Time dependent aggregation induced emission enhancement and the study of molecular packing in closely related azo-phenol BODIPY species

Article abstract of DOI:10.1039/c7dt04047d

Fluorescent azo-phenol BODIPYs (1-3) have been obtained by the substituent (-OCH₃/-CH₃) directed synthesis of ortho (L1) and para (L2-L3) azo-phenol aldehydes. These display aggregation caused quenching (ACQ, 1) and aggregation induc

Full text of DOI:10.1039/c7dt04047d

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Time dependent aggregation induced emission enhancement and study of
molecular packing in closely related azo-phenol BODIPY species
Rajendra Prasad Paitandi, Roop Shikha Singh, Bhupendra Kumar Dwivedi, Vishwa Deepak
Singh, and Daya Shankar Pandey*
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi – 221 005,
India.
Abstract
Fluorescent azoꢀphenol BODIPYs (1−3) have been obtained by substituent (−OCH3/−CH3)
directed synthesis of ortho (L1) and para (L2−L3) azoꢀphenol aldehydes. These display
aggregation caused quenching (ACQ, 1) and aggregation induced emission enhancement (AIEE,
2 and 3) depending upon the position of azo relative to phenolic hydroxyl group. An intriguing
time dependent morphological transition from nanosphere to ordered nanorods and subsequent
emission changes in AIEE active azoꢀphenol BODIPYs have been successfully realized by time
dependent fluorescence, scanning electron (SEM), transmission electron (TEM) and fluorescence
optical microscopic (FOM) studies. The existence of one dimensional (1D) nanorods as ultimate
species in these compounds (2−3) has been supported by crystal packing patterns. Diverse
aggregated forms and hierarchical nanostructures have been related to variable extent of
fluorescence enhancement. Plausible charge transfer process and its role in AIEE have been
supported by DFT studies.
*Author to whom all the correspondence should be addressed; Eꢀmail ID: dspbhu@bhu.ac.in

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Introduction
Azobenzene derivatives have attracted substantial current interest because of their potential
applications in diverse areas, including development of the molecular devices and functional
materials.1−6 These display photochromism due to cis−trans isomerizations, nevertheless very
efficient photoisomerization causes a harmful effect on emission characteristics by limiting the
radiative deactivation of photoexcited states.7−9 Usually, azobenzene derivatives are nonꢀ
10−11
fluorescent in solution,
yet selfꢀassembled aggregates having amphiphiles, orthometalated
azobenzenes or those in a rigid matrix at low temperature display substantial fluorescence.12−21
Further, aggregation induced emission enhancement (AIEE) offers a novel route for emission
redemption in conventional πꢀconjugated organic fluorophores suffering from selfꢀ
quenching.22−23 It mainly arises due to hindered molecular rotation causing discrete molecular
arrangements viz, Jꢀaggregate formation, headꢀtoꢀtail arrangement etc.24−25 A large surge in AIEE
materials has stimulated the scientific community to fabricate structurally wellꢀdefined systems.
In this regard, oneꢀdimensional (1D) nanomaterials based on organic systems have drawn special
attention owing to their versatile applications.26−28 In addition, recent studies have shown that
emission enhancement in linear azobenzene derivatives containing long alkyl chain can occur via
selfꢀassembly under the influence of prolonged UV irradiation.29−31 Han et al. have investigated
the effect of selfꢀassembly on emission enhancement in diverse azobenzene based nanoꢀ/micro
structures.32−33 However, azobenzene derivatives exhibiting AIEE are rather scarce and need
further explorations. 32,34
Furthermore, borondipyrromethenes (BODIPY) represent highly luminescent materials
exhibiting excellent photophysical properties and are amenable to structural modifications.35−37
Due to ease of synthesis and high modification ability azobenzene based BODIPYs hold great
promise and may find potential applications in light emitting devices, fluorescent probes etc.,
provided that these display remarkable emission in solution and solidꢀ/aggregated state. Despite
possible emission enhancement arising from N–B dative bond in azoꢀBODIPYs all the efforts in
this direction has proved to be futile due to their inability to restrain photoisomerization of the
azo core.38−39 Some selfꢀassembled aggregates based on BODIPY dyes display intense solid state
emission due to AIEE via formation of Jꢀaggregates, introduction of AIE active or asymmetric
substituents at appropriate position.40−47 Moreover, it has been shown that emission enhancement

