Bigger and Brighter Fluorenes: Facile π-Expansion, Brilliant Emission and Sensing of Nitroaromatics

Article abstract of DOI:10.1002/asia.201601359

π-Expanded butterfly-like 2D fluorenes and 3D spirobifluorenes 1–5 were synthesized via a DDQ-mediated oxidative cyclization strategy with a high regioselectivity. Through structural modification via π-expansion, it was possible to achieve near-ultraviolet absorption, bright-blue emission, very high near-unity fluorescence quantum yields in solution as well as in film states, and deep-lying HOMO energy levels with excellent thermal stabilities. Furthermore, these electron-rich compounds displayed a notable behavior towards sensing of nitroaromatic explosives, such as picric acid, up to a detection limit of 0.2 ppb.

Full text of DOI:10.1002/asia.201601359

CHEMISTRY
AN ASIAN JOURNAL
www.chemasianj.org
Accepted Article
Title: Bigger and Brighter Fluorenes: Facile π-Expansion, Brilliant
Emission and Sensing of Nitroaromatics
Authors: Venkatakrishnan Parthasarathy and Ramakrishna Jagarapu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
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
published online in Early View as soon as possible and may be different
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. Asian J. 10.1002/asia.201601359
Link to VoR: http://dx.doi.org/10.1002/asia.201601359
A Journal of
A sister journal of Angewandte Chemie
and Chemistry – A European Journal

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
Bigger and Brighter Fluorenes: Facile π-Expansion, Brilliant
Emission and Sensing of Nitroaromatics
Ramakrishna Jagarapu,[a] and Venkatakrishnan Parthasarathy*[a]
Dedication ((optional))
Abstract: π-Expanded butterfly-like two-dimensional (2D) fluorenes
and three-dimensional (3D) spirobifluorenes 1-5 were synthesized
via oxidative cyclization strategy with a high regioselectivity. Through
structural modification via π-expansion, it was possible to achieve
near-ultraviolet absorption, bright-blue emission, very high close-to-
other hand, small molecule-based approaches[5] is promising in
terms of direct synthesis, easy purification, better reproducibility
and high fluorescence quantum yields. These advantageous
properties associated with small molecules spurred our interest
on the development of fluorenes as suitable candidates for
unity fluorescence quantum yields in solution as well as in film states, sensor applications. Several electron rich simple, small-molecule
and deep-lying HOMO energy levels with excellent thermal stabilities. polycyclic aromatic hydrocarbons (PAHs), such as, pyrene,[6]
Further, these electron-rich compounds revealed notable behaviour
towards sensing of nitroaromatic explosives, such as, picric acid, up
to a detection limit of 0.2 ppb.
triphenylene,[7] truxene,[8] fluoanthrene,[9] pentiptycene,[10]
hexabenzocoronene,[11] etc. have been attempted for the
detection of nitroaromatic compounds, due to favourable donor-
acceptor interactions.[6-11]
Hence, to tap the fullest potential of the small less-explored
fluorene skeleton (molecular length and width ca. 0.69 and 0.37
nm, respectively) for sensing applications, appropriate structural
modification is essential. Structural modification so as to improve
the optical and electronic characteristics via extension of the π-
conjugation of fluorene is commonly accomplished either
through its 2,7- or 3,6-positions,[12] but a least attention is
devoted on the approach through expansion of the π-system of
fluorene. We surmised, π-expansion, which leads to a larger
skeleton with extensive π-delocalization, could be an attractive
strategy to influence upon the electronic as well as optical
behaviour of fluorenes, especially the photoluminescence
quantum yields (PLQEs) and sensitivity of detection.
One of the ways to expand the π-system of fluorene is by
annulation with aromatic or heteroaromatic units at the 2,3 and
6,7-faces (Figure 1). This would lead to a larger, planar, rigid,
conjugated and electron-rich fluorene system which may have
better optoelectronic properties. It is understandable that
annulation at other faces of fluorene may lead to π-expanded
fluorenes with non-planar geometry (helicenes) and limited
conjugation, due to steric reasons.[13] Initially, Mullen's group
have successfully exploited the most popular method—Scholl
cyclization (Clar's method), i.e., oxidative cyclization strategy—to
form π-expanded various 2D graphene-type architectures,[14]
including "super-fluorenes" that possess limited solubility.[15]
Later, Müllen et al. have synthesized n-type phthalimide fused
one-dimensional (1D) fluorene derivatives and demonstrated
their field-effect transistor (FET) behaviour.[16] Recently, Cheng's
group have reported fluorenes fused with heteroaromatic
thiophene and selenophene units to generate a new class of 1D
pentacyclic dithienofluorene (DTF) and diselenofluorene (DSF)
derivatives via sulphur-mediated thermal cyclization for better
optoelectronic properties;[17] however, photoluminecsnece (PL)
as well as the sensing behaviour of any of the above set of
fluorenes have not been explored.[15,16,17] In the latter instances,
π-expansion via ring-annulation/fusion at the 2,3 and 6,7-faces
of fluorene was accomplished essentially by prior introduction of
the right functional groups, such as, halogens, alkyls, etc., at the
2,3 and 6,7 positions. From this, it is clear that effecting multiple
substitutions simultaneously at all the 2,3,6 and 7 positions of
fluorene is challenging, tedious and involves several steps.[18] As
In recent years, organic luminescent materials have emerged as
inextricable materials for application in organic light emitting
diodes (OLEDs),[1a] organic light emitting transistors (OLETs),[1b]
light emitting electrochemical cells (LEECs),[1c] solid-state lasing
and sensing,[1d,e] and in bio-imaging,[1d] etc.[1] In particular,
fluorenes
and
their
derivatives,
oligofluorenes
and
spirobifluorenes have been shown to exhibit excellent
fluorescence behaviour, both in solution and film/solid states.[2]
Advantageously, fluorene-based compounds possess high
thermal stability, good processability (via dialkyl substitution at
the 9-position), and ready availability. However, their huge band
gaps and deep highest occupied molecular orbital (HOMO)
energy levels are limiting factors to a considerable extent for
their effective utilization as functional materials. It may be partly
originated from the unique structural attribute of fluorene.
As such, fluorene is a simple planar, rigid molecule with a
methylene bridge that connects the 2,2'-positions of the biphenyl
leading to a slightly bent geometry (162°) of banana-type. It is
interesting to note that, though planar, the emission spectrum of
simple fluorene (em in cyclohexane = 306 (sh), 315 nm)
highlights a significant blue-shift when compared to that of the
parent biphenylene (em in cyclohexane = 324, 330 (sh) nm)
whose tilt angle due to the sterics imparted by ortho hydrogens
of the phenyls is ca. 45 (Figure 1).[3] Because of their emission
in the ultra-violet region, simple fluorenes/fluorene derivatives
have not been investigated so far for sensor applications. In
contrast, electrospun nanofibres, membranes and films, and
polymer nanoparticles of poly(fluorenes) have been utilized for
sensing purposes due to their long range exciton migration
pathway and better electronic communication with the
analytes.[4] Generally, synthesis of polymers suffers from batch-
to-batch variation, reproducibility and tedious purification. On the
[a]
Dr. V. Parthasarathy, R. Jagarapu
Department of Chemistry
Indian Institute of Technology Madras
Chennai - 600 036, India
E-mail: pvenkat@iitm.ac.in
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
discussed earlier, fluorenes can be functionalized at 2,7 or 3,6
positions very readily.
