Synthesis, chemicaleoptical characterization and solvent interaction effect of novel fluorene-chromophores with DeAeD structure

Article abstract of DOI:10.1016/j.dyepig.2012.12.029

The synthesis, chemical characterization and optical studies of three novel fluorene derivatives is reported. These compounds comprise a DeAeD architecture with fluorene moieties as donor groups and fluorenone or benzothiadiazole derivatives as acceptor groups. Theoretical analysis confirmed the existence and the nature of two principal electronic transitions (π → π* and intramolecular charge transfer). Spectroscopic studies in solution revealed that the intramolecular charge transfer character, and in turn the two-photon activity i.e., the fluorescence induced by two-photon absorption, is strongly affected by solvent polarity. The influence of specific solventesolute interactions over emission properties was also studied through LipperteMataga plot. Evaluation of the two-photon absorption cross-sections, gave a maximum value of 105 GM (1 GM = 10^-50 cm⁴ s) in toluene and a minimum value of 23 GM in THF solutions at 750 nm for the fluorenone derivative, a molecule with poor intramolecular charge transfer character and thus weak two-photon absorption; however the benzothiadiazole derivative, with stronger intramolecular charge transfer transition, produced a maximum value of 1000 GM in THF and a minimum value of 236 GM in methanol. Fluorescence quantum efficiency of these compounds was also affected by the medium, with fluorescence quenching in protic solvents such as methanol due to specific solvent interactions (i.e., hydrogen-bonding). Nevertheless, in a more polar medium such as water, nanoaggregates synthesized from the benzothiadiazole derivative exhibited good two-photon activity, i.e., w500 GM and fluorescence quantum efficiency of 0.83. Furthermore, these nanoaggregates exhibited more resistance against photodegradation processes than any of the organic solutions tested.

Full text of DOI:10.1016/j.dyepig.2012.12.029

Dyes and Pigments 98 (2013) 31e41
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, chemicaleoptical characterization and solvent interaction
effect of novel fluorene-chromophores with DeAeD structure
,
b
Jesús Rodríguez-Romero a, Laura Aparicio-Ixta b, Mario Rodríguez b , Gabriel Ramos-Ortíz ,
*
José Luis Maldonado b, Arturo Jiménez-Sánchez a, Norberto Farfán c, Rosa Santillan a
a Departamento de Química, Centro de Investigación y de Estudios Avanzados del IPN, 07000, Apdo. Postal. 14-740, México D.F., Mexico
,
*
b Centro de Investigaciones en Óptica A.P. 1-948, 37000 León, Gto., Mexico
c Facultad de Química, Departamento de Química Orgánica, Universidad Nacional Autónoma de México, 04510, México, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis, chemical characterization and optical studies of three novel fluorene derivatives is
reported. These compounds comprise a DeAeD architecture with fluorene moieties as donor groups and
fluorenone or benzothiadiazole derivatives as acceptor groups. Theoretical analysis confirmed the
Received 15 October 2012
Received in revised form
22 December 2012
Accepted 27 December 2012
Available online 17 January 2013
existence and the nature of two principal electronic transitions (p / p* and intramolecular charge
transfer). Spectroscopic studies in solution revealed that the intramolecular charge transfer character,
and in turn the two-photon activity i.e., the fluorescence induced by two-photon absorption, is strongly
affected by solvent polarity. The influence of specific solventesolute interactions over emission
properties was also studied through LipperteMataga plot. Evaluation of the two-photon absorption
cross-sections, gave a maximum value of 105 GM (1 GM ¼ 10ꢀ50 cm4 s) in toluene and a minimum value
of 23 GM in THF solutions at 750 nm for the fluorenone derivative, a molecule with poor intramolecular
charge transfer character and thus weak two-photon absorption; however the benzothiadiazole deriv-
ative, with stronger intramolecular charge transfer transition, produced a maximum value of 1000 GM in
THF and a minimum value of 236 GM in methanol. Fluorescence quantum efficiency of these compounds
was also affected by the medium, with fluorescence quenching in protic solvents such as methanol due to
specific solvent interactions (i.e., hydrogen-bonding). Nevertheless, in a more polar medium such as
water, nanoaggregates synthesized from the benzothiadiazole derivative exhibited good two-photon
activity, i.e., w500 GM and fluorescence quantum efficiency of 0.83. Furthermore, these nano-
aggregates exhibited more resistance against photodegradation processes than any of the organic
solutions tested.
Keywords:
Fluorene
Donoreacceptor interactions
Solvatochromism
Intramolecular charge transfer
Two-photon absorption
Organic nanoparticles
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
a DeAeD (donoreacceptoredonor) architecture, with Intra-
molecular Charge Transfer (ICT) process from the edges to the core
of the molecular structure, have shown high TPA response of
quadrupolar origin [8e10]. In these molecular systems, parameters
like planarity and extended conjugation have been investigated
with the aim to improve their ICT character [1], whereas various
types of electronic donors (D) are utilized including triarylamine,
carbazole, and fluorene derivatives; the commonly used electronic
acceptors (A) are benzothiazole, triazine, benzothiadiazole, carba-
zole, coumarins, porphyrins and fluorene derivatives linked to D by
The design and synthesis of organic conjugated molecules with
efficient Two-Photon Absorption (TPA) properties is an area in full
expansion [1,2]. These materials find applications in laser scanning
fluorescent microscopy [3], optical limiting [4] three dimensional
optical data storage [5] and microfabrication [6]. A great variety of
organic chromophores have been prepared to investigate the
structureeproperty relationships that are capable of providing
exceptional two-photon absorption cross-sections (dTPA) values
[1,7,8]. Specially,
p-conjugated organic molecules that incorporate
a
p
bridge [8,11].
A parameter that plays a significant role on the TPA properties
exhibited by organic chromophores is the molecular environment;
this is because in solutions the dipole moments for ground and
excited states are affected by solvent polarity. The effect of solvent
polarity on TPA is not well understood yet, although some
* Corresponding authors.