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and aggregate build up in selfꢀassembled BODIPY nanoaggregates can be tuned by suitable
substituents.48−49 However, efficacy of BODIPYs toward development of azoꢀphenol BODIPYs
exhibiting superior emission in aggregated and solid state with detailed mechanistic insights has
not been explored.50ꢀ51 In this direction an attempt has been made to develop efficient solid state
redꢀemitting azoꢀphenol BODIPYs. Structural diversity brought about by the position of diazo
moiety in these azo−phenol BODIPYs led to distinct aggregation and emission characteristics in
solution as well as solidꢀstate.
Through this work synthesis and thorough characterization of three novel fluorescent azoꢀ
phenol BODIPYs (1−3) achieved by varying position of diazo moiety and substituents at
azobenzene core have been described. Also, we present herein our detailed investigations on
strong positioning effect of the diazo moiety, vital role of substituents affecting the course of
reaction and controlling molecular stacking via intermolecular interactions. To our best, this is
the first report fluorescent azoꢀphenol BODIPYs exhibiting 1D selfꢀassembly with time
dependent AIEE attribute.
Experimental details
Reagents
The solvents were dried and distilled prior to use following standard literature procedures.52
Pyrrole, trifluoroacetic acid, pꢀtoluidine, anisidine, 2,3ꢀdichloroꢀ5,6ꢀdicyanoꢀ1,4ꢀbenzoquinone
(DDQ), triethylamine and boron trifluoride diethyl etherate were procured from Sigma Aldrich,
India and used as received without further purifications.
General information
Elemental analyses for C, H, and N were obtained on an Elementar Vario EL III Carlo Erba 1108
from microanalytical laboratory of the Sophisticated Analytical Instrumentation Facility (SAIF),
Central Drug Research Institute (CDRI), Lucknow, India. Electronic absorption spectra were
acquired on Shimadzu UVꢀ1601 spectrophotometer. Fluorescence spectra at room temperature
1
was acquired on a PerkinꢀElmer LS 55 Fluorescence spectrometer. H, 11B, 13C and 19F and
spectra were acquired on a JEOL AL 500/300 FTꢀNMR spectrometer using tetramethylsilane
Si(CH3)4 as an internal reference. Electrospray ionization mass spectroꢀmetric (ESIꢀMS)
measurements were made on a Bruker Daltonics Amazon SL ion trap mass spectrometer. SEM

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and TEM images were acquired on a JEOL JSM 840 Scanning and EMꢀ410 LS Transmission
Electron Microscope, respectively.
X-ray structure determinations
Crystal data for 1−3 were collected on a dual source Bruker D8 QUEST CMOS diffractometer at
low (2) and room temperature (1 and 3 ) with MoꢀKα radiation (λ = 0.71073 Å). The structures
were solved by direct methods (SHELXS 97)53ꢀ54 and refined by fullꢀmatrix leastꢀsquares
methods against F2 with version 2017/1 of ShelXL55 using Olexꢀ2 software.56 Nonꢀhydrogen
atoms were refined with anisotropic thermal parameters. Hydrogen atoms were geometrically
fixed and refined using a riding model. Computer program PLATON was used for analyzing the
interaction and stacking distances.57−58 The CCDC deposition Nos. 1518524 (1), 1520438 (2) and
1518525 (3), contain supplementary crystallographic data for this paper.
Theoretical studies
Quantum chemical calculations were performed at B3LYP Density Functional Theory (DFT)
level using B3LYP/6ꢀ31G** for 1−3.59−60 The geometry optimizations and frequency
calculations were performed using Gaussian 09.61
Syntheses
Synthesis of 5-[2-hydroxy-3-((4-methoxyphenyl)diazenyl)]dipyrromethane (L4)
A round bottom flask containing L1 [2ꢀhydroxyꢀ3ꢀ((4ꢀmethoxyphenyl)diazenyl)benzaldehyde,
0.5 g, 1.95 mmol] was charged with pyrrole (8.0 mL) and catalytic amounts of trifluoroacetic
acid (3 drops). The contents of the flask were stirred at room temperature for 24 h and after
completion of the reaction (monitored by TLC) ensuing solution was concentrated to dryness
under reduced pressure. Crude product thus obtained was purified by column chromatography
(SiO2; ethylacetate/hexane) to afford L4 as an off white solid. Yield: 60% (0.43 g), Anal. Calc.
1
for C22H20N4O2: Calcd: C, 70.95; H, 5.41; N, 15.04. Found: C, 70.88; H, 5.35; N, 14.97%. H
NMR (CDCl3, 300 MHz, δ ppm): 3.89 (s, 3H, OCH3), 6.45 (s, 1H, H5), 6.71ꢀ7.02 (m, 4H, H2,3),
7.11−7.25 (m, 4H, H1,14), 7.90 (d, 3H, J = 9.0 Hz, H10,13), 8.08 (d, 1H, J = 8.7 Hz, H11), 8.65 (d,
1H, H9), 9.62 (br, 2H, NH), 12.22 (s, 1H, OH). IR (KBr pellet, cmꢀ1): 3372, 1637, 1556, 1487,
1423, 1334, 1152, 1084, 1026, 817, 782 and 729.
Synthesis of 5-[2-hydroxy-5-((4-methoxyphenyl)diazenyl)]dipyrromethane (L5)

