An acido-triggered reversible luminescent and nonlinear optical switch based on a substituted styrylpyridine: EFISH measurements as an unusual method to reveal a protonation-deprotonation NLO contrast

Article abstract of DOI:10.1039/c3cc48149b

Diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine (DPVPA) constitutes a novel acido-triggered reversible luminescent and nonlinear optical switch. Remarkably, for the first time the Electric-Field Induced Second Harmonic generation (EFISH) technique is used to reveal a protonation-deprotonation NLO contrast. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cc48149b

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An acido-triggered reversible luminescent and
nonlinear optical switch based on a substituted
styrylpyridine: EFISH measurements as an unusual
method to reveal a protonation–deprotonation
NLO contrast†
Cite this:DOI: 10.1039/c3cc48149b
Received 24th October 2013,
Accepted 3rd December 2013
DOI: 10.1039/c3cc48149b
Elena Cariati,*abc Claudia Dragonetti,abc Elena Lucenti,*bc Filippo Nisic,ab
Stefania Righetto,ab Dominique Roberto*abc and Elisa Tordinab
www.rsc.org/chemcomm
Diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine of weak organic acids in solution by Hyper Rayleigh Scattering
(DPVPA) constitutes a novel acido-triggered reversible luminescent (HRS)4 intensity measurements. Successively, different second-
and nonlinear optical switch. Remarkably, for the first time the order NLO contrasts have been studied by the HRS technique. For
Electric-Field Induced Second Harmonic generation (EFISH) technique instance, a series of acidochromes based on benzimidazolo[2,3-b]-
is used to reveal a protonation–deprotonation NLO contrast.
oxazolidines,5 benzazolo-oxazolidines6 and indolino[2,1-b]oxazo-
lidine7 moieties show a remarkable contrast in the NLO response
due to the reversible transformation of the oxazolidine closed form
into a colored zwitterionic open form by acidification. Also, a series
of push–pull molecules containing a 4,5-dicyanoimidazole acceptor
(A) unit, an N,N-dimethylamino donor (D) group, and p-conjugated
linkers (p) were reported as pH-triggered NLO switches; protonation
occurs at the N,N-dimethylamino function, which leads to disruption
of the electronic charge-transfer delocalization: the A–p–D pattern of
the neutral species with large quadratic hyperpolarizability b values,
as determined by HRS, was transformed into an A–p–A0 pattern,
which displayed much smaller b values.8
The high increase of the quadratic hyperpolarizability of variously
substituted styryl pyridines upon coordination to a metal center9 or
alkylation of the pyridine moiety10 prompted us to investigate the
possibility of switching their second-order NLO response by the
protonation–deprotonation reaction. In addition, since p-delocalized
systems are often characterized by intense photoluminescence
and in view of the recent and increasing interest in luminescent
organic solids showing acidochromic switch of the emission,11
we also studied the transformation in solid state and in solution
by emission spectroscopy. The molecular target chosen was the
diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-vinyl}-phenyl)-amine
(DPVPA), a known luminescent p-delocalized chromophore,12,13
which second-order NLO properties had never been studied. In
this communication, we present the results of this investigation
that led to a new acido-triggered reversible luminescent and NLO
switch. It is also shown for the first time that Electric-Field-
Induced Second Harmonic generation (EFISH) measurements
can be used as a convenient method to reveal a protonation–
deprotonation NLO contrast.
Compounds with second-order nonlinear optical (NLO) properties
are important as molecular building block materials for optical
communications, optical data processing and storage, or electro-
optical devices.1 Among them, molecular species with commutable
NLO properties are of growing interest.2 In fact, modulating the
electronic and optical properties using an external trigger has been
extended to the field of nonlinear optics and, in recent years, there
has been significant interest in the ability to switch the second-order
NLO response at the molecular level. The quadratic hyper-
polarizability of chromophores may be manipulated by reversibly
modifying the properties of specific parts of active molecules. The
on–off switching may involve reducing the donor capacity of the
electron-rich fragment of a typical donor–acceptor species, D–linker–
A, by oxidation or protonation. Conversely, the acceptor behaviour of
A can be altered by reduction, or by deprotonation. Alteration of the
quadratic hyperpolarizability may also involve structural or chemical
modification of the bridging group, thereby interfering with the
communication between D and A. Thus, NLO switches can be
achieved by a pH variation, by a redox process or by interaction
with electromagnetic radiation.2
Of particular interest are organic acidochromes which display
substantial NLO variations due to their ability to alternate between
two distinct chemical forms having different absorption spectra in
response to protonation–deprotonation.
The first studies on the NLO properties of acid–base couples were
reported by Das,3 who elegantly obtained the dissociation constants
a
`
Dipartimento di Chimica dell’Universita degli Studi di Milano, via Golgi 19,
20133 Milano, Italy. E-mail: elena.cariati@unimi.it, dominique.roberto@unimi.it
b UdR INSTM di Milano, via Golgi 19, 20133 Milano, Italy
c ISTM-CNR, via Golgi 19, 20133 Milano, Italy. E-mail: e.lucenti@istm.cnr.it
DPVPA was easily prepared using a slightly modified version
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3cc48149b of the procedure reported in the literature (see Scheme 1).12,13
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Scheme 1 Synthesis of diphenyl-(4-{2-[4-(2-pyridin-4-yl-vinyl)-phenyl]-
vinyl}-phenyl)-amine (DPVPA). (i) tBuOꢀK+, THF, RT, 16 h, 85%; (ii) vinylpyridine,
