Photophysical properties of some novel tetraphenylimidazole derived BODIPY based fluorescent molecular rotors

Article abstract of DOI:10.1039/c9dt04177j

The strategic design, synthesis and thorough characterization of four novel hydroxyl-substituted tetraphenylimidazole (HPI) based boron dipyrromethene (BODIPY) fluorophores (HPIB1-HPIB4) have been reported. Single crystal X-ray structure determination unveiled non-planar twisted orientations for these molecules. The non-planar orientations entirely restrict detrimental π-π interactions and avoid the non-radiative relaxation pathway for excited states in the solid/aggregated state and make them AIE active. The AIE characteristics of these compounds have been related to fine J-aggregation (evident from their crystal structures) along with restricted intra-molecular rotations (RIRs). These compounds display significant sensitivity toward viscosity and can serve as fluorescent molecular rotors due to multiple phenyl groups around the imidazole ring, which has been confirmed by measuring fluorescence quantum yields and lifetimes.

Full text of DOI:10.1039/c9dt04177j

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Photophysical properties of some novel
tetraphenylimidazole derived BODIPY
based fluorescent molecular rotors†
Cite this: Dalton Trans., 2020, 49,
438
Bhupendra Kumar Dwivedi, Vishwa Deepak Singh, Yogesh Kumar and
Daya Shankar Pandey
*
The strategic design, synthesis and thorough characterization of four novel hydroxyl-substituted tetraphe-
nylimidazole (HPI) based boron dipyrromethene (BODIPY) fluorophores (HPIB1–HPIB4) have been
reported. Single crystal X-ray structure determination unveiled non-planar twisted orientations for these
molecules. The non-planar orientations entirely restrict detrimental π–π interactions and avoid the non-
radiative relaxation pathway for excited states in the solid/aggregated state and make them AIE active. The
AIE characteristics of these compounds have been related to fine J-aggregation (evident from their crystal
structures) along with restricted intra-molecular rotations (RIRs). These compounds display significant sensi-
tivity toward viscosity and can serve as fluorescent molecular rotors due to multiple phenyl groups around
the imidazole ring, which has been confirmed by measuring fluorescence quantum yields and lifetimes.
Received 28th October 2019,
Accepted 27th November 2019
DOI: 10.1039/c9dt04177j
rsc.li/dalton
sity.5 Furthermore, intramolecular H-bonding also enhances
the emission properties of the compounds in dilute solution
Introduction
Fluorescent organic materials have fascinated the scientific and in the aggregated state by inducing planarity in the
community because of their utility in various areas including systems.7
biology, chemistry, materials science, etc.1 The majority of
An extensive literature survey on the emission behavior of
organic fluorophores display fluorescence quenching (ACQ) AIE materials revealed the prominent role of J-aggregation
in the aggregated state or viscous medium which limits their which not only boosts the emission but also causes shifts of
possible applications in diverse areas.2 In this context, aggre- the emission maxima to long wavelengths (red/NIR region).8
gation-induced emission (AIE) has been attractive as it over- Thus to develop systems with J-type molecular packing, it is
comes ACQ and makes the system highly emissive in the essential to modify the molecules in such a way that they
aggregated state.3 The photophysical properties of planar adopt non-planar orientations and boost quantum efficiency
luminogens may be controlled by adopting various strategies4 in the aggregated state. Furthermore, the utility of various
including introduction of bulky rotors,5 charge transfer6 and organic fluorophores with a variety of functionalities exhibit-
excited-state intramolecular proton transfer7 (ESIPT) pro- ing AIE has been explored.3 Among these, BODIPYs have fasci-
perties. Introduction of rigid and bulky aromatic units as nated researchers due to their excellent properties and appli-
rotors can be a good move toward converting ACQ systems cations in diverse areas.9 Usually the quantum efficiencies of
into AIE fluorophores.5 Essentially, dynamic intramolecular the BODIPYs are high in dilute solution; however, they suffer
motions/rotations of the bulky and rigid aromatic core utilize from fluorescence quenching in the aggregated state.8c,10
the major fraction of the excitation energy and make it non- These compounds hardly fluoresce in the solid state due to
or weakly emissive in dilute solution. However, in the aggre- ACQ arising from π–π stacking in dense medium.3,11 In this
gated state or viscous medium suppression of the active direction, in order to suppress detrimental π–π stacking, Tang
motions/molecular rotations increases radiative channels and et al. introduced a known AIE system tetraphenylethylene
therefore leads to the enhancement of the fluorescence inten- (TPE) into the BODIPY core.12a,b The triphenylamine (TPA)
system has also been integrated into the BODIPY core to
achieve emission in the solid/aggregated state.12c,d These
systems motivated the design of the BODIPYs possessing
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi -
221 005, UP, India. E-mail: dsprewa@yahoo.com, dspbhu@bhu.ac.in
bulky phenyl groups which can show non-planar orientations
†Electronic supplementary information (ESI) available: CCDC 1945394–1945397.
and avoid detrimental π–π stacking in the aggregated state.12
For ESI and crystallographic data in CIF or other electronic format see DOI:
Furthermore, multiple bulky phenyl rotors linked via single
10.1039/C9DT04177J
438 | Dalton Trans., 2020, 49, 438–452
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bonds may cause viscosity-dependent emission changes and Single crystal X-ray analyses
can serve as fluorescent molecular rotors (FMRs). Viscosity is
Crystal data for HPIB1–HPIB4 were acquired on a Bruker APEX
II CCD diffractometer at room temperature with Mo-Kα radi-
ation (λ = 0.71073 Å). The structures were solved by direct
methods (SHELXS 97) and refined by full-matrix least squares
on F2 (SHELX 97) with version 2016/1 of SHELXL.15 Non-hydro-
gen atoms were refined anisotropically and hydrogen atoms
an important parameter that determines the diffusion rate of a
species in various environments. Also, minute viscosity
changes are related to diseases and malfunctions at the cellu-
lar level in biological systems.13 Thus, such molecular rotors
may play a vital role in viscosity sensing in various environ-
ments and biological systems including subcellular organelles
and related processes.13
Considering these points, in this work, four BODIPY fluoro-
phores based on hydroxyl-substituted tetraphenylimidazole
(HPI) derivatives have been described. It is presumed that the
phenyl rotors of HPI may favor radiative decay pathways in the
solid state or viscous medium due to the suppression of active
intramolecular rotations and the compounds may serve as
FMRs. The phenyl rotors may also facilitate the entire molecule
to lose planarity and enable it to avoid detrimental π–π stacking
and display appropriate interactions causing J-aggregates
required for good emission in the aggregated state. Herein, we
present the efficient AIE, good solid state fluorescence, and
impressive viscosity-dependent emission behavior of hydroxyl-
substituted tetraphenylimidazole based BODIPY fluorophores.
geometrically fixed and refined using
a riding model.
