Solid state and solution fine tuning of the linear and nonlinear optical properties of (2-pyrene-1-yl-vinyl)pyridine by protonation-deprotonation reactions

Article abstract of DOI:10.1039/c4cc05891g

The unexpected and acido-triggered reversible luminescence and nonlinear optical properties of (2-pyrene-1-yl-vinyl)pyridine, a simple and highly transparent chromophore, are studied both in solution and in the solid state. Remarkably, for the first time the acidomodulation of the NLO response of a poled thin film is reported. This journal is

Full text of DOI:10.1039/c4cc05891g

ChemComm
View Article Online
View Journal
COMMUNICATION
Solid state and solution fine tuning of the linear
and nonlinear optical properties of (2-pyrene-1-
yl-vinyl)pyridine by protonation–deprotonation
reactions†
Cite this:DOI: 10.1039/c4cc05891g
Received 29th July 2014,
Accepted 23rd September 2014
E. Cariati,*abc C. Botta,d S. G. Danelli,a A. Forni,*c A. Giaretta,a U. Giovanella,d
DOI: 10.1039/c4cc05891g
E. Lucenti,*bc D. Marinotto,ab S. Righettoab and R. Ugoab
www.rsc.org/chemcomm
The unexpected and acido-triggered reversible luminescence and non- measurements can be used as a convenient alternative to the
linear optical properties of (2-pyrene-1-yl-vinyl)pyridine, a simple and hyper Rayleigh scattering (HRS) technique to reveal a protonation–
highly transparent chromophore, are studied both in solution and in the deprotonation NLO contrast. A similar interconversion process was
solid state. Remarkably, for the first time the acidomodulation of the also induced by exposure of powders or thin films of DPVPA
NLO response of a poled thin film is reported.
dispersed in a polymethylmethacrylate (PMMA) matrix to acid–base
vapours producing a reversible modification of the solid state
While the research on compounds with commutable emissive emissive properties. On the other hand, no solid state NLO acid-
properties started some years ago, the possibility to trigger the ochromic switch was reported. The modulation of the second-order
NLO properties by an external stimulus has been evaluated more NLO properties of dispersed thin films is in fact a far more difficult
recently.1 Of particular interest from both points of view are organic task to be accomplished and remains a challenge requiring a
acidochromes which display substantial emissive and NLO varia- reversible control of the noncentrosymmetric alignment of the
tions due to their ability to alternate between two distinct chemical NLO moieties. Solid state switching of second harmonic generation
forms in response to the protonation–deprotonation process.2
(SHG) has been previously demonstrated in poled polymers
In this regard, some of us have recently reported the solution containing photochromic dyes,4 organic photochromic crystals,5
acidochromic switch of the photoluminescence and of the second- and Langmuir–Blodgett thin films of organometallic complexes.6
order NLO properties of diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)- However, to the best of our knowledge no acidochromic SHG
phenyl]-vinyl}-phenyl)-amine (DPVPA) upon exposure to HCl and switch has been reported for thin films yet.
ammonia vapours.3
In this communication we report our results on the reversible
acidochromic behaviour both in solution and in the solid state of
(2-pyrene-1-yl-vinyl)pyridine, 1, a simple and highly transparent
chromophore characterized by unexpected and tunable second-
order NLO properties. Pyrene and its derivatives have demonstrated
to exhibit excellent chromophore features, thanks to their extended
p-electron delocalized systems.7 In addition, the high rigidity of
their polyaromatic systems should assure larger chemical and
thermal stability with respect to the most commonly used
chromophores, such as stilbenes and azobenzenes.8 For the
same reason, pyrene-based chromophores should show further
enhanced stability with respect to the recently investigated class
of azaphenanthrene and azachrysene derivatives.9 Compound 1
was prepared in high yield and in one step only by Heck coupling
(see Scheme 1) rather than following published procedures
which require multiple steps.10
The alteration of the linear and nonlinear optical properties of
the chromophore was induced by the modulation of its internal
charge-transfer due to the external stimulus. In addition, this
molecular target was chosen to demonstrate for the first time
that electric-field-induced second harmonic (EFISH) generation
a
`
Dipartimento di Chimica dell’Universita degli Studi di Milano, via Golgi 19,
20133 Milano, Italy. E-mail: elena.cariati@unimi.it
b UdR INSTM di Milano, Via Golgi 19, 20133 Milano, Italy
c ISTM-CNR, Via Golgi 19, 20133 Milano, Italy.
