Linear and Nonlinear Optical Properties of Tricyanopropylidene-Based Merocyanine Dyes: Synergistic Experimental and Theoretical Investigations

Article abstract of DOI:10.1002/cphc.201701143

New merocyanines dyes with tricyanopropylidene-based acceptor units connected to dihexylaminophenyl or dihexylaminothiophenyl donor moieties through polyenic bridges of different lengths have been designed. All derivatives exhibited a strong dipolar character and showed a typical intramolecular charge transfer (ICT) transition. NMR spectroscopy experiments combined with DFT calculations demonstrated that both the nature of the donor–acceptor pair and the length of the conjugated linker strongly impact the electronic structure of the dyes and induce alteration in the bond-length alternation (BLA) and marked shifts in the ICT absorption bands. Hyper-Rayleigh scattering experiments revealed an exponential increase in the second-harmonic generation response as the polyenic chain length was increased. Strikingly, the largest chromophores with the strongest donor–acceptor pair exhibited a very high first hyperpolarizabilty together with a cyanine-like electronic structure, which apparently contradicts the paradigm of optimal BLA predicted by the two-state model. Although it decreased as the polyenic chain length increased, all dyes also exhibited high thermal stability, which demonstrates their potential for applications in nonlinear optical devices.

Full text of DOI:10.1002/cphc.201701143

Accepted Article
Title: Linear and Nonlinear Optical Properties of Tricyanopropylidene-
based Merocyanine Dyes: Synergistic Experimental and
Theoretical Investigations
Authors: Venkatakrishnan Parthasarathy, Ravindra Pandey, Puspendu
Kumar Das, Frederic Castet, and Mireille Blanchard-Desce
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10.1002/cphc.201701143
ChemPhysChem
Linear and Nonlinear Optical Properties of Tricyanopropylidene-
based Merocyanine Dyes: Synergistic Experimental and Theoretical
Investigations
Venkatakrishnan Parthasarathy,a,b Ravindra Pandey,c,d Puspendu Kumar Das,*,c
Frédéric Castet,*,e Mireille Blanchard-Desce*,b,e
a Department of Chemistry, Indian Institute of Technology Madras, Chennai – 600 036, India.
pvenkat@iitm.ac.in
b Chimie et Photonique Moléculaire (CNRS, UMR 6510), Université de Rennes 1,35042
Rennes, France.
c Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore,
560012, India.
d Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur,
Kolkata 700032, India
e University of Bordeaux, Institut des Sciences Moléculaires (CNRS, UMR 5255), 33405
Talence, France.
pkdas@ipc.iisc.ernet.in
frederic.castet@u-bordeaux.fr
mireille.blanchard-desce@u-bordeaux.fr
Abstract
New merocyanines dyes including tricyanopropylidene-based acceptor units connected to
dihexylaminophenyl or dihexylaminothiophenyl donor moieties through polyenic bridges of
different lengths have been designed. All derivatives exhibit a strong dipolar character and
show a typical intramolecular charge transfer (ICT) transition. NMR experiments combined to
1
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DFT calculations demonstrate that the nature of the donor/acceptor pair, as well as the length
of the conjugated linker, strongly impact the electronic structure of the dyes, inducing
alteration in the bond length alternation (BLA) and marked shifts in the ICT absorption bands.
Hyper-Rayleigh scattering experiments reveal an exponential increase of the second-harmonic
generation response when enlarging the polyenic chain length. Strikingly, the largest
chromophores that incorporate the strongest donor/acceptor pair exhibit giant first
hyperpolarizabilty in combination to a cyanine-like electronic structure, which apparently
contradicts the paradigm of optimal BLA predicted by the two-state model. Although it
decreases when increasing the polyenic chain length, all dyes also exhibit high thermal
stability, which demonstrates their potential for application in nonlinear optical devices.
I. Introduction
The design of organic chromophores displaying large nonlinear optical (NLO) responses have
attracted much attention in the chemistry community, due to their potential use as molecular
components in optoelectronic devices,1 or as nonlinear probes for cellular imaging.2 For
instance, second harmonic scattering, which produces a scattered wave with frequency twice
of that of the incident light, is currently exploited in laser components, optical
telecommunications, information and signal processing, data storage, 3 as well as in
bioimaging.4 Although large NLO properties have been demonstrated in octupolar systems,5
the design of efficient NLO-phores mostly relies on push–pull molecules that contain an
electron-donating group (D) interacting with an electron-withdrawing group (A) through π-
conjugation. Among them, push-pull polyenes motivated a lot of studies that allowed
establishing structure-property relationships linking the magnitude of the second-order NLO
response (i.e. the quadratic hyperpolarizability !) to the chemical structure of the molecules,
highlighting the key role of the relative strength of the D and A groups as well as of the length
2
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10.1002/cphc.201701143
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of the polyenic chain.6 Simple conceptual guidelines for designing such D-π-A polyenic dyes
are provided by the two-state model, in which the normalized ground-state wavefunction can
be written as a linear combination of the !" and !!!! resonance forms:
!! = 1 − ! !" + ! !!!!
(1)
with ! the formal charge transfer between D and A. Within this two-state approximation
(TSA), the static first hyperpolarizability tensor is expressed as:
!"#
!
= 6 !!!"Δ!!" ΔΕ!" = 9! Δ!!" Δ!!!"
(2)
!!!