Interaction and stacking distances were analyzed using
PLATON.16 The CCDC deposition No. 1945394 (HPIB1),
1945395 (HPIB2), 1945396 (HPIB3) and 1945397 (HPIB4)
contain supplementary crystallographic data for this paper.†
Theoretical studies
Quantum chemical calculations on HPIB1–HPIB4 were carried
out at the B3LYP Density Functional Theory (DFT) level using
B3LYP/6-31G**.17 Geometry optimization and frequency calcu-
lations were performed using the Gaussian 09 suite of
programs.18
Syntheses
Preparation of 2-hydroxy-3-(1,4,5-triphenyl-1H-imidazol-2-
yl)-benzaldehyde (4a). Compound 3a (776.46 mg, 2 mmol) and
an equivalent amount of hexamethylenetetramine (280.37 mg,
2 mmol) were added to a round bottom (RB) flask containing
trifluoroacetic acid (10 mL). Contents of the flask were refluxed
at 100 °C for 12 hours under stirring and the progress of the
reaction was monitored by TLC. After complete consumption
of 3a the reaction mixture was allowed to cool to room temp-
erature. It was then treated with 4 M HCl (30 ml) solution and
stirring was continued for an additional one hour to obtain a
yellow solid. The solid was washed with an excess of diethyl
ether and the solid powder was subjected to column chromato-
graphy (silica gel) using dichloromethane/n-hexane (1 : 9) as
Experimental details
Reagents
Solvents were dried and distilled following literature pro-
cedures prior to use.14 Reagents like benzil, aniline, p-tolui-
dine, 4-chloroaniline, 2-amino-5-nitrobenzophenone, hexam-
ine, triethylamine, salicylaldehyde, 5-chlorosalicylaldehyde, tri-
fluoroacetic acid, pyrrole, 2,3-dichloro-5,6-dicyano-1,4-benzo-
quinone (DDQ), and boron trifluoride diethyletherate were
purchased from Sigma Aldrich, India and used as received.
General information
the eluent. The first fraction afforded the desired product as a
1
1H [500 MHz; (CH3)4Si], 11B [160.4 MHz; BF3·OEt2], 13C yellow solid. Yield: 400 mg (48%). H NMR (500 MHz, CDCl3)
[125 MHz; (CH3)4Si] and 19F [470.6 MHz; CF3COOH] NMR δ = 10.41 (s, 1H), 7.63 (s, 1H), 7.52 (d, J = 7 Hz, 3H), 7.44–7.39
spectra were acquired on a JEOL AL 500 FT-NMR spectrometer (m, 3H), 7.3–7.25 (m, 6H), 7.18–7.14 (m, 4H), 6.83 (s, 1H). 13C
at room temperature. Electrospray ionization mass spec- NMR (125 MHz, CDCl3) δ = 190.08, 160.05, 142.14, 135.63,
troscopy (ESI-MS) studies were performed on an Agilent 134.80, 132.47, 131.23, 131.11, 130.83, 129.94, 129.91, 129.17,
Technologies (1260 Infinity) mass spectrometer. Elemental 129.03, 128.68, 128.49, 128.29, 128.13, 127.80, 127.10, 124.47,
analyses (C, H, and N) were performed on an Elementar Vario 123.51, 115.63 ppm.
EL III Carlo Erba 1108 in the micro-analytical laboratory of the
Preparation of 5-chloro-2-hydroxy-3-(1,4,5-triphenyl-1H-imid-
Sophisticated Analytical Instrumentation Facility (SAIF), azol-2-yl)-benzaldehyde (4b). Compound 4b was prepared fol-
Central Drug Research Institute (CDRI), Lucknow, India. lowing the above procedure described for 4a using 3b
Electronic absorption spectra were acquired on a Shimadzu (845.82 mg, 2 mmol) and hexamethylenetetramine (1122 mg;
UV-1800 and fluorescence spectra on a PerkinElmer LS 55 8 mmol) in trifluoroacetic acid (15 ml) and the reaction
spectrometer. Field emission scanning electron microscopic mixture was refluxed for 36 hours at 100 °C. It was isolated as
(FESEM) studies were performed on a Nova NanoSEM 450 a yellow solid. Yield: 750 mg (83.16%). 1H NMR (500 MHz,
scanning electron microscope. Time-resolved fluorescence CDCl3) δ = 10.51 (s, 1H), 7.65 (s, 1H), 7.53 (d, J = 7 Hz, 2H),
(TRF) decay experiments were performed on a TCSPC system 7.47–7.42 (m, 3H), 7.31–7.25 (m, 7H), 7.21–7.16 (m, 4H), 6.74
(Horiba Yovin; Delta Flex). In these studies, the samples were (s, 1H). 13C NMR (125 MHz, CDCl3) δ = 189.56, 160.32, 142.62,
excited using a picosecond diode laser (Model: Delta Diode) 135.97, 135.27, 131.86, 131.24, 131.17, 129.99, 129.92, 128.92,
and data analysis was performed using EzTime (Horiba 128.79, 128.66, 128.47, 128.26, 127.64, 127.03, 124.78, 123.18,
Scientific) decay analysis software.
115.62 ppm.
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Preparation of 5-chloro-3-(4,5-diphenyl-1-(p-tolyl) -1H-imid- 2H), 7.26–7.23 (m, 6H), 7.20 (d, J = 7.5 Hz, 2H), 7.13 (d, J = 7.5
azol-2-yl) -2-hydroxy-benzaldehyde (4c). This compound was Hz, 2H), 7.04 (d, J = 8 Hz, 2H), 7.00 (d, J = 2 Hz, 1H), 6.70 (d,
prepared following the above procedure described for 4b using J = 7 Hz, 2H), 6.39 (d, J = 2 Hz, 1H). 6.15 (d, J = 8.5 Hz, 2H),
3c (874 mg, 2 mmol). Yield: 800 mg (86.03%). 1H NMR 5.98 (s, 2H), 5.82 (s, 1H), 2.38 (s, 3H). 13C NMR (125 MHz,
(500 MHz, CDCl3) δ = 10.29 (s, 1H), 7.63 (s, 1H), 7.50 (t, J = 2 CDCl3) δ = 154.39, 143.87, 139.77, 134.99, 133.86, 132.65,
Hz, 3H), 7.31–7.25 (m, 8H), 7.16–7.14 (m, 4H), 7.03 (d, J = 8.5 132.19, 131.82, 131.24, 130.96, 130.38, 129.72, 129.45, 128.55,
Hz, 3H), 2.34 (s, 3H). 13C NMR (125 MHz, CDCl3) δ = 190.95, 128.49, 128.28, 128.14, 127.17, 126.94, 124.48, 122.62, 117.00,
159.72, 141.75, 140.27, 134.17, 133.56, 132.50, 131.32, 131.07, 114.08, 108.23, 106.73, 39.30, 21.02 ppm.
130.39, 129.14, 128.83, 128.71, 128.54, 128.27, 128.06, 127.74,
127.65, 127.23, 123.95, 123.80, 115.46, 21.20 ppm.
Preparation of 4-chloro-2-(1-(4-chlorophenyl)-4,5-diphenyl-
1H-imidazol-2-yl)-6-(di(1H-pyrrol-2-yl)methyl)-phenol (5d).