According to its reduced conjugation length, comparable to that
of the classical stilbene system,11 1 is characterized by a high optical
gap, with an absorption edge in CHCl3 at 378 nm (Fig. 1). This
absorption band is associated with an intramolecular charge transfer
(ICT) transition emanating from pyrene towards the pyridine moiety
E-mail: alessandra.forni@istm.cnr.it, e.lucenti@istm.cnr.it
d ISMAC-CNR, Via Bassini 15, 20133 Milano, Italy
† Electronic supplementary information (ESI) available: Computational and
poling details. See DOI: 10.1039/c4cc05891g
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun.

View Article Online
Communication
ChemComm
Scheme 1 Synthesis of (2-pyrene-1-yl-vinyl)pyridine 1. (i) Bis(tri-t-
butylphosphine)-palladium(0)/toluene, 80 1C, 40 h, 90%.
Fig. 2 Plots of the optimized PBE0/6-311++G(d,p) geometry of 1 in its
neutral (above) and protonated (below) forms in CHCl3, along with their
dipole moment of the ground state computed at the same level of theory
(left) and of the excited state computed at the TD-PBE0/6-311++G(d,p)
level (right). The center of mass is chosen as the origin of the dipole.
(see the ESI† for a HOMO–LUMO plot together with the full set
of theoretical results). The solution was prepared using CHCl3
treated overnight with basic Al2O3, otherwise an easy protonation of
the pyridine fragment was observed (see the following). Upon
exposure of the CHCl3 solution of 1 to HCl vapours for few seconds,
the absorption maximum is red shifted to 443 nm (due to charge
transfer in the same direction as the neutral form) with a shoulder
at 395 nm. The reverse transformation can be accomplished by
treatment of the solution with NH3 vapours. Interestingly, the
protonation–deprotonation process is accompanied by a macro-
scopic variation in the emissive behaviour. In fact, 1 in CHCl3
displays an intense emission centred at 450 nm (quantum yield,
QY, equal to 65% using diphenylanthracene as the standard) which
is shifted to 550 nm upon exposure to HCl (QY 19% using quinine
sulfate in H2SO4 0.1 M as the standard) and restored after treat-
ment with NH3 vapours. The time required for the protonation–
deprotonation process is related to the solution concentration:
higher concentration solutions require longer time exposure
to acid–base vapours. Fig. 1 shows the absorption and emission
spectra recorded every 3 seconds before and after exposure to
HCl vapours. Thanks to the higher sensitivity of photoluminescence
spectroscopy, the complete transformation between protonated
and deprotonated forms of 1 is safely assessed. In fact, while
the transformation seems to be complete after 6 seconds by the
absorption spectroscopy, it is necessary to wait for 3 more
seconds, as evidenced by the emission spectra, for the process
to be completely accomplished.
The measurement of mbl of 1 was carried out in CHCl3
(treated with basic Al2O3) solutions at 1907 nm non-resonant
wavelength by the EFISH method. In spite of its relatively low
dipole moment, m (3.9 D, well reproduced by PBE0/6-311++G(d,p)
calculations in chloroform, 4.4 D, and directed from pyridine to
pyrene, see Fig. 2), 1 gives rise to unexpectedly large mbl values
(1500 ꢀ 10ꢁ48 esu at 10ꢁ4 M concentration), much higher than
the most efficient push–pull 4,40-disubstituted stilbenes.12
Such results can be interpreted in terms of the phenomenological
Oudar two-level model,13 giving the CT contribution to the quadratic
hyperpolarizability, bCT p (fDmeg/DEeg2), wherein f is the oscillator
strength, Dmeg is the difference between the dipole moment moduli
of the excited and ground states, and DEeg is the excitation energy.
TD-PBE0/6-311++G(d,p) calculations in chloroform provide in fact a
quite large oscillator strength for the lower energy transition, 1.27,
and an increase of the excited state dipole moment with respect to
the ground state (Dmeg = 7.1 D), because the charge transfer involved
in the excitation (see above) increases the negative charge localized
on the pyridine moiety (see Fig. 2).