1p: 79% (R = C6H13), 2p: 75% (R = Ph) and 3p: 80% (R =
spiro(2,2'-biphenyl)).
Similarly,
2,2',7,7'-tetrabromospiro-
Herein, we report the synthesis of π-expanded soluble 2D
fluorenes/3D spirobifluorenes (1-5, Figure 1) starting from the
readily obtainable 2,7-disubstituted fluorenes/spirobifluorenes
via oxidative cyclization strategy under DDQ/MeSO3H
conditions; the design of 2D (planar) and 3D (orthogonal) π-
expanded electron-rich fluorenes was aimed at tweaking the
solution/solid state PL properties of fluorenes which in turn may
be beneficial for effective sensing of nitroaromatics. Through π-
expansion, we have structurally enlarged fluorenes (molecular
length, ca. 1.71 nm and width, ca. 0.78 nm, as deduced from X-
ray structural analysis) introducing extensive delocalization to
achieve (i) absorption in the near-ultraviolet region (ca. 390 nm),
(ii) very high PLQEs in solution (close to unity) as well as in film
(up to 0.88) states, (iii) very good thermal stabilities (ca. 540 °C),
and (iv) better optical band-gaps (~ 3.1 eV) with deep HOMOs
(as low as –5.80 eV). These electron-rich bigger and brighter
butterfly-like fluorene skeletons have been shown to act as
excellent fluorescent sensors for nitroaromatic explosives with a
detection limit upto 0.2-2.0 ppb.
bifluorene 4k19c was subjected to four-fold Suzuki cross-coupling
reaction with excess of biphenyl-2-boronic acid pinacol ester to
deliver 4p in 60% yield (Scheme 1).
Our synthetic approach to achieve target fluorenes 1-5 is shown
in Scheme 1. Initially, we prepared the desired key
intermediates–‒9,9-dihexyl-2,7-dibromofluorene 1k,[19a] 9,9-
diphenyl-2,7-dibromofluorene
tetrabromospirobifluorene
2k[19b]
and
modified
2,2',7,7'-
literature
4k[19c]–‒by
procedures, cf. SI.[19] The dibromofluorene derivatives (1k and
2k) and commercially purchased 2,7-dibromospirobifluorene 3k
Scheme 1. (a) Biphenyl-2-boronic acid pinacol ester, Pd(PPh3)4, K2CO3,
toluene:ethanol (3:1), 90 °C, 12 h for 1p: 79%; 24 h for 2p: 75%; 3p: 80%, and
4p: 60%.
Having obtained the precursors 1-4p in hand, we next targeted
the π-expanded derivatives of fluorene (2D) and spirobifluorene
(3D) systems 1-5. The biphenyl units in compounds 1-4p are
appropriately placed for the structural π-elaboration of fluorenes
by oxidative cyclization strategy. Generally, oxidative Scholl
cyclization reactions can be carried out by a variety of oxidizing
agents, viz., CuCl2, Cu(OTf)2 and AlCl3, MoCl5, FeCl3, SbCl5,
PIFA, DDQ/MeSO3H, etc.[20] Our initial attempts were with the
most popular reagent, FeCl3.[21] When compound 1p was
subjected to oxidative cyclization condition in presence of FeCl3,
we observed the formation of a novel undesired ring chlorinated
product 3, but not the dicylized product 1, in a regioselective
fashion within 10 min (86% yield, Scheme 2). Compound 3 was
thoroughly characterized, and X-ray crystal structure analysis of
single crystals of 3 grown from chloroform or toluene solutions
unequivocally confirmed that it is a trichlorinated derivative
(Scheme 2) possessing rigid, coplanar structure with extensive
π-conjugation (see SI for structural details). Such regioselectivity
in oxidative cyclization reactions in presence of FeCl3, has been
observed earlier.[22] Also, ring chlorination has been observed as
one
of
the
major
side
reactions,
besides
oligomerization/polymerization, during oxidative cyclization
reaction with FeCl3.[23] It is generally overcome by introducing
stable blocking groups at the active positions of the starting
material.[24] Several optimizations performed on the above
reaction, viz., reducing FeCl3 equivalents, reaction time, varying
concentration of the reaction medium, bubbling the reaction
mixture with nitrogen gas to remove the dissolved HCl, etc., to
obtain 1 were not successful. To the best of our knowledge, 3
Figure 1. Chemical structures of target π-expanded 2D fluorenes 1-3 and 3D
spirobifluorenes 4-5.
were subjected to Pd(0) catalyzed two-fold Suzuki cross
coupling reaction with biphenyl-2-boronic acid pinacol ester to
produce the precursors in reasonably good yields (Scheme 1):
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
serves as the first structural evidence for the chlorinated product
obtained from the oxidative cyclization reaction of fluorenes. The
pattern of substitution of chlorine atoms on the aromatic ring
may provide valuable information about the mechanism of
formation of such chlorinated products in Scholl reaction. In view
of the beneficial use of chlorine in polycyclic aromatics, that
provides effective molecular packing, we believe these
candidates are of prominence in organic electronics.[25]
Scheme 3. (a) DDQ, DCM:MeSO3H (10:1), 0 °C to rt, 2.5 h, 1: 76%; 12 h, 2:
60%; 5 h, 4: 75%; 6 h, 5: 73%.
Scheme 2. (a) FeCl3, DCM, rt, 10 min, 86%.