Preparation of 5-chloro-3-(1-(4-chlorophenyl)-4,5-diphenyl- Compound 5d was prepared following the above procedure for
1H-imidazol-2-yl)-2 hydroxybenzaldehyde (4d). Compound 4d 5a using 4d (1942 mg, 4 mmol) in place of 4a. Yield: 80%
was prepared following the above procedure described for 4b (1920 mg). 1H NMR (500 MHz, CDCl3) δ = 8.43 (s, br, 2H), 7.48
1
using 3d (915 mg, 2 mmol). Yield: 779 mg (80.24%). H NMR (d, J = 7 Hz, 2H), 7.40–7.38 (m, 2H), 7.32–7.22 (m, 6H),
(500 MHz, CDCl3) δ = 10.50 (s, 1H), 7.65 (d, J = 2.5 Hz, 1H), 7.12–7.11 (m, 4H), 7.04 (d, J = 3 Hz, 1H) 7.71 (d, J = 6.5 Hz,
7.54 (d, J = 7.5 Hz, 1H), 7.50 (d, J = 7 Hz, 2H), 7.39 (d, J = 9 Hz, 2H), 6.41 (d, J = 2 Hz, 1H), 6.16 (d, J = 9 Hz, 2H), 5.99 (s, 2H),
2H), 7.33–7.25 (m, 5H), 7.14–7.11 (m, 4H), 6.75 (d, J = 2.5 Hz, 5.84 (s, 1H). 13C NMR (125 MHz, CDCl3) δ = 154.34, 143.81,
1H). 13C NMR (125 MHz, CDCl3) δ = 189.40, 160.21, 142.58, 135.68, 135.49, 135.11, 132.57, 132.39, 131.70, 131.23, 130.82,
135.91, 135.85, 134.68, 132.14, 131.47, 131.14, 130.22, 129.53, 130.09, 130.00, 129.76, 129.11, 128.90, 128.75, 128.35, 127.39,
129.05, 128.82, 128.76, 128.66, 128.45, 128.32, 127.94, 127.59, 126.97, 124.39, 122.85, 117.08, 113.77, 108.33, 106.81,
126.93, 125.00, 123.44, 116.07 ppm.
Preparation of 2-(di(1H-pyrrol-2-yl)methyl)-6-(1,4,5-tri-
39.18 ppm.
Preparation of 5,5-difluoro-10-(2-hydroxy-3-(1,4,5-triphenyl-
phenyl-1H-imidazol-2-yl)-phenol (5a). Catalytic amounts of tri- 1H-imidazol-2-yl)-phenyl)-5H-dipyrrolo-[1,2-c:2′,1′-f][1,3,2]dia-
fluoroacetic acid (2 drops) were added under stirring to a solu- zaborinin-4-ium-5-uide (HPIB1). To a solution of 5a (2131 mg,
tion of 4a (1666 mg, 4 mmol) in pyrrole (10.0 mL) and the reac- 4 mmol) in dry dichloromethane (50.0 mL) and THF (10 ml), a
tion mixture was stirred for 12 hours at room temperature. solution of an equivalent amount of 2,3-dichloro-5,6-dicyano-
After complete consumption of the aldehyde (ensured by TLC), benzoquinone (908 mg; 1 eq.) dissolved in benzene was added
the excess of pyrrole was removed under reduced pressure on a dropwise over an hour and the reaction mixture stirred for an
rotatory evaporator. The crude product was subjected to silica additional 3 hours. After the completion of the reaction, the
gel column chromatography (ethyl acetate/hexane, 1 : 9) to resulting reaction mixture was dried under reduced pressure
1
obtain 5a as a white solid. Yield: 75.09% (1600 mg). H NMR using a rotatory evaporator. The crude product was dissolved
(500 MHz, CDCl3) δ = 8.47 (s, br, 2H), 7.50 (d, J = 7.5 Hz, 2H), in a minimum amount of dichloromethane (DCM) and filtered
7.29–7.20 (m, 10H), 7.15 (d, J = 6.5 Hz, 2H), 7.06–7.02 (m, 3H), to remove any solid impurities. The filtrate was treated with tri-
6.72 (s, 2H), 6.39 (d, J = 2 Hz, 1H), 6.17 (d, J = 2.5 Hz, 2H), 6.00 ethylamine (3.0 mL) followed by BF3·Et2O (3.0 mL) and the
(s, 2H). 5.83 (s, 1H). 13C NMR (125 MHz, CDCl3) δ = 154.39, reaction mixture allowed to stir for 30 minutes at room temp-
143.87, 139.78, 135.00, 133.86, 132.65, 132.18, 131.83, 131.25, erature. It was then subjected to dryness under reduced
130.97, 130.39, 129.75, 129.46, 128.56, 128.50, 128.29, 128.14, pressure and the crude product thus obtained was purified by
127.18, 126.96, 124.49, 122.63, 117.02, 114.10, 108.24, 106.74, silica gel column chromatography (DCM/hexane 40 : 60) to
39.27 ppm.
obtain the desired product as an orange-red solid. Yield: 13%
Preparation of 4-chloro-2-(di(1H-pyrrol-2-yl)methyl)-6-(1,4,5- (301 mg). Anal. calcd for C36H25BF2N4O: C, 74.75; H, 4.36; N,
triphenyl-1H-imidazol-2-yl)-phenol (5b). Compound 5b was 9.69. Found: C, 74.35; H, 4.40; N, 9.63. 1H NMR (500 MHz,
prepared following the above procedure for 5a using 4b CDCl3) δ = 7.92 (s, 2H), 7.49–7.45 (m, 5H), 7.29–7.15 (m, 12H),
(1804 mg, 4 mmol) in place of 4a. Yield: 76.48% (1735 mg). 6.93 (d, J = 3 Hz, 2H), 6.59 (s, 1H), 6.50 (s, 2H). 13C NMR
1H NMR (500 MHz, CDCl3) δ = 8.47 (s, br, 2H), 7.50 (d, J = (125 MHz, CDCl3) δ = 155.09, 144.47, 143.03, 142.72, 136.42,
8.5 Hz, 2H), 7.45–7.40 (m, 3H), 7.28–7.22 (m, 6H), 7.19–7.17 135.41, 135.33, 132.18, 131.32, 131.23, 131.15, 130.81, 130.07,
(m, 2H), 7.15–7.13 (m, 2H), 7.02 (d, J = 2.5 Hz, 1H), 6.72 (d, J = 129.94, 129.10, 128.84, 128.62, 128.48, 128.35, 127.43, 127.04,
1.5 Hz, 2H), 6.35 (d, J = 2 Hz, 1H), 6.16 (d, J = 3.5 Hz, 2H), 6.00 126.85, 123.52, 122.12, 118.41, 114.56 ppm. 11B NMR
(s, 2H), 5.83 (s, 1H). 13C NMR (125 MHz, CDCl3) δ = 154.39, (160.4 MHz, CDCl3) δ = −0.661 (t, 1B) ppm; 19F NMR
143.82, 136.58, 132.29, 131.81, 131.26, 130.92, 129.83, 129.62, (470.6 MHz, CDCl3): δ = −143.42 (dq, 1F), −146.68 (dq, 1F) ppm;
129.36, 128.64, 128.53, 128.32, 127.26, 126.98, 124.49, 122.66, IR (KBr pellet): 3445, 3107, 2925, 2485, 1590, 1557, 1493, 1455,
117.04, 113.99, 108.27, 106.76, 39.30 ppm.