Upon exposure of the solution to HCl vapours until no
presence of unreacted 1 is evident in the emission spectrum, an
inversion of the sign of mbl is observed (ꢁ950 ꢀ 10ꢁ48 esu). This
result was unexpected on the basis of previous studies performed
by some of us on several methylpyridinium salts of stilbazolic
precursors. Alkylation of the pyridinic nitrogen provided invariably
positive values of mbl, increasing with the enhancing electron-
withdrawing character of the positively charged nitrogen atom.14
In the present case, the surprising change in the sign of mbl of 1
upon protonation can be explained by a negative Dmeg value
(ꢁ11.9 D from TD-PBE0/6-311++G(d,p) calculations in chloroform,
which provide a reduced oscillator strength, 1.19, compared to 1, in
agreement with the observed spectra).15 In fact, the charge transfer
associated with excitation of the protonated molecule is opposite to
the direction of the large ground state dipole moment (20.8 D),
which is oriented from pyrene to pyridine (see Fig. 2). The different
behavior of the neutral and protonated species can be appreciated
by looking at the trend of the respective electrostatic potential maps
on going from the ground to the excited state (see Fig. S3, ESI†),
showing an increased polarization for the former and the opposite
for the latter. These results demonstrate not only the interesting
properties of pyrene as an NLO chromophore, but also its
Fig. 1 Absorption (up) and emission (down) spectra of 1 in CHCl3 solution
at different times of exposure to HCl vapours.
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2014

View Article Online
ChemComm
Communication
Fig. 4 Acidochromic SHG switch of the poled thin film.
Fig. 3 Absorption and emission spectra of the 1/PMMA pristine film and
after exposure to HCl vapours.
recorded after each cycle confirm this behavior. In fact, the increase
of the integrated area under absorption spectra (see Fig. S8, ESI†)
indicates a loss of orientation of the chromophore during the
switches and well correlates with the drop in the NLO response.
This loss of the SHG signal has already been observed in other
photochromic switches in the PMMA matrix.4,5,24 Besides, the
integrity of the film after the repeated switches was also established
by emission spectroscopy and by proving the invariance of the film
thickness by profilometry (see Table S2, ESI†).
In this communication we report the large second-order
NLO response of the highly transparent chromophore 1. We
also evidence the ambivalent donor–acceptor role of the pyrene
unit resulting in an interesting acidochromic emissive and NLO
switch both in solution and in the solid state. To our knowledge, this
study represents the first demonstration of the acidomodulation
of the NLO response of a thin film, opening a new avenue for
preparation of convenient reversible-NLO switches.
ambivalent donor–acceptor role. It is to be noted that a related
phenomenon has been previously observed in some pyrrole-(p-
bridge)-pyridine-based chromophores, which showed sign inversion
of solvatochromism after alkylation of the pyridinic nitrogen.16
To study the solid state interconversion process, thin films
of 1 dispersed in polymethylmethacrylate (PMMA) were prepared by
spin coating few drops of a dichloromethane solution (1/PMMA =
2 wt%; PMMA = 10 wt% with respect to the solvent) on a glass
substrate.17 By exposure of the 1/PMMA film to HCl vapours for 60 s,
the initial absorption maximum is shifted from 378 nm (neutral
form; e = 4610 cmꢁ1) to lower energy with one absorption at 396 nm
and a broad band at 434 nm (e = 3915 cmꢁ1) and the emission
maximum shifts from 446 nm (69% QY) to 546 nm (32% QY) (see
Fig. 3). The reverse reaction is induced by exposure of the film to
ammonia vapours for 30 s.
This interconversion can be exploited to produce a solid
state second-order NLO acidochromic switch. Corona poling18
on the 1/PMMA pristine film was carried out at 9.7 kV while
increasing the temperature at a rate of 1.8 1C minꢁ1 up to 60 1C.
The temperature was maintained at 60 1C for 90 min and then
gradually decreased to room temperature while monitoring the
SHG response of the film (see Fig. S5, ESI†).19 A second-order
NLO coefficient, d33, value of the 1/PMMA poled film equal to
0.55(ꢂ0.11) pm Vꢁ1 was obtained.20 The film was then stored in
the dark for at least 72 h after poling, in order to ensure that the
surface charges resulting from the deposition of ions on the film
surface during the corona poling process21 were neutralized and
a more stable SHG signal was reached (22 a.u.). The decay of the
SHG signal after poling was determined by monitoring the film
response for about 180 h (see Fig. S7, ESI†).
