White-light emission from a structurally simple hydrazone

Article abstract of DOI:10.1039/c9cc03912k

Two hydrazones featuring a unique excitation wavelength-dependent dual fluorescence emission have been developed. The mixing extent of the two emission bands can be modulated by tuning the excitation wavelength, resulting in multicolor and even white light emission from structurally simple hydrazones.

Full text of DOI:10.1039/c9cc03912k

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White-Light Emission from a Structurally Simple Hydrazone
Baihao Shao,a Nell Stankewitz,a Jacob A. Morris,b Matthew D. Liptakb and Ivan Aprahamian*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Two hydrazones featuring
a
unique excitation wavelength- pH and light activated switches.17 As part of these efforts, we
dependent dual fluorescence emission have been developed. The report here on the development of two hydrazones (1 and 2;
mixing extent of the two emission bands can be modulated by Fig. 1) that exhibit excitation-dependent emission, the linear
tuning the excitation wavelength resulting in multicolor and even mixing of which results in the emission of a variety of colors,
white light emission from structurally simple hydrazones.
including white light in the case of 1. Based on spectral
evidence along with fluorescence decay features, we
hypothesize that the excitation-dependent emission results
from conformational changes that lead to two distinct
emissive states in 1 and 2.16d,18
Excitation wavelength-dependent fluorescence emission is a
sought-after property as it results in fluorophores with tunable
optical properties without the need for changes in molecular
structure, mixture composition ratio, or molecular
R
O
interactions.1
photoluminescence
Furthermore
in
excitation-dependent
single-component system
Br
Br
O
H
a
N
N
Na2S2O5,
i. n-BuLi, THF, -78 °C
O
tremendously simplifies device fabrication, thus increasing
their practicality and potential use in advanced lighting
N
H
H2O, 140 °C
Sealed tube
83 %
R
3
applications2 and optical sensing devices. However, and
ii.
O
N
OH
based on Kasha’s rule,4 emission from fluorophores (e.g.,
organic dyes,5 inorganic metal complexes6 and quantum dots7)
is rarely dependent on excitation wavelength. Nonetheless,
effort has been made to discover and elucidate excitation-
dependent optical profiles in systems including graphene8 and
its derivatives9, organic nanoparticles10 and nanowires11,
carbon nanotubes12 and nanodots.13 Quantum confinement
effects caused by uneven size distributions14 lead to the
coexistence of discrete electronic transitions of varying
energies in such systems, leading to the observed excitation-
dependent photoluminescence. Although this phenomenon is
relatively common in large scaffolds, it is rarely seen in small
molecular architectures (i.e., low molecular weight organic
molecules),15 which are usually easier to access and
manipulate synthetically.
N2
5
1: R= -H
2: R= -NO2
THF, -78 °C to rt
N
Fig. 1 The synthetic procedure for accessing hydrazones 1 and 2.
Hydrazones 1 and 2 were synthesized in a straightforward
manner, involving in its main step the coupling between 2-
diazo-2-phenylacetate or ethyl 2-diazo-2-(4-nitrophenyl)
acetate with (6-(dimethylamino) naphthalen-2-yl) lithium
under inert conditions (Fig. 1). After column chromatography,
1 and 2 were isolated as orange and red solids in 85 and 25 %
yield, respectively, and characterized by NMR spectroscopy
(Fig. S4-7, ESI†), mass spectrometry and X-ray crystallography
(Fig. S19 and S20, ESI†). Both hydrazones show the
characteristic low-field shifted proton signal ( = 12.4 and 12.7
ppm in CD3CN, respectively) assignable to the H-bonded (with
the carbonyl group) hydrazone N-H proton, implying that they
adopt the Z configuration in solution. The crystal structures of
1 (N-H···O=C, 2.505(4) Å, 136.22)19 and 2 (N-H···O=C, 2.578(2)
Å, 135.24) show that this bond is also persistent in the solid-
state.