1412, 1381, 1387, 1272, 1259, 1225, 1181, 1147, 1131, 1113,
Preparation of 4-chloro-2-(di(1H-pyrrol-2-yl)methyl)-6-(4,5- 1075, 1052, 942, 767, 734, 709, 671, 584, 552, 408 cm−1; HRMS
diphenyl-1-(p-tolyl)-1H-imidazol-2-yl)-phenol (5c). Compound (ESI): m/z calcd for C36H25BF2N4O [M + Na]+: 601.2; found: 601.1.
5c was prepared following the above procedure for 5a using 4c
Preparation of 10-(5-chloro-2-hydroxy-3-(1,4,5-triphenyl-1H-
(1860 mg, 4 mmol) in place of 4a. Yield: 76.57% (1780 mg). 1H imidazol-2-yl)phenyl)–5,5-difluoro-5H-dipyrrolo[1,2-c:2′,1′-f][1,3,2]
NMR (500 MHz, CDCl3) δ = 8.44 (s, br, 2H), 7.49 (d, J = 7 Hz, diazaborinin-4-ium-5-uide (HPIB2). HPIB2 was prepared fol-
440 | Dalton Trans., 2020, 49, 438–452
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lowing the above procedure described for HPIB1 using 5b 741, 704, 548, 412 cm−1
;
HRMS (ESI): m/z calcd for
(2268 mg, 4 mmol) in place of 5a. Yield: 15.50% (380 mg). C37H26BClF2N4O [M + H]+: 627.2; found: 627.2.
Anal. calcd for C36H24BClF2N4O: C, 70.55; H, 3.95; N, 9.14.
Found: C, 70.31; H, 4.02; N, 9.11. H NMR (500 MHz, CDCl3) δ 1H-imidazol-2-yl)-2-hydroxy-phenyl)-5,5-difluoro-5H-dipyrrolo
Preparation of 10-(5-chloro-3-(1-(4-chlorophenyl)-4,5-diphenyl-
1
= 7.89 (s, 2H), 7.63–7.57 (m, 3H), 7.44 (d, J = 7.5 Hz, 2H), [1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (HPIB4). HPIB4
7.37–7.23 (m, 7H), 7.16 (t, J = 7 Hz, 2H), 6.94 (d, J = 7 Hz, 4H), was prepared following the above procedure described for
6.49 (d, J = 2.5 Hz, 3H). 13C NMR (125 MHz, CDCl3) δ = 152.88, HPIB1 using 5d (2406 mg, 4 mmol] in place of 5a. Yield: 15%
144.60, 141.15, 139.30, 135.35, 134.38, 134.19, 133.19, 132.93, (389 mg). Anal. calcd for C36H23BCl2F2N4O: C, 66.80; H, 3.58;
1
131.46, 131.20, 130.98, 130.76, 130.21, 129.38, 129.16, 128.59, N, 8.66. Found: C, 66.57; H, 3.61; N, 8.55. H NMR (500 MHz,
128.20, 127.94, 127.63, 126.27, 125.99, 123.46, 118.52, CDCl3) δ = 7.93 (s, 2H), 7.47–7.45 (m, 4H), 7.35–7.26 (m, 3H)
112.09 ppm. 11B NMR (160.4 MHz, CDCl3) δ = −0.709 (t, 1B) 7.22 (m, 6H), 7.15 (d, J = 7.5 Hz, 2H), 6.90 (s, 2H), 6.71 (s, 1H),
ppm; 19F NMR (470.6 MHz, CDCl3): δ = −141.93 (dq, 1F), 6.51 (d, J = 3 Hz, 2H). 13C NMR (125 MHz, CDCl3) δ = 154.89,
−147.86 (dq, 1F) ppm; IR (KBr pellet): 3420, 3107, 2924, 2884, 144.56, 142.89, 142.37, 136.07, 135.37, 134.77, 131.26, 131.20,
1563, 1560, 1505, 1445, 1412, 1389, 1260, 1228, 1113, 1079, 131.10, 130.31, 129.70, 129.16, 128.86, 128.65, 128.42, 127.69,
1045, 1010, 950, 924, 871, 772, 997, 552, 417 cm−1; HRMS (ESI): 127.24, 126.93, 123.92, 122.48, 118.49, 114.33. 11B NMR
m/z calcd for C36H24BClF2N4O [M + H]+: 613.2; found: 613.2.
(160.4 MHz, CDCl3) δ = −0.681 (t, 1B) ppm; 19F NMR
Preparation of 10-(5-chloro-3-(4,5-diphenyl-1-(p-tolyl)- (470.6 MHz, CDCl3): δ = –143.62 (dq, 1F), −146.51 (dq, 1F)
1H-imidazol-2-yl)-2-hydroxy-phenyl)-5,5-difluoro-5H-dipyrrolo ppm; IR (KBr pellet): 3407, 2924, 2856, 2884, 1602, 1590, 1560,
[1,2-c:2′,1′-f][1,3,2]diazaborinin-4-ium-5-uide (HPIB3). HPIB3 1537, 1493, 1458, 1412, 1387, 1353, 1260, 1181, 1107, 1074,
was prepared following the above procedure described for 988, 942, 980 cm−1
;
HRMS (ESI): m/z calcd for
HPIB1 using 5c (2325 mg, 4 mmol) in place of 5a. Yield: C36H23BCl2F2N4O [M + H]+: 647.1; found: 647.1.
16.63% (417 mg). Anal. calcd for C37H26BClF2N4O: C, 70.89; H,
4.18; N, 8.94. Found: C, 70.85; H, 4.21; N, 8.93. 1H NMR
(500 MHz, CDCl3) δ = 7.92 (s, 2H), 7.47–7.44 (m, 2H), 7.31–7.26
Results and discussion
Synthesis and characterization
(m, 5H) 7.21–7.13 (m, 8H), 7.94 (d, J = 5 Hz, 2H), 6.64 (d, J =
2.5 Hz, 1H), 6.51 (d, J = 4 Hz, 2H), 2.43 (s, 3H). 13C NMR
(125 MHz, CDCl3) δ = 155.11, 144.44, 143.13, 142.81, 140.18, A simple synthetic route adopted for preparation of HPIB1–
135.44, 135.24, 133.76, 132.27, 131.39, 131.25, 131.17, 130.74, HPIB6 is shown in Scheme 1. Compounds 3a–3d were
130.64, 129.25, 128.77, 128.60, 128.33, 128.16, 127.37, 127.07, obtained by the reaction of the respective salicylaldehyde
126.86, 123.48, 122.10, 118.41, 114.68, 21.30 ppm. 11B NMR derivatives [R1 = H (1a), Cl (1b)] and phenylamine counterparts
(160.4 MHz, CDCl3) δ = −0.709 (t, 1B) ppm; 19F NMR [R2 = H (2a), CH3 (2b), Cl (2c)] with benzyl in the presence of
(470.6 MHz, CDCl3): δ = −141.93 (dq, 1F), −147.89 (dq, 1F) ammonium acetate following literature procedures.19
ppm; IR (KBr pellet): 3435, 2925, 2851, 1558, 1516, 1412, 1388,
Formylation of 3a–3d following the Duff reaction gave
1299, 1258, 1144, 1108, 1077, 1042, 1009, 916, 916, 874, 787, 4a–4d with free formyl groups.20 The aldehydes 4a–4d reacted
Scheme 1 The synthetic procedure adopted for HPIB1–HPIB4.