E-mail addresses: mrodri@cio.mx (M. Rodríguez), rsantill@cinvestav.mx
(R. Santillan).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2012.12.029

32
J. Rodríguez-Romero et al. / Dyes and Pigments 98 (2013) 31e41
theoretical and experimental results show that the solvent effect on
dTPA values for De eD and DeAeD systems displayed a non-
monotonic behavior with respect to solvent polarity [12,13]. In
general, for these -systems the highest dTPA values are obtained
using solvents with low or intermediate polarity, although some-
times the highest dTPA values are observed in polar solvents, as in
the case of dibenzothiophene core-branched structures [14].
with fluorene for the preparation of highly fluorescent green flu-
orophores [30] and two-photon absorption dyes [31].
p
In this work, we report the synthesis of novel fluorescent mol-
ecules with DeAeD architecture based on fluorenes moieties
linked to the acceptors 9-fluorenone and benzothiadiazole rings,
for compounds 5 and 6 respectively (see Scheme 1). In the design of
5 and 6 we chose the fluorene ring as donor group since it exhibits
excellent photon harvesting and good charge transfer properties
toward the acceptor core. We utilized 4 to be a reference material
without an acceptor group and in consequence, with no possible
ICT character. The aim of our work is to study the effect that has the
solventesolute interaction on the ICT character of our compounds
and consequently on their luminescent and non-linear optical
properties. The ICT transition for compounds 4e6 was evaluated by
solvatochromic studies through steady-state absorption and fluo-
rescence spectroscopy. Then, the strength of the third-order optical
nonlinearities that resulted for the ICT character of each compound
was first screened by third-harmonic generation experiments and
subsequently evaluated in a more detailed way by two-photon
absorption studies. Our results demonstrate that the ICT character
of these compounds, and the fluorescence induced by TPA, is
strongly affected by the utilized solvent, without a monotonic trend
with respect to solvent polarity. Such two-photon activity tends to
be significantly quenched with solvents of high polarity and with
the capacity of producing hydrogen-bonding esuch as watere
limiting some of the biomedical applications for these compounds,
i.e., as labels in fluorescence microscopy. To overcome this
p
Electronic
p-systems containing fluorene rings represent an
attractive class of aromatic compounds to build different molecular
structures, including those with a DeAeD arrangement [2]. Fluo-
rene has been largely used as donor in DeA materials due to its
interesting properties which are a consequence of a large HOMOe
LUMO energy gap [15,16]. Moreover, fluorene based systems pos-
sess unique photophysical properties such as high fluorescent
quantum yield, large optical nonlinearities, large photostability,
and excellent hole-transporting properties [17e20]. Due to these
properties fluorene derivatives have been used extensively as
functional materials for organic light-emitting diodes [21], photo-
dynamic therapy photosensitizers [22], solar cells [23] and fluo-
rescence microscopy [24]. On the other hand, with the oxidation of
a fluorene ring it is possible to obtain fluorenone derivatives, which
act as acceptor groups favoring the ICT process [25]. This moiety has
been also employed for tuning the electronic properties of D‒A
chromophores [26] and for photovoltaic applications [27e29].
Another moiety of our current interest to tune the photophysical
properties in DeAeD systems is the 2,1,3- benzothiadiazole ring.
This ring has been widely utilized as acceptor group in combination
Scheme 1. a) 1-Bromooctane, KOH, DMSO; b) Me3SiChCH, Pd(CH3CO2)2, CuI, PPh3, DIPA; c) K2CO3, MeOH/Et2O; d) 2,7-Dibromofluorene, PdCl2(PPh3)2, t-BuNH2/H2O; e) 4,7-
Dibromobenzo[c]-1,2,5-thiadiazole, PdCl2(PPh3)2, t-BuNH2/H2O; f) Cs2CO3, DMSO.

J. Rodríguez-Romero et al. / Dyes and Pigments 98 (2013) 31e41
33
limitation, we prepared an aqueous suspension of nanoparticles
synthesized from 6 through the re-precipitation method. We
demonstrate that our nanoparticles suspended in water exhibit
good two-photon activity (without quenching effects) similar to the
activity of solutions prepared with a solvent of intermediate
polarity.
(t, J ¼ 8.2 Hz, 4H), 1.25e1.04 (m, 20H), 0.81 (t, J ¼ 7.2 Hz, 6H), 0.64e
0.56 (m, 4H) ppm. 13C NMR (125 MHz, CDCl3):
192.4, 151.1, 150.9,
d
143.2, 141.9, 140.3, 137.7, 134.6, 130.8, 127.7, 127.4, 126.9, 126.1, 124.8,
123.0, 120.9, 120.6, 120.1, 119.8, 92.8, 88.7, 55.2, 40.4, 31.8, 30.1, 29.8,
29.3, 23.8, 22.6, 14.1 ppm. IR (nmax/cmꢀ1) 2923, 2848 (CeH
stretching), 1716 (C¼O), 1465 (phenyl nucleus). APCIeMS calcu-
lated for C75H89O [M þ H]þ 1005.6907, found 1005.6907 [M þ H]þ
(ppm error of ꢀ0.0940).
2. Materials and experimental section
2.1. Materials
2.3.3. 4,7-bis((9,9-dioctyl-2-fluorenyl)ethynyl)-2,1,3-
benzothiadiazole (6)
All starting materials were purchased from SigmaeAldrich and
utilized without further purification. The solvents were purified by
distillation over appropriate drying agents.
Compound 3 (0.15 g, 0.37 mmol), 4,7-dibromobenzo[c]-1,2,5-
thiadiazole (0.05 g, 0.17 mmol) and PdCl2(PPh3)2 (9 mg,
0.01 mmol) were dissolved in a 1:1 mixture of t-BuNH2/H2O (6 mL)
and the mixture was stirred at room temperature for 48 h. The
completion of the reaction was verified by TLC. The organic phase
was extracted with ethyl acetate (20 mL) and was washed with
water (3 ꢁ 20 mL). The resulting organic solution was dried over
anhydrous Na2SO4 and the solvent was eliminated under vacuum.
An orange solid was obtained in 50% yield (82 mg) by preparative
silica TLC using hexane:ethyl acetate (96:4) as eluent. M.p. ¼ 67e
2.2. Equipment
1H, 13C and two-dimensional NMR spectra were recorded on
a Jeol ECA þ500 MHz Spectrometer, using deuterated solvents.