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This compound was prepared following the above procedure for L4 using L2 [2ꢀhydroxyꢀ3ꢀ((5ꢀ
methoxyphenyl)diazenyl)benzaldehyde), 1.0 g, 3.9 mmol]. Yield: 70% (1.02 g), Anal. Calc. for
C22H20N4O2: Calcd: C, 70.95; H, 5.41; N, 15.04; Found: C, 70.86; H, 5.53; N, 14.95%.1H NMR
(CDCl3, 500 MHz, δ ppm): 3.87 (s, 3H, OCH3), 5.64 (s, 2H, H5, OH), 6.06 (s, 2H, H2), 6.18 (d,
2H, J = 2.0 Hz, H3), 6.75 (s, 2H, H1), 6.97 (t, 3H, J = 9.5 Hz, H14,8), 7.71 (s, 1H, H11), 7.76 (m,
1H, H9), 7.84 (d, 2H, J = 8.5 Hz, H13), 8.20 (br, 2H, NH). 13C NMR (CDCl3, 125 MHz, δ ppm):
40.6 (C5), 55.6 (C16), 107.4 (C3), 108.8 (C2), 114.2 (C14), 118.1 (C6), 118.3 (C1), 123.1 (C13),
124.5 (C8), 125.2 (C9), 128.6 (C11), 130.3 (C4), 147.0 (C12), 147.4 (C10), 156.1 (C7), 161.7
(C15). IR (KBr pellet, cmꢀ1): 3376, 1639, 1558, 1485, 1425, 1332, 1153, 1084, 1026, 819, 783
and 729.
Synthesis of 5-[2-hydroxy-5-(p-tolyldiazenyl)]dipyrromethane (L6)
It was prepared following the above procedure for L4 using L3 [2ꢀhydroxyꢀ5ꢀ(pꢀtolyldiazenyl)ꢀ
benzaldehyde, 1.87 g, 8.00 mmol]. Yield: 82%. (1.53 g). Anal. Calc. for C22H20N4O: Calcd: C,
1
74.14; H, 5.66; N, 15.72; Found: C, 74.07; H, 5.58; N, 15.65%. H NMR (CDCl3, 500 MHz, δ
ppm): 2.34 (s, 3H, CH3), 5.56 (s, 1H, H5), 5.64 (s, 1H, OH), 5.99 (s, 2H, H2), 6.11 (q, 2H, H3),
6.67 (q, 2H, H1), 6.88 (d, 1H, J = 8.5 Hz, H8), 7.19 (d, 2H, J = 8.5 Hz, H14), 7.65−7.72 (m, 4H,
13
H
9,11,13), 8.12 (br, 2H, NH). C NMR (CDCl3, 125 MHz, δ ppm): 21.5 (C16), 40.5 (C5), 107.4
(C3), 108.7 (C2), 118.0 (C6), 118.2 (C1), 122.6 (C14), 123.2 (C8), 125.5 (C9), 128.7 (C11),
129.7 (C13), 130.4 (C4), 141.1 (C15), 147.4 (C10), 150.8 (C12), 156.5 (C7). IR (KBr pellet, cmꢀ
1): 3384, 2921, 1595, 1494, 1425, 1462, 1341, 1266, 1194, 1094, 1079, 1027, 824, 735 and 706.
Synthesis of 1
To a stirred solution of L4 (0.5 g, 1.34 mmol) in dichloromethane (50.0 mL) a solution of DDQ
(0.31 g, 1.34 mmol) in benzene (25.0 mL) was added drop wise over 1 h and reaction mixture
stirred for an additional 4 h. After completion of reaction the contents of the flask were
concentrated to dryness under reduced pressure. Crude product was dissolved in DCM and
filtered to remove any solid impurities. Triethylamine (1.0 mL) and BF3.Et2O (3.0 mL) were
successively added to this solution and reaction mixture stirred for 15 min at room temperature.
Progress of the reaction was monitored by TLC. After completion of the reaction it was filtered
and the filtrate washed thrice with water, extracted with dichloromethane and concentrated to
dryness under reduced pressure. Crude product thus obtained was charged on a flash column
(SiO2; CH2Cl2/hexane). Dark orangeꢀred band was collected and concentrated to dryness to