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with an excess of pyrrole in the presence of catalytic amounts may probably arise due to conformational isomerization owing
of trifluoroacetic acid to afford dipyrromethane derivatives 5a– to intramolecular rotations (Fig. S23†).23 Absorption and emis-
5d.21 Ultimately, boron dipyrromethene derivatives HPIB1– sion spectra have also been acquired by varying the concen-
HPIB4 were prepared by oxidation of the respective dipyrro- tration of the compounds. Emission intensity gradually
methanes 5a–5d with 2,3-dichloro-5,6-dicyano-benzoquinone enhanced with increasing concentrations of the compounds
(DDQ) followed by addition of triethylamine (Et3N) and up to 50 μM and thereafter it diminished (Fig. S28–S31†). In
BF3·Et2O successively.22 The compounds (HPIB1–HPIB4) were addition, emission maxima displayed continual red-shift with
thoroughly characterized by elemental analyses, IR, ESI-MS increasing concentrations (up to 25–30 nm on going from
and NMR (1H/13C/11B/19F) studies (see ESI†). The 10 μM to 150 μM). The observed fluorescence quenching may
molecular structures of HPIB1–HPIB4 have also been unequi- be due to short-range intermolecular interactions as well as
vocally determined by X-ray single crystal analyses. The com- active intramolecular rotation/molecular flexibility.22b The red-
pounds under investigation showed good solubility in di- shift of the emission maxima clearly indicated short-range
chloromethane (DCM), chloroform, dimethyl sulfoxide intermolecular interactions.22b However, the intensity of
(DMSO), dimethylformamide (DMF) and acetonitrile, however, absorption maxima augmented with increasing concentration
poor solubility in methanol, ethanol, hexane, toluene, benzene without any shifts in their positions (Fig. S24–S27†).
and water.
The absorption and emission spectra of these compounds
have also been acquired in various solvents (Fig. 2 and
Fig. S32–S34†). The examination of the emission spectra
Absorption and emission spectroscopy
The electronic absorption spectra of HPIB1–HPIB4 (DMF) dis- revealed that the intensity of the bands in non-polar solvents
played sharp absorption at ∼505 nm [λabs,max = 505; HPIB1, is high relative to polar solvents (Fig. 2 and Fig. S32–S34†).
507; HPIB2, 506; HPIB3, and 505 nm; HPIB4] along with a High emission intensity in non-polar solvents may be related
shoulder at 480 nm owing to 0–0 and 0–1 vibrational bands of to strong intramolecular H-bonding inducing rigidity in the
S0–S1 transition in the BODIPY core (Fig. 1 and Fig. S21†). system. On the other hand, low emission intensity may be
Also, these compounds displayed
a
weak band at ascribed to intermolecular H-bonding and/or other inter-
∼340–350 nm due to the meso-phenyl-BODIPY core assignable actions with solvent molecules allowing the system to lose
to S0–S2 transitions.22a The absorption bands are in good energy non-radiatively and making it less fluorescent.
agreement with reported values for phenyl BODIPY systems.22a
Furthermore, the emission spectra of the compounds in
Their emission spectra (DMF) showed a single emission band solvents like toluene and benzene showed intense and slightly
at ∼535 nm [λem,max = HPIB1, 536; HPIB2, 535; HPIB3, 535; red-shifted maxima relative to other solvents. This is in con-
and HPIB4, 536 nm] (Fig. 1 and Fig. S22†).
trast to the usual blue-shifted emission in non-polar solvents
It has been observed that the emission maxima of these like benzene and toluene. Most probably this may be due to
compounds lie at almost the same position; however, the emis- aggregation as these solvents can serve as non-solvents (poor
sion intensities of HPIB2 and HPIB3 are high relative to HPIB1 solvents) for the aggregation process.24 A quantitative idea
and HPIB4. The lower fluorescence intensities of HPIB1 and about the emission behaviour of HPIB1–HPIB4 in various sol-
HPIB4 may be attributed to the more active intramolecular vents has further been deduced from fluorescence quantum
rotations in these systems. In addition, fluorescence excitation yields (Table S1†). Notably, the absorption spectra did not
spectra at the respective emission maxima of these com- show any appreciable changes with the polarity of the solvent
pounds resemble the absorption spectra, except for a small and the absorption maxima appeared at almost the same posi-
red-shift (Fig. S23†). Small mismatch of the excitation spectra tion as in DMF (Fig. S35–S38†).
Fig. 1 Absorption (a) and emission spectra (b) of HPIB1–HPIB4 in DMF (c; 50 μM, DMF, λex; 505 nm).
442 | Dalton Trans., 2020, 49, 438–452
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Fig. 2 Emission spectra (a) of HPIB2 and its normalized representation (b) in different solvents with varying polarities (c; 50 μM, λex; 505 nm); [THF =
tetrahydrofuran, DCM = dichloromethane, DMSO = dimethyl sulfoxide, DMF = N,N-dimethylformamide, MeCN = acetonitrile, MeOH = methanol].
Aggregation-induced emission
Emission spectra have also been acquired in the same
solvent system. As shown in Fig. 4 and Fig. S42,† emission
The aggregation-induced emission behaviour of HPIB1–HPIB4 intensity enhanced with increasing fw without any change in
has been investigated by acquiring the absorption spectra of the position of the emission maxima (λem = ∼535 nm) up to an
these compounds in DMF/water mixtures with varying water fw of 50%. An increase of fw to 60% led to small red-shifted
content ( fw). As depicted in Fig. 3 and S39–S41,† the absorp- emission with higher emission intensity relative to that in
tion spectra did not show any appreciable changes with DMF along with a new band at ∼615 nm. A further increase of
increasing fw (up to 50%). However, at an fw of 60% they fw (>60%) led to apparent dual emission at ∼560 and ∼610 nm
showed broadening of the bands and lowering of the absorption compared to emission maxima at ∼535 nm (in pure DMF). The
intensity. A further increase of the water fraction led to an inces- emission behavior at a high water fraction may be attributed to
sant decrease of intensity with an appreciable red-shift, and ulti- AIE.3 Furthermore, emission behavior has also been investi-
mately at an fw of 99% the intensity significantly went down and gated by estimation of fluorescence quantum yields (Φf) at
the absorption band radically broadened. Such a change in the various water fractions ( fw: 0% and 60%–99%). Calculated Φf
intensity of the band and broadening may be related to the are high at higher water fractions ( fw: >50%) relative to those
aggregation process25 and may arise due to Mie scattering. In in DMF ( fw: 0%) for the respective compounds (Table 1). This
fact, the aggregation process leads to transparent nanoparticle clearly indicated the occurrence of AIE for all these derivatives
suspensions which scatter light and cause the broadening of at high fw (Table 1). High emission intensity for HPIB1–HPIB4
the absorption bands with an apparent red-shift (level-off in the aggregated state ( fw: >50%) has also been supported by
tails).25 Moreover, the red-shift of the absorption maxima at time-resolved fluorescence studies. The time-resolved emission
high water fractions ( fw > 50%) suggested J-aggregation.25b
spectra were acquired in DMF/water mixtures with varying
Fig. 3 Absorption spectra (a) of HPIB1 and its normalized representation (b) in DMF/water mixtures with increasing water volume fractions
(c; 50 μM).