!"
where ΔΕ!"is the excitation energy between the ground state (g) and the dipole-allowed
excited state (e), !!" is the transition dipole associated to the excitation, ! = !! ΔΕ!"!!!" is
!"
the oscillator strength, and ∆!!" = !! − !! is the change in dipole moment between the two
electronic states.This latter quantity can be simply expressed as ∆!!" = 1 − 2! !!, where
!! is the dipole moment of the !!!! resonance structure. Therefore, ∆!!"cancels out in the
so-called cyanine limit, where the resonant basis states !" and !!!! have identical
weights in the ground state wavefunction (! = 1/2 in Eq. 1). Highly conjugated cyanine-like
chromophores showing negligible bond length alternation (BLA) along the polyenic chain are
thus expected to display weak second-order NLO responses. In contrast, chromophores
exhibiting optimal BLA (≈ 0.05 Å) are expected to behave as effective NLO scatterers.4c
However, with the design of original polyenic push-pull derivatives bearing a phthalimide-
substituted tricyanopropylidene terminal segment (series II’ in Chart 1), we demonstrated
recently that this BLA/β paradigm could apparently be overcome.7 The longest derivatives
were indeed found to display record hyperpolarizabilities (up to 3900 × 10-30 esu from hyper-
Rayleigh scattering (HRS) measurements carried out at 1907 nm) although having vanishing
BLA, in contrast to previously reported push-pull compounds displaying large β values.4c,8
These strongly dipolar dyes were also demonstrated to exhibit good thermal stabilities
3
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ChemPhysChem
together with narrow absorption spectra (and thus improved transparency). Moreover, they
were shown to undergo a base-promoted intramolecular cyclization that further enhances their
quadratic NLO response and thermal stability, as compared to their non-cyclized analogs
having the same number of conjugated double bonds in the polyenic linker.9
In this work, we extend this family of compounds by designing new derivatives that involve
either a 4-(dihexylamino)phenyl (D1) or a 2-(dihexylamino)thiophenyl (D2) function as donor
unit, and a tricyanopropylidene-based acceptor unit incorporating an amine (A1) or a
phthalimide (A2) substituent. The connection of these push and pull moieties through
polyenic conjugated chains of variable length gives rise to the four series depicted in Chart 1.
The linear and nonlinear optical properties, as well as the thermal stability of the three new
series I, I’ and II, are described herein in comparison to those of their II’ analogs, reported
previously7. Quantum chemical calculations are also conducted in order to provide an in-
depth understanding of the experimental measurements.
II. Experimental and computational details
II.1 Synthesis of the poly[n]enic chromophores
The synthesis of the push-pull chromophores is described in Scheme 1. The required
aldehydes, viz., 4-dihexylaminobenzaldehyde and 5-bromo-2-thiophenecarboxaldehyde, were
obtained starting from the 4-dihexylaminobenzene and 5-bromo-2-thiophenecarboxaldehyde
by Vilsmeier-Haack formylation and amination, respectively. The vinylogation of these
aldehydes was attempted by two different methods: A) by repetitive Wittig oxypropenylation
using tributyl-[1,3]-dioxolan-2-ylmethylphosphonium bromide under NaH/THF conditions at
optimum temperature followed by removal of the dioxolane moiety by acid hydrolysis, and B)
by Wittig oxypropenylation using phosphonium ionic liquid, which can be obtained from
Wittig phophonium salt as shown in Scheme 1 (see the Supporting Information for details),
under neat/melt (100 °C) conditions. As the aldehyde functionality in 3a or 3b is strongly
4
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deactivated due to the presence of the N,N-dihexylamino unit at the donor site, a highly
reactive phosphorane was considered necessary so as to have better reactivity. Hence, a
slightly modified Spangler's strategy (method A) using tributyl-[1,3]-dioxolan-2-
ylmethylenephosphorane instead of triphenyl-[1,3]-dioxolan-2-ylmethylenephosphorane was
optimized for series 3a[n] and was adapted further for series 3b[n]. Indeed, this reaction
proceeded in an expeditious and facile fashion to attain better yields with complete
conversion without any higher homologs or undesired products. Such a transformation is
highly encouraged in multistep synthesis due to the minimum or no loss of the precursor
enals; the products obtained must be able to be taken to the next step conveniently without
any further purification. The method B that involves phosphonium ionic liquid also resulted in
reasonable yields. Each olefin-homologated aldehyde (enals), thus obtained at this stage, was
further subjected to Knoevenagel condensation with 3-amino-2-cyano-pent-2-endiamine to
provide the desired amines (series I and I'); the condensation proceeded smoothly without
any added base. The resulting extended amines were further condensed with phthalic
anhydride in presence of triethylamine to afford the desired phthalimide-anchored extended
push-pullchromophores, II and II' (see Supporting Information for details). The polyenic
push-pull chromophores (both amines and imides) thus obtained were strongly colored. The
results of the above steps have been consolidated in Table 1. All the above compounds were
1
13
thoroughly characterized by IR, H and C NMR spectroscopy and mass spectrometry, as
reported in the Supplementary Material.