Electronic absorption spectra were obtained with a Perkin Elmer
LAMBDA 2S UVeVis spectrophotometer. The solution emission
spectra were measured on a Varian Cary Eclipse fluorescence
spectrometer. Infrared spectra were recorded on an FTeIR Varian
ATR spectrometer. High resolution mass spectra were obtained
with an Agilent G1969A spectrometer.
68 ꢂC. 1H NMR (500 MHz, C6D6):
d
7.88 (d, 4J ¼ 1.0 Hz, 1H), 7.70
(dd, 3J ¼ 7.7 Hz, 4J ¼ 1.0 Hz, 1H), 7.49e7.47 (m, 1H), 7.41 (d,
3J ¼ 7.7 Hz, 1H), 7.30 (s, 1H), 7.18e7.13 (m, 3H), 1.90 (m, 4H), 1.15e
1.09 (m, 4H), 1.07e0.97 (m, 12H), 0.93e0.87 (m, 4H), 0.77 (t,
J ¼ 7.2 Hz, 6H), 0.79e0.71 (m, 4H) ppm. 13C NMR (125 MHz, C6D6):
2.3. Synthesis of compounds 1e6
d 154.7, 151.2, 151.1, 142.4, 140.5, 131.9, 131.3, 127.8, 127.1, 126.3,
122.8, 121.6, 120.3, 120.1, 117.5, 98.8, 86.7, 55.2, 40.4, 31.8, 30.1, 29.4,
29.3, 23.9, 22.7, 14.0 ppm. IR (nmax/cmꢀ1) 2921, 2852 (CeH
stretching), 2200 (CeC triple bond), 1736 (C¼N), 1464 (phenyl).
ESI-MS calculated for C68H85N2S [M þ H]þ 961.6427, found
961.6428 [M þ H]þ (ppm error of 0.0003).
Compounds (1e3) were synthesized according to reported
methodologies [32e34].
2.3.1. 2,7ebis((9,9-dioctyl-2-fluorenyl)ethynyl) fluorene (4)
Compound 3 (0.26 g, 0.63 mmol), 2,7-dibromo fluorene (0.10 g,
0.31 mmol) and PdCl2(PPh3)2 (17 mg, 0.02 mmol) were dissolved in
a 1:1 mixture of t-BuNH2/H2O (6 mL) and the mixture was stirred at
room temperature for 48 h. The completion of the reaction was
verified by TLC. The organic phase was extracted with ethyl acetate
(20 mL) and was washed with water (3 ꢁ 20 mL). The resulting
organic solution was dried over anhydrous Na2SO4 and the solvent
was eliminated under vacuum. The crude product was purified by
silica gel column chromatography eluting with hexane. Compound
4 was obtained as a colorless oil which solidified adding methanol
thereby obtaining a white solid in 15% yield (0.07 g). M.p. ¼ 109e
2.3.4. Fabrication of aqueous suspensions of organic nanoparticles
Nanoparticles from our compounds were prepared by using the
re-precipitation method: small volumes (200e500 mL) of THF so-
lution (5 ꢁ 10ꢀ4 M) were injected into aqueous solution of Cetyl-
trimethylammonium bromide (CTAB) (10 mL) at concentration of
8 ꢁ 10ꢀ4 M under ultrasonic stirring. Molecules began to aggregate
at once, and a colloidal suspension of nanoparticles in water was
obtained. Excess of THF was eliminated by evaporation under
vacuum. The surfactant CTAB was needed to avoid coalescence of
nanoparticles. Special care was taken to eliminate large particles by
110 ꢂC. 1H NMR (500 MHz, CDCl3):
d
7.78e7.77 (m, 2H), 7.71e7.68
filtering the suspensions with a membrane filter with 0.2 mm cutoff.
The resultant samples exhibited high colloidal stability.
(m, 2H), 7.61 (dd, 3J ¼ 7.7 Hz; 4J ¼ 1.5 Hz, 1H), 7.55e7.53 (m, 2H),
7.36e7.31 (m, 3H), 3.95 (s, 1H), 1.98 (t, J ¼ 8.3 Hz, 4H), 1.23e1.05 (m,
20H), 0.82 (t, J ¼ 7 Hz, 6H), 0.69e0.59 (m, 4H) ppm. 13C NMR
2.3.5. Determination of quantum yield
(125 MHz, CDCl3):
d
151.1, 150.8, 143.6, 141.5, 141.2, 140.5, 130.7,
The fluorescence quantum yield (f) for samples was measured
130.6, 128.2, 127.5, 126.9, 126.0, 122.9, 122.0, 121.5, 120.2, 120.0,
119.7, 91.0, 90.1, 55.2, 40.4, 36.6, 31.8, 30.1, 29.3, 23.8, 22.6, 14.1 ppm.
IR (nmax/cmꢀ1) 2923, 2852 (CeH stretching), 1465 (phenyl). APCIe
MS calculated for C75H91 [M þ H]þ 991.7115, found 991.7111
[M þ H]þ (ppm error of ꢀ0.4336).
using the integrating sphere method [35], through one-photon
excitation provided by a diode-laser (375 nm). The fluorescence
quantum yields of samples in solutions were calculated using as
reference Rhodamine 6G (fref ¼ 0.95) dissolved in methanol.