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Fig. 4 Emission spectra of HPIB1 (a), HPIB2 (b), HPIB3 (c) and HPIB4 (d) in DMF/water mixtures with increasing water volume fractions (c; 50 μM,
λex; 505 nm).
Table 1 Fluorescence lifetimes (τf)a and quantum yields (Φf)b at varying fw for HPIB1–HPIB4
HPIB1
HPIB2
HPIB3
HPIB3
Water content ( fw)
τf/ns
Φf
τf/ns
Φf
τf/ns
Φf
τf/ns
Φf
0%
1.07
1.87
1.91
2.34
2.53
2.52
0.039
0.077
0.081
0.099
0.110
0.107
1.21
2.57
3.85
3.96
4.17
3.89
0.053
0.059
0.210
0.207
0.240
0.200
1.17
2.49
2.50
2.67
3.57
3.37
0.059
0.197
0.219
0.223
0.229
0.216
1.04
2.55
2.67
2.73
2.96
2.85
0.033
0.057
0.059
0.071
0.074
0.070
60%
70%
80%
90%
99%
a The average lifetime is given; ns = nanosecond; the excitation wavelength is 482 nm. b Fluorescence quantum yields were determined using the
rhodamine 6G dye (H2O, λex = 530 nm, λem = 552 nm, Φf = 0.95) as the standard; stock solutions of HPIB1–HPIB4 were prepared in DMF.
water fraction ( fw: 0% and 60%–99%) and best fitted to a bi- 500–700 nm in the aggregated state and among these HPIB1
exponential model as depicted in Table 1 and Fig. S43.† The and HPIB4 exhibited prominent emission at ∼616 nm with a
observed lifetimes in the aggregated state ( fw: >50%) are high shoulder at ∼560 nm; however, HPIB2 and HPIB3 showed pro-
relative to those in DMF ( fw: 0%) for all these derivatives and minent emission at ∼560 nm with a shoulder at ∼610 nm.
are in good agreement with available data on AIE lumino- Furthermore, HPIB2 and HPIB3 displayed high emission
gens.25 Hence, the high fluorescence lifetime may be related to intensity with a relatively small red-shift in the aggregated
the aggregation-induced emission behavior of these state, while HPIB1 and HPIB4 showed relatively weak emission
compounds.25
with large red-shifted maxima (Fig. 4). The anomalies in the
Overall, data obtained from the absorption and emission emission behavior of these compounds may be related to the
spectra as well as measured fluorescence lifetimes and orientation of the respective compounds causing distinct
quantum yields suggested AIE behavior for these compounds. molecular packing in the aggregated state which has been
These compounds displayed dual emission in the range of explained (vide-infra) on the basis of crystal structures.
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Aggregate morphology and size
The restricted intramolecular rotations (RIRs) and the poss-
ible role of these fluorophores as FMRs have been assessed by
viscosity-dependent absorption and emission spectral studies
in methanol/glycerol mixtures by varying the viscosity of the
medium from 0.61 (CH3OH; fg 0%) to 793.0 cP ( fg 90%).26
Absorption spectra did not show any significant changes in
the intensity and position of the absorption maxima
(Fig. S45†). However, with an increase in viscosity from 0.6 to
793 cP, the emission intensity enhanced appreciably (30;
HPIB1, 25; HPIB2, 16; HPIB3 and 25-fold; HPIB4) without
changes in the position of emission maxima at ∼532 nm [532;
HPIB1, 532; HPIB2, 534; HPIB3 and 534 nm; HPIB4] (Fig. 6a
and Fig. S46a–S48a†). The time-resolved emission decay pro-
files of these derivatives at varying viscosities revealed the
gradual increase of the fluorescence lifetime (τf) with increas-
ing viscosity of the medium as depicted in Fig. 6b, Fig. S46b–
S48b and Table S2.† Also, fluorescence quantum yields (Φf)
appreciably enhanced from ∼1% [0.9%; HPIB1, 1.2%; HPIB2,
1.5%; HPIB3, and 1.2%; HPIB4] at a viscosity of 0.6 cP in
methanol to ∼70% [67%; HPIB1, 69.7%; HPIB2, 67.3%; HPIB3
and 69.3%; HPIB4] at 793 cP in 90% glycerol fractions
(Table S3†). Hence, it may be concluded that these derivatives
are sensitive toward the viscosity of the medium and these
observations are consistent with earlier reports.13,26
The effects of variation of substituents on aggregation behavior
and their co-relation with optical responses have been examined
by subjecting the aggregates (DMF/water mixture; fw, 99%) to
field emission scanning electron microscopy (FESEM) (Fig. 5).
The FESEM images revealed that each of these compounds
(HPIB1–HPIB4) exhibit different aggregate morphology which
has been related to the changes in packing arrangement of
molecules due to variation of the substituents. As shown in
Fig. 5, HPIB1 gave Thuja leaf shaped nanoclusters, while HPIB2
rope-like fibrous nanoclusters. On the other hand, HPIB3 and
HPIB4 gave nano-sphere aggregates. Thus, it can be concluded
that the nano-aggregates of these derivatives display unique
morphologies which strongly depend on the substituents under
analogous conditions. This confirmed the distinct emission be-
havior of these compounds. Furthermore, the size of the aggre-
gate has been determined by dynamic light scattering (DLS)
studies under analogous conditions (at an fw of 99%) and it lies
in the range of 89 nm to 462 nm [89 nm, HPIB1; 462 nm,
HPIB2; 341 nm, HPIB3; and 178 nm; HPIB4] (Fig. S44†).
