Fine-tuning polyoxometalate non-linear optical chromophores: a molecular electronic "Goldilocks" effect

Article abstract of DOI:10.1039/c8dt01491d

A new aryl-imido polyoxometalate non-linear optical chromophore (POMophore) with a diphenylamino donor group attains the highest β_{zzz, 0} value (196 × 10^-30 esu by Hyper-Rayleigh Scattering, HRS), and best transparency/non-linearity t

Full text of DOI:10.1039/c8dt01491d

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Fine-tuning polyoxometalate non-linear optical
chromophores: a molecular electronic
“Goldilocks” effect†
Cite this: DOI: 10.1039/c8dt01491d
Received 16th April 2018,
Accepted 15th May 2018
Ahmed Al-Yasari,a,e Philip Spence, a Hani El Moll,a Nick Van Steerteghem,b
DOI: 10.1039/c8dt01491d
a
Peter N. Horton,c Bruce S. Brunschwig, d Koen Claysb and John Fielden
*
rsc.li/dalton
A new aryl-imido polyoxometalate non-linear optical chromo- fast responses associated with molecular donor–acceptor
phore (POMophore) with a diphenylamino donor group attains the systems (vs. traditional extended inorganic solids) is critical to
highest βzzz, 0 value (196 × 10−30 esu by Hyper-Rayleigh Scattering, development of advanced applications in telecommunications,
HRS), and best transparency/non-linearity trade off yet for such optical/electro-optical computing and imaging.4 We recently
materials. Stark spectroscopic and DFT investigation of this com- showed that the strong POM-organic communication in donor-
pound, plus NMe2 and carbazole analogues, show that its high functionalized arylimido-Lindqvist ([Mo6O19]2−) anions results
performance results from
electronic transitions, and strong electronic communication across perties.5 Specifically, these POM acceptors combined with
a
combination of strongly dipolar in promising 2nd order NLO (laser frequency doubling) pro-
the π-system.
short (phenyl) π-bridges give rise to a better combination of
transparency and 2nd order NLO activity than planar, dipolar
organic systems. Thus, they may help overcome the challenge
of obtaining materials that have high NLO coefficients (β),
while avoiding problems for efficiency and stability that result
from reabsorption of second harmonic (SH) light. As yet
though, absolute β-values for these POMophores fall far short
of records and extension of the organic π-system – a classic
means of increasing β – has been seen to sacrifice any advan-
tage over purely organic (nitro) analogues.5b
Herein, we investigate the effect of using the weaker, but
more conjugated donor groups carbazole (cbz) and diphenyla-
mino (NPh2) on the behaviour of extended POMophores. We
find that, of the donor groups tested so far, NPh2 is the best
adapted for use in POMophores – producing the highest
β0 and best transparency/non-linearity trade-off. This results
from a combination of stronger electronic communication,
and strong, relatively high energy electronic transitions that
maintain a highly dipolar nature.
The chemistry of covalent polyoxometalate (POM) “hybrid”
compounds has advanced rapidly in recent years. Organic
groups can be connected to many of the common POM cluster
anions, and an increasing range of post-synthetic modifi-
cations enable incorporation of POMs into complex organic
architectures.1 However, study of the physical properties,2 and
associated applications3 of these structures is still in its
infancy. Relatively little is known about the interaction
between the POM and organic subunit, and indeed in most
systems electronic communication between them appears to
be weak.
With strong electronic communication between an acceptor
(e.g. a POM) and more electron rich moieties, many useful
optical and photophysical properties can emerge – such as
non-linear optical (NLO) effects. NLO materials are used to
manipulate laser light, and the tunable properties and strong,
The synthetic approach to 1 to 3 (Fig. 1) centres on the
DCC-mediated coupling of anilines with [NBu4]2[Mo6O19].1c,5
For both 1 and 2, we found the most expeditious route was to
first synthesize the precursor diphenylamino and cbz functio-
nalized ligands, before reaction with hexamolybdate. The syn-
aSchool of Chemistry, University of East Anglia, Norwich, NR4 7TJ, UK.