2.3.6. Determination of two-photon cross-sections
2.3.2. 2,7-bis((9,9-dioctyl-2-fluorenyl)ethynyl)-9-fluorenone (5)
Two-photon excitation spectra of molecules dissolved in dif-
ferent solvents (1 ꢁ 10ꢀ5 M) were measured using the two-photon
excited fluorescence technique (TPEF) [36] employing a mode
locked Ti:Sapphire laser (Tsunami Spectra-Physics) as excitation
source. The laser provided pulses of 100 fs with a pulse repetition
frequency of 80 MHz and was tunable in the wavelength range of
750e820 nm. The laser radiation was focused onto a quartz cell of
1 cm of path length (containing the organic solutions or aqueous
suspensions of nanoparticles) using a focal lens (5 cm). To avoid
linear absorption effects, the beam was focused close to the cell
wall. Then the fluorescence emission from solutions of compounds
Compound
4 (47 mg, 0.05 mmol) and Cs2CO3 (46 mg,
0.14 mmol) were dissolved in DMSO (3 mL) and kept at 80 ꢂC
overnight. The completion of the reaction was verified by TLC. The
organic phase was extracted with ethyl acetate (10 mL) and was
washed with water (3 ꢁ 10 mL). The resulting organic solution was
dried over anhydrous Na2SO4 and the solvent was eliminated under
vacuum. An orange solid was obtained in 60% yield (28 mg) by
preparative silica TLC using hexane:ethyl acetate (96:4) as eluent.
M.p. ¼ 98e99 ꢂC. 1H NMR (500 MHz, CDCl3):
d
7.86(d, 4J ¼ 1.0 Hz,
1H), 7.71e7.68 (m, 3H), 7.54e7.52 (m, 3H), 7.36e7.30 (m, 3H), 1.97

34
J. Rodríguez-Romero et al. / Dyes and Pigments 98 (2013) 31e41
was focused and recorded on a spectrometer (Ocean Optics
USB4000). The measurements were performed in a low excitation
intensity regime where the fluorescence signal showed a quadratic
dependence on the intensity of the excitation beam. A methanol
solution of Rhodamine 6G whose cross-sections values have been
completely characterized in the literature [37], was employed as
a reference for calculations. The TPA cross-sections were calcu-
lated at each wavelength according to Eq. (1), where C(Cref) and
n(nref) are the concentration and refraction index of the sam-
ple(reference), respectively, and F(Fref) is the integral of the TPEF
spectrum.
[42], the photostability of compounds 6 in solution and colloidal
suspension was evaluated (vide infra).
3.2. IR spectroscopy
The infrared spectra of 4, 5 and 6 show characteristic bands for
each compound that corroborate the formation of ethynylfluorene
derivatives. For compound 4 only the aliphatic groups near
3000 cmꢀ1 and the aromatic moiety at 1465 cmꢀ1 are observed. For
compound 5 the characteristic band for the carbonyl group appears
at 1716 cmꢀ1. Finally, the spectrum of compound 6 shows the band
at 1736 cmꢀ1 that is attributed to C¼N stretching and the alkyne
stretching observed at 2200 cmꢀ1 (Fig. S4).
fref Cref nref
F
dTPA
¼
dref
(1)
f
C
n Fref
3.3. Linear absorption and fluorescence spectra
Absorption spectra of compounds 4e6 were obtained in differ-
ent solvents such as hexane, toluene, dioxane, tetrahydrofuran,
acetone, acetonitrile and methanol to evaluate the influence of the
polarity of the medium over their photophysical properties. In
particular, we were interested in studying the ICT transition in 5
and 6 (each compound with different donoreacceptor capabilities).
The linear absorption spectra of compounds 4, 5 and 6 are
shown in Fig. 1 and the main characteristics are summarized in
Table 1 (compounds 4 and 5 are insoluble in polar solvents due to
their hydrophobic character). In the absorption spectrum of 4 there
is one intense band in the range 369e374 nm. This band is assigned
2.3.7. Third-harmonic generation (THG) experiments
Non-linear optical properties of our compounds in solid films
were evaluated by the well-known THG Maker fringe technique.
Polystyrene thin films doped with these chromophores (30 %wt)
were deposited over glass substrates using the spin-coating
method. Briefly, the THG experiments consist on focusing infrared
laser pulsed irradiation (nanosecond pulses at a repetition rate of
10 Hz) into the films. Then the intensity of the third-harmonic
beam that emerges from the films is measured and the third-
order non-linear susceptibility is calculated according to THG
Maker fringes approach [38]. For details of our THG Maker fringe
setup see previous work [39].
to a
p / p* electronic transition and its shape is very similar to the
one produced by fluorene [43]. For compound 5, although four
bands are distinguished, our interest was focused on those at 366e
370 nm (
compound 6, a “camel back” spectrum shape is observed with two
strong absorption bands around 319e321 ( *) and 431e
p / p*) and 439e449 nm, respectively. In the case of
3. Results and discussion
p
/ p
3.1. Synthetic strategy
441 nm. According to previous reports [44,45], the intense bands
observed in the range 431e449 nm in 5 and 6 should correspond to
an ICT transition that occurs between the fluorene edge units and
the fluorenone and benzothiadiazole central cores, respectively.
This affirmation is supported by the absence of an absorption band
at visible wavelengths e assigned in 5 and 6 as ICT transition bands e
in the spectrum of compound 4 due to the absence of an acceptor
group, so that ICT is improbable in such compound. Then, the in-
clusion of stronger acceptor groups in 5 and 6 within the electronic
The alkylation reaction of 2-bromofluorene with 1-
bromooctane using KOH as base in DMSO gave 1 in excellent
yield (Scheme 1). Subsequent treatment of compound 1 with
ethynyltrimethylsilane under Sonogashira cross-coupling condi-
tions produced compound 2. The hydrolysis reaction of 2 to obtain
3 was carried out with K2CO3 in a MeOH/Et2O solvent mixture.
Sonogashira cross-coupling reaction of
3
and 2,7-
dibromofluorene or 4,7-dibromobenzo[c]-1,2,5-thiadiazole using
pesystem induces a decrease in the intensity of the p / p* tran-
PdCl2(PPh3)2 as catalyst, in the absence of copper (to prevent
Glasser-type oxidative homocoupling) with solvent mixtures of -
sition (see molar absorption coefficients for compounds 5 and 6
compared with 4 in Table 1) and also leads to the appearance of the
t
BuNH2/water [40] afforded 4 and 6. Compound 5 was obtained in
60% yield by oxidation of the central methylene group in 4 with
Cs2CO3 in DMSO [41]. The structures of compounds 4, 5, and 6 were
established using 1H and 13C NMR spectra and two-dimensional
(COSY, HETCOR and HMBC) experiments. The 13C NMR spectra of
compounds 4 to 6 show characteristic signals. The 13C NMR spectra
of compound 4 (see Fig. S1 of supporting information) shows
a signal at 36.6 ppm that corresponds to the CH2 moiety, whereas
for compound 5 the signal at 192.4 ppm confirms the oxidation of
the methylene moiety to the corresponding keto group (Fig. S2).