II.2 Quantum chemical calculations
Molecular structures were optimized using density functional theory (DFT) together with the
B3LYP exchange-correlation functional (XCF) and the 6-311G(d) basis set. Although the
B3LYP XCF has been shown to underestimate the bond length alternation values in long
conjugated oligomers,10 this level of calculation provides reliable general trends when
5
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comparing analogous systems. To save computational time, the N,N-dihexylamino units at the
donor site of the compounds were replaced by N,N-dimethylamino units in all geometry
optimizations. Each structure was characterized as a minimum of the potential energy surface
on the basis of its real harmonic vibrational frequencies. Vertical excitation energies and
excited state properties were determined using time-dependent density functional theory
(TDDFT) with the CAM-B3LYP XCF and the larger 6-311+G(d) basis set. The CAM-
B3LYP XCF adds a long-range correction to B3LYP using the Coulomb-attenuating
method,11 and includes 19% of HF exchange at short-range and 65% at long-range (with a
-1
range separating parameter µ = 0.33 a0 ). This long-range correction is known to improve the
description of low-lying charge-transfer excited states12. Dynamic (frequency-dependent) first
hyperpolarizabilities were calculated at the same level of approximation using an incident
wavelength of 1907 nm, since it was shown to fairly reproduce the second-order NLO
properties of a large variety of organic chromophores,13 including the enhancement of the
HRS hyperpolarizability upon increasing the polymethine chain length in the II’ series7 and in
their cyclized derivatives.9 Solvent (chloroform) effects were included both in geometry
optimizations and calculations of the optical properties by using the Integral Equation
Formalism (IEF) version of the Polarizable Continuum Model (IEF-PCM).14 All calculations
were performed using the Gaussian 09 package.15
III. Geometry and ground state electronic structure of push-pull dyes
The geometry and ground-state electronic structure of the various chromophores were
investigated by combining 1H NMR experiments and DFT calculations. The 1H NMR spectra
recorded in CDCl3 revealed an all-trans configuration of the protons of the polyenic
1
backbone. The H–1H coupling constants between vicinal protons of the conjugated chain
1
(3JHH) deduced from H–1H COSY experiments, as well as their average differences (ΔJav),
are reported in Table 2. It is known that protons in anti configuration linked through a pure
6
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3
double carbon bond have a typical coupling constant of 16.5 Hz, while JHH = 10.0 Hz for
protons linked through a pure single C–C bond.16 The coupling constants reported in Table 2,
ranging between these two limit values, are typical of conjugated polyenic systems with
strong electron delocalization. The average difference values ΔJav reveal that the oscillatory
3
behavior of the JHH values (see also Figure 1) is more pronounced in compounds I and II
than in compounds I’ and II’, which provides evidence for a smaller bond length alternation
along the conjugated chain in chromophores incorporating the strongest donor group D2. As
previously reported,7 the II’ series involving both the strongest donor (D2) and acceptor (A2)
groups exhibits negligible ΔJav values, and thus appears to be close to the cyanine limit, i.e.
with a BLA close to zero. Moreover, one observes that the ΔJav values increase upon
increasing the length of the conjugated backbone in series I and I’, while they decrease in
series II. In the case of II’, ΔJav lies close to zero independently on the chain length.
To examine the alteration in coupling constants with respect to the solvent polarity,
additional 1H NMR experiments were performed in C6D6 (less polar) and C3D6O (more polar)
for the poly[n]enic analogs with n = 1 as representative cases (Table 3). For I and I', the ΔJav
values display a non-monotonic behavior with respect to solvent polarity. For the II[1]
chromophore, the ΔJav values decrease with increasing solvent polarity, evidencing a
reduction of BLA. In contrast, the ΔJav values of II’ remain close to zero independently on the
solvent polarity, which further confirms the closeness of the II' series to the cyanine limit,
with no alteration in BLA.
The bond length alternations calculated at the B3LYP/6-311G(d) level in chloroform
(Table 4) are fully consistent with the NMR measurements. For a conjugated chain containing
N carbon atoms, the BLA is calculated as:
!!!
!-!
!!!
BLA!…! = 1 N-2
d!!!,!!!-d!,!!! × -1
(3)
1
where !!,! is the interatomic distance between carbons i and j. Consistently with H NMR
7
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experiments, the series I and II that involve the weaker donor group D1 exhibit larger BLA
than I’ and II’, in which the donor is stronger. For a given length of the polyenic chain, the
BLA values evolve following the order I>II>I’>II’, supporting the NMR data. In particular,
the II’ series display much smaller BLA values than the other series, confirming its closeness
to the cyanine limit. However, within a series, the computed BLA decreases as the polyenic
chain length increases until saturation for the longer chains, which contradicts the
experimental results for chromophores I[n] and I’[n].
Table 4 also reports the calculated ground state dipole moments, the HOMO and
LUMO energies and the resulting energy gap, EGAP = ELUMO– EHOMO. As expected, increasing
the polyenic chain length shifts up and down the HOMO and LUMO levels, respectively,
resulting in a lowering of the electronic gap. The ground-state dipole also progressively
increases with chain length. Consistent with the relative strengths of the donor and acceptor
units, dipole values follow the I<II<I’<II’ ordering, i.e. reverse of the BLA values, and reach
values exceeding 30 D for the longest derivative of the II’ series.