aromatic hydrocarbons (PAHs).[27] This is in excellent agreement
with the rigid and near-planar (tilt angle, ca. 3.36) structure
observed by 3 from X-ray structural analysis. All of them reveal a
low energy absorption band corresponding to π-π* transition in
the range of 375–380 nm, in addition to the high energy
absorption bands at ca. 285 nm (not shown in Figure 2) and at
ca. 344 nm (Table 1).[28] While substitution of diphenyl (as in 2,
log ε = 4.50) and chlorine atoms (as in 3, log ε = 4.79) red-shifts
the absorption maximum by 2–6 nm when compared to that of
the dihexyl derivative (as in 1, log ε = 5.01), we observe only
marginal differences in the absorption spectrum when we move
from 2→4→5 (2D→3D). The inductive effect due to the ring
chlorine atoms and the effect of substituents at the 9-position in
the absorption spectrum is noteworthy here. It is notable that the
absorption spectrum of 2 reveals a tail at around 400 nm, while
To avoid ring chlorination and achieve the target compound 1,
we followed Rathore's oxidative cyclization strategy using
DDQ/MeSO3H[26] with 1-4p that contain no blocking groups. At
first, we optimized the reaction conditions with 1p. To our
gratification, we observed complete dicyclization (1, 100%
conversion) regioselectively in 1-2 h, when 4.0‒5.0 equivalents
of DDQ were used. Using the above optimized conditions,
compounds 2 (76%) and 4 (60%) were synthesized in very good
yields (Scheme 3). Under similar conditions (DDQ, 16.0 equiv.),
the tetracyclized compound 5 was obtained in 73% yield
(Scheme 3). It is important to note that the oxidative cyclization
using 2-biphenyls at 2 and 7 positions of fluorene (1-4p), as
above, is always observed to be regioselective, i.e., cyclization
occurs only at 3 and 6 positions of fluorene. Aryl fusion at 1,2
and 7,8 faces of fluorene leading to a symmetrical regioisomer
as well as 1,2 and 6,7 faces of fluorene leading to a cross one
were not observed at all under the above experimental
conditions (SI). Thus, the above method of achieving π-
expansion in fluorenes starting from 2,7-disubstituted derivatives
such
a feature is absent in other derivatives. A UV-vis
absorption spectrum recorded at 80-90 C in toluene with same
concentration, indicated that it is an inherent molecular
absorbance and not due to aggregation. This further signifies the
role of diphenyl substituents in suppressing molecular
aggregation. Interestingly, π-expansion in all the derivatives
contributes to shifting the absorption maximum close to the
visible region with absorption tailing all the way into the visible
region; for comparison, λmax (abs) of triphenylene in methanol is
ca. 247 nm,[3] λmax (abs) of fluorene in CHCl3 is ca. 300 nm[28]
and λmax (abs) of DTF/DSF in CHCl3 is ca. 340 nm.[17] To see, if
the molecular properties (solution) of 1-5 are preserved in films,
we carried out absorption spectral studies in a inert polymer,
polystyrene (ps), matrix.[29] The absorption spectrum of ps films
of 1-5 (0.1 wt%) exhibited a very similar behaviour to their
solutions in toluene; notably, 2 exhibited a slight bathochromic
shift (2‒9 nm) when compared to other fluorenes (SI). This
indicates that the intermolecular interactions are quite similar in
both solution and film states under the above experimental
conditions.
is more advantageous than starting from 2,3,6 and
7
tetrasubstituted derivatives. As expected, compounds 1-5 exhibit
excellent solubility in common organic solvents, such as
chloroform, dichloromethane, tetrahydrofuran, ethyl acetate,
toluene, chlorobenzene, dichlorobenzene, etc. All the starting
materials, intermediates and final compounds were thoroughly
1
characterized by infra-red, H and 13C NMR spectroscopic data,
and high resolution mass spectrometry (see SI for details).
To examine the influence of π-expansion on the optical
properties
of
fluorene,
UV-vis
absorption
and
photoluminescence spectrum of the π-expanded fluorene
derivatives 1-5 were examined in dilute toluene solutions (ca. 1
× 10-5 M) at room temperature. The UV-vis absorption spectral
profiles of compounds 1-5 (Figure 2) display typical vibronic
features similar to those observed for rigid coplanar polycyclic
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
Table 1. Optical, electrochemical and thermal properties of π-expanded 2D fluorenes 1-3 and 3D spirobifluorenes 4 and 5.
[b]
[g]
Compound
λmax abs (nm)/log ε
Eg
λmax em (nm) at λexc = 342 nm
PLQE[d]
soln/film
τ[a]
HOMO[e]
LUMO[f]
(eV)
Td
(M-1cm-1) at 379 nm
soln[a]
(eV)
soln[a]
ps film[c]
(ns)
(eV)
(C)
1
2
3
4
5
342, 357, 375/5.01
344, 360, 378/4.50
348, 356, 379/4.79
344, 360, 379/4.64
344, 361, 380/4.91
3.18
3.17
3.12
3.15
3.13
380, 400, 422
381, 403, 424
389, 405, 430
382, 402, 425
384, 404, 424
378, 400, 426
381, 403, 427
386, 406, 415
381, 403, 426
383, 405, 429
1.00/0.70
0.95/0.78
0.04/0.35
0.84/0.86
0.78/0.88
2.19
2.57
–5.76
–5.82
–5.85
–5.80
–5.79
–2.57
–2.65
–2.73
–2.65
–2.66
406
434
412
456
542
0.21, 1.85
2.59
2.72
[a] Measured from toluene solutions. [b] Optical band gap derived from the onset of absorption. [c] Measured from ps films. [d] Measured from toluene solutions
[31]
using anthracene in ethanol (Φ = 0.28) as the standard,[30] and for films, diphenylanthracene in ps films (Φ = 0.98) is used as the reference;
λ
exc
= 342 nm, error
<5%. [e] Calculated from cyclic voltammetry using dichloromethane solutions of 1-5 (1.0 mM) and 0.1 M Bu4NPF6 as the supporting electrolyte using the relation
EHOMO = –(Eox–E(Fc+/Fc)+ 4.8) eV. [f] Calculated from the optical band gap and HOMO energy level using the relation ELUMO = EHOMO + Eg eV. [g] Td-thermal
decomposition temperature at 5% weight loss.
1
1.0
0.8
0.6
0.4
0.2
0.0
when we move from 2D to 3D. The compound 1 exhibited the
highest PLQE of unity in the solution state with the following
order for others: 1>2>4>5>3 (Table 1). It appears that expansion
of the π-system of fluorenes (as in 1-5) enhances the
luminescence efficiencies (up to ca. 32-40%) in solution, but
does not affect the same; for comparison, Φ (fluorene, EtOH) =
0.68 and Φ (triphenylene, EtOH) = 0.60.[35] It is noteworthy to
mention here that the PLQEs measured for helical fluorenes or
helical sila-fluorenes or helical bitriphenylenes (arenes fused at
3,4 and 5,6 faces of fluorenes) are ca. 60-80% lower than that of
the present fluorene system.[13] Usually, compounds with larger
π-system tend to aggregate resulting in diminished PLQEs
(aggregation-caused quenching, ACQ).[27] A molecular design
(π-expansion with appropriate substituents at the methylene
position), as shown above, paves way for fluorenes with stable
blue emission and improved luminescence properties to serve
as potential candidates in optoelectronic applications. The
lowest PLQE observed with the trichloro compound 3 may be
attributed to the enhanced non-radiative decay pathways
probably caused by the chlorine atoms. In ps films, the PLQE
order is as follows: 5>4>2>1>3; the spirocompounds 4 and 5
exhibit higher PLQEs than planar fluorenes 1–3 (Table 1). It is
noteworthy that PLQE of films of 4 remains almost the same as
that of the one in solution, while 5 improves drastically. It is
evident that the spiro-configuration in 5, due to its unique
orthogonal planes, clearly reduces intermolecular interactions
impeding self-quenching, even after π-expansion.[36] The
improvement of PLQE of 3 in the ps film (when compared to that
in solution) could probably be attributed to the restricted motions,
and hence reduced non-radiative decay pathways.[37] Further,
studies on the PL lifetimes (Table 1) reveal a single exponential
decay (ns) for compounds other than 3, which probably is due to
different excited state species arising from the polarity variation
caused in its excited states.[38] Indeed, π-expansion, rigidity and
planarity of these new fluorenes, and the orthogonal nature of
spirobifluorenes improve the emission (blue) characteristics
without hampering the PLQEs. The near unity PLQEs (1)
achieved in solution as well as a PLQE of 0.90 (5) in a ps film is
1.0
0.8
0.6
0.4
0.2
0.0
1
2
3
4
5
2
1
2
3
4
5
3
4
5
350
400
450
500
320
360
400
Wavelength, nm
Wavelength, nm
Figure 2. UV-vis absorption (left) and photoluminescence (right) spectra of 1-5
in toluene solutions. Inset: Photographs of toluene solutions and polystyrene
films of 1-5 under room light (top) and under UV light (λexc = 365 nm, bottom).