bis(tri-tert-butylphosphine)palladium(0), toluene, 80 1C, 24 h, 90%.
The UV-visible absorption spectrum of DPVPA in CHCl3
shows one major band at 410 nm which can be attributed to an
intramolecular charge transfer transition (ICT) emanating from the
NPh2 donor-end of the molecule. This absorption is red shifted
with respect to that of dialkylamino styrylpyridine (lmax at 381 nm),
in agreement with a higher p-delocalisation. Upon exposure of a
CHCl3 solution of DPVPA to HCl vapours for 1 h, the absorption
maximum is shifted to 485 nm, a red-shift that can be attributed
to protonation of the pyridine moiety with formation of
[DPVPAH]+Clꢀ, as confirmed by 1H NMR spectroscopy (see
Fig. 1 and ESI†). The reverse transformation can be induced
faster (about 30 min) by treatment of the solution with NH3
vapours, as confirmed by 1H NMR and absorption spectroscopy.
Interestingly, the protonation–deprotonation process is
accompanied by a macroscopic variation in the emissive behaviour.
In fact, DPVPA in CHCl3 displays an intense emission centred at
506 nm which is shifted to 665 nm upon exposure to HCl and
restored after treatment with NH3 vapours. A similar, but much
faster, interconversion process can be induced by exposure of
powders or thin films of DPVPA dispersed in a polymethylmetha-
crylate (PMMA) matrix to HCl and ammonia vapours. The initial
558 nm emission of DPVPA powder is, in fact, shifted to 675 nm
after less than 5 minutes of exposure to HCl vapours and restored by
treatment with ammonia vapours (less than 5 min). For PMMA thin
films, the protonation is accomplished in less than 2 minutes, with
Fig. 2 Absorption (up) and emission (down) spectra of a DPVPA–PMMA
thin film before (blue) and after (red) exposure to HCl vapours for 2 min. A
photograph of the protonated and unprotonated thin films under UV light
is shown in the inset.
absorption maxima shifting from 399 nm (not protonated form) to
434 nm and emission maxima going from 497 nm to 599 nm (see
Fig. 2). Even in this case, the reverse reaction is induced by exposure
to ammonia vapours (less than 2 min).
The observation of a 75 nm red-shift of the absorption band
upon exposure of the CHCl3 solution to HCl vapours prompted us to
monitor the second-order NLO properties during the DPVPA/
[DPVPAH]+Clꢀ interconversion. In fact, the [DPVPAH]+Clꢀ low
energy absorption band should be characterized by a signifi-
cant increase in the dipole moment upon excitation from the
ground to the excited state (Dmeg)14 and therefore be respon-
sible for a higher second-order NLO response of [DPVPAH]+Clꢀ
with respect to DPVPA.15
Protonation–deprotonation NLO switches in solution are
commonly studied by the HRS technique2 which, however,
suffers the limitation of possible overestimation of the value
of the quadratic hyperpolarizability due to multiphoton fluores-
cence. To our knowledge, the EFISH method16 has never been
applied in such investigations, being usually considered off-
limits for ionic species. However, it appears that the EFISH
technique can be applied for the determination of the second-
order NLO response of ionic species by working in a solvent of a
low dielectric constant such as CHCl3 which favours ion-
pairing.17 The observation of the easy interconversion between
DPVPA and [DPVPAH]+Clꢀ upon exposure of CHCl3 solutions to
acid–base vapours prompted us to apply the EFISH method to
study the NLO response of the DPVPA system upon protonation–
deprotonation cycles.
Fig. 1 1H NMR of DPVPA and [DPVPAH+]Clꢀ in CDCl3 (inset: chemical
structures of the two compounds).
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Scheme 2 An acido-triggered NLO switch based on DPVPA.
It is known that the EFISH technique16 can provide direct infor-
mation on the intrinsic molecular NLO properties through eqn (1):
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gEFISH = (mbEFISH/5kT) + g(ꢀ2o; o, o, 0)
(1)
´
ˇ
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ability at frequency o of the incident light, is a purely electronic cubic
contribution to gEFISH which can usually be neglected when studying
the second order NLO properties of dipolar compounds.
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mbEFISH (690 ꢁ 10ꢀ48 esu; the positive value of mbEFISH is in
agreement with an increase of the excited state dipole moment with
respect to the ground state), working in CHCl3 at a concentration of
10ꢀ4 M with a non-resonant incident wavelength of 1.907 mm,
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The mbEFISH of DPVPA increases by a factor of 1.5 (mbEFISH
=
1025 ꢁ 10ꢀ48 esu) upon protonation in the presence of HCl
vapours, in agreement with the red shift of the absorption
band, whereas further exposure to NH3 vapours restores (after
filtration of the solution to remove NH4Cl which muddies the
solution) the original value (Scheme 2).
In conclusion, DPVPA constitutes a new acido-triggered
reversible luminescent and nonlinear optical switch. The altera-
tion of the nonlinear properties is induced by the modulation of
the internal charge-transfer due to the response of the molecule
to protonation–deprotonation as the external stimulus. Remark-
ably, this communication unveils that the EFISH technique is a
convenient and novel method to reveal a protonation–deproto-
nation NLO contrast. Moreover, based on the recent results
reported by some of us on the application of the electric poling
technique18 for hybridizing thin films of ionic NLO chromo-
phores,19 the use of this latter technique to detect acidochromic
NLO contrasts for bulk materials can be envisaged. Studies on
the reversible protonation of thin films of DPVPA dispersed in
PMMA are currently under investigation in our laboratories.
This work was supported by MIUR (FIRB 2003:
RBNE033KMA and FIRB 2004: RBPR05JH2P) and CNR.
`
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Notes and references
1 J. Zyss, Molecular Nonlinear Optics: Materials, Physics and Devices,
Academic Press, Boston, 1994.
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