The SHG efficiency of the initial poled film and its switching
performances in the presence of HCl/ammonia vapours were
then monitored ex situ using a Maker fringe set-up22 at a fixed
angle of 601 using a p–p polarization. After exposure to HCl
vapours the SHG signal of the film increases up to 61 a.u., and
then decreases to about 4 a.u. after treatment with ammonia
vapours indicating a first acidoswitch with a SHG drop of about
93.5% (see Fig. 4).23 The observed decrease of the SHG signal
during the switches is probably due to the partial irreversible loss of
orientation through the acid–base treatment. Absorption spectra
Notes and references
1 B. J. Coe, in Non-Linear Optical Properties of Matter, ed. M. G.
Papadopoulos, A. J. Sadlej and J. Leszczynski, Springer Verlag,
Berlin, 2006, p. 571; V. Guerchais, L. Ordronneau and H. Le Bozec,
Coord. Chem. Rev., 2010, 254, 2533; K. A. Green, M. P. Cifuentes,
M. Samoc and M. G. Humphrey, Coord. Chem. Rev., 2011, 255, 2530;
F. Castet, V. Rodriguez, J. L. Pozzo, L. Ducasse, A. Plaquet and
B. Champagne, Acc. Chem. Res., 2013, 46, 2656.
2 P. K. Das, J. Phys. Chem. B, 2006, 110, 7621; L. Sanguinet, J.-L. Pozzo,
M. Guillaume, B. Champagne, F. Castet, L. Ducasse and E. Maury,
J. Phys. Chem. B, 2006, 110, 10672; F. Mancois, L. Sanguinet, J.-L.
Pozzo, M. Guillaume, B. Champagne, V. Rodriguez, F. Adamietz,
L. Ducasse and F. Castet, J. Phys. Chem. B, 2007, 111, 9795; F. Mancois,
J.-L. Pozzo, J. Pan, F. Adamietz, V. Rodriguez, L. Ducasse, F. Castet,
A. Plaquet and B. Champagne, Chem. – Eur. J., 2009, 15, 2560; A. Plaquet,
´
ˇ
B. Champagne, J. Kulhanek, F. Bures, E. Bogdan, F. Castet, L. Ducasse
and V. Rodriguez, ChemPhysChem, 2011, 12, 3245; D. Liu, Z. Zhang,
H. Zhang and Y. Wang, Chem. Commun., 2013, 49, 10001; C. H. Wang,
N. N. Ma, X. X. Sun, S. L. Sun, Y. Q. Qiu and P. J. Liu, J. Phys. Chem. A,
2012, 116, 10496.
3 E. Cariati, C. Dragonetti, E. Lucenti, F. Nisic, S. Righetto, D. Roberto
and E. Tordin, Chem. Commun., 2014, 50, 1608.
4 J. Boixel, V. Guerchais, H. Le Bozec, D. Jacquemin, A. Amar,
A. Boucekkine, A. Colombo, C. Dragonetti, D. Marinotto, D. Roberto,
S. Righetto and R. De Angelis, J. Am. Chem. Soc., 2014, 136, 5367;
¨
R. Loucıf-Saibi, K. Nakatani, J. A. Delaire, M. Dumont and Z. Sekkat,
Chem. Mater., 1993, 5, 229; Y. Atassi, J. A. Delaire and K. Nakatani, J. Phys.
Chem., 1995, 99, 16320.
5 K. Nakatani and J. A. Delaire, Chem. Mater., 1997, 9, 2682; M. Sliwa,
´
S. Letard, I. Malfant, M. Nierlich, P. G. Lacroix, T. Asahi,
H. Masuhara, P. Yu and K. Nakatani, Chem. Mater., 2005, 17, 4727.
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun.

View Article Online
Communication
ChemComm
6 L. Boubekeur-Lecaque, B. J. Coe, K. Clays, S. Foerier, T. Verbiest and
I. Asselberghs, J. Am. Chem. Soc., 2008, 130, 3286.
7 T. M. Figueira-Duarte and K. Mu¨llen, Chem. Rev., 2011, 111, 7260.
chromophore loadings reported in Fig. S4 (ESI†). Aggregation is
instead present at 4% w/w with a 40 nm red shift of the emission
band.
