An organic white light-emitting dye: Very small molecular architecture displays panchromatic emission

Article abstract of DOI:10.1039/c0cc02598d

The synthesis and photophysical characterization of a new white-light fluorophore is described. The optimization of excitation wavelengths allows the naphthalimide (NI) dyes to display blue, green or white light emission depending on the excitation wavelength.

Full text of DOI:10.1039/c0cc02598d

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An organic white light-emitting dye: very small molecular architecture
displays panchromatic emissionw
Premchendar Nandhikonda and Michael D. Heagy*
Received 16th July 2010, Accepted 8th September 2010
DOI: 10.1039/c0cc02598d
The synthesis and photophysical characterization of a new
white-light fluorophore is described. The optimization of excita-
tion wavelengths allows the naphthalimide (NI) dyes to display
blue, green or white light emission depending on the excitation
wavelength.
Organic fluorophores have at times been criticized for their
wide band emission. In the rapidly developing field of white
organic light-emitting devices (WOLEDs)1 however, such
features have become exalted in the search for dyes with
panchromatic emission.2 To date, very few small molecule
dyes have been reported to display white light emission.
Gratzel recently reported a cyclometallated iridium(III) com-
¨
plex emitting from an excited state of mixed character.3
Among metal-free organic systems, Strongin and coworkers
have developed a white-light emitting system composed of
benzo[a]- and [b]xanthene derivatives.4 Advantages of this
system include water-solubility with simple synthetic design.
More recently, Park and coworkers prepared a white-light
emitting molecule utilizing excited state properties via so-called
frustration of ESIPT relaxation mechanism.5 Lastly, Hou
demonstrated the brightest white-light organic dyes to date
using p-conjugated carbazole-substituted phenyl enynes.6 In
solution, these dyes emit blue-green emission, however upon
vapor deposition, an orange-red band appears that is attributed
to excimer formation. Therefore, the synthesis of efficient and
color-stable white-light emitting single molecules not affected
by the environment represents a current challenge.
Scheme
1 Synthesis of di-substituted-2,3-naphthalic anhydride.
Reagents and conditions: (a) ethylene glycol, toluene, reflux, overnight,
95%; (b) dry DMSO, KOH, ClCH2SO2Ph, overnight, 24 1C, 75%;
(c) AcOH/H2O 4 : 1, reflux, 6 h, 95%; (d) diethyl maleate, 18-crown-6,
K2CO3, CH3CN, 24 1C, then reflux, 6 h, 70%; (e) p-phenylenediamine,
pyridine, reflux, 12 h; (f) p-anisidine, pyridine, reflux, 12 h.
with respect to environmental concerns during processing and
(3) perhaps most importantly, a versatile synthetic plan giving
several structural scaffolds for a variety of synthetic handles as
well as photooptical properties.
A synthesis that generates various 5- and 6-substituted 2,3-
naphthalic anhydrides bearing electron-withdrawing substituents
is critical for panchromatic emission. 6-Choloro-7-nitro-2,3-
NI dyes can be obtained from 4-chloro-3-nitrobenzaldehyde
as starting material (Scheme 1). Protection of this substrate
(ketalization with ethylene glycol) is required for the key step
known as ‘‘vicarious nucleophilic substitution of aromatic
hydrogen’’ involving chloromethyl phenylsulfone.9 This
functionality along with the protected benzaldehyde under-
goes basic elimination in order to react with diethylmaleate
giving diethyl ester 4. Simple condensation yields compounds
I and II.
In an earlier report, we implemented a synthetic matrix
approach to discover dual-fluorescent dyes7 by preparing a
nine element matrix of N-aryl-1,8-naphthalimides.8 Based on
those findings, we report the synthesis and optical measure-
ments of two 2,3-NI systems I and II whereby a balance
between two excited states is achieved via donation into the
N-arene and electron withdrawal from the naphthalimide; a
requirement for two-color spectra. A key hypothesis we sought
to verify involves the alteration of structure with symmetry