The optical band gap values derived from the onset of the
absorption spectrum of films of 1-5 (Table 1) highlights a wide
band gap energy (ca. 3.1–3.2 eV) attesting to their
semiconducting nature. The band gap values obtained here are
similar to tricyclic fluorene (truxene ca. 3.1 eV)[32] or much
smaller than those of DTF/DSF derivatives (ca. 3.61 eV)
reported earlier,[17] indicative of the extensive π-delocalization in
these systems. Such wide band gap materials with brilliant blue
emission are useful in white light generation with suitable
dopants via fluorescence resonance energy transfer (FRET)
mechanisms in OLEDs.[33]
As mentioned, 9,9-dialkylfluorenes are highly fluorescent
compounds, and spirobifluorenes that possess orthogonal
planes exhibit excellent fluorescence emission in the solid/film
state. Compounds characterized by orthogonal planes are
known to impede self-quenching mechanisms due to approach
problems for aggregation.[34] The fluorescence quantum yields of
compounds 1-5 were determined in toluene solutions (ca. 1 ×
10-6 M) and in inert polystyrene films in order to understand their
fluorescence behaviour. As shown in Figure 2, all compounds 1-
5 (in both solution and films) emit in the blue region (380–430
nm), which is characteristic of fluorenes. While the emission
spectrum or peak positions do not change significantly, one may
notice a considerable modification in the trend with PLQEs,
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
one of the highest for any compound reported in
fluorene/spirobifluorene series (for comparison chart, see SI).
In order to understand the effect of π-expansion and introduction
of spiro-units on the thermal properties of compounds 1-5, and
to gauge their functional utility in thin-film devices, thermal
stabilities of 1-5 were estimated by thermogravimetric analyses
(TGA). All these compounds 1-5 were found to exhibit
extraordinary thermal stabilities (Table 1); thermal decomposition
temperatures (Td) of 1-5 are greater than 400 °C with maximum
(542 °C) being exhibited by 5. This may be readily discernible
from the rigid, planar, π-expanded fluorene/spiroconnected
bifluorene unit.[34] In addition, the thermal stability trend,
5<4<2<3<1, suggests, (i) longer alkyl chains decrease thermal
stability (due to their flexible thermal motions), (ii) substitution of
alkyl to aryl to spiro substituents at the methylene carbon bridge
increases the thermal stability, (iii) incorporation of chlorine
atoms into the skeleton (as in 3) indeed improves the thermal
stability.
Having developed π-rich fluorenes with excellent optical and
thermal characteristics, the electron donor tendency of these
novel polycyclic aromatic hydrocarbons (PAHs) were
investigated for their utility as sensors. The HOMO and LUMO
energy levels of the newly synthesized compounds 1-5 were
extracted from cyclic voltammetry (CV) experiment in
dichloromethane (Figure 3a) solutions. Compounds 1-5
displayed only reversible one electron oxidation potentials in
support of their electron rich/donor nature. The HOMO energy
levels calculated from the differential pulse voltammogram
(DPV) are -5.76 (1), -5.82 (2), -5.85 (3), -5.80 (4), and -5.79 (5)
eV, and the LUMO energy levels were derived from the
corresponding optical band-gap (Eg) values and the HOMO
energy levels (Table 1). The HOMO energy levels of 2-5 are
slightly deeper (ca. 0.16‒0.25 eV) than those of the previously
reported thiophene/selenophene-fused fluorenes (-5.60 to -5.75
eV),[17] besides lowering of the LUMO energy levels (ca. 0.5‒0.7
eV), which showcases the effect of extended π-conjugation as
well as the air-stability gained by these systems.[39] We
postulated that the donor ability (better oxidizing behaviour)
arising from the delocalized π* excited state would facilitate
exciton delocalization (lifetime) which in turn might increase the
electrostatic interaction between the donor fluorophore and
electron deficient nitroaromatic analyte. In addition, the well-
positioned LUMO energy levels of the fluorophores may facilitate
photo-induced electron transfer (PET) to the electron poor
aromatic analytes (Figure 3b).
Various electron-rich PAHs have been attempted for the
detection of nitroaromatic compounds.[5,6-11,40] The highly
sensitive and explosive nature, and very low vapour pressure
(5.8 × 109 Torr at 25 C)[40b] of PA, when compared to other
nitrophenols and TNT, especially, make it an important and
challenging candidate for detection which warrants development
of efficient sensors. As mentioned earlier, fluorenes and their
analogues have not been tested as sensors for nitroaromatics
(NAs), since most of them emit in the UV-region. The excellent
fluorescent properties (as evidenced by very high PLQEs) as
well as the improved donor behaviour (as evidenced by their
oxidation curves in cyclic voltammetry) exhibited by the new set
of structurally modified fluorenes 1-5, herein prompted us to
explore their functional utility further as sensors for the detection
of electron-deficient nitroaromatics (explosives), such as, picric
acid (PA), 2,4-dinitrophenol (DNP), p-nitrophenol (PNP), phenol
(P), 2,4-dinitrotoluene (DNT), p-nitrotoluene (PNT), and
nitrobenzene (NB). The quenching experiments were conducted
on 2D fluorene 2 and 3D fluorenes 4, as representative cases.
As expected, titration experiments of 2D fluorene 2 and 3D
fluorene 4 with all the nitroaromatics exhibited fluorescence
quenching to a greater extent without change in emission profile
and energy (SI). The quenching of fluorescence was found to be
negligible for other electron deficient aromatics, such as,
dichlorobenzene (CB), benzoic acid (BA), etc., and phenol (see,
SI). Interestingly, quenching was found to be significant in the
case of p-quinone (Q). The Figure 4a typically reveals a
decrease in fluorescence intensity of 2 (1 × 10-6 M, toluene)
upon addition of varying amounts of PA (1 mM) in toluene
solution; about 74% PL quenching was observed upon addition
of 200 equivalents of PA. Under the above conditions, the spiro-
configured 4 revealed a similar PL quenching behaviour (ca.
72%), but 5 revealed a marginal increase in its response (ca.
79%). The quenching of fluorescence in both the cases were
easily detected by a naked eye upon UV light (λexc = 365 nm)
illumination.