8 Photoreactive Organic Thin Films, ed. Z. Sekkat and W. Knoll, Academic 18 In situ SHG measurements were carried out using a Q-switched
Press, San Diego, California, 2002.
9 V. Calabrese, S. Quici, E. Rossi, E. Cariati, C. Dragonetti, D. Roberto,
Nd:YAG (Quanta System HYL-101) laser operating at 1064 nm with
20 Hz repetition rate, using a p–p polarization.
E. Tordin, F. De Angelis and S. Fantacci, Chem. Commun., 2010, 19 A higher poling temperature, i.e. 85 1C resulted in an unstable and
46, 8374. lower SHG response, see Fig. S6 (ESI†).
10 N. B. Sankaran, A. Das and A. Samanta, Chem. Phys. Lett., 2002, 20 The d33 value was calculated following the standard Maker fringe
351, 61; Y. Singh, A. Gulyani and S. Bhattacharya, FEBS Lett., 2003,
541, 132.
11 PBE0/6-311++G(d,p) geometry optimizations of 1, its analogous with
benzene instead of pyrene, and stilbene provide essentially the same
length of the central double bond, i.e., 1.344, 1.342 and 1.343 Å,
respectively.
12 L.-T. Cheng, W. Tam, S. H. Stevenson, G. R. Meredith, G. Rikken and
S. R. Marder, J. Phys. Chem., 1991, 95, 10631.
13 J. L. Oudar and D. S. Chemla, J. Chem. Phys., 1977, 66, 2664.
14 F. Tessore, E. Cariati, F. Cariati, D. Roberto, R. Ugo, P. Mussini,
C. Zuccaccia and A. Macchioni, ChemPhysChem, 2010, 11, 495.
technique; see i.e. W. N. Herman and L. M. Hayden, J. Opt. Soc. Am.
B, 1995, 12, 416; the thickness of the 1/PMMA poled film was
measured by profilometry, see Table S2 (ESI†). For comparison,
the d33 value of dispersed red one calculated under similar poling
conditions is equal to 33 pm Vꢁ1, see D. Marinotto, S. Proutiere,
`
C. Dragonetti, A. Colombo, P. Ferruti, D. Pedron, M. C. Ubaldi and
S. Pietralunga, J. Non-Cryst. Solids, 2011, 357, 2075.
21 D. Li, N. Minami, M. A. Ratner, T. J. Marks, J. Jang and G. K. Wong,
Synth. Met., 1989, 28, D585; M. A. Pauley, H. W. Guan and C. H. Wang,
J. Chem. Phys., 1996, 104, 6834; H. W. Guan, C. H. Wang and S. H. Gu,
J. Chem. Phys., 1994, 100, 8454.
15 D. Roberto, R. Ugo, S. Bruni, E. Cariati, F. Cariati, P. C. Fantucci and 22 D. Marinotto, R. Castagna, S. Righetto, C. Dragonetti, A. Colombo,
I. Invernizzi, Organometallics, 2000, 19, 1775.
16 A. Facchetti, A. Abbotto, L. Beverina, M. E. van der Boom, P. Dutta,
C. Bertarelli, M. Garbugli and G. Guglielmo, J. Phys. Chem. C, 2011,
115, 20425.
G. Evmenenko, T. J. Marks and G. A. Pagani, Chem. Mater., 2002, 23 The opposite switch trend observed in solution and in the solid state
14, 4996; A. Facchetti, L. Beverina, M. E. van der Boom, P. Dutta,
G. Evmenenko, A. D. Shukla, C. E. Stern, G. A. Pagani and T. J. Marks,
J. Am. Chem. Soc., 2006, 128, 2142.
maybe partially due to the concentration effect which is currently
under investigation in our laboratory.
24 Y. Atassi, J. Chauvin, J. A. Delaire, J. F. Delouis, I. Fanton-Maltey and
K. Nakatani, Pure Appl. Chem., 1998, 70, 2157; S. Hosotte and
M. Dumont, Synth. Met., 1996, 81, 125.
17 At this chromophore concentration aggregation phenomena are
negligible as evidenced by the emission spectra of films at different
Chem. Commun.
This journal is ©The Royal Society of Chemistry 2014