Restricted intramolecular rotations (RIRs) and viscochromism
The compounds presented in this study have been designed in
such a way that they can serve as FMRs wherein four bulky
phenyl rings are present around the imidazole moiety along
with the BODIPY core connected through a single bond. In
such systems, the rotations of the constituent units are active in
low-viscous medium/dilute solution promoting the non-radia-
tive decay process and thereby weakening the fluorescence
intensity. On the other hand, intra-molecular rotations get
restricted in viscous medium populating the radiative channels
causing large emission enhancements.3,13a,25
Furthermore, radiative (kr) and non-radiative (knr) rate con-
stants calculated using measured Φf and τf revealed that kr
remains almost constant (0.06–0.15 ns−1) at various viscosities
of the medium under investigation (Table S4†). On the other
hand, knr sharply decreases (0.90 to 0.07) with increasing vis-
cosity of the medium (Table S4†). The decrease in knr values
clearly confirmed that significant enhancement in quantum
efficiency and the average lifetime at higher viscosity is due to
Fig. 5 The FESEM image of the nano-aggregates of HPIB1–HPIB4 formed in DMF/water mixtures at an fw of 99% (c; 50 μM).
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Fig. 6 (a and b) Emission spectra (c; 50 μM, λex; 505 nm) (a) and time-resolved fluorescence decay profiles (b) (λex = 482 nm and λem = ∼535 nm), at
different viscosities of the medium (in methanol/glycerol mixtures); (c) a plot of fluorescence quantum yield (Φf) as a function of viscosity; the inset
shows the linearity in the entire range of viscosity under investigation obtained in the log plot; (d) a plot of the fluorescence lifetime (τf) as a function
of viscosity; the inset shows the linearity in the entire range of viscosity under investigation obtained in the log plot according to the Förster–
Hoffmann equation for HPIB1.
the suppression of non-radiative decay processes with increas- Fig. 46d–S48d†).27 Thus, it is concluded that these compounds
ing viscosity of the medium and the observations are in accord serve as FMRs in various media including biological systems.
with the Förster–Hoffmann theory.13c,26,27 Furthermore, a plot
of fluorescence quantum yields (Φf)/lifetimes (τf) vs. viscosity
(η) has been obtained following the Förster–Hoffmann
equations (Fig. 6 and Fig. S46–S48†).26,27
Crystal structures, packing diagrams and the mechanism of
AIE
In order to gain deep insight into AIE behavior and its relation-
ship with molecular packing, the structures of HPIB1–HPIB4
have been determined by X-ray single crystal analyses (Fig. 7).
Suitable single crystals for these compounds were grown by
slow evaporation of DCM/methanol solution. The crystal data
and refinement parameters are summarized in Table 3 and
Tables S5–S7 in the ESI.†
logðΦf Þ ¼ C þ x logðηÞ
ð1Þ
ð2Þ
logðτf Þ ¼ C′ þ y logðηÞ
where C/C′ and x/y represent constants.
As expected, a plot of log Φf vs. log η showed a straight line
Careful examination of the dihedral angles (Table 2)
with a slope of 0.593 (HPIB1), 0.560 (HPIB2), 0.550 (HPIB3) revealed highly twisted and non-planar orientations for these
and 0.581 (HPIB4), respectively. These are consistent with derivatives. The phenol (ring A) and imidazole rings are
earlier reports on BODIPY based FMRs (Fig. 6c and Fig. S46c– almost co-planar as evidenced by small dihedral angles (3.08°;
S48c†).26,27 Furthermore, a similar observation has also been HPIB1, 6.34°; HPIB2, 12.86°; HPIB3 and 3.13°; HPIB4) due to
made from a plot between log τf and log η, which showed a intramolecular H-bonding. The intramolecular H-bond dis-
slope of 0.167 (HPIB1), 0.190 (HPIB2), 0.176 (HPIB3), and tances (–O1–H⋯N3) are small and fall in the range of
0.133 (HPIB4). Notably, these are also analogous to the typical 1.711–1.770 Å [1.766; HPIB1, 1.711; HPIB2, 1.770; HPIB3, and
slopes (0.2–1.4) obtained in earlier reports (Fig. 6d and 1.759 Å; HPIB4], while –O1–H⋯N3 angles are in range of
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Fig. 7 ORTEP views of (a) HPIB1, (b) HPIB2, (c) HPIB3 and (d) HPIB4 at 50% ellipsoid probability (hydrogen atoms have been omitted for clarity).
Table 2 Dihedral angles between phenyl and imidazole rings as well as
the phenyl ring and the BODIPY core, extracted from single crystal XRD
data for HPIB1–HPIB4
Furthermore, the twisted and non-planar orientation
enables the system to avoid detrimental π–π stacking that
creates excimers/exciplexes responsible for ACQ.29 In fact, the
crystal structures of HPIB1–HPIB4 completely lack detrimental
π–π stacking. Instead, these exhibited C–H⋯π, B–F⋯H–C and
B–F⋯π interactions favoring J-type packing responsible for
emission enhancement in the aggregated state with an
appreciable red-shift.8 Also, these interactions interlock mole-
cules and provide physical restraints on intramolecular
rotations in aggregated/dense medium and, consequently,
emission is enhanced.
The crystal structures of HPIB1 and HPIB2 involve C–H⋯π
and B–F⋯H–C interactions and yield J-type aggregates. In
HPIB1, C–H⋯π (2.817 Å) forms long ordered J-type packing
which is further linked by B–F⋯H–C (2.348 Å) to create a 2D
network (Fig. S49†). HPIB2 showed J-type packing due to both
C–H⋯π (2.663 Å) and B–F⋯H–C (2.571 Å) interactions (Fig. 8
and Fig. S50†). On the other hand, the crystal structure of
HPIB3 involves B–F⋯H–C (2.511 Å) and B–F⋯π (3.120 Å) inter-
actions instead of C–H⋯π interactions to form J-clusters
(Fig. 9 and Fig. S51†). Furthermore, HPIB4 involves C–H⋯π
(2.755 Å) and B–F⋯π (3.143 Å) interactions and displays
J-type packing similar to other derivatives (Fig. 10 and
HPIB1 HPIB2 HPIB3 HPIB4
Between the BODIPY core and
phenyl ring A
Between the imidazole ring and
phenyl ring A
80.93° 61.85° 60.68° 71.22°
3.08° 6.34° 12.86° 3.13°
Between the imidazole ring and
phenyl ring B
Between the imidazole ring and
phenyl ring C
86.68° 83.83° 73.94° 87.05°
76.34° 72.21° 50.57° 78.27°
Between the imidazole ring and
phenyl ring D
3.39°
3.34° 32.16° 23.21°
140.94–148.98° [148.45°; HPIB1, 140.94°; HPIB2, 149.62°; Fig. S52†).