The chemical shift for the acetylene fragment signals in 6 (98.8 and
86.7 ppm) shows a large difference with respect to the same signals
in derivatives 4 (91.0 and 90.1 ppm) and 5 (92.8 and 88.7 ppm),
probably due to the more electronegative nature of the benzo-
thiadiazole ring compared to the fluorene/fluorenone core. The
signal at 154.7 ppm was assigned to the C¼N fragment of benzo-
thiadiazole (Fig. S3). The derivatives 4e6 were obtained as stable
solids and they were soluble in common organic solvents. Since
previous reports have provided evidence that some ethy-
nylbenzothiadiazole derivatives are unstable at room conditions
ICT band [46]. Table 1 also shows that the position of the p / p*
electronic transitions in different solvents are practically indepen-
dent of solvent polarity. As for the ICT bands, it is known that they
usually show bathochromic shifts when the solvent polarity is
increased, however, in the case of derivatives 5 and 6 only slight
shifts were recorded with a non-monotonic behavior. In any case, it
is known that the absorption spectra are less sensitive to polarity
changes in comparison with the emission spectra [2,47].
The photoluminescence (PL) spectra for 4, 5 and 6 derivatives
were also obtained in different solvents using 360, 310 and 320 nm
as excitation wavelengths, respectively (see Fig. 2). The corre-
sponding wavelengths of maximum emission, intensity of emission
and Stokes shifts are summarized in Table 2, as well as the quantum
yield values for 6. The well-defined vibronic PL spectrum (blue light
emission) of 4 is typically observed in fluorene derivatives [49]; this
PL spectrum from 4 did not change significantly in different sol-
vents. In contrast, no significant PL generated by fluorene moieties
was observed from 5 and 6. The quenching of the emission of the
fluorene moiety is compatible either with a charge transfer process
from the fluorenes to the acceptor core center [44,50], or with an

J. Rodríguez-Romero et al. / Dyes and Pigments 98 (2013) 31e41
35
Fig. 1. UVeVis normalized absorption spectra of a) 4, b) 5 and c) 6 in different solvents (molar absorption spectra for 6 are shown in Fig. S5). Inset in b) shows an enlargement of the
region where the ICT band appears.
energy transfer process, although the latter is rather related to
bimolecular systems [51]. For derivative 5 a very weak blue emis-
sion was detected (inset of Fig. 2b), this behavior was associated
with an incomplete ICT process. Indeed, as shown in Figs. 1 and 2,
there is an overlap between the emission spectrum of 4 (band
generated by the fluorene moiety) and the ICT band in the ab-
sorption spectra of 5 and 6. Thus, it can be concluded that the
acceptor moieties in 5 and 6 are responsible for the luminescent
processes. For these two compounds, slightly structured emission
bands are observed in hexane solution at 507 and 495 nm,
respectively, but when moderate polar solvents are used, a broad
and unstructured band is recorded. It is noteworthy that the PL
spectra from compound 6 are strongly dependent on the solvent
and show a monotonic bathochromic shift through the solvent
polarity, going from greenish emission in hexane solution to red-
dish in methanol (see Table 2 and Fig. 2c and d).
emission was recorded with the excitation from 320 to 340 nm,
corroborating that the PL properties of compound 6 are due to the
ICT. In general, the wavelength of maximum emission for this de-
rivative shows a monotonic shift effect when the solvent polarity is
increased. This bathochromic shift was accompanied with changes
in the fluorescence quantum yield (see Table 2). Results show
a significant reduction for this photophysical property going from
non-polar aprotic to polar protic solvents. Acetone, acetonitrile and
THF solutions gave quantum yields higher than 0.85, however, in
methanol, there was a drastic PL quenching (
f
¼ 0.3) which was
assigned to hydrogen-bonding interactions between the solvent
and benzothidiazole core [47,52,53]. The quenching was also
observable in mixtures of solvents. For instance, Fig. 3b displays the
PL spectra of compound 6 (at concentration of 6 ꢁ 10ꢀ5 M) in
mixtures of THF-methanol. The data shows that as the ratio of
methanol present in the solution is increased, the emission band
undergoes a bathochromic shift. A reduction of the PL intensity is
also clearly observed. From this, one can infer that in a protic polar
solvent like water, the fluorescent properties of compound 6 would
be also quenched, precluding then its application in areas for which
aqueous solutions of these type of molecular systems are of major
Compound 6 exhibited the most intense PL and significant ICT
character, so that we selected it for additional spectroscopic studies.
Firstly, to verify the ICT character of this molecule we obtained its
excitation spectrum. Fig. 3a presents the PL intensity detected at
511 nm for 6 dissolved in toluene. As expected, the largest green
Table 1
One-photon absorption data for compounds 4e6 obtained in different solvents.
Solvent
P. I.a
Compound 4
Compound 5
Compound 6
lp
/
p* (ε)c
b
lp
/
p* (ε)c
b
lICTb (ε)c
lp
/
p*b (ε)c
lICTb (ε)c
Hexane
Toluene
Dioxane
THF
Acetone
Acetonitrile
Methanol
0.009
0.099
0.164
0.207
0.355
0.460
0.762
369(85,100)
374(84,000)
372(96,000)
372(57,500)
366(7900)
370(8780)
368(7040)
369(8350)
444(820)
449(1040)
439(840)
445(830)
320(43,450)
320(47,600)
320(43,550)
321(51,950)
320(87,200)
319(36,560)
320(25,300)
441(33,000)
441(34,750)
438(31,200)
441(37,500)
435(63,400)
431(23,900)
439(16,700)
a
Polarity Index see ref [48].
b
c
Value expressed in nanometers.
Value expressed in Mꢀ1 cmꢀ1
.