IV. Thermal properties
Thermal stability of a chromophore is one of the key requirements for its practical
applications. In order to test the potential of the newly synthesized push-pull systems, the
thermal properties of the push-pull polyenic chromophores I, I', II and II' were probed by
differential scanning calorimetry (DSC), and the transition temperatures (glass transition
temperature Tg, crystallization temperature Tc, melting temperature Tm, onset of
decomposition Tod, and decomposition exotherm, Td) derived from the thermograms are
presented in Table 5. The DSC thermograms for the secondary analogs (n = 1) are reported in
Figure 2. All merocyanine dyes exhibit a sharp endothermic melting transition and few of
them reveal a glass transition. A perusal of the data from Table 5 indicates that the shortest
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poly[n]enes generally possess better thermal stabilities (onset of decomposition) than more
elongated polyenes, and that the simplest analogs I, I’ and II’[0] display decomposition
temperature as high as ca. 270 °C. This suggests that shortening the chain length increases the
steric congestion and thus reduces the flexibility (or increases the rigidity) around the
polyenic backbone, which has for consequence to enhance the thermal stability. The
thiophene-based push-pull poly[n]enes incorporating a phthalimide function (II’) possess
higher thermal stabilities than their amine counterparts I’. As previously observed in various
molecular systems and polymers,17 the phthalimide unit affords rigidity to the molecule, locks
the geometry and thereby restricts the molecular motion in the solid state, thus resulting in a
better stability. However, phenyl-based polyenes do not follow such a trend. Moreover, it is
noteworthy that phenyl-based push-pull systems I and II show better thermal properties than
their thiophene analogs I’ and II’.
V. Linear optical properties
V.1 Absorption properties in chloroform
The UV-vis spectra of the different chromophores were recorded in chloroform (ca. 10-5 M).
As representative examples, chromophores of the I' and II' series are shown in Figure 3,
while the maximum absorption wavelengths measured for all compounds are gathered in
Table 6. Depending on the nature of the donor and acceptor groups and on the length of the
polyenic chain, the push-pull chromophores exhibit a wide range of absorption maxima, going
from the visible to the near-IR region. Within a given series, a marked bathochromic shift is
observed upon increasing the conjugation length. In addition, a considerable broadening of
the main absorption band is observed as increasing n within the series I, I' and II.
Intriguingly, the opposite trend is observed for the series II', in which the full-width at half
maximum (FWHM) falls as the length of the conjugated chain increases.
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Moreover, upon elongation of the backbone, the absorption maximum tends to saturate
in the cases of I and I’. In the II and II’ series, each vinylogation induces a redshift of the
absorption maximum by ca. 100 nm, without any saturation. Most notably, the chromophores
II'[3] and II'[4] reveal a near-IR absorption at 916 and 1033 nm, respectively. Figure 4
illustrates the dramatic variation in the absorption maximum within the series I → I' → II →
II' for the shortest merocyanine derivatives (n = 0).
Although the maximal absorption wavelengths are systematically underestimated, TDDFT
calculations conducted at the CAM-B3LYP/6-311+g(d) level fairly reproduce their evolution
with respect to the nature of the donor and acceptor units, as well as the relative amplitude of
the redshift when increasing the polyenic chain length. Consistent with UV/Vis.
measurements, the calculated maximal absorption wavelengths evolve in the order I[n] <I'[n]
<II[n] <II'[n] for the same chain length n. As expected for conjugated dyes, the lowest-
energy excited state is dominated by a HOMO to LUMO electronic excitation. Both MOs
extend over the whole conjugated backbone of the molecules, the LUMOs being shifted
towards the tricyanopropylidene withdrawing unit compared to the HOMOs (the shape of the
MOs are provided in the supporting information for all compounds). In the I’ and II’ series,
the frontier MOs do not spread over the phthalimide substituent, since the latter makes an
angle of about 60° with the tricyanopropylidene fragment due to steric hindrance. We note
also that, within a same series, the weight of the H→L excitation in the first allowed optical
transition slightly decreases with increasing n. For a same chain of length n, the H→L
contribution is larger in compounds I’ and II’ than in their I and II analogs. The calculated
oscillator strengths are larger in I-II than in I’-II’, and progressively increase with chain
length, as a result of the gradual increase of the transition dipole moment. This trend is not
fully observed experimentally, where the oscillator strengths, evaluated as !!"# ≈ 4.3×
10!!×!×!"#$, decrease for the larger chains.
10
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In addition to the theoretical transition wavelengths and oscillator strengths, Table 6
also reports the dipole moment variation between the ground state and the optically active
excited state (∆!!"). From the analysis of the total electron densities of the two electronic
states, ∆!!" is further decomposed as ∆!!" = !!"× !!", where !!" is the photo-induced
charge transfer, i.e. the global amount of charge transferred upon light excitation, and !!" is
the distance over which this charge is transferred.18 For identical chain lengths n, the ∆!!"
values decrease in the order I[n] >II[n] >I’[n] >II’[n], except for n = 0 for which the ∆!!"
values of II[0] and I[0] are inverted. Strikingly, the II’ series displays very weak dipole
moment variations compared to the other series. As indicated by the !!" values collected in
Table 6, these weak ∆!!" values result from small charge transfer distances, the latter being
the consequence of a larger spatial overlap between the HOMO and LUMO, and thus to more
localized changes in the electron density upon the S0!S1 transition (see the density difference
maps in the supporting information). The smaller ∆!!" values obtained for the II’ series also
confirm that these chromophores are closer to the cyanine limit. As mentioned in the
introduction, the cyanine limit refers to push-pull dyes for which the ground-state
wavefunction has equal weight of the !" and !!!! resonance forms. Within the two-state
approximation, the ground and excited state dipole moments are respectively expressed as
!! = !!! and !! = 1 − ! !!, where ! is the formal ground state charge transfer between
the D and A units (see Eq. 1), and !! is the dipole moment of the !!!! resonance form. In
the cyanine limit, the amount of intramolecular CT between the D and A units in the ground
state is formally equal to 0.5, so that ∆!!" = 0. The formal ground-state charge transfer
values between D and A, evaluated from the ratio between the ground and excited state dipole
moments as ! = !! !! + !! , are reported in the last column of Table 6. The ! values,
close to 0.5 and constant when increasing the chain length, further illustrate the better
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conjugation in chromophores belonging to the II’ series. All other compounds display smaller
! values, which progressively decrease with n in series I and I’.