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afford the desired product. Yield: 10% (0.056 g). Anal. Calc. for C22H17BF2N4O2: Calcd: C,
1
63.18; H, 4.10; N, 13.40; Found: C, 63.10; H, 4.03; N, 13.33%. H NMR (CDCl3, 500 MHz, δ
ppm): 3.90 (s, 3H, OCH3), 6.50 (d, 2H, J = 1.5 Hz, H2), 6.91 (d, 2H, J = 4.0 Hz, H3), 7.03 (m,
2H, H14), 7.18 (t, 1H, J = 7.5 Hz, H10), 7.38 (q, 1H, H11), 7.86 (q, 2H, H1), 7.92 (s, 2H, H13),
13
8.08 (q, 1H, H9), 13.55 (br, 1H, OH). C NMR (CDCl3, 125 MHz, δ ppm): 55.7 (C16), 114.7
(C14), 118.3 (C2), 119.1 (C6), 122.7 (C10), 124.3 (C13), 131.1 (C3), 134.0 (C4), 134.4 (C11),
11
135.5 (C9), 136.8 (C8), 142.9 (C5), 144.1 (C12), 144.3 (C1), 150.8 (C7), 162.7 (C15). B NMR
(160.4 MHz, CDCl3, δ ppm): −0.68 (t, 1B). 19F NMR (470.6 MHz, CDCl3, δ ppm): −143.83 and
−146.35 (octet, 2F). ESIꢀMS. (Calcd, found, m/z) 441.1310, 441.1343 [M + Na]+. IR (KBr
pellets, cmꢀ1): 3106, 2929, 1592, 1557, 1487, 1411, 1387, 1357, 1257, 1140, 1108, 975 and 764.
Synthesis of 2
It was prepared following the above procedure for 1 using L5 (1.0 g, 2.68 mmol) in place of L4.
Yield: 35% (0.39 g). Anal. C22H17BF2N4O2: Calcd: C, 63.18; H, 4.10; N, 13.40; Found: C,
1
63.12; H, 4.06; N, 13.35%. H NMR (CDCl3, 500 MHz, δ ppm): 3.87 (s, 3H, OCH3), 5.98 (br,
1H, OH), 6.55 (d, J = 4.0 Hz, 2H, H2), 6.98 (t, 4H, J = 8.0 Hz, H3,14), 7.14 (d, 1H, J = 9.0 Hz,
13
H8), 7.87 (q, 3H, H1,11), 7.96 (s, 2H, H13), 8.01 (d, 1H, J = 8.5 Hz, H9). C NMR (CDCl3, 125
MHz, δ ppm): 55.7 (C16), 114.4 (C14), 117.6 (C2), 119.3 (C8), 120.4 (C6), 124.8 (C13), 126.3
(C11), 126.4 (C9), 131.4 (C3), 135.0 (C4), 141.4 (C5), 145.4 (C1), 146.6 (C12), 146.8 (C10),
11
155.3 (C7), 162.2 (C15). B NMR (160.4 MHz, CDCl3, δ ppm): −0.68 (t, 1B). 19F NMR (470.6
MHz, CDCl3, δ ppm): −143.52 and −146.01 (octet, 2F). ESIꢀMS. (Calcd, found, m/z) 441.1310,
441.1346 [M + Na]+. IR (KBr pellets, cmꢀ1): 3482, 2923, 1582, 1555, 1456, 1389, 1261, 1112,
1074, 955 and 781.
Synthesis of 3
It was prepared through the adaptation of the synthetic procedure above for 1 where L6 (1.0 g,
2.80 mmol) in place of L4. Yield: 30% (0.32 g). Anal. Calc. for C22H17BF2N4O: Calcd: C, 65.70;
1
H, 4.26; N, 13.93; Found: C, 65.62; H, 4.20; N, 13.85%. H NMR (CDCl3, 500 MHz, δ ppm):
2.43 (s, 3H, CH3), 5.83 (br, OH), 6.55 (d, 2H, J = 3.0 Hz, H2), 6.99 (d, 2H, J = 4.5 Hz, H3), 7.16
(d, 1H, J = 8.5 Hz, H8), 7.30 (d, 2H, J = 7.5 Hz, H14), 7.79 (d, 2H, J = 7.5 Hz, H13), 7.93 (d, 2H,
13
J = 3.0 Hz, H1), 7.97 (s, 1H, H11), 8.04 (q, 1H, H9). C NMR (CDCl3, 125 MHz, δ ppm): 21.6
(C16), 117.5 (C2), 119.3 (C8), 120.3 (C6), 122.8 (C14), 126.4 (C11), 126.5 (C9), 129.8 (C13),
131.4 (C3), 135.0 (C4), 141. 3 (C5), 141.7 (C10), 145.4 (C1), 146.4 (C15), 150.6 (C12), 155.4

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(C7). 11B NMR (160.4 MHz, CDCl3, δ ppm): −0.67 (t, 1B). 19F NMR (470.6 MHz, CDCl3, δ
ppm): −143.52 and −145.95 (octet, 2F). ESIꢀMS. (Calcd, found, m/z) 425.1361, 425.1393 [M +
Na]+. IR (KBr pellets, cmꢀ1): 3484, 2924, 2854, 1583, 1553, 1456, 1390, 1260, 1112, 1077, 957
and 781.
Results and discussion
Synthesis and characterization of 1−3
Syntheses of the aldehydes L1−L3 have been achieved by following literature procedures.62
Notably −OCH3 containing diazonium moiety upon coupling with salicylaldehyde provided both
ortho (L1) and para (L2) azo phenol aldehydes whereas those with −CH3 gave only para (L3)
azo phenol derivative. It may be attributed to higher stability of the diazonium salt involving
−OCH3. The aldehydes L1−L3 reacted with an excess of pyrrole in presence of catalytic
amounts of trifluoroacetic acid to afford dipyrromethanes L4−L6. The azoꢀphenol BODIPYs
1−3 have been synthesized using L4−L6 via a two step reaction, oxidation to corresponding
dipyrrins with DDQ followed by treatment with BF3.Et2O in presence of triethylamine (Scheme
S1†). Formation of the compounds has been confirmed by satisfactory elemental analyses, IR,
13
11
19
NMR (1H, C, B, F), HRMS, electronic absorption and emission spectroscopic studies (Fig.
19
S1−S10†). The F NMR spectra of 1−3 displayed two sets of octets at ~ −143 and −146 ppm
19
due to presence of two nonꢀequivalent fluorine atoms. F NMR showed that each fluorine atom
is coupled with boron and gives a quartet which is further coupled with nonꢀequivalent fluorine
and appeared as an octet. This observation is consistent with other reported hydroxy BODIPY
derivatives.63ꢀ64
Crystal structures
Structures of 1−3 have been unambiguously authenticated by single crystal Xꢀray analyses (Fig.
1). Compound 1 crystallizes in a triclinic system with 'Pꢀ1' space group, while 2 and 3 in
monoclinic system with 'P21/c' and 'P21/n' space groups, respectively. Important solution and
refinement parameters are summarized in Table 1 while geometrical parameters are gathered
through Tables S1−S2†. Boron atom in 1−3 lies within the dipyrrin plane and adopted usual
tetrahedral geometry. The N–B–N [105.6; 1, 106.6; 2, 106.5° 3] and F–B–F angles in these
compounds [109.1; 1, 108.4; 2, 108.6°; 3] are comparable to other related systems.48−49