36
J. Rodríguez-Romero et al. / Dyes and Pigments 98 (2013) 31e41
Fig. 2. Emission spectra of a) 4, b) 5 and c) 6 in different solvents; d) photograph of solvatochromism effect for compound 6 under UV lamp excitation: from left to right hexane,
toluene, dioxane, THF, acetone, acetonitrile (ACN) and methanol solutions.
interest, i.e., for fluorescent microscopy. Nevertheless, we demon-
strate in Section 3.6 that in mixtures of THF-water the quantum
yield of 6 can be conserved after molecular aggregation.
determined by chemical properties of the fluorophore and the
solvent, including hydrogen-bonding, preferential solvation and
molecular charge transfer interactions [53]. The LipperteMataga
plot shows the relation between Stokes shift and the solvent po-
larity according to [54]:
3.4. General solvent effects evaluated by LipperteMataga relation
ꢀ
ꢁ
2
In order to explain the effect of the polarity of the medium, and
to determine whether general and/or specific solvent effects [47]
are responsible for the spectral shifts in the emission of com-
pound 6, we applied the LipperteMataga relation. For general type
effects, the fluorophore is considered as a dipole in a continuous
medium of uniform dielectric constant. The general effects are
determined by the electronic polarizability of the solvent; those
effects are described by the refractive index and the molecular
polarizability, which results from reorientation of solvent dipoles.
Molecular polarizability is a function of the static dielectric con-
stant, ε. In contrast, specific solvent effects are interactions
me
ꢀ
mg
2
hc
Dn
¼
yA
ꢀ
yF
¼
ð
D
f Þ
þ C
(2)
a3
where yA and yF are the wavenumbers (in cmꢀ1) of the absorption
and emission bands, respectively, h ¼ 6.6256 ꢁ 10ꢀ27 J s is Planck’s
constant, c ¼ 2.9979 ꢁ 1010 cm/s is the speed of light, a is the
Onsager radius [55], and Dm
¼
me e mg is the dipole moment dif-
ference between the ground and excited states. The solvent polarity
f is defined in terms of the dielectric constant (ε) and the refractive
index (n) of solvent as (Eq. (3)):
D
Table 2
Emission data for derivatives studied in different solvents.
Solvent
Compound 4a
Compound 5b
Compound 6c
lemd (I. E.)e
Stokes shiftf
lemd (I. E.)e
Stokes shiftf
lem (I. E.)e
fg
Stokes shiftf
d
Hexane
Toluene
Dioxane
THF
Acetone
Acetonitrile
Methanol
393(230)
398(180)
397(190)
397(170)
1655
1612
1693
1693
507(370)
529(180)
538(120)
536(160)
7599
8123
8587
8443
495(670)
511(530)
515(530)
525(540)
541(720)
545(440)
582(120)
0.85
0.76
11,048
11,681
11,833
12,105
12,766
12,999
14,068
0.87
1
0.9
0.3
a
Concentration 1.0 ꢁ 10ꢀ5 M. lex ¼ 360 nm. The slit aperture was of 1.5 for excitation and 2.5 for emission.
Concentration 5.2 ꢁ 10ꢀ5 M. lex ¼ 310 nm. The slit aperture was of 2.5 for excitation and emission.
Concentration 5.2 ꢁ 10ꢀ5 M. lex ¼ 320 nm. The slit aperture was of 2.5 for excitation and emission.
Value expressed in nm.
b
c
d
e
f
Intensity emission expressed in arbitrary units.
Value expressed in cmꢀ1
.
g
Rhodamine 6G methanol solution (
f
¼ 0.95) was utilized as reference.

J. Rodríguez-Romero et al. / Dyes and Pigments 98 (2013) 31e41
37
Fig. 3. a) Excitation spectrum from 6 in toluene solution. The PL is detected at 511 nm b) PL spectra of compound 6 in mixtures of THF-methanol. The concentration of 6 in these
mixtures was kept constant (6 ꢁ 10ꢀ5 M). Inset photography of the PL exhibited by the mixtures.
ꢂ
ꢃ
ꢀ
ꢁ
ε ꢀ 1
n2 ꢀ 1
f ¼ f ðεÞ ꢀ f n2
¼
ꢀ
(3)
dipole moment difference between the ground and excited state
D
2n2 þ 1
ꢀ
2ε þ 1
(Dm) to be 15.3 D, taking an Onsager radius of 6.33 A estimated by
a Density Functional Theory (DFT) calculation using Gaussian 03
[58]. In this procedure the tight option was selected for a better
It is known that when LipperteMataga plots exhibit a linear
behavior the general solvent effects are dominant in the spectral
shifts. On the other hand, when Lippert plots are non-linear, the
specific solvent effects govern the spectral shifts and it is regarded
as evidence of specific fluorophoreesolvent interactions. Fig. 4
presents the Stokes shift as a function of the solvent polarity for
compound 6. Clearly, data in this plot deviates from linearity (line
not shown in the figure with slope ¼ 6020 ꢃ 1485, R2 ¼ 0.72), and
a bilinear behavior is observed: i) the first linearity for the non-
ꢀ
computational accuracy. Onsager radii 0.5 A longer than the com-
puted radii are chosen to account for the van der Waals radii of the
surrounding solute molecules. This Dm value is only an approx-
imation, but is comparable with those reported in the literature (3e
20 D) for other solvatochromic fluorophores [59].
3.5. Theoretical studies
polar region (
D
f < 0.03; slope ¼ 38,449 ꢃ 7926, R2 ¼ 0.918) in-
cludes hexane, toluene and dioxane, and ii) the second linearity for
With the purpose of corroborating the origin of the electronic
transitions involved in the studied compounds, we calculated
Frontier Molecular Orbitals (FMOs) obtained by the Density Func-
tional Theory (DFT) at the B3LYP/6-31G* level of theory using the
Gaussian 03 code [58]. The optimized ground-state geometry and
the corresponding FMOs are shown in Fig. 5. It can be seen that the
three modeled molecules exhibit practically a planar geometry. We
assumed three possible conformations: a) with the methylene
bridge of the terminal fluorenes in different directions (one of them
parallel to the core); b) both methylene bridges of the fluorenes
parallel to the core and c) both methylene bridges of the fluorenes
opposite to the core, ensuring by frequency analysis that all these
conformers are local minima, for which no imaginary points were
found. Fig. 5 shows the FMOs of the three derivatives in the more
stable conformation and Table 3 summarizes the most important
results for electronic excitations and oscillator strength values. For
compound 4, we found that HOMO / LUMO was the transition
the polar region (
D
f > 0.2; slope ¼ 9286 ꢃ 395, R2 ¼ 0.996) includes
only THF, acetone and acetonitrile (ACN). The nonlinearity observed
as a general trend going from hexane to acetonitrile can be related
to ICTas specific interaction while methanol deviation was assigned
to intermolecular hydrogen-bonding [56] in addition to ICT.