VI. Nonlinear optical properties
The evolution with respect to the length of the polyenic chain of the total HRS
hyperpolarizabilities (!!"#), as well as of the main diagonal component of the β tensor
(evaluated as !!!! ≈ 35 6 !!"# ), is reported in Figure 5 for the four series of
chromophores. The numerical values of !!"#and !!!!are given in Table 7, together with the
results obtained from TDDFTcalculations. All β values are given according to the convention
of Ref. 19. The experimental HRS data evidence a strong enhancement of βHRS and βzzz with
increasing chain length, which is particularly pronounced for chromophores bearing the
strong acceptor group A2 implying a phthalimide substituent. The nature of the donor group
has a smaller impact on the NLO enhancement, although, for the same polyenic chain length,
the βHRS responses of chromophores having the stronger donor D2 (series I’ and II’) are larger
than those involving D1 (series I and II). This results in the remarkable enhancement within
the II’ series (see !!"#[!] !!"# 0 ratios in Table 7), and to the record βHRS response of
II’[4]. As already noticed in our previous paper,7 this huge HRS response is striking for a
cyanine-like derivative, which is expected to show a very low quadratic NLO response due to
vanishing Δµ and BLA values. This apparent breaking of the 2-state model should however
be nuanced because the magnitude of the NLO response partly originates from resonance
effects, the second-harmonic wavelength (954 nm for laser pulses at 1907 nm) lying within
the main absorption band of the largest compounds (λmax = 916 nm for II'[3], see Table 6 and
Figure 3). To estimate the intrinsic β responses of the dyes, the HRS hyperpolarizabilities
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measured at 1907 nm were extrapolated to the static limit (i.e. at infinite wavelength), by
using a homogeneously damped frequency dispersion factor expressed as:20
ΔE2ge ΔE −iΓ 2
(
)
ge
F(ω )=
(4)
2
2
2
2
⎡
⎤⎡
⎤
ΔE −iΓ − !ω
ΔE −iΓ − 2!ω
(
)
(
)
(
)
(
)
⎢
⎥⎢
⎥
ge
ge
⎣
⎦⎣
⎦
where!ω is the photon energy of the incident beam, and Γ a homogeneous broadening
parameter. To account for the change in the absorption bandwidth from one compound to
another, the value of the damping factor Γ was set equal to 1.2 times the half width at half
maximum of the first absorption band (HWHM = FWHM/2, see Table 6), in line with
previous studies.21 The static HRS responses, estimated as !!"#,! = !!"# !(!) (Table 7) are
smaller than the dynamic ones (! ! > 1), and also show significant enhancement when
increasing the chain length. However, the frequency dispersion factors estimated in the
II’series are larger than in the other series (increasing with chain length from 1.5 to 3.0), so
that removing frequency dispersion effects in this series induces a larger decrease of the !
response than in the others. As a result, for a same chain length (n = 2), the static !!"#,!
values involve in the order I≈I’<II’<II, differing from that of the dynamic ones (I<I’<II<II’).
The progressive increase with chain length of the dynamic HRS hyperpolarizabilities
is qualitatively reproduced by the TDDFT calculations. However, the calculated enhancement
factors !!"# ! !!"# 0 (Table 7) are overestimated compared to experiments in series I and
I’, due to the systematic overestimation of the transition energies by the CAM-B3LYP XCF
(see Table 6). In series II and II’, the TD-DFT calculations instead underestimate the
enhancement factors beyond n = 2, leading to the II’<I’<I<II hierarchy for the computed
dynamic !!"# values of polye[n]enic compounds with n = 2. This hierarchy is preserved
when considering the static !!"# values (!!!""##!,!, see Table 6) and differs from that obtained
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from HRS measurements, which demonstrates that the discrepancies between experiments
and theory cannot be only ascribed to frequency resonance effects. Note that, although
underestimated, the static HRS responses estimated using the 2-state approximation
!"#
63 35 ! , with !
evaluated using the quantities gathered in Table 6 following
!!!
!"#
!"#
(!
=
!"#
!!!
Eq. 2), leads to a similar hierarchy to that found from full TDDFT calculations, demonstrating
that the 2-state approximation provides reasonable trends within the investigated series.
Finally, TDDFT calculations also evidence that increasing the conjugation length reinforces
the 1D character of the NLO response, as indicated by the progressive increase of the
!"#!
!!
!
!
ratios (see Table 6).
!!!
!!!
VII. Conclusions
New dipolar tricyanopropylidene-based merocyanine dyes containing different pairs of
push/pull units linked through polyenic bridges of different lengths have been synthesized.
Within each series, the evolution of the structural as well as of the linear and nonlinear optical
properties with respect to the polyenic chain length have been characterized by combining
complementary spectroscopy techniques and first principle calculations. 1H NMR
measurements revealed that chromophores incorporating a phthalimide substituent within the
tricyanopropylidene withdrawing moiety (II and II’) display smaller bond length alternation
along the conjugated linker than those including a smaller amine function (I and I’).