The Stern-Volmer (SV) curves were generated by plotting the
change in fluorescence intensity of the fluorophore against
concentration of the quenchers added.[41] Indeed, in all cases, at
lower concentrations (0.15–0.20 mM), the SV plots were found
to be linear, but adapted a hyperbolic behaviour at higher
concentration of the quenchers. This suggests an involvement of
both static and dynamic quenching behaviour at higher
concentration of the analytes.[42] Hence, the Stern-Volmer
constants (Ksv) for compounds 2, 4 and 5 were derived from the
linear regime (up to ca. 0.2 mM) and were found to be 9.59 ×
103 M-1, 11.9 × 103 M-1 and 12.6 × 103 M-1, respectively, for PA
(Figure 4b). These values suggest that the sensitivity of 5 is
relatively better, and they are in good agreement with other
PAHs carrying no substituents reported in the literature (see
comparison chart SI). The relative fluorescence quenching
order based on the Stern-Volmer constants derived from PL
quenching experiments and upon addition of 120 equivalents
(for example) of various electron deficient aromatics was found
to be: PA>DNP>Q>PNP>DNT>PNT>NB in 2 as well as 4 (cf. SI).
Within a class, it appears that the quenching efficiency increases
with the number of nitro groups. It may be readily noticeable that
PA has greater quenching efficiency when compared to other
NAs; it is ca. 4.5 times higher when compared to that of
nitrobenzene. We also found that these larger fluorophores (2
a)
b)
1
3
5
4
3
↑
LUMO
-2.65 eV
DNT
-3.39 eV
2
PA
1
-4.31 eV
2
4
0
↑
HOMO
-1
0.0
0.4
0.8
1.2
1.6
Voltage, V
Figure 3. a) Cyclic voltammograms of 1, 3 and 5 recorded in dichloromethane
solution. b) Possibility of photoinduced electron transfer in 2 and 4 upon
titration with DNT and PA.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
and 4) are selective to nitrophenols when compared to
nitrotoluenes; that is, DNP is 3.5 times effective in quenching the
fluorescence when compared to DNT. The kq values derived
from the Stern-Volmer equation (τ (2/4) = 2.57 ns, τ (5) = 2.72
ns) for 2, 4 and 5 in case of PA, are found to be 3.73 × 1012 M-1s-
1, 4.63 × 1012 M-1s-1 and 4.66 × 1012 M-1s-1, respectively. These
values are two order of magnitude higher than those of the
typical diffusion-controlled quenching process (ca. 1010 M-1s-1),
and hence we attribute this quenching to a static quenching
process.[8,9,43] We believe that such high values of kq resulting in
a static mechanism, is originated from the introduction of larger
π-system for extended electron (or exciton) delocalization. To
confirm this static behaviour, we varied the quencher
concentration in the fluorophore solution (both 2 and 4) and
measured the corresponding fluorescence lifetimes. A straight
line with no variation in lifetime (τ0/τ = 1) was observed,
confirming that it follows a typical static quenching mechanism
(Figure 4b).[43] This conveys probably a complex (donor-acceptor
type) formation between the electron poor PA and the electron
rich fluorophores in the ground-state. The absorption spectrum
recorded for the fluorophores with varying concentration of the
quencher (PA) clearly indicated a tailing effect without change in
the absorption spectrum supporting the formation of a non-
fluorescent ground-state complex (SI).[42c,43a] A slight increment
in kq values in the cases of 4 and 5 supports enhanced ground
state interaction for complex formation possibly through its more
contact area (3D). Additionally, fluorescence titrations of 2 and 4
with addition of varying amounts of PA at various excitation
wavelengths (320, 342, 360 and 385 nm) were conducted to
check the masking effect ("inner-filter effect") of PA (as PA and
2/4 have absorption in the same range of the absorption
spectrum). Though the emission intensities observed at various
excitation wavelengths were different, their emission profile and
stabilized due to π-electron delocalization and are very well
positioned with the LUMO values of DNT (-3.39 eV), PA (-4.31
eV), etc. for an efficient photo-induced electron transfer (PET) to
occur from the electron-rich fluorescent sensor to electron-
deficient NACs (Figure 3b). The thermodynamic driving force for
this process is calculated to be ca. -1.66 eV and -0.74 eV for PA
and DNT, respectively. The observed exergonicity (ΔGPET
=
negative) in the above process, in combination with the high
quenching rate constants (kq) support the existence of PET
mechanism. Moreover, this corroboration is further confirmed by
non-overlapping emission spectrum of the fluorophore and the
absorption spectrum of PA (see, SI). Thus, PL quenching with
various nitrophenols/nitrotoluenes observed here could be
attributed to the donor-acceptor-type charge transfer interaction
between the electron rich fluorene and the electron deficient
nitroaromatics and not due to the energy transfer mechanism. 1H
NMR spectrum of
a mixture of 4 and PA (1:50 equiv.,
respectively) in CDCl3 displayed a slight upfield shift for the
protons of PA and 4, consistent with the π-π interaction,
attesting to a typical charge-transfer interaction (SI).
Another important criterion for sensing of analytes is the limit of
detection (LOD). The detection limit of these fluorophores (2 and
4) for sensing PA is calculated by plotting the change in
fluorescence intensity against [PA]. By curve fitting procedure,
LOD can be obtained using the formula: 3 × SD/m; where SD
stands for standard deviation and m for the slope value of the
calibration curve.[43a] LOD calculated for compounds 2 and 4 is
3.8 ppm, and for 5 is 1.8 ppm. The above set of sensing
experiments suggests better sensitivity/performance for a spiro-
fluorophore (5, 3D) when compared to the planar derivative 2
(2D), probably because of its possibility to offer large-area (3D)
multi-modal interactions (via π-π) with the electron poor
aromatics. Fluorescence titrations of the fluorophores 2 and 4
with picric acid (of several dilutions, 10-2 to 10-8 M) also clearly
suggests that these π-expanded fluorophores have the ability to
detect as low concentration of PA as 0.2 ppb or 0.2 ng/mL,
which is in comparison to the triptycene-based nanographene
sensors.[11] It is interesting to notice a marginal preference
shown by 3D skeletons in the sensing of nitroaromatics in
solution, within this unique set of π-expanded fluorenes.
Visual sensing of nitroaromatic explosive vapors using a solid-
matrix is so important that it can have immediate application in
military/defence.[44] As these fluorophores 1-5 show excellent PL
in ps films, sensing properties (NAs) in solution, we investigated
the vapour phase detection of NAs by monitoring the
fluorescence quenching behaviour in films of approximately
same thickness. The spin-coated ps films of compounds 2
(average film thickness ca. 1.60 μm), 4 (average film thickness
ca. 1.56 μm) and 5 (average film thickness ca. 1.58 μm) were
exposed to the saturated vapours of PNT (due to its high vapour
pressure at room temperature, 4.89 × 10-2 Torr at 25 C)[40b] for 5
min. The films of the fluorophores 2, 4 and 5 experienced
quenching of fluorescence by PNT vapours to a greater extent
(up to 87% in 2, 90% in 4 and 94% in 5, respectively, Figure 5a)
when compared to that in solution, where contribution of solvent
effects and interactions are pivotal. We evaluated the quenching
performance of the films of 2 by varying the film thickness. A
thinner film of 2 (ca. 1.40 μm) exhibited fluorescence quenching
up to 98%, while its thicker film (ca. 3.0 μm) exhibited
10
a) 20
b)
9
8
7
6
5
4
3
2
1
0
L
2.0
0 μL
1.5
1.0
10
500 L
500 μL
0
0.10
0
350
0.00
0.05
[ PA] mM
400
450
500
Wavelength (nm)
Figure 4. a) Changes in fluorescence emission intensity of 2 upon addition of
varying amounts of PA. Inset photographs (top) show the fluorescence
observed at the initial (0 μL, under room light and UV light illumination) and
final (500 μL) stages of the titration with PA (under UV light illumination). b)
Stern-Volmer plot from the changes in fluorescence intensity (black dots) and
the changes in lifetime (blue dots) during the titration of PA with 2.
percentage quenching was found to remain unaltered. Based on
this result, we rule out the possibility of fluorescence quenching
due to masking by PA.[42c,43b]
To further obtain insights into the quenching mechanism, the
LUMO level and absorption spectrum of the quencher were
compared with the LUMO level and emission spectrum of the
fluorophore, respectively. From the electrochemical data, it is
clear that the LUMO values of the π-expanded fluorene
fluorophores 2 (-2.65 eV), 4 (-2.65 eV) and 5 (-2.66 eV) are now
fluorescence quenching up to 64%.