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Fig. 1 ORTEP views of 1 (a), 2 (b) and 3 (c) at 50% ellipsoidal probability (H atoms and solvent
molecules are omitted for clarity).
Photophysical properties: aggregation induced emission enhancement/aggregation caused
quenching
Optical properties of BODIPYs 1−3 have been investigated in methanol (c, 50 ꢀM) at room
temperature. Absorption spectra of these compounds displayed maxima due to S0−S1 transitions
at almost the same position (501, 1; 500, 2 and 502 nm, 3) while S0–S3 transitions appeared
higher in energy in the spectrum at ~350 nm (362, 1; 349, 2; and 351 nm, 3).49,65−66 Noticeably,
high energy band due to π− π∗ transitions of the azo moiety in 1 red shifted by ~12 nm relative
to 2 and 3 due to intramolecular hydrogen bonding.67 Further, upon illumination at 500 nm
compounds 1−3 displayed emission maxima at ~530 nm assignable to BODIPY chromophore
(Fig. 2).49,61 Notably significant emission shown by these compounds in solution is in contrary to
other reported azoꢀBODIPYs.38The presence of ortho and para phenol groups in 1−3 enables
these to exist in tautomeric hydrazone form which are emissive due to restriction of cis−trans
isomerization.68 Notably, these compounds showed low fluorescence intensity in polar solvent
relative to nonꢀpolar solvents (Fig. S13†).

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Fig. 2 UVꢀvis (a) and fluorescence (b) spectra of 1−3 (c, 50 ꢀM) in methanol.
On the other hand, absorption spectra did not show any noteworthy change with solvent
polarity. An examination of their excitation spectra revealed that it does not overlap with the
absorption spectra (Fig. S14†). It further suggested polarization of the excited state which
eventually causes nonꢀradiative decay of the excited state to some extent while ground state
remains unaffected. The AIEE characteristics of 1−3 have been affirmed by monitoring
photophysical properties in water–methanol mixture with varying water fractions. Absorption
spectra of these compounds exhibited similar absorption profile with increasing water content.
Upon addition of water at fw 80% it showed a decrease in absorbance along with a levelꢀoff tail in
visible region which is a common feature of nanoaggregate suspension,69 as well, colour of the
solution turned red from yellow (Fig. S11a and S12†). Excited state behaviour of the compounds
during aggregation has been followed by acquiring photoluminescence (PL) spectra of 1–3 in the
aforesaid solvent system.

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Fig. 3 Emission spectra of 2 (a) and 3 (b) in methanolꢀwater mixture with different water
fractions (c, 50 ꢀM).
Upon increasing the water content fw to 60%, 1 showed regular decrease in emission intensity
however, at fw > 70% a new small band emerged at 570 nm and solution turned almost nonꢀ
luminescent which is a typical characteristic of ACQ (Fig. S11b†). On the other hand, at fw 60%
2 exhibited considerable emission quenching until commencement of the aggregate formation
(Fig. 3). With further increase in fw to 80% emission maxima red shifted with appearance of new
bands at 575 and 610 nm. On the other hand at fw 80% 3 showed a single band at 585 nm. Due to
strong electron donating nature of OCH3 relative to CH3 the lone pairs on oxygen (OCH3 and
OH) at either end may push electron density into the ring and which may affect overall charge
transfers process in the excited state causing dual emission in 2.70 Further, emission assumed
maximum intensity at fw 90% indicating enhancement of nanoaggregate formation.
To gain deep insight into fluorescence enhancement time dependent studies have been
performed on the nanoaggregates of 2 and 3 (Fig. 4). Interestingly, addition of poor solvents like
water (for organic compounds) could not induce any enhancement in the emission instantly;
however it led to revival of AIEE after 60 minutes. This delayed response prompted us to
monitor the emission of nanoꢀaggregates with time (up to 120 min.). From Fig. 4 it is clear that
instantly formed nanoꢀaggregates of 2 and 3 (90% water) are weakly fluorescent and gain
significant emission with time. It may be attributed to probable changes in the packing of dye
molecules with time during aggregation (videꢀinfra).71 A small decrease in emission intensity
observed after 60 minute and also at fw 100% may be attributed to reduced solubility. Multiple
emission bands upon aggregation suggested occurrence of various aggregated species in solution
which is consistent with time dependent changes in AIEE. Noticeably, with increasing water
content emission profile of 3 displayed features similar to that for 2. On going from solution to
aggregated form 2 and 3 displayed ~ 4 and 8 fold emission enhancement.
To further confirm AIEE, solid state emission spectra of 2−3 have been acquired which
showed redꢀshifted emission with maxima at 618 and 635 nm (Fig. S15†). Red shift in the
position of emission maxima on going from solution to solid state via aggregates indicated
creation of ordered structures which can further be supported by Xꢀray diffraction studies.

