramolecular charge transfer (TICT) •
Viscosity sensor.
Conclusions
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studies. Interestingly, fluorescence of dyad m-AD (in 10/90 v/v
THF/water) revealed a near white light emission with CIE
chromaticity coordinates (x, y) of (0.32, 0.29). Furthermore,
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10.1002/chem.201902952
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Aswathy P. R., S. Sharma, N. P.
Tripathi, S. Sengupta*
Molecular Sensors: Regioisomeric
BODIPY-Benzodithiophene dyads and
Page No. – Page No.
triads
show
either
twisted
intramolecular charge transfer (TICT)
with red emission or aggregation in
solution. These rotors exhibit tunable
Regioisomeric BODIPY-
Benzodithiophene Dyads and Triads
with Tunable Red Emission as
Ratiometric Temperature and
Viscosity Sensors.
red
emission,
pronounced
temperature and viscosity sensing
and near-white light emission that are
promising for bioimaging and sensing
applications.
This article is protected by copyright. All rights reserved.