A similar trend was
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
observed for the films of 4 and 5 as well. These results definitely
point to a direct correlation between performance variation and
film thicknesses. It has been known from several reports that
thinner the film, better the sensitivity is, which is in line with our
observation.[29a,45] To our excitement, when these quenched
films were exposed to saturated hexane vapours for a period of
5 min, the fluorescence was regenerated effectively (84%
recovery in case of 2, 67% recovery in case of 4, and 64%
recovery in case of 5). The lower recovery observed in the cases
of films of fluorenes 4 and 5, when compared to 2, may possibly
be attributed to the differences in their molecular structure (or
shape, 2D vs 3D) and in their abilities for multi-modal interaction.
Indeed, a larger contact area (3D spiro-type, orthogonal π-
expanded planes in 4 and 5) that provides better platform for
multiple donor-acceptor interactions for binding could probably
be the reason for the slow recovery observed for the films of 4
and 5. The above method of regeneration of fluorescence
(visible to naked-eye) by exposing the film to solvent vapours,
instead of dipping in a solvent, is beneficial as it prevents
contamination of solvents via dipping process. To see, if the
fluorescence regeneration can be quantitative, the films (2/4/5)
after exposing to PNT vapours (quenched) were dipped into
hexane, and the fluorescence was regenerated very efficiently
(ca. 99%). Thus, nitroaromatics can be detected reversibly using
these films, in multiple cycles, which is one of the prime criteria
for recyclability.
oxidative cyclization, without the need for blocking groups.
Through π-expansion, we have improved the absorption
characteristics (ε) by shifting the absorption maximum close to
the visible region. Importantly, the π-expansion of fluorene
moves the emission into the blue region, but does not affect the
PLQEs in solution (near unity); PLQE of polystyrene films of
spirocompound 5 reached ca. 0.90, which is the highest for any
compound in fluorene/spirobifluorene series. The rigidity,
coplanarity/spiro-linkage and the larger π-system in 1-5 lead to
reduction in bandgap, deepened HOMOs, and high thermal
stabilities (up to 540 °C). The synthesized compounds reported
herein constitute a new set of π-expanded fluorenes, in that, to
our excitement, 3D skeletons reveal
a
slightly better
performance in the sensing behavior, and detect nitroaromatic
compounds (PA) at a ppb level. Besides understanding the
regioselectivity in fluorenes, we are presently exploring the
potential of these bigger and brighter fluorenes in organic
electronics, due to their suitable and advantageous
characteristics when compared to their high molecular weight
oligofluorene/spirobifluorene counterparts. The π-enlarged
fluorene/spirobifluorene skeletons reported here may find
potential application in sensory devices for homeland security,
military operations, etc. due to their high sensitivity and ease of
synthesis.
Experimental Section
5x105
a)
b)
4x105
The detailed procedure for the synthesis of 2D fluorenes (1-3), 3D
spirobifluorenes (4 and 5), their intermediates (1k-4k and 1p-4p) and
their complete characterization details, and other photophysical, thermal
and electrochemical characterization and single crystal X-ray diffraction
details were provided in the supplementary information. Single crystal X-
ray diffraction structure of compound 3 can be obtained from CCDC, No:
1505272.
hexane
PNT
3x105
2x105
vapours
5 min
vapours
5 min
c)
1x105
0
400
450
500
550
Wavelength (nm)
Acknowledgements
Figure 5. a) Fluorescence spectrum recorded for ps film of 2 before (blue line)
and after (red line) exposure to PNT vapours (for 5 min). Inset photographs
show blue emission of ps films of 2 and its PL quenching upon exposure to
PNT vapors (5 min) under UV-light illumination. Regeneration of fluorescence
is noticed upon exposing the films to hexane vapours (5 min). b) Complete PL
quenching of the bright blue circle (left, obtained from 2) on TLC plate by dip-
coating PA solution (right). c) Complete PL quenching of the scripts (IITM)
written with solution 2 (top) on TLC plate by overwriting with dinitrotoluene
DNT solution (bottom, left).
P.V.K is thankful to DST-SERB, CSIR and IITM India for funding,
and R.J. to CSIR for a fellowship. We thank Profs. A. K. Mishra
and E. Prasad for photophysical measurements and fruitful
discussions, Dr. R. Jagan for X-ray structural analysis, Prof.
Somnath Chanda Roy of Department of Physics for thermal
analyses, and Department of Chemistry, IIT Madras for the
facilities.
The detection of nitroaromatics were also tested on a solid-
support, such as, silica-gel TLC plate. Initially, the fluorophore
was adsorbed on the silica-gel surface of TLC plate by dip-
Keywords: π-Expansion • Fluorenes • Photoluminescence •
Nitroaromatics • Sensing
painting the fluorophore solution using
a
paint-brush
[1]
a) Organic Light Emitting Devices. Synthesis, Properties and
Applications (Eds.: K. Müllen and U. Scherf), Wiley-VCH: Weinheim,
2006; b) C. Zhang, P. Chen, W. Hu, Small 2016, 12, 1252; c) S. B.
Meier, D. Tordera, A. Pertegás, C.-R. Carmona, E. Ortí, H. J. Bolink,
Mater. Today 2014, 17, 217; d) I. D. W. Samuel, G. A. Turnbull, Chem.
Rev. 2007, 107, 1272; e) S. W. Thomas Iii, G. D. Joly, T. M. Swager,
Chem. Rev. 2007, 107, 1339; f) J.-L. Li, H.-C. Bao, X.-L. Hou, L. Sun,
X.-G. Wang, M. Gu, Angew. Chem. Int. Ed. 2012, 52, 1830.
(photographs, Figures 5b,c). When the quencher solution
(DNT/PA) was repainted/dip-coated over the fluorophore traces
on TLC, the fluorescence was quenched fully very immediately.
This quenching of fluorescence can be immediately detectable
by the naked eye under UV-light (λexc = 365 nm) illumination.
The fluorescence quenching mechanism in films/solid-supports
of 2/4/5 may be viewed as an adsorption of NACs on the surface
of the solid matrix via facile donor-acceptor interactions.