E-mail: John.Fielden@uea.ac.uk
bDepartment of Chemistry, University of Leuven, Celestijnenlaan 200D,
B-3001 Leuven, Belgium
cUK National Crystallography Service, School of Chemistry, University of
Southampton, SouthamptonSO17 1BJ, UK
dBeckman Institute, MC 139-74, California Institute of Technology,
1200 East California Blvd, Pasadena, CA 91125, USA
eCollege of Pharmacy, University of Kerbala, Kerbala, Iraq
†Electronic supplementary information (ESI) available: Synthetic and other
experimental/computational details, CIF files. CCDC 1837358 and 1837405. For
ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c8dt01491d
Fig. 1 Arylimido Lindqvist based chromophores 1 to 3.
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thesis of 3 was previously published.5b Notably, both 1 and 2
(most of all 2) are more vulnerable to hydrolysis than 3 and
other previously studied arylimido-POMs (a-POMs), and
require careful handling. As there is no difference in steric pro-
tection of the imido-bond, this observation implies a differ-
ence in electronic structure that increases its reactivity. UV-vis
spectra (Table 1, Fig. S1 in ESI†) show that compared to 3,
these compounds both show blue shifts in Emax of the intra-
hybrid charge transfer (IHCT5b) band, consistent with their
Table 2 HRS data at 1064 and 1100 nm for 1 to 3
a
b
a
b
βzzz
,
β0, 1064
βzzz,
1100
β0, 1100
β0/N3/2 c
λmax (nm) (10−31006e4su)
1
2
3
388
415
421
150 36
590 20
440 55
62 15
196
139 17
190 28
—
—
82 12
—
1.57
3.75
2.65
7
—
a βzzz calculated assuming a single dominant tensor component,
measured using 1064 or 1100 nm fundamental laser beams. The
quoted units (esu) can be converted into SI units (C3 m3 J−2) by divid-
weaker electron donors. However, they also absorb more ing by a factor of 2.693 × 1020
.
b Non-resonant, static β estimated from
βzzz using the two state model.6 Data for 3 are from ref. 5b. c N =
number of π-electrons in bridge.7
strongly across the range from 300 to 400 nm, likely a result of
stronger π-conjugation. Despite the weaker donors, the
reduction potentials of the {Mo6} unit in 1 and 2 are at a
similar potential to that of 3 (Table 1), suggesting that com-
munication between the POM and NMe2 donor is weaker than
with NPh2 or cbz.
HRS-determined β-values for 1 to 3 are shown in Table 2.
For 1, the higher value obtained at 1100 nm is likely more
accurate, as at 1064 nm the analysis was complicated by a
need to deconvolute HRS and fluoresence signal. Even so,
both resonance corrected static β-values (β0) for 1 are substan-
tially lower than that of 3, commensurate with the lower donor
strength. For 2, β0 is around 40% higher than for 3, and com-
bined with its lower λmax, this makes it the best performing
POMophore described to date. Compound 2 is also the first
extended POMophore that clearly outperforms comparable
purely organic systems, as shown by an intrinsic β value that
breaks through empirical performance limits (defined by
intrinsic β vs. λmax) defined by Kuzyk for planar, dipolar organ-
ics (Fig. 2).7 Although POM electrons are not included in the N
used to calculate intrinsic β, this analysis clearly shows that
the POM must influence the NLO properties of the organic
system. Moreover, the POM is broadly analogous to the aryl
remote acceptor in N-aryl stilbazolium chromophores – the
electrons of this aryl group are not included in N for these
systems, and yet N-aryl stilbazoliums do not exceed the appar-
ent limit in Fig. 2.7b
Fig. 2 Intrinsic β vs. λmax for 1, 2 and 3 vs. short (phenyl bridged)
POMophores,5 and planar, dipolar organic chromophores.7 Compound
2 is the first POMophore to clearly breach Kuzyk’s apparent limit for the
performance of planar, dipolar organic chromophores.