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Presumably, variable degree of emission enhancement for these compounds may originate from
substituent effect which may also alter aggregate buildꢀup (videꢀinfra). To have a quantitative
picture of AIEE, fluorescence quantum yields (ΦF) for these luminogens have been determined
in solution and solid state (Table S3†). The ΦF values for 1, 2 and 3 in methanol are low and fall
in the range of 1–3%. On the other hand in solid state, ΦF ehanced for 2 and 3 [2, 2−8; 3,
2.5−11%] and further authenticated occurrence of an effective AIEE in these systems. On the
other hand, 1 remained nonꢀfluorescent in solid state due to ACQ.
Fig. 4 Fluorescence spectra of 2 (a) and 3 (b) (c, 50 ꢀM) at λex 500 nm in fw = 90% with
different time interval.
Aggregation induced emission enhancement vs restriction of intramolecular rotation
RIR (restricted intramolecular rotation) is believed to be the main cause of AIE in most of the
BODIPY derivatives. To investigate this phenomenon in 2 and 3 viscosity experiments have
been performed. In this regard emission spectra of 2 and 3 have been acquired by varying
glycerol fractions (fg) in methanolꢀglycerol mixture (c, 50 ꢀM). Emission intensity of 2 and 3
remarkably enhanced with an increase in glycerol fraction (fg) (Fig. 5) while insignificant
changes occurred for 1 (Fig. S16†) which strengthened the supposition of occurrence of an
effective RIR in 2 and 3.

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Fig. 5 Fluorescence spectra of 2 (a) and 3 (b) (c, 50 ꢀM) at λex 500 nm in methanolꢀglycerol
mixture with different volume fractions of glycerol (fg).
Aggregation and AIEE in these systems have been further affirmed by timeꢀresolved
fluorescence studies (Fig. 6). On going from solution to aggregated state fluorescence life time
enhanced at fw 90% [2, 2.00−3.88; 3, 1.24−4.95 ns] (Table S4†). It may be concluded that active
intramolecular rotation for 2 and 3 in methanol get restricted upon aggregate formation at fw 90%
which influences the nonꢀradiative deactivation pathway of the excited state leading to emission
enhancement, thus fluorescence life time.
Fig. 6 Fluorescence lifetime spectra of 2−3 in different fraction of water.

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Powder X-ray diffraction studies
The aggregate build up has further been followed by comparing the powder Xꢀray diffraction
patterns (PXRD) for crystalline material with aggregates of 2 and 3 at fw 70% (required for
aggregation initiation) and 90% (required for completion of aggregation and to achieve
maximum emission). An examination of the PXRD data of the aggregates obtained by
centrifugation method at fw 70% and 90% (after 48 h) revealed that pattern of the latter one
(90%) are close to the crystalline material and showed well defined peaks. This reinstates the fact
that an increase in water content leads to increase in crystallinity of aggregates. However, it is
worth mentioning that PXRD data for aggregates at 90% water contents are a bit broader than
that of crystalline material which suggested ordered structure for the crystalline material relative
to the aggregates. It further accounted well for difference in fluorescence wavelengths between
aggregates and solid state (videꢀsupra) (Fig. 7).
(a)
(b)
Fig. 7 Powder XRD Pattern of 2 and 3.
Morphological analysis
To gain deep insight into time dependent AIEE and its influence on molecular selfꢀassembly,
aggregates of 2 and 3 were subjected to scanning electron (SEM), fluorescence optical (FOM),
and transmission electron microscopic (TEM) studies. Selfꢀassembly for both 2 and 3 after