Moreover, using a dihexylaminothiophenyl fragment (I’ and II’) as a push moiety further
reduces the BLA compared to substitution using a dihexylaminophenyl fragment (I and II).
As a consequence, the largest compounds of the II’ series incorporating the most efficient
donor/acceptor pair exhibit a cyanine-like electronic structure, with negligible BLA. The
alteration of the BLA with the nature of the donor/acceptor units induces marked shifts in the
intramolecular charge transfer absorption bands. Within a series, successive vinylogations
14
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10.1002/cphc.201701143
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also induce significant redshifts of the absorption maximum, associated with large increases
of the second-harmonic responses. Despite displaying negligible BLA, compounds of the II’
series exhibit the highest second-order hyperpolarizabilities for a given chain length, which
apparently breaks the paradigm of optimal BLA predicted by the 2-state approximation. DFT
calculations qualitatively reproduce the evolution of the linear and nonlinear optical
properties with chain length, and provide fundamental insights onto the relationship between
the chemical structure of the dyes and their optical properties. In particular, the computed
geometrical structures and dipole moment variations confirm the cyanine-like structure of the
II' series. However, DFT calculations do not reproduce the experimental measurements
regarding the relative ordering of the hyperpolarizabilities of the different series, and more
specifically the particularly marked enhancement of the NLO response of II', which partly
originate from frequency dispersion effects. Finally, DSC measurements show that all
derivatives display reasonable thermal stabilities. Therefore, owing to their large NLO
responses, reasonable thermal stabilities and full transparency at telecommunication
wavelengths, these dipolar dyes appear highly promising for various NLO applications.
Supporting Information
Synthesis and characterization of merocyanines dyes of series I and II. HOMO and LUMO
shapes and density difference maps. Hyper-Rayleigh scattering experimental details.
Acknowledgements
MBD and PKD thank the Indo-French Centre for Promotion of Advanced Research
(IFCPAR)/Centre Franco-Indien pour la Promotion de la RechercheAvancée (CEFIPRA)
(Project: 3505-1, Supramolecular Interactions in Multichromophores) for financial support,
15
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10.1002/cphc.201701143
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and a PDF to VP. MBD gratefully acknowledges financial support from Conseil Regional
d'Aquitaine (Chaire d'accueil grant). The calculations were performed on the “Mésocentre de
Calcul Intensif Aquitain” (MCIA) of the University of Bordeaux, financed by the Conseil
Régional d'Aquitaine and the French Ministry of Research and Technology.
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Table 1. Summary of the results of vinylogation and condensation reactions.
Aldehyde Yielda (%) Chromophore Yieldb (%) Chromophore
Yieldc(%)
Ar
n
0
1
2
3a[0]
3a[1]
3a[2]
-
I[0]
I[1]
I[2]
62
85
69
II[0]
II[1]
II[2]
80
83
66
76 68d
55 74d
a
0
1
2
3
4
3b[0]
3b[1]
3b[2]
3b[3]
3b[4]
-
I'[0]
I'[1]
I'[2]
I'[3]
I'[4]
65
74
58
71
65
II'[0]
II'[1]
II'[2]
II'[3]
70
65
97
88
83
76
72e
50e
S
b
b
a Isolated yield of the vinylogation step (method A): 3[n-1] → 3[n]. Isolated yield in chromophore I
and I'. c Isolated yield in case of chromophore II and II'. dIsolated yields by method B. e Accompanied
by a cyclization side-product9.
17
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10.1002/cphc.201701143
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1
Table 2. H-1H coupling constants (3JHH, Hz) between vicinal protons along the polyenic
chain of the push-pull poly[n]enes,a and average difference in the 3JHH values of adjacent C=C
and C–C bonds (ΔJav, Hz).
Compd
I[0]
J12
J23
J34
J45
J56
J67
J78
J89
ΔJav
-
I[1]
I[2]
14.7
15.1
11.7
10.9
3.0
3.2
-
1.4
1.6
2.1
2.4
-
2.1
1.4
-
0.0
0.2
0.0
14.0
11.8
I'[0]
I'[1]
I'[2]
I'[3]
I'[4]
II[0]
II[1]
II[2]
II'[0]
II'[1]
II'[2]
II'[3]
13.5
14.0
14.4
14.4
12.3
11.9
11.5
11.7
13.3
13.9
14.5
12.1
12.1
11.4
13.4
13.9
11.8
12.0
13.7
11.9
14.3
14.7
12.2
11.9
13.1
13.1
13.0
12.8
13.6
13.0
12.5
13.6
13.0
13.0
13.0
13.0
13.8
13.8
a the atom numbering starts from the donor end to the acceptor end along the polyenic chain.
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10.1002/cphc.201701143
ChemPhysChem
Table 3. Variation ofΔJav (Hz) with solvents of varying polarity for I, I', II, II'[1].
Solvent
C6D6
CDCl3
C3D6O
I[1]
3.4
3.0
3.3
I'[1]
2.1
1.4
II[1]
2.6
2.1
II'[1]
0.0
0.0
1.9
2.0
0.0
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10.1002/cphc.201701143
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Table 4. Bond Length Alternation along the conjugated carbon chain (Å), HOMO and
LUMO energies and gaps (eV), and ground state dipole moments (D), as calculated at the
B3LYP/6-311G(d) level in chloroform for the merocyanine dyes I, I'[n] and II, II'[n].