NO2 acceptor,8 where NMe2 outperforms NPh2 with both
alkyne and alkene bridges. This suggests that the POM accep-
tor must alter the electronic structure of the organic donor
and bridge, and since NPh2 is a weaker donor than NMe2, it
must increase β by influencing charge separation and com-
munication across the bridge. Indeed, the X-ray crystal struc-
tures of both 1 and 2 vs. 3 (Fig. 3, see ESI† for full details) are
consistent with stronger conjugation. Whereas that of 3 shows
a ca. 86° twist between the planes of the two phenyl rings,5b in
Thus, it appears that the POM contributes more to the NLO
properties of 2, than it does to those of 3, leading to a reversal
of the trend seen for structurally analogous systems with the
Table 1 UV-vis absorption and electrochemical data for 1 to 3 in acetonitrile
λmax/nma (ε, 103 M−1 cm−1
)
Emax (eV)
Assignment
E1/2 vs. Ag/AgCl/Vb (ΔE/mV)
−0.503 (64)
1
227 (70.4)
292 (48.4)
327 (31.3)
341 (33.7)
385 (43.4)
292 (41.2)
327 (36.2)
414 (45.3)
248 (36.2)
292 (44.5)
421 (41.2)
5.28
4.25
3.78
3.63
3.20
4.28
3.74
2.98
5.02
4.24
2.94
O → Mo and π → π*
O → Mo and π → π*
O → Mo and π → π*
O → Mo and π → π*
IHCT
O → Mo and π → π*
O → Mo and π → π*
IHCT
2
3
−0.503 (64)
−0.498 (61)
O → Mo and π → π*
O → Mo and π → π*
IHCT
a Concentrations ca. 10−5 M in MeCN. b Solutions ca. 10−3 M in analyte and 0.1 M in [NBu4][BF4] at a glassy carbon working electrode with a scan
rate of 100 mV s−1. Ferrocene internal reference E1/2 = 0.46 V, ΔEp = 80 mV. Data for 3 are from ref. 5b.
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band (16 to 35 nm). This can be an indicator of dipolar elec-
tronic transitions.10 The Stark-derived Σβ0 values for the series
(Table 3, see ESI† for details of fitting and extraction of charge
transfer parameters) follow a similar trend to HRS β0, although
measurement and fitting uncertainties are large and in all
cases values are larger than HRS β0 – a common observation in
prior work.5b,7,11 Similar to 3, the fit of the main low energy
peak in 2 consists of three Gaussians, each at slightly higher
energy than those of 3. Dipole moment changes Δµ and
charge transfer distances r associated with these peaks in 2
and 3 are within experimental error of one another (though
consistently slightly higher for 2), as are most other measured
parameters. However, an additional peak in 2 requires a 4th
Gaussian for an adequate fit, and while its transition is less
dipolar in nature (lower Δµ values) it shows strong electronic
coupling (higher Hab) and a high value of µ12 means that it
contributes significantly to Σβ0. In 1, the four Gaussian com-
ponents have less dipolar character their counterparts in 1 and
3, and generally electronic coupling is stronger than in either 2
or 3. The overall picture suggests that 2 hits a sweet spot where
transitions are just as dipolar than those of 3, yet electronic
communication across the diphenylacetylene bridge is strong.
TD-DFT calculations of the electronic spectra and β values
of 1 to 3 in acetonitrile solvent have been performed using the
ADF program, with the SAOP functional and TZ2P basis set.
Previously,5b we showed that a solvent forcefield is necessary
for these calculations to reflect experimental trends, although
the β-values computed were much larger (up to 30×) than
those observed experimentally. The methods used here are an
incremental improvement on those previously published.
Values are closer to experiment, but still high (reflecting low
computed transition energies, see ESI†), and the trend is an
excellent match for Stark and HRS findings. βzzz, 0 of 960 (1),
1300 (3) and 2160 × 10−30 (2) are obtained, and normalising all
three techniques to 2 as 100% (Table S2†) makes it clear just
how good the agreement in trend is between experiment and
theory. A qualitative interpretation of the TD-DFT-computed
orbital-to-orbital transitions is thus justified, and reveals two
key differences between 2 and 3. Firstly, the HOMO level of 2 is
concentrated on the NPh2 group and immediately appended
aryl ring (Fig. 4), whereas in 3 it spreads across the entire
organic system – as does the HOMO−1 for both systems. This
may increase the dipole moment change associated with
strong, low energy transitions to the LUMO+6 and LUMO+8,
which spread across POM and imido-ring, and is reflected in
the very slightly higher Δµ values found for 2 by Stark
(although they are within experimental error of those for 3).