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injecting water (5 min) created nanospheres (50−100 nm) at 90% water content (Fig. 8a and Fig.
8c). However, water injection after 60 min. enabled the nanospheres to grow into larger thick
rods (~500 nm) of diverse lengths from 2–6 ꢀm (Fig. 8b and Fig. 8d).
Fig. 8. SEM images showing nanospheres in 2 (a), 3 (c) after 5 min and nanorods in 2 (b), 3 (d)
and TEM image 2 (e) and 3 (f) after 60 min of water injection respectively (fw = 90%; (c, 50
ꢀM).
The FOM (Fig. S17†) and TEM (Fig. 8eꢀf) images matched well with SEM and revealed that
nanorod/ꢀnanostick can be easily visualized from orange and red fluorescence for 2 and 3
(excited with green light after injecting water for 60 min). DLS studies further indicated an
increase in the size of nanoaggregates with time (Fig. S18†). The nanoaggregates for 3 (390 nm)
are large relative to 2 (302 nm) which may be related to greater aggregation and emission
enhancement for this molecule.
Theoretical studies
To have clear understanding about the relationship between structure of the luminogens and their
photophysical properties, theoretical calculations have been performed (Fig. S19†). Notably,
HOMO energy levels for 1−3 are dominated by orbitals from azo moiety, while LUMOs mainly
by BODIPY units. The computed (TDꢀDFT) electronic absorption spectra of 1−3 showed low
energy bands at ~412 nm arising from (HOMO→LUMO) and high energy ones at ~300 nm from
HOMO(−1)→LUMO(+1) transitions (3.44; 1, 3.62; 2 and 3.75 eV, 3) (Fig. S20† and Table

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S5†). Noticeably lower energy difference for HOMO(−1)→LUMO(+1) transition is responsible
for red shifting of the high energy band in 1 relative to 2 and 3 (310; 1, 297; 2 and 293 nm, 3).
Although, these values are relatively lower than the experimental ones which may be due to the
differential electron correlation.72 Further, distribution of the HOMO and LUMO for 2 and 3 are
very similar which rationalizes well their comparable AIEE (Fig. 9). On the other hand, complete
overlap between HOMO and LUMO makes the compound 1 an ACQ dye.73 The electrostatic
surface potential (ESP) for 1 showed positive charge on phenolic azobenzene core and negative
on the BODIPY core. On the other hand, for 2 and 3 positive charge on phenolic hydroxyl group
and partial negative on BODIPY core (Fig. S21†). Localized frontier orbitals on 2 and 3 clearly
indicated that these may redistribute charge in the excited state which have also been confirmed
by PL studies using solvents with diverse polarity. Calculated HOMO−LUMO gap for 1−3 are
almost similar (3.06 eV) and in good agreement with those obtained from UVꢀvis studies.
LUMO(+1)
−2.51 eV
−2.61 eV
−2.25 eV
−2.68 eV
−2.13 eV
LUMO
−2.64 eV
HOMO
−5.67 eV
−5.71 eV
−5.75 eV
HOMO(−1)
−6.00 eV
−5.95 eV
−5.75 eV
(a)
(b)
(c)
Fig. 9 Selected ground state MOs of 1−3 (a−c, respectively).

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Molecular level understanding
PXRD established that nanoaggregates have ordered structure which in turn, suggested that basic
aggregation mechanism can be elucidated by cautious analysis of molecular arrangements. In
this direction, supramolecular crystal packing modes have been thoroughly analyzed.
Fig. 10 Crystal packing in 1 (a) via π−π interaction and 3 (b) via C−Hꢁꢁꢁπ interactions.
Notably, structure of 1 revealed strong intramolecular Hꢀbonding interactions between
phenolic ꢀOH and azo nitrogen. Further, parallel boraindacene units stack in headꢀtoꢀhead
manner with the adjacent ones through two π−π (3.328 Å ) and six C–Hꢁꢁꢁπ interactions along the
crystallographic aꢀaxis (Fig. S22†). However, 1 did not show any intermolecular interactions in
other two directions. On the other hand, crystal structure of 2 unvealed that it possesses two
π−π (3.328 Å ) and eight intraꢀ/interchain C–Hꢁꢁꢁπ interactions through which adjacent molecules
stack strictly in headꢀtoꢀtail fashion and form a dimer (Fig. S23†). The dimer is further
strengthened by two (NꢁꢁꢁH−C) (2.626 Å) Hꢀbonding interactions between diazo nitrogen of one
molecule and C−H of boraindacene from the adjacent one (Fig. 11a). This interaction is believed
to be responsible for restriction of the photoisomerization in aggregated state. Apparantly headꢀ
toꢀtail stacking in 2 is closer to Jꢀaggregate like structure relative to headꢀtoꢀhead one in 1 which
may account for AIE in 2 and ACQ in 1. Similarly in the crystal structure of 3 the neighbouring
units stack strictly in headꢀtoꢀtail fashion via eight intraꢀ/interchain C–Hꢁꢁꢁπ interactions and
form a dimer (Fig. 10b). In addition, packing in 3 acquired further rigidity via weak
intermolecular interactions with other unit involving two each of (NꢁꢁꢁH−C) (2.674 Å),
B−FꢁꢁꢁH−C (2.537 Å) and B−FꢁꢁꢁH−O (2.021 Å) (Fig. S24c†). On the other hand, in 2 as all these