From the LipperteMataga plots, the Dm values can be obtained
to allow an estimation of the dipole moment of the emitting excited
state. It should be also noted that according to the DeAeD char-
acter of 6, the states involved in absorption transitions have no
significant permanent ground-state dipole moment, so that the
values of Dm are mostly related to polar emitting excited states.
Then, from the slope of the polar region [57] on the LipperteMataga
plot e the region with the best approximation e we estimated the
(p / p*) with the highest contribution. For compound 5, analysis
showed the existence of three principal transitions:
HOMO / LUMO þ 2, HOMO / LUMO þ 1 and HOMO / LUMO at
353, 358 and 432 nm, respectively; while for derivative 6, the
principal transitions are: HOMO / LUMO þ 1 and HOMO / LUMO
located at 361 and 551 nm respectively. These results clearly
demonstrate that for both 5 and 6 the transitions at low energies
are associated to HOMO levels with a
p type bonding character and
electronic density delocalized on the complete molecular frame;
whereas for the LUMO levels, the electronic density was redis-
tributed and confined to the central acceptor moieties. Therefore
the HOMO / LUMO transitions involve an ICT process. At higher
energies such ICT process is no longer observed and it is replaced by
p
/
p* transitions.
The oscillator strength values for compounds 4, 5 and 6 were
also estimated in the framework of ZINDO without taking into ac-
count solvent effects (see Table 3). The computed values on the
semiempirical optimized geometry for the HOMO / LUMO
Fig. 4. LipperteMataga plot of 6 in seven pure solvents.

38
J. Rodríguez-Romero et al. / Dyes and Pigments 98 (2013) 31e41
Fig. 5. Gas-phase orbital pictures for 4, 5 and 6 in the frontier region. For compounds 5 and 6 it is evident that the lower transitions, i.e., HOMO / LUMO undergo a redistribution of
electronic density from the entire frame to only the acceptor core center.
electronic transition for
4
and HOMO
/
LUMO and
of colloidal nanoparticles of compound 6 (6NP) was prepared using
a modification of the re-precipitation process reported by Naka-
nishi and coworkers [60]. The surfactant agent (CTAB) was
employed to reduce the surface tension of the nanosystem. Fluo-
rescent nanoparticles with diameter smaller than 200 nm were
obtained (see inset in Fig. 6b). The suspension was stable at room
temperature for several months.
HOMO / LUMO þ 1 for 5 and 6 resulted overestimated, observing
a red-shift compared to the experimental absorption bands. These
red-shifted values can be related to a complete delocalization in the
system, which is a consequence of a totally planar arrangement.
However, these bathochromic shifts are observed for the three
molecules showing the same trend in the oscillator strength
estimation.
The absorption and PL spectra from aqueous suspensions of 6NP
are shown in Fig. 6. A comparison of the 6NP spectrum with those
obtained for 6 in solution (Fig. 1) reveals that the
p / p* transition
3.6. Photophysical properties from aqueous solutions of
nanoparticles
appears again at 319e320 nm, however, the peak for ICT transition
is now slightly red shifted to 447 nm. The PL spectrum of 6NP
shows an unstructured band at 522 nm, a wavelength than lies
between that observed for the PL peaks in solutions of dioxane
(515 nm) and THF (525 nm). The quantum yield of 6NP in water was
The alkyl chains present in derivatives 4e6 provide a hydro-
phobic character that favors the self agglomeration and formation
of nanoparticles (nanoagglomerates) in water. Aqueous suspension
surprisingly high, giving a value of
f
¼ 0.83; such a value is close to
the values determined in THF (
f
¼ 0.87) and in hexane (
f
¼ 0.85)
solutions (see Table 2). The optical properties were maintained for
several months without degradation, since the hydrophobic envi-
ronment created by the molecules of surfactant that surround the
6NP provided stabilization. We also prepared aqueous suspensions
of 6NP without the use of surfactant, obtaining nanoparticles that
were stable only for a few days, and after that time the formation of
a precipitate was observed. Nevertheless, we could observe that the
optical properties of the nanoparticles synthesized with and
without surfactant were the same.
As far as the fluorophores are concerned, one of the most
important properties is their photostability. In order to evaluate the
photostability of derivative 6, we carried out a continuous UVeVis
irradiation on a THF solution under aerobic conditions over 2 h. As
irradiation source we used a Xenon lamp (150 W). In addition, the
Table 3
Oscillator strength of simulated electronic spectra and major transition contribu-
tions for compounds 4e6.
Compound Experimental Theoretical Oscillator Transition
absorptiona absorptiona strength
Contribution
(%)
4
369
391
2.4654
H ꢀ 1 / Lþ1
H / L
H / L
H ꢀ 1 / L þ 2
H / L þ 1
H ꢀ 1 / L
H / L þ 2
H / L
8
33
35
9
27
11
17
43
27
5
444
366
432
358
0.9111
1.525
353
0.1043
6
441
320
551
361
0.9115
1.0249
H / L þ 1
a
Value expressed in nm.

J. Rodríguez-Romero et al. / Dyes and Pigments 98 (2013) 31e41
39
The samples were pumped at 1200 nm corresponding to a THG
wavelength of 400 nm. For fluorene derivative 5 the THG response
was lower than our experimental detection limit, while derivative
6, having a DeAeD structure with a stronger ICT character, pro-
duced an easily detectable THG signal. This clearly demonstrates
how the ICT contributes to enhance the nonlinearities. The cubic
non-linear susceptibility
c
3(ꢀ3
u; u, u, u) from thin films of 6 was
3 ꢁ 10ꢀ12 esu, calculated by means of THG Maker Fringes formal-
ism. This value is comparable to the value of 4.3 ꢁ 10ꢀ12 esu
reported for fluorene polymers containing similar electronic
structure [61].