Secondly, 3 has a weak transition from an imido-phenyl based
HOMO−5, to a POM-based LUMO+7, at a high computed
energy (3.39 eV, Table S5 and Fig. S7†). In 2, a similar tran-
sition is observed from the HOMO−9 to LUMO+1 (Table S4
and Fig. S6†), but the calculated energy is 0.7 eV lower and cal-
culated fos nearly 3× higher – possibly due to involvement of
the alkyne bridge. This seems consistent with the fourth
Gaussian peak used to fit the Stark spectrum of 2, which has
both moderately high Hab and Δμ, and appears responsible for
Fig. 3 (a) ORTEP representation of the molecular anions in 1 and 2.
Disordered parts omitted for clarity, thermal ellipsoids are at the 30%
probability level, C atoms are grey; Mo, green; O, red; N, blue. H atoms
are white spheres of arbitrary radii. (b) Crystal packing in 2 viewed along
the crystallographic a-axis. For clarity, NBu4 cations are colored blue
and [Mo6O18NPhCCPhNPh2]2− anions are colored green.
+
2 this is only ca. 22°, and in 1 19°. Moreover, the imido (accep-
tor) ring in 2 shows a shortening of the ortho-to-meta C–C
bonds (mean distance 1.36(2) Å vs. 1.42(2) Å for ortho/meta-to-
para), suggesting a significant contribution from a quinoidal
resonance form. This is not present in the imido ring of 3,
while in 1 disorder and application of various restraints pre-
cludes such in depth analysis. However, it should be noted the
weak rotational barrier of unhindered phenylacetylenes (ca.
1 kcal mol−1)9 means that these differences will not persist in
solution. The X-ray structure of 2 also reveals that it is the first
POMophore to crystallize in a non-centrosymmetric space
group (Pna21), with polar packing of chromophore dipoles.
These are oriented at ca. 90° to one another, giving the struc-
ture net polarity (Fig. 4b) and making it a potential bulk NLO
material. Instability resulting from large solvent occupied void
spaces has prevented further investigation of this, but the
result suggests that adding steric bulk to the organic system
may be one way to counteract the influence of the POM in the
crystallization process and encourage formation of polar
materials.
Evidence that differences in electronic structure between 1,
2 and 3 persist in glasses and solution is provided by a combi-
nation of Stark spectroscopy (see ESI† for details), and DFT
calculations. In Stark, the low temperature (77 K) electronic
absorption spectra in butyronitrile have a similar overall form
to the room temperature spectra in MeCN, but all three com-
pounds show substantial red shifts in the low-energy IHCT
Fig. 4 TDF-DFT calculated HOMOs for 1 to 3 (bottom) and most
important acceptor orbitals LUMO+x (in terms of oscillator strength
(top)).