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(NꢁꢁꢁH−C, B−FꢁꢁꢁH−C and B−FꢁꢁꢁH−O) interactions lie along the direction of C–Hꢁꢁꢁπ interactions
(Fig. S24b†), therefore it cannot provide 3D rigidity to the crystal lattice. It may be one of the
reasons behind intense emission of 3 in aggregated state relative to 2.
Fig. 11 Crystal packing showing N···H−C interactions in 2 (a) and 3 (b).
It is known that morphology and soft surface of the organic functional nanoaggregates can
modulate emission behaviour. Hence, it is presumed that when nanoꢀaggregates are soft (initial
stage), due to the presence of strong intramolecular hydrogen bonding in aggregates (1), weak
intermolecular hydrogen bonding (C−Hꢁꢁꢁπ, C−HꢁꢁꢁO) might not have been operative leading to
ACQ. However, for 2 and 3 absence of intramolecular Hꢀbonding initially formed soft spherical
nanoaggregates which transform into hard nanorod (1D) via intermolecular Hꢀ bonding,
rigidified, and showed time dependent AIEE. Also, due to the presence of intramolecular Hꢀ
bonding in 1, the −OH group is not available for interactions with water molecules while 2 and 3
do not show such interactions leading to rigorous aggregation. Based on aforementioned
observations it can be concluded that intermolecular interactions play a crucial role in restricting

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the intraꢀmolecular motions and photoisomerization by creating a rigid 3D network, thereby an
increase in emission intensity in aggregated/solid state.
In summary, through this work novel fluorescent azoꢀphenol BODIPYs (1−3) have been
described. Notably, emission of 1 quenched upon aggregation via ACQ, while for 2 and 3
enhanced, articulating intriguing time dependent AIEE phenomenon. Photophysical properties of
these compounds are directed by substituents, as aggregated states of 3 bearing −CH3 exhibited
greater AIEE relative to 2 having –OCH3. Time dependent fluorescence and morphological study
indicated formation of soft nanoaggregates, getting converted to hard nanorod (1D) with
increasing time. The size of selfꢀassembled nanoaggregates varied with extent of aggregation
pronouncing dominance of substituent effect on overall aggregation and emission enhancement.
The ACQ (1) and AIEE (2−3) have been rationalized by crystal packing which showed absence
of intramolecular Hꢀbonding and extensive intermolecular interactions in 2 and 3 relative to 1.
Specific NꢁꢁꢁH−C interꢀchain interaction has been identified as the driving force for restriction of
cis−trans isomerization in aggregated state. This work pioneers successful development of azoꢀ
phenol BODIPYs with intense emission in solution and aggregated state. The idea of combining
AIE/AIEE BODIPY and diazo chromophores to obtain fluorescent phenolic azobenzene dye
may broaden the application of these dyes in diverse areas.
Acknowledgements
We acknowledge Science and Engineering Research Board (SERB), New Delhi, India for
providing financial support through the Scheme EMR/2015/001535 and also to Council of
Scientific and Industrial Research (CSIR), New Delhi, India for the award of a senior Research
Fellowship to RPP (No. 09/013(0514)ꢀ/2013ꢀEMRꢀI). We are also grateful to Dr. S. M. Mobin,
Indian Institute of Technology, Indore, India for providing single crystal Xꢀray data.
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Table 1. Selected crystallographic parameters for 1−3.
1
2
3
Crystal data
Empirical formula
Crystal system
C22H17BF2N4O2
triclinic
C22H19BF2N4O3
monoclinic
C22H17BF2N4O
monoclinic
Pꢀ1
7.2565(5)
7.8314(5)
17.8401(11)
101.755(2)
93.007(2)
101.674(2)
967.38(11)
2
P 21/c
6.3281(8)
10.0875(1)
31.947(4)
90
P21/n
6.9013(6)
18.4057(17)
15.0143(14)
90
Space group
a (Å)
b (Å)
c (Å)
α (°)
90.08
91.438(3)
90
β (°)
90
γ (°)
V (Å3)
2039.3(4)
4
1906.6(3)
4
Ζ
432.2377
1.4356
904
832
F(000)
1.421
1.401
ρ
calc (Mg mꢀ3)
T (K)
293(2)
150(2)
0.108
273(2)
0.102
ꢁ (mmꢀ1)
0.107
5023
5212
4366
refln collected
GOF on F2
1.0797
1.076
1.014
0.0437
0.0360
0.0465
Final R1 on
observed data
0.0968
0.0899
0.0885
Final wR2 on
observed data

Dalton Transactions
Page 24 of 24
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DOI: 10.1039/C7DT04047D
Graphical Abstract
Time dependent aggregation induced emission enhancement and study of
molecular packing in closely related azo-phenol BODIPY species
Rajendra Prasad Paitandi, Roop Shikha Singh, Bhupendra Kumar Dwivedi, Vishwa Deepak
Singh, and Daya Shankar Pandey*
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi - 221005,
India
Novel fluorescent azo-phenol BODIPYs (13) achieved by substituent (OCH3/CH3) directed
fabrication of ortho (L1) and para (L2L3) azo-phenol aldehydes. The position of diazo group
relative to phenol moiety determines the aggregation behavior of 13.