related 2,3-NI systems. This study allows us to probe the
effects of the rotationally dynamic N-aryl-2,3-naphthalimide
junction in comparison to similar 1,8 NI systems. The antici-
pated advantages of these new dyes over the aforementioned
systems include (1) reduced molecular weight for improved
vacuum deposition, (2) water solubility as an improvement
As shown in Fig. 1, compound I displays a three-color
emission over the entire visible region. When excited at 423 nm
dye I displays three-color peaks, blue, green and orange
(450–700 nm) with high emission intensities (see inset Fig. 1
middle photo of white light emission). Upon shorter wave-
length excitation (390 nm), I displays intense blue color
emission whereas longer wavelength excitation at 450 nm, I
displays intense green color emission. Although dye II displays
dual fluorescence, it did not exhibit white light emission.
Nevertheless, excitation of II at specific wavelengths promoted
the fluorescence emission of colors similar to I. The origin
of these bands in emission spectra, as fully described in a
recent paper, results from a mix of p–p* and n–p* excited
Department of Chemistry, New Mexico Institute of Mining and
Technology, Socorro, USA. E-mail: mheagy@nmt.edu;
Tel: +1 575-835-5417
w Electronic supplementary information (ESI) available: Data in this
section includes optical absorbance spectra, fluorescence spectra as
well as NMR data for the reported compounds. See DOI: 10.1039/
c0cc02598d
c
8002 Chem. Commun., 2010, 46, 8002–8004
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Table 1 Summary of fluorescence, molar absorptivity and quantum
yield F
Entry Excitation/nm Emission/nm ha
Fb
p - p*
n - p*
I
II
423
390
450
405
6.5 Â 104 3.4 Â 104 0.46
1.4 Â 105 5.5 Â 104 0.36
a
Molar absorptivity I: p–p* 350 nm, n–p* 422 nm; II: p–p* 390 nm,
b
n–p* 422 nm. Quantum yields are calculated based on quinine
sulfate.
This observation lends evidence against any excimer formation
as being responsible for the panchromatic features.10 As
shown in Table 1, both dyes exhibit high fluorescence quantum
yields. In addition, the optical spectra for these dyes indicate
high molar absorptivities. We tentatively assign the shortwave
and more intense absorption as p–p* and the long wavelength
less intense absorption as n–p*.11
In conclusion, we have synthesized two dual fluorescent
2,3-NI dyes using vicarious nucleophilic substitution at the
aromatic C–H bond to deliver the required Diels–Alder synthons.
Our hypothesis that 2,3-NI dyes would exhibit similar photo-
physics as symmetry related 1,8-NI proved to be a correct one
with the added advantage that the 5-ring carboximide/6-ring
N-aryl juncture permits broader fluorescence emission than
the more restrictive 6-ring carboximide/6-ring N-aryl juncture
of 1,8 NI. In addition these dyes exceeded our goals prescribed
earlier such as water solubility, reduced chromophore size and
molecular weight as well as a synthetic path that allows
synthetic diversity of substituents within the 2,3-NI frame-
work. Plans to utilize this synthetic approach for a variety of
panchromatic fluorophores are currently underway. Lastly,
experiments involving solid state optical properties using
electroluminescence are planned in the near future.
Fig. 1 The emission spectra of 20 mM compound I in water with
0.10% DMSO at different excitation wavelengths 390 nm, 423 nm and
450 nm, corresponding to blue, white and green photographs, respec-
tively. Colored light of cuvette is due to output of excitation light.
Fig. 2 Excitation–emission matrix of 20 mM compound I in DMSO
with 0.25% phosphate buffer (35 mM, pH 7).
Notes and references
states.8 An excitation–emission matrix of I in DMSO are
shown in Fig. 2. This figure shows broad spectrum of
wavelengths from 450 nm to 700 nm.
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Fig. 3 Fluorescence spectra of I at different concentrations (1 Â 10À5
,
6 Â 10À5 and 1 Â 10À3 M in water with 0.10% DMSO). The excitation
wavelength was 423 nm.
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8004 Chem. Commun., 2010, 46, 8002–8004
This journal is The Royal Society of Chemistry 2010