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Table 3 ICT absorption and Stark spectroscopic data for 1 to 3
c
d
e
f
g
h
j
k
Amp
(norm)
Fwhh
(eV)
μ12
Δμ12
Δμab
r12
(Å)
rab
Hab
β0
Σβ0
λmaxa (nm)
Emaxa (eV)
fos
(D)
(D)
(D)
(Å)
(103 cm−1
)
cb
(10−30 esu)
(10−30 esu)
180
b
2 i
1
2
3
404 (433)
(405)
(366)
(342)
444 (475)
(451)
(425)
(355)
456 (482)
(456)
3.07 (2.86)
(3.06)
(3.39)
(3.62)
2.79 (2.61)
(2.75)
(2.92)
(3.50)
2.72 (2.57)
(2.72)
0.13
0.56
0.16
0.15
0.22
0.29
0.30
0.20
0.30
0.41
0.29
0.16
0.44
0.28
0.28
0.18
0.28
0.42
0.55
0.20
0.28
0.35
0.06
0.82
0.16
0.16
0.13
0.28
0.46
0.51
0.16
0.34
0.31
2.4
8.4
3.6
3.4
3.6
5.2
6.4
6.2
4.0
5.7
5.3
12.5
17.7
9.1
13.4
24.4
11.6
—
23.7
26.3
28.1
20.0
23.1
26.0
26.8
2.6
3.7
1.9
—
4.7
5.0
5.2
3.3
4.5
4.9
5.1
2.8
5.1
2.4
—
4.9
5.5
5.9
4.2
4.8
5.4
5.6
4.2
8.5
8.4
—
3.2
4.4
5.4
8.7
3.7
4.9
4.7
0.97
0.87
0.90
—
0.98
0.96
0.94
0.90
0.97
0.95
0.96
11
155
12
—
0
51
22.5
24.2
25.0
15.8
21.6
23.3
24.6
350
280
101
142
57
64
121
96
(426)
(2.91)
a In butyronitrile at 77 K; observed absorption maxima with maxima for Gaussian fitting functions in brackets. Data in all subsequent columns
relate to fitted curves. b Obtained from (4.6 × 10−9 M cm2) εmax × fw1/2 where εmax is the maximal molar extinction coefficient and fw1/2 is the full
width at half height (in cm−1). c Calculated using eqn (2), ESI. d Calculated from fintΔμ12 using fint = 1.33. e Calculated from eqn (1), ESI.
f Delocalized electron-transfer distance calculated from Δμ12/e. g Effective (localized) electron-transfer distance calculated from Δμab/e.
h Calculated from eqn (3), ESI. i Calculated from eqn (4), ESI. j Calculated from eqn (5), ESI. k Sum of the β0 vales from each individual Gaussian
function, to 2 s.f. Estimated errors are ca. 20% on β0, μ12 and Δµ12
,
30% on Δμab, Hab and 50% cb2 and are further discussed in the ESI.
most of the β enhancement vs. 3. In compound 1, the HOMO
and HOMO−1 (Fig. 4 and S5†) are both distributed across the
Acknowledgements
entire organic fragment, similar to 3, but in addition the We thank the UK EPSRC National Mass Spectrometry Facility
imido-phenyl based HOMO−3 also spreads across the alkyne for MS. This work was supported by: the Iraqi Government
to the donor ring (Fig. S5†). This strengthened conjugation (HCED scholarship to AAY), Royal Society of Chemistry, EPSRC
across the bridge means that the most important acceptor (EP/M00452X/1) and Fonds voor Wetenschappelijk Onderzoek-
orbitals (LUMO+8, +15 and +18, Fig. 4 and S5†) also spread Vlaanderen (FWO-V, PhD fellowship for NVS). BSB acknowl-
onto the donor phenyl ring, as well as POM and phenylimido edges support from the Beckman Institute of the California
groups, and transitions to entirely POM-based orbitals are very Institute of Technology. Stark data were collected and analysed
weak compared to both of the other POMophores (Table S3–S5 at the Molecular Materials Resource Center of the Beckman
and Fig. S5–S7†). This change in electronic structure and the Institute of the California Institute of Technology.
nature of the transitions is entirely consistent with the smaller
dipole moment changes and stronger electronic coupling
found for this compound by Stark spectroscopy.
In addition to the supplementary information and deposited
cif files, data can be accessed by contacting the corresponding
In conclusion, we have synthesised the highest perform- author.
ance, POM-based molecular 2nd order NLO chromophore
known to date, and crystallised it in a polar space group indi-
cating a potential path to bulk materials. A combined spectro-
Notes and references
scopic and computational approach has revealed that its high
β0 and high visible transparency result from a moderate-
strength conjugated donor group that increases electronic
communication (vs. alkylamino donors) without sacrificing
charge separation. These findings are relevant not only to
molecular design of new POM-based NLO chromophores, but
also to systems for light energy conversion by charge separ-
ation. Moreover, the near-identical trends in β between compu-
tation and experiment, and the qualitative consistency of the
computed transitions and Stark measurements, suggests our
TD-DFT approach is a valid tool to help understand these
systems, although much further work is needed to improve its
quantitative accuracy.
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