ELUMO
BLA
EHOMO
EGAP
µg
I[0]
I[1]
I[2]
0.071
0.053
0.048
0.045
0.032
0.030
0.030
0.031
0.055
0.035
0.030
0.023
0.010
0.008
0.008
-5.850
-5.605
-5.442
-5.714
-5.442
-5.279
-5.143
-5.022
-5.905
-5.687
-5.524
-5.823
-5.578
-5.388
-5.252
-2.667
-2.884
-3.020
-2.667
-2.857
-2.993
-3.102
-3.162
-3.184
-3.320
-3.401
-3.129
-3.238
-3.347
-3.401
3.184
2.721
2.422
3.048
2.585
2.286
2.041
1.860
2.721
2.367
2.122
2.694
2.340
2.041
1.850
13.49
16.29
18.89
14.86
18.73
22.11
25.19
27.98
13.24
17.00
20.68
15.29
20.64
25.49
30.24
I'[0]
I'[1]
I'[2]
I'[3]
I'[4]
II[0]
II[1]
II[2]
II'[0]
II'[1]
II'[2]
II'[3]
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ChemPhysChem
Table 5. Thermal stability data for the push-pull polyenes I, I', II and II' from DSC
experiments.a
Compd
Tg
Tc
Tm
Tod
Td
I[0]
I[1]
I[2]
/
/
/
194
158
176
103
146
191
131
145
171
185
148
154
160
205
192
258
203
206
242
195
196
147
149
185
191
152
250
234
207
204
275
>220
235
275
>200
207
166
158
196
200
>160
269
254
>215
210
138
/
/
/
/
/
/
/
101
/
/
I'[0]
I'[1]
I'[2]
I'[3]
I'[4]
II[0]
II[1]
II[2]
II'[0]
II'[1]
II'[2]
II'[3]
138
101
/
120
100
104
110
115
98
/
/
/
132
140
/
128
aTg-glass transition temperature; Tc-crystallization temperature; Tm-melting temperature; Tod-onset of
decomposition and Td-decomposition exotherm. The experiment was run under nitrogen gas
atmosphere at a heating rate of 10 °C per minute.
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Table 6. Linear optical properties of chromophores I, I', II, II'.Maximal absorption
wavelength (λmax, nm), oscillator strengtha (ƒmax), molar extinction coefficient (logεmax), and
full width at half maximum (FWHM, cm–1) measured in CHCl3. Maximal absorption
wavelength (λge, nm), oscillator strength (ƒge), weight of the HOMO to LUMO excitation
(%H→L),transferred charge (qCT, |e|), charge transfer distance (dCT, Å), dipole moment
b
variation (∆!!", D), and formal ground-state charge transfer (δ, |e|), calculated at the
TDDFT/CAM-B3LYP/6-311+G(d) level.
Cpd
FWHM fmax
fge
qCT
dCT
2.28
2.92
λmax logεmax
λge
µge
%H→L
95
Δµge
5.80
7.81
δc
I[0]
I[1]
I[2]
470
529
565
4.62
4.68
4.62
4.87
5.00
4.79
4.70
4.57
4.88
5.03
4.87
5.05
5.23
5.43
5.24
3400
4700
5100
1900
1700
3000
4500
5100
2200
2050
2800
1300
900
0.61 410 10.31 1.22
0.97 478 13.12 1.69
0.91 537 15.70 2.16
0.61 448 10.21 1.09
0.73 527 13.47 1.62
0.80 599 16.48 2.13
0.97 663 19.23 2.62
0.81 720 21.72 3.08
0.72 474 11.31 1.27
0.94 553 14.41 1.77
0.89 622 17.25 2.25
0.63 508 11.34 1.19
0.66 593 14.76 1.73
0.96 675 17.99 2.25
0.60 756 21.11 2.77
0.53
0.56
0.59
0.44
0.46
0.50
0.55
0.60
0.57
0.56
0.58
0.45
0.45
0.47
0.49
0.41
0.40
0.39
0.47
0.46
0.44
0.42
0.40
0.39
0.41
0.41
0.47
0.48
0.48
0.48
94
92
97
96
95
94
93
89
93
93
97
96
95
94
3.61 10.18
I'[0] 515
I'[1] 614
I'[2] 704
I'[3] 712
I'[4] 720
II[0] 566
II[1] 668
II[2] 759
II'[0] 581
II'[1] 690
II'[2] 803
II'[3] 916
1.05
1.49
2.36
3.30
2.23
3.32
5.68
8.67
4.28 12.27
2.57
2.79
3.20
0.86
0.62
0.79
1.14
6.99
7.48
8.85
1.84
1.33
1.78
2.67
830
800
a evaluated as !!"# ≈ 4.3×10!!×!×!"#$.b evaluated as ! = !! !! + !! .
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10.1002/cphc.201701143
ChemPhysChem
Table 7. Nonlinear optical properties of the push-pull chromophores in CHCl3. All β values
are given in 10-30esu. The ratios ! = !!"#[!] !!"# 0 measure the relative enhancement of
βHRS with chain length.
!"#
!"#,!
!"#
!"#
!"#
b
!"#
!!!
a
d
%1D g
c
e
d
f
!"#!
!"#!
d
!"#!
!"#
!"#!