[2]
[3]
M. Shimizu, T. Hiyama, Chem. Asian J. 2010, 5, 1516.
R. Dabestani, I. N. Ivanov, Photochem. Photobiol. 1999, 70, 10.
In summary, we have successfully demonstrated a strategy for
expansion of the π-system of fluorenes via DDQ-mediated
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
[4]
a) Y. Y. Long, H. B. Chen, Y. Yang, H. M. Wang, Y. F. Yang, N. Li, K. A.
Li, J. Pei, F. Liu, Macromolecules 2009, 42, 6501; b) H. H. Nguyen, X.
Li, N. Wang, Z. Y. Wang, J. Ma, W. J. Bock, D. Ma, Macromolecules
2009, 42, 921; c) S. W. Thomas, J. P. Amara, R. E. Bjork, T. M. Swager,
Chem. Commun. 2005, 4572; d) P. Marks, S. Cohen, M. Levine, J.
Polym. Sci., Part A: Polym. Chem. 2013, 51, 4150.
Kaner, Chem. Sci. 2015, 6, 7190; e) X.-G. Li, Y. Liao, M.-R. Huang, R.
B. Kaner, Chem. Sci. 2015, 6, 2087.
[24] B. T. King, J. Kroulík, C. R. Robertson, P. Rempala, C. L. Hilton, J. D.
Korinek, L. M. Gortari, J. Org. Chem. 2007, 72, 2279.
[25] M. L. Tang, Z. Bao, Chem. Mater. 2011, 23, 446.
[26] a) L. Zhai, R. Shukla, R. Rathore, Org. Lett., 2009, 11, 3474; b) L. Zhai,
R. Shukla, S. H. Wadumethrige, R. Rathore, J. Org. Chem. 2010, 75,
4748.
[5]
For small molecule-based approaches via self-assembly, see: (a) K. K.
Kartha, A. Sandeep, V. C. Nair, M. Takeuchi, A. Ajayaghosh, Phys.
Chem. Chem. Phys. 2014, 16, 18896; (b) T. Naddo, Y. Che, W. Zhang,
K. Balakrishnan, X. Yang, M. Yen, J. Zhao, J. S. Moore, L. Zang, J. Am.
Chem. Soc. 2007, 129, 6978; (c) K. K. Kartha, S. S. Babu, S.
Srinivasan, A. Ajayaghosh, J. Am. Chem. Soc. 2012, 134, 4834; (d) A.
Sandeep, V. K. Praveen, K. K. Kartha, V. Karunakaran, A. Ajayaghosh,
Chem. Sci. 2016, 7, 4460; (e) K. K. Kartha, A. Sandeep, V. K. Praveen,
A. Ajayaghosh, Chem. Rec. 2015, 15, 252; (f) N. Dey, S. K. Samanta,
S. Bhattacharya, ACS Appl. Mater. Interfaces, 2013, 5, 8394.
[27] J. B. Birks, Photophysics of Aromatic Molecules; Wiley: New York,
1970.
[28] A. Bree, R. Zwarich, J. Chem. Phys. 1969, 51, 903.
[29] a) G. B. Demirel, B. Daglarac, M. Bayindir, Chem. Commun. 2013, 49,
6140; b) P. Beyazkilic, A. Yildirim, M. Bayindir, ACS Appl. Mater.
Interfaces. 2014, 6, 4997; c) Y. Wang, A. La, Y. Ding, Y. Liu, Y. Lei,
Adv. Funct. Mater. 2012, 22, 3547.
[30] W. H. Melhuish, J. Phys. Chem. 1961, 65, 229.
[6]
[7]
a) J. V. Goodpaster, V. L. McGuffin, Anal. Chem. 2001, 73, 2004; b) K.
S. Focsaneanu, J. C. Scaiano, Photochem. Photobiol. Sci. 2005, 4,
817; c) P. Singla, P. Kaur, K. Singh, Tetrahedron Lett. 2015, 56, 2311.
a) H. Wang, X. Xu, A. Kojtari, H. F. Ji, J. Phys. Chem. C 2011, 115,
20091; b) Y. Z. Liao, V. Strong, Y. Wang, X. G. Li, X. Wang, R. B.
Kaner, Adv. Funct. Mater. 2012, 22, 726; c) X. G. Li, Y. Liao, M. R.
Huang, V. Strong, R. B. Kaner, Chem. Sci. 2013, 4, 1970.
[31] a) T. Serevičius, R. Komskis, P. Adomėnas, O. Adomėnienė, V.
Jankauskas, A. Gruodis, K. Kazlauskas, S. Juršėnas, Phys. Chem.
Chem. Phys. 2014, 16, 7089; b) M. P. Aldred, C. Li, G.-F. Zhang, W.-L.
Gong, A. D. Q. Li, Y. Dai, D. Ma, M.-Q. Zhu, J. Mater. Chem. 2012, 22,
7515.
[32] J. Luo, Y. Zhou, Q.-Z. Niu, Q.-F. Zhou, Y. Ma, J. Pei, J. Am. Chem. Soc.
2007, 129, 11314.
[8]
[9]
P. Sam-ang, D. Raksasorn, M. Sukwattanasinitt, P. Rashatasakhon,
RSC Adv. 2014, 4, 58077.
[33] For example: a) X. H. Zhang, M. W. Liu, O. Y. Wong, C. S. Lee, H. L.
Kwong, S. T. Lee, K. S. Wu, Chem. Phys. Lett. 2003, 369, 478; b) O. K.
Cheon, J. Shinar, Appl. Phys. Lett. 2002, 81, 1738.
N. Venkatramaiah, S. Kumar, S. Patil, Chem. Commun. 2012, 48, 5007.
[10] a) G. V. Zyryanov, M. A. Palacios, P. Anzenbacher, Org. Lett. 2008, 10,
3681; b) P. Anzenbacher, L. Mosca, M. A. Palacios, G. V. Zyryanov, P.
Koutnik, Chem. Eur. J. 2012, 18, 12712.
[34] T. P. I. Saragi, T. Spehr, A. Siebert, T. Fuhrmann-Lieker, J. Salbeck,
Chem. Rev. 2007, 107, 1011.
[35] W. R. Dawson, W. M. Windsor, J. Phys. Chem. 1968, 72, 3251.
[36] For other designs of ours that impede self-quenching, see: a) J. N.
Moorthy, P. Venkatakrishnan, P. Natarajan, D. F. Huang, T. J. Chow, J.
Am. Chem. Soc. 2008, 130, 17320; b) J. N. Moorthy, P.
Venkatakrishnan, P. Natarajan, D. F. Huang, T. J. Chow, Org. Lett.,
2007, 9, 5215; c) J. N. Moorthy, P. Venkatakrishnan, P. Natarajan, D. F.
Huang, T. J. Chow, Chem. Commun. 2008, 2146; d) J. N. Moorthy, P.
Venkatakrishnan, P. Natarajan, Z. Lin, T. J. Chow, J. Org. Chem. 2010,
75, 2599.
[11] P.-C. Zhu, L.-N. Luo, P.-Q. Cen, J.-T. Li, C. Zhang, Tetrahedron Lett.