TPA properties, associated to the cubic nonelinear susceptibility
tensor of type
u
3(ꢀ
u
;
u
, ꢀ
u, u), were also evaluated. Taking
advantage of the PL exhibited by solutions of our fluorene de-
rivatives, we obtained the TPA cross-sections (dTPA) for compounds
5 and 6 using the TPEF technique with femtosecond laser pulses, as
described in the experimental section. Table 4 summarizes a com-
parison between the dTPA values measured at 750 nm using differ-
ent solvents (for compound 5, TPA properties were studied only in
four solvents due to insolubility characteristics). From this table we
clearly observe how the strong ICT over the electronic DeAeD
structure of 6 impacts the nonlinearities: the maximum dTPA
observed in 6 (w1000 GM in THF) is about ten times larger with
respect to the maximum value in 5 (105 GM in hexane). Such a TPA
effect in 6 is comparable with recent reports on DeAeD structures.
For instance, Cheng et al. [62] observed values of dTPA w 1600 GM
when they used diphenylamine and benzothiadiazole moieties as D
and A, respectively, and fluorene units as
Wang et al. measured dTPA values up to 2103 GM (with
THF solutions of a DeAeD compound containing a benzothiadia-
zole core and fluorene units as a -bridges [31a]; note that in this
case the two-photon activity (the product of dTPA) is similar to
p
-bridges. Similarly,
f
w 0.36) in
p
f
ꢁ
the activity of 6. The enhanced TPA properties observed in 6 can
also be visualized by comparison with other DeAeD systems that
employed benzothiadiazole as acceptor group but lack the presence
of fluorene units in their structure, i.e., in the representative works
performed by Kato et al. in a variety of such compounds they
obtained dTPA in the range 43e330 GM [31b]. Small chromophores
Fig. 6. a) Absorption and PL spectra of 6NP. b) Percentage photodecomposition of
compound 6 in THF solution (filled squares) and aqueous nanoagglomerates (open
circles). Inset. TEM micrographs of 6NP obtained from suspension processed using
solution of CTAB at 0.8 mM.
with DepeA structure comprising fluorene units also display
similar dTPA, although the TPA effect can be certainly improved
incorporating fluorene moieties into multibranched chromophores
to reach values of dTPA in the order of 600e3000 GM [63].
photostability of 6NP in water was also evaluated under the same
experimental conditions. The photodegradation of compound 6 in
solution and 6NP was quantified by monitoring the decrease of
absorption intensity at 447 nm (see Fig. 6b). The THF solution
showed significant photodegradation, with almost 90% of photo-
decomposition observed after 40 min of irradiation; in contrast, the
suspension of 6NP showed only 33% of degradation after 120 min.
Clearly the experimental results indicate that the formation of
nanoagglomerates leads to high photostability.
Fig. 7a display the dispersion curves of dTPA measured for 6 in
different solvents and 6NP in water. The recorded data indicate that
for all solvents the maximum TPA cross-sections for 6 is obtained at
750 nm, with a maximum value of nearly 1000 GM in THF solution.
However, the non-linear response decreases notably in hexane,
acetonitrile and methanol solutions, with measured dTPA values of
317, 238 and 236 GM, respectively. Meanwhile, the TPA spectrum of
6NP in water shows a maximum dTPA value of 514 GM at 750 nm.
This value is approximately half of the maximum value recorded in
THF solution. To summarize these results, Fig. 7b presents the
maximum value of dTPA for 6 as a function of solvent dielectric
constant. We can see that dTPA has a non-monotonic behavior with
respect to the dielectric constant. It is evident that the interaction
between the chromophore and solvent molecules produced
enhanced ICT process (which is the basis of TPA response) with
3.7. Non-linear optical characterization
Coherent frequency conversion experiments were implemented
to obtain a first insight on the non-linear optical properties of our
compounds. In particular, we detected the THG produced from
solid polystyrene thin films (w150 nm) doped with either 5 or 6.
Table 4
Summary of dTPA values of compounds 5 and 6 at 750 nm.
Compound
Hexane
Toluene
THF
23
1000
Acetone
Acetonitrile
Methanol
Nanoparticles
5
6
105
317
81
819
e
e
e
e
660
238
236
514
TPA cross-section given in GM units (1 GM ¼ 10ꢀ50 cm4 s).

40
J. Rodríguez-Romero et al. / Dyes and Pigments 98 (2013) 31e41
Fig. 7. a) TPA spectra of compound 6 dissolved in hexane, toluene, THF, acetone, methanol, acetonitrile. For comparison, the spectrum corresponding to 6NP dispersed in water is
included. b) Non-monotonic behavior of the maximum dTPA of 6 as a function of dielectric constant for each solvent.
medium polar solvent such
a
THF, but such process was
Jiménez-Sánchez thank CONACyT for PhD fellowships. The authors
thank Martin Olmos for technical assistance, QFB. T. Cortez and Q. V.
González for determination of NMR spectra and G. Cuéllar for
HRMS spectra.
considerably reduced in non-polar (hexane) and polar (methanol)
solvents. For the case of methanol, chromophoreesolvent
interactions were also responsible for strong fluorescence
quenching effects. In view of the dTPA values exhibited by 6 in dif-
ferent solvents, it is noteworthy that 6NP tends to retain good dTPA
values in spite they are immersed in a high polar medium such as
water. Furthermore, it can be pointed out that the fluorescence
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2012.12.029.
quantum yield measured from 6NP resulted to be
f
¼ 0.83. The
relatively large dTPA values and high quantum yield exhibited by
6NP are attractive properties; in fact the two-photon activity of our
nanoagglomerates is larger than those measured for commercial
biomarkers, i.e., Rhodamine-Phalloidin with w80 GM, utilized in
fluorescence microscopy [13].
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