!"
!
Compd
!
!
!
!"#,!
!
!
!"#
!
!
!
!!!
!
!"#
!!!
I[0]
I[1]
12
19
37
18
22
51
/
10
16
30
14
15
32
29
46
61
/
64
125
367
888
88
65
147
405
937
100
288
826
85
91
95
88
92
95
1.6
168
388
41
2.8
6.4
/
165
352
45
193
455
29
I[2]
3.1
/
89
I'[0]
I'[1]
I'[2]
43
1.2
2.8
53
119
342
881
2029
110
283
648
52
2.9
8.3
122
314
711
1404
110
259
529
56
265
788
105
348
886
1886
126
299
640
38
123
357
2090
106
I'[3]
I'[4]
II[0]
148
865
44
8.2
48.1
/
107
714
32
21.5
49.5
/
2076
4904
207
2129
4902
266
97
100
78
II[1]
II[2]
II'[0]
II'[1]
II'[2]
II'[3]
56
574
47
1.3
13.0
/
36
344
32
135
2.6
5.9
/
587
1425
92
685
1565
127
86
91
72
72
78
86
1386
114
78
1.7
44
188
102
235
611
2.0
4.5
11.8
105
213
456
176
441
1272
64
246
630
13.4
83.0
282
1522
9420
165
427
568
3900
1280
1476
a derived from HRS measurements at 1.907 µm in chloroform. b extrapolated using the 2-state
!"#
!"#
damped model. c evaluated as !!!!"!#
≈
35 6 !
.
d calculated at the CAM-B3LYP/6-311+G(d)
e
f
!"#
!"#!
!"#
!!
level in chloroform. evaluated using !
=
6! 35 ! . evaluated as !
≈
!!!
35 6 !
.
!!!
!"#
!"#
g
!"#!
!!
evaluated as !
!!!! ×100.
!!!
23
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ChemPhysChem
A1
A1
D1
D2
n
n
NC
NC
NC
CN
NH2
NC
CN
NH2
n
n
Hex2N
Hex2N
S
I
I'
I[0], n = 0
I[1], n = 1
I[2], n = 2
I'[0], n = 0
I'[1], n = 1
I'[2], n = 2
I'[3], n = 3
I'[4], n = 4
A2
A2
D2
D1
n
n
NC
NC
NC
CN
NC
CN
O
O
N
N
n
n
Hex2N
O
O
Hex2N
S
II
II'
II[0], n = 0
II[1], n = 1
II[2], n = 2
II'[0], n = 0
II'[1], n = 1
II'[2], n = 2
II'[3], n = 3
Chart 1. Structures of the merocyanine derivatives investigated in this study.
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10.1002/cphc.201701143
ChemPhysChem
O
O
i
O
O
PBu3 Br
PBu3 NTf2
2
1
method A
1
n x (iia,b)
n x (iic,b)
CHO
n
Hex2N
Hex2N
CHO
Ar
Ar
3a[n]
3b[n]
3a[0]
3b[0]
2
method B
(when Ar = 1,4-C6H4)
NC
NC
CN
NH2
iii
NC
CN
NC
4
NH2
n
Hex2N
Ar
I[n]
I'[n]
NC
NC
O
CN
O
O
iv
N
n
Hex2N
Ar
O
O
5
II[n]
II'[n]
Ar
=
S
Reagents and conditions: (i) LiNTf2, H2O, rt, 8 h; (ii)a) 1 (1.1 equiv), NaH, THF,
rt, overnight; b) HCl, THF, rt; c) 2 (1.1 equiv), NaH, 100 °C, 1-2 h; (iii) 4 (1.0
equiv), EtOH, reflux, 2-4 h; (iv) 5 (3.0 equiv), Et3N (3.0 equiv), CH2Cl2, 50-60 °C,
3-6 h.
Scheme 1. Synthesis of series of merocyanine derivatives I, I’, II and II’.
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10.1002/cphc.201701143
ChemPhysChem
I[1]
15
14
13
12
11
I'[1]
II[1]
II'[1]
I[2]
15
14
13
12
11
I'[2]
II[2]
II'[2]
I'[3]
II'[3]
15
14
13
12
11
Figure 1. Oscillatory behavior of the coupling constants of the push-pull poly[n]enes with n =
1 (top), n = 2 (middle), and n = 3 (bottom).
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Td = 200 °C
I[1]
Td = 254 °C
I'[1]
II[1]
Tm = 146 °C
II'[1]
Tm = 156 °C
Tm = 185 °C
Tm = 160 °C
50
100
150
200
250
300
Temperature (°C)
Figure 2. DSC scans of the push-pull poly[n]enes I, I', II and II' with n = 1.
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10.1002/cphc.201701143
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I'[2]
I'[1]
I'[3]
I'[0]
I'[4]
400 500 600 700 800 900 1000 1100
wavelength (nm)
II'[1]
II'[2]
II'[0]
II'[3]
400 500 600 700 800 900 1000 1100
wavelength (nm)
Figure 3. UV-vis absorption spectra of the push-pull poly[n]enes I’ (top) and II’ (bottom) in
chloroform.
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10.1002/cphc.201701143
ChemPhysChem
I'[0]
II'[0]
II[0]
I[0]
400
500
600
700
wavelength (nm)
Figure 4. UV-vis absorption spectra of push-pull poly[n]enes (n = 0) I, I’, II, and II’ in
chloroform.
29
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