Our group is interested in developing functional
hydrazone-based materials, ranging from fluorescent dyes16 to
a
Department of Chemistry, Dartmouth College, Hanover, New
The photophysical properties of hydrazones 1 and 2 were
studied in toluene using UV/Vis and fluorescence
spectroscopies. An equilibrated solution of 1 in toluene
exhibits a broad absorption band with a maximum (max) at
415 nm ( = 24000 M-1cm-1; Fig. 2a).20 A small shoulder is
Hampshire 03755, USA. E-mail: ivan.aprahamian@dartmouth.edu
Department of Chemistry, University of Vermont, Burlington, Vermont
05405, USA.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
b
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observed at around  = 380 nm, indicating the presence of a highlighting the accessible range of excitation-dependent
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DOI: 10.1039/C9CC03912K
second electronic transition. Surprisingly, the solution of 1 multicolor emission.
gives two well-separated emission bands (em = 423 and 550
Unlike compound 2 that features two completely
nm) upon excitation (ex) at 380 nm (Fig. 2b). This observation separated fluorescence bands (em = 200 nm), compound 1
led us to explore the emission of 1 under variable excitation shows two partially overlapping emission bands (Fig. 2b),
wavelengths. When excited at a short wavelength (ex = 340 which coincidently cover the whole visible range (i.e., 400700
nm) hydrazone 1 exhibits an emission band at 423 nm (FL
=
nm). By tuning the excitation wavelength, the toluene solution
1.09  0.07%), along with a relatively weaker one at 550 nm. of 1 undergoes a color transition from blue (0.19, 0.09) to
Upon switching excitation to a longer wavelength (ex = 420 yellow (0.42, 0.54) (Fig. 3b). As seen in the CIE diagram, 1
nm) the emission band at 550 nm (FL = 0.10  0.01%) exhibits an extensive range of color variation, which covers
increases in intensity, while the intensity of the band at 423 even the central part of the CIE diagram (i.e., the white light
nm drastically diminishes. Such a variation in emission band as space; Fig. 3a). Excitation of 1 at 395 nm leads to roughly equal
a
function of excitation wavelength in small organic intensity emission bands and results in the best coverage of
compounds is highly unusual. 21
the visible range, leading to white light emission with CIE
coordinates of (0.31, 0.35), which is very close to pure white
emission (0.33, 0.33).
While we observed fluorescence emission from other
hydrazones before,16f such an unusual excitation-dependent
emission profile is unique to 1 and 2, which contain a 6-
(dimethylamino) naphthalen-2-yl subunit as part of the overall
structure. Consequently, we studied (Fig. S13, ESI†) the
photophysical properties of the starting naphthyl material (5)
and realized that it emits blue fluorescence (em = 405 nm)
upon excitation at 365 nm (max). This emission feature
corresponds well with the bluish components found in the
spectra of 1 and 2 (i.e., 423 nm and 415 nm, respectively).
Based on this observation we conclude that the shorter
wavelength emission in 1 and 2 mainly emanates from the
naphthyl moiety, while the reddish components (i.e., 550 nm
and 615 nm) stem from the whole hydrazone, in agreement
with previous observations.16f
Fig.2 (a) UV/Vis absorption spectrum of 1 (5  10-6 M) in toluene; (b)
Normalized fluorescence emission spectra of 1 (5  10-6 M) in toluene,
under different excitation wavelengths (340 to 420 nm); (c) UV/Vis
absorption spectrum of 2 (5  10-6 M) in toluene; (d) Normalized
fluorescence emission spectra of 2 (5  10-6 M) in toluene, where
excitation wavelengths ranging from 340 to 460 nm were applied.
Compared to 1, compound 2 exhibits a more structured
absorption band, which is red-shifted to max = 453 nm ( =
19800 M-1cm-1; Fig. 2c) as a result of the push-pull character
of the molecule.22 Moreover, a weaker absorption band at max
= 340 nm ( = 8800 M-1cm-1) is observed (Fig. 2c) suggesting
that there is a second electronic transition in the system.
Excitation of 2 with 340 nm light results in an intense emission
band at 415 nm (FL = 0.28  0.01%), accompanied with a
comparably weak emission at 615 nm (Fig. 2d). When longer
wavelengths are used for excitation the intensity of the
emission band at 415 nm decreases, whereas the one at 615
nm is enhanced (FL = 0.10  0.01%).
The tunability of the relative intensities of the two
emission bands in 1 and 2 allowed us to access multiple color
emissions (i.e., other than the blue and red ones mentioned
above) from structurally simple molecules.23 Excitation of 2 at
340 nm leads to a violet color (Fig. 3d) with a CIE coordinate of
(0.26, 0.13). Lowering the excitation energy red-shifts the
emission band, resulting in a color transition from blue to blue-
orange and finally reaching orange red (0.56, 0.42; Fig. 3d).