HPIB3, and 148.98°; HPIB4]. The observed parameters clearly
Thus it is concluded that J-type packing in the aggregated
revealed strong intramolecular H-bonding and offered necessary state is responsible for emission enhancement (i.e. AIE) in
co-planarity between two units (phenol and imidazole rings) for addition to the RIR in these systems. From the crystal struc-
these derivatives.28 Moreover, these derivatives showed distinct tures, it can be seen that each derivative exhibits a distinct site
dihedral angles for each ring probably due to substituent vari- of interaction between molecules and leads to a different
ation and their electronic and steric effects (Table 2).
packing pattern and, accordingly, they exhibited diverse emis-
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Table 3 Selected crystallographic parameters for HPIB1–HPIB4
Crystal data
HPIB1
HPIB2
HPIB3
HPIB4
Empirical formula
C36H25BF2N4O
Triclinic
P1
C36H24BClF2N4O
Monoclinic
P21/n
9.6016(3)
25.7083(6)
12.6994(3)
90
105.329(3)
90
3023.21(1)
4
C37H26BClF2N4O
Monoclinic
P21/c
9.1437(2)
14.2235(3)
24.9508(5)
90
97.853(2)
90
3214.55(1)
4
C36H23BCl2F2N4O
Triclinic
P1
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
ˉ
ˉ
10.208(2)
10.769(2)
14.485(3)
111.348(5)
90.210(5)
97.478(5)
1468.3(5)
2
9.5668(7)
13.2433(9)
13.9708(9)
104.534(2)
103.397(2)
104.401(2)
1575.80(2
2
γ (°)
V (Å3)
Z
F (000)
598.0
1.306
273(2)
0.088
1264
1.346
150(2)
0.176
1296
1.295
150(2)
0.167
664
1.364
273(2)
0.254
ρcalc (g cm−3
T (K)
)
μ (mm−1
)
Reflns collected
GOF on F2
Final R1
32 481
1.008
0.1069
0.1635
26 415
1.017
0.0602
0.1602
47 535
1.026
0.0576
0.1463
38 778
1.007
0.0485
0.1084
Final wR2
Fig. 8 (a and b) J-type packing via C–H⋯π and B–F⋯H–C interactions respectively extracted from single crystal XRD data for HPIB2.
Fig. 9 (a and b) J-type packing due to B–F⋯H–C and B–F⋯π interactions extracted from single crystal XRD data for HPIB3.
sions. Furthermore, the extent of emission enhancement and intensity owing to loss of excitation energy in non-radiative
red shift can be rationalized on the basis of twisting of struc- pathways.29 It can be seen from the dihedral angles (Table 2)
tural entities. Usually systems exhibiting twisting to a greater of these derivatives that HPIB1 and HPIB4 are relatively more
extent display red-shifted emission to a greater extent with low twisted and have greater dihedral angles. In contrast to HPIB1
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Fig. 10 (a and b) J-type packing structures due to C–H⋯π and B–F⋯π interactions respectively extracted from single crystal XRD data for HPIB4.
Fig. 11 (a) Emission spectra in the solid state (powder, λex = ∼505 nm); (b) a summary of emission maxima in solution, aggregated and solid states;
and (c) photographs of HPIB1–HPIB4 under UV irradiation (λex, 365 nm) for solution ( fw of 0%), aggregated ( fw of 99%) and solid states (powder).
and HPIB4, for HPIB2 and HPIB3 the dihedral angles are that the red-shift of emission maxima in the solid/aggregated
small between BODIPY and the phenyl ring (ring A) as well as state is related to the extent of twisting in the solid or crystal-
for other rings, and therefore these compounds displayed line state of the respective compounds.29,30 Furthermore, it is
strong emission with a small red-shift.
worth noting that these compounds are good solid state emit-
ters as a result of J-type molecular packing and the lack of det-
rimental π–π stacking as suggested by their crystal structures.
Solid state emission
As shown in Fig. 11, HPIB1–HPIB4 displayed a single emission It is observed that HPIB1–HPIB4 displayed dual-emission in
band with maxima between 649 and 606 nm [649; HPIB1, 610; the aggregated state (in DMF/water mixtures at high fw) com-
HPIB2, 606; HPIB3 and 640 nm; HPIB4]. Among these HPIB1 pared to the apparent single emission band in the solid state.
(λem; 649 nm) and HPIB4 (λem; 640 nm) showed a large red The dual-emission in the aggregated state is most probably
shifted band relative to other compounds probably due to due to conformation isomerization and it is presumed that
greater twisting.29,30 Furthermore, the observed dihedral two types of conformational isomers can exist in the DMF/
angles between the BODIPY core and phenyl ring A [Table 2: water mixture due to single bond rotations. However, in the
(80.93°; HPIB1, 61.85°; HPIB2, 60.68°; HPIB3 and 71.22°; solid state only one conformer exists and shows a single emis-
HPIB4) for these compounds clearly confirm our viewpoint. sion band. Hence, from available data on HPIB1–HPIB4 in the
Other rings around imidazole also showed large dihedral solid/aggregated state it can be concluded that emission be-
angles with respect to the central imidazole ring for HPIB1 havior can be vastly tuned even by small structural changes in
and HPIB4 confirming red-emission. Thus, it is concluded closely related molecules.
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Fig. 12 Frontier molecular orbitals for HPIB1–HPIB4.
Theoretical considerations
in viscous medium and therefore the systems become sensitive
toward the viscosity of the medium and may find application
in viscosity sensing. Theoretical data including electronic tran-
sitions deduced from DFT calculations for these derivatives
match well with the experimental data. These HPI based
BODIPYs displaying absorption and emission in the visible
region [λabs ∼505 nm, λemi ∼535 nm (solution), λemi ∼615 nm
(aggregated state), and λemi ∼649 nm (solid state)] may serve as
good candidates for various applications including opto-
electronics and sensing especially for biological purposes.
To understand the orientation, photophysical properties and
their correlation, DFT calculations have been performed on
HPIB1–HPIB4 using the B3LYP 6-31G** method (Fig. 12 and
Fig. S53†). The orientation of DFT optimized structures
resembles the orientation observed for the crystal structures.
This has been confirmed by the comparison of the dihedral
angles obtained from DFT optimized structures to those
obtained from crystal structures (Table S7†). Furthermore, geo-
metrical parameters like bond lengths and bond angles
obtained from DFT optimized structures also showed good
agreement with the structural data (Tables S5 and S6†). As
depicted in Fig. 12, HOMO energy levels are spread over the
phenyl-BODIPY core, imidazole and phenyl rings (attached to
the imidazole), except for HPIB4 wherein the HOMO is distrib-
uted only on the phenyl-BODIPY core, while the LUMO is
mainly distributed over phenyl-BODIPYs. The HOMO–LUMO
separations are almost similar (−3.08; HPIB1, −3.06; HPIB2,
−3.06; HPIB3 and −3.05 eV; HPIB4) and lie in the decreasing
Conflicts of interest
The authors declare no conflict of interest.
Acknowledgements
order from un-substituted (HPIB1) to substituted compounds The authors gratefully acknowledge the Science and
(HPIB4). The energy difference lies in the descending order Engineering Research Board (SERB), New Delhi, India for pro-
with increasing electron-withdrawing ability of R2 (R2; see viding financial assistance through the scheme (EMR/2015/
Scheme 1). Furthermore, calculated electronic transitions 001535). B. K. D. acknowledges the University Grants
(time-dependent DFT) are in good agreement with experi- Commission, New Delhi, India for
a Senior Research
mental results (Fig. S54 and Table S8†).
Fellowship (22/6/2014(i)EU-V).
Conclusions
References
In summary, in this work, four novel hydroxyl-substituted tet-
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