2014, 55, 6277.
[12] For example: a) S. H. Lee, T. Nakamura, T. Tsutsui, Org. Lett. 2001, 3,
2005; b) S. Beaupre, P. L. Boudreault, M. Leclerec, Adv. Mater. 2010,
22, E6.
[13] Annulation at the 3,4 and 5,6-faces of fluorene leading to a helicene
has been reported very recently, see: a) O. Hiromi, A. Midori, N. Koji, N.
Masanobu, N. Kazuyuki, N. Kyoko, Org. Lett. 2016, 18, 3654; b) H.
Oyama, K. Nakano, T. Harada, R. Kuroda, M. Naito, K. Nobusawa, K.
Nozaki, Org. Lett. 2013, 15, 2104; For helical bitriphenylenes: c) Y.
Sawada, S. Furumi, A. Takai, M. Takeuchi, K. Noguchi, K. Tanaka, J.
Am. Chem. Soc. 2012, 134, 4080.
[37] a) M. S. Kwon, Y. Yu, C. Coburn, A. W. Phillips, K. Chung, A. Shanker,
J. Jung, G. Kim, K. Pipe, S. R. Forrest, J. H. Youk, J. Gierschner, J.
Kim, Nature Commun. 2015, 6, 8947; b) J. L. Banal, J. M. White, K. P.
Ghiggino, W. W. H. Wong, Scientific Reports 2014, 4, 4635.
[38] Excited States in Photochemistry of Organic Molecules (Eds.: M.
Klessinger, J. Michl), VCH: New York, 1995.
[14] J. Wu, W. Pisula, K. Mullen, Chem. Rev. 2007, 107, 718.
[15] J. Wu, M. D. Watson, N. Tchebotareva, Z. Wang, K. Müllen, J. Org.
Chem. 2004, 69, 8194.
[39] F. Valiyev, W.-S. Hu, H.-Y. Chen, M.-Y. Kuo, I. Chao, Y. T. Tao, Chem.
Mater. 2007, 19, 3018.
[16] D. Xia, T. Marszalek, M. Li, X. Guo, M. Baumgarten, W. Pisula, K.
Müllen, Org. Lett. 2015, 17, 3074.
[40] a) S. Shanmugaraju, P. S. Mukherjee, Chem. Commun. 2015, 51,
16014; b) X. Sun, Y. Wang, Y. Lei, Chem. Soc. Rev. 2015, 44, 8019.
[41] V. O. Stern, M. Volmer, Physik. Zeitschr. 1919, 20, 183.
[42] a) J. R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer,
3rd edn, 2010; b) D. Zhao, T. M. Swager, Macromolecules 2005, 38,
9377; c) V. Bhalla, A. Gupta, M. Kumar, D. S. Rao, S. K. Prasad, Appl.
Mater. Interfaces 2013, 5, 672; d) J. Wang, J. Mei, W. Z. Yuan, P. Liu,
A. J. Qin, J. Z. Sun, Y. G. Ma, B. Z. J. Tang, Mater. Chem. 2011, 21,
4056; e) J.-F. Xiong, J.-X. Li, G.-Z. Mo, J.-P. Huo, J.-Y. Liu, X.-Y. Chen,
Z.-Y. Wang, J. Org. Chem. 2014, 79, 11619.
[17] C.-H. Lee, Y.-Y. Lai, S.-W. Cheng, Y.-J. Cheng, Org. Lett. 2014, 16,
936.
[18] S. Kumar, D. Karthik, K. R. J. Thomas, M. S. Hundal, Tetrahedron Lett.
2014, 55, 1931.
[19] a) G. Xu, L. Ping, Y. Ma, Polymer 2014, 55, 3083. b) K. T. Wong, Z. J.
Wang, Y. Y. Chien, C.-L. Wang, Org. Lett. 2001, 3, 2285; c) F. Moreau,
N. Audebrand, C. Poriel, M.-B. Virginie, J. Ouvry, J. Mater. Chem.,
2011, 21, 18715.
[20] M. Grzybowski, K. Skonieczny, H. Butenschön, D. T. Gryko, Angew.
Chem. Int. Ed. 2013, 52, 9900.
[43] a) N. Venkatesan, V. Singh, P. Rajakumar, A. K. Mishra, RSC Adv.
2014, 4, 53484; b) V. Vij, V. Bhalla, M. Kumar, Appl. Mater. Interfaces,
2013, 5, 5373; c) V. Bhalla, H. Arora, H. Singh, M. Kumar, Dalton Trans.
2013, 42, 969.
[21] A. A. O. Sarhan, C. Bolm, Chem. Soc. Rev. 2009, 38, 2730.
[22] For example: a) Y. Zhou, W.-J. Liu, W. Zhang, X.-Y. Cao, Q.-F. Zhou, Y.
Ma, J. Pei, J. Org. Chem. 2006, 71, 6822; b) A. Pradhan, P.
Dechambenoit, H. Bock, F. Durola, Angew. Chem. Int. Ed. 2011, 50,
12582; c) M. Danz, R. Tonner, G. Hilt, Chem. Commun. 2012, 48, 377.
[23] a) F. Dötz, J. D. Brand, S. Ito, L. Gherghel, K. Müllen, J. Am. Chem.
Soc. 2000, 122, 7707; b) C. D. Simpson, J. D. Brand, A. J. Berresheim,
L. Przybilla, H. J. Rader, K. Müllen, Chem. Eur. J. 2002, 8, 1424; c) X.-
G. Li, Y.-W. Liu, M.-R. Huang, S. Peng, L.-Z. Gong, M. G. Moloney,
Chem. Eur. J. 2010, 16, 4803; d) X.-G. Li, Y. Liao, M.-R. Huang, R. B.
[44] a) S. Shanmugaraju, S. A. Joshi, P. S. Mukherjee, J. Mater. Chem.
2011, 21, 9130; For reviews, see: b) R. Miao, J. Peng, Y. Fang, Mol.
Syst. Des. Eng. DOI: 10.1039/c6me00039h; c) W. Guan, W. Zhou, J.
Lu, C. Lu, Chem. Soc. Rev. 2015, 44, 6981.
[45] J.-S. Yang, T. M. Swager, J. Am. Chem. Soc. 1998, 120, 11864.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.

10.1002/asia.201601359
Chemistry - An Asian Journal
COMMUNICATION
Entry for the Table of Contents (Please choose one layout)
Layout 1:
COMMUNICATION
Text for Table of Contents
Author(s), Corresponding Author(s)*
Page No. – Page No.
Title
((Insert TOC Graphic here))
Layout 2:
COMMUNICATION
Ramakrishna Jagarapu,
Venkatakrishnan Parthasarathy*
Page No. – Page No.
Bigger and Brighter Fluorenes: Facile
π-Expansion, Brilliant Emission and
Sensing of Nitroaromatics
A bigger glow makes sense: Highly emissive π-expanded fluorenes as well as
spirobifluorenes obtained via a regioselective oxidative cyclization procedure were
shown to sense nitroaromatics at a ppb level.
For internal use, please do not delete. Submitted_Manuscript
This article is protected by copyright. All rights reserved.