The trend of color changes as a function of excitation energy is
summarized in the CIE diagram depicted in Figure 3c,
Fig. 3 (a) CIE chromaticity diagram of 1 labelled with CIE coordinates
calculated from the spectra in Fig. 2b, where white light (0.31, 0.35) is
generated using 395 nm light excitation; (b) Photos of compound 1 in
toluene using various excitation wavelengths (i.e., 340, 365, 394 and
410 nm); (c) CIE chromaticity diagram of 2 showing the color changes
as a function of excitation wavelength, based on the fluorescence
emission spectra shown in Fig. 2d; (d) Selected photos for toluene
solutions of 2 under excitation at 340, 365, 394 and 442 nm light. The
red arrows in Fig. 3a and 3c represent the changes in CIE coordinates
(color variation) as the excitation wavelength shifts bathochromically.
2 | J. Name., 2012, 00, 1-3
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Next, we measured the fluorescence excitation spectra and systems, including white emission in 1. ThVieew AdrtiicflfeeOrnelinnet
DOI: 10.1039/C9CC03912K
fluorescence lifetimes of 1 and 2 (using time-correlated single fluorescence decay profiles and excitation spectra of the two
photon counting (TCSPC)) to garner further insights into the emission bands, which are temperature dependent, allow us
nature of the two emissive states. In 1, the excitation spectrum to conjecture that there are two conformationally accessible
(em = 550 nm; Fig. 4a) corresponds to the absorption band emissive states24 in the hydrazones leading to the excitation-
(max = 415 nm) confirming the origin of the reddish dependent fluorescence property. This unique characteristic of
component in 1. The excitation spectrum of the bluish 1 and 2 make them promising candidates for advanced full-
component (em = 423 nm) of 1 on the other hand has a color display25 and ratiometric sensing26 applications, among
maximum at 360 nm (Fig. 4a) that does not correspond to others.
bands in the absorption spectrum. Moreover, the two
emissions have different fluorescence decay profiles (Fig. S14,
S15, and Table S1) illustrating that they originate from
Conflicts of interest
different excited states. Similar fluorescence excitation (Fig.
4b) and fluorescence decay profiles (Fig. S16, S17 and Table
S1) were measured for the two emissions (em = 415 and 615
nm) of compound 2. The fluorescence excitation spectrum
(Fig. 4b) of the 615 nm emission matches the main absorption
band (max = 453 nm), whereas the other one (em = 415 nm)
fits the weaker absorption band (max = 340 nm) measured in
2. These results lead us to conclude that there are two distinct
emissive states present in hydrazones 1 and 2 giving rise to the
excitation wavelength-dependent emission profiles.
There are no conflicts to declare.
Acknowledgements
This work was supported by the generous support of the
National Science Foundation (DMR-1506170 for IA, and DMR-
1506248 for MDL). N. S. acknowledges the support of the
Deutscher Akademischer Austauschdienst (DAAD) RISE
program. We are grateful to Prof. Richard Staples (Michigan
State University) for the X-ray crystallography data.
Notes and references
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Fig. 4 (a) Normalized UV/Vis spectrum (orange) and fluorescence
excitation spectra of 1 in toluene monitored at the emission
wavelengths of 423 (red) and 550 nm (blue), respectively; (b)
Normalized UV/Vis spectrum (orange) and fluorescence excitation
spectra of 2 in toluene monitored at the emission wavelengths of
415 (red) and 615 nm (blue), respectively.
To further elaborate on the nature of the two emissive states
we measured the emission spectra of 1 and 2 in toluene at 77K
(Figs. S21- S23, ESI†). Neither of the hydrazones exhibits
excitation-dependent emission at low temperature, i.e., there
is no blue-shifted emission. This result is similar to what happens
in the solid-state (Fig. S12, ESI†), i.e., the emission band of 1 in
the solid-state (em = 570 nm; FL = 3.3  0.1%) does not
change upon excitation with different wavelengths of light
(340, 394 and 410 nm). We hypothesize that in the rigid
environments the hydrazones are locked in
a planar
conformation (Figs. 19 and 20, ESI†) that prohibits the
population of the higher excited state, and hence the blue
emission is quenched. This in turn means that in solution, the
rotation of the naphthyl fragment, results in a conformation
(e.g., perpendicular) that decouples it from the hydrazone core
thus populating the higher emissive state and leading to blue
emission.
To conclude, we synthesized two structurally simple
hydrazone-based fluorophores (1 and 2) that exhibit
excitation-dependent emission. By fine-tuning the excitation
wavelengths, multicolor emissions can be observed in both
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