Substituent effect on quantum efficiency in 4-aryloxy-N-(2′,6′- diisopropylphenyl)-1,8-naphthalimides: Experimental and computational investigations

Article abstract of DOI:10.1016/j.dyepig.2014.05.007

A series of potential optical brightening agents - 4-aryloxy-N-(2′, 6′-diisopropylphenyl)-1,8-naphthalimides - are synthesized and their photo-physical properties are experimentally determined. Interestingly, it is found that their quantum efficiencies or

Full text of DOI:10.1016/j.dyepig.2014.05.007

Accepted Manuscript
Substituent effect on quantum efficiency in 4-aryloxy-N-(2’,6’-diisopropylphenyl)-1,8-
naphthalimides: Experimental and computational investigations
Tianchai Choppawa , Mongkol Sukwattanasinitt , Somboon Sahasithiwat , Vithaya
Ruangpornvisuti , Paitoon Rashatasakhon
PII:
S0143-7208(14)00187-9
10.1016/j.dyepig.2014.05.007
DYPI 4379
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 4 February 2014
Revised Date: 10 April 2014
Accepted Date: 6 May 2014
Please cite this article as: Choppawa T, Sukwattanasinitt M, Sahasithiwat S, Ruangpornvisuti V,
Rashatasakhon P, Substituent effect on quantum efficiency in 4-aryloxy-N-(2’,6’-diisopropylphenyl)-1,8-
naphthalimides: Experimental and computational investigations, Dyes and Pigments (2014), doi:
10.1016/j.dyepig.2014.05.007.
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ACCEPTED MANUSCRIPT
Substituent effect on quantum efficiency in 4-aryloxy-N-(2’,6’-diisopropylphenyl)-1,8-
naphthalimides: Experimental and computational investigations
Tianchai Choppawa,1 Mongkol Sukwattanasinitt,1,2 Somboon Sahasithiwat,3 Vithaya
Ruangpornvisuti,1 Paitoon Rashatasakhon1,2*
1Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330,
Thailand
2Nanotec-CU Center of Excellence on Food and Agriculture, Department of Chemistry, Faculty
of Science, Chulalongkorn University, Bangkok 10330, Thailand
3National Metal and Materials Technology Center, 114 Thailand Science Park, Pahol-yothin Rd.,
Klong 1, Khlong Luang, Pathumthani 12120, Thailand
Corresponding author: Paitoon Rashatasakhon
Email Address: paitoon.r@chula.ac.th

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Abstract
A series of potential optical brightening agents - 4-aryloxy-N-(2’,6’-diisopropylphenyl)-1,8-
naphthalimides - are synthesized and their photo-physical properties are experimentally
determined. Interestingly, it is found that their quantum efficiencies or quantum yields are
exclusively dependent on the substituents on the aryloxy group at the 4-position. It is postulated
that the electronic effect of the substituents influences the photo-induced electron transfer
process, which causes a variation in quantum efficiencies. Due to analysis of charge transfer
from 1,8-naphthalimide molecular part to its substituents in dichloromethane based on the time-
dependent density functional theory at Becke 3-parameters Lee-Yang-Parr level coupling with
the 6-31G(d) basis set in the conductor-like polarizable continuum model with the united atom
topological model computations, it is found that computed charge transfers of these serial
compounds correspond well with their experimental quantum yields. Validation of the
calculation method is performed by experimental determination of the quantum yield of the
nitro-substituted compound.
Keywords:
N-Aryl naphthalimide
Quantum yield
Optical brightener
Photo-induced electron transfer
TDDFT

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1. Introduction
With their good photo- and thermo stabilities, 1,8-naphthalimide derivatives have been used
as optical brighteners (OB) or fluorescent brightening agents (FBA) [1] - compounds that can
improve whiteness of other materials or surfaces by compensating the blue defects with their
blue fluorescent light [2]. Several derivatives of 1,8-naphthalimides with noble fluorescence
property have also been applied as signal transducers in fluorescent chemosensors [3] and
materials in optoelectronic devices [4]. In our research on synthesis and study of fluorescent
compounds, we became interested in the 4-substituted-1,8-naphthalimide derivatives due to their
generally high emission quantum efficiencies. We herein reported our synthesis and
photophysical properties of a series of N-aryl-4-aryloxy-1,8-naphthalimide. The quantum
efficiencies of these compounds depend on the electronic property of the aryloxyl moiety at the
4-position which can be related to photo induced electron transfer (PET) process. The density
functional theory (DFT) and time-dependent density functional theory (TDDFT) calculations
[5,6] yielded a linear relationship between the fluorescence quantum efficiency and the degree of
charge transfer which is useful for the quantum efficiency prediction.
2. Experimental
2.1 Chemicals and instruments
4-Bromo-1,8-naphthalic anhydride, 4-methoxyphenol, 4-tert-butylphenol, 4-chlorophenol,
methyl 4-hydroxybenzoate, 4-cumylphenol, and 2,6-diisopropyl aniline were purchased from
Sigma-Aldrich (USA) and used without purification. Phenol, 1-butanol, potassium hydroxide,
and potassium carbonate were purchased from Fluka (Switzerland) and used without
purification. Acetic acid and DMF were reagent grade purchased from Aldrich (USA). They

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were stored over 4Å molecular sieve with N2 bubbling for 1 h prior to use. Commercial n-
hexane, ethyl acetate and methanol were fractionally distilled before use.
1
The H NMR experiments were preformed on a Varian Mercury NMR spectrophotometer
13
(Varian, USA) at 400 MHz with chemical shifts reported as ppm in CDCl3. The C NMR was
measured on Bruker Mercury NMR spectrophotometer (Bruker, Germany) which equipped at
100 MHz with chemical shifts reported as ppm in CDCl3. A UV-2550 UV-visible
spectrophotometer (SHIMADZU, Japan) was used for the absorption studies. Emission spectra
were acquired by a Carry Eclipse Fluorescence Spectrophotometer (Agilent Technologies).
Fluorescent lifetimes were measured by a Horiba Jobin Yvon FluoroMax 4 (Horiba Jobin Yvon,
France) using the time correlated single photon counting method with the following settings:
excitation source, 370 nm NanoLED; time range, 100 ns; peak preset 10,000 counts; repetition
rate at 1 MHz; coaxial delay of 60ns. Cyclic voltammograms were obtained from an Autolab
voltammetry instrument.
2.2 Synthesis
4-bromo-N-(2’,6’-diisopropylphenyl)-1,8-naphthalimide (2)
4-Bromo-1,8-naphthalic anhydride (1) (1.0 g, 1.07 mmol) and 6 mL glacial acetic acid
were added into a round bottom flask, then 2,6-diisopropylaniline (0.33 mL, 0.31 g, 1.75 mmol)
was added and the mixture was heated under refluxing conditions for 12 h. After the reaction
was allowed to cool to room temperature, the mixture was poured into 30 g of ice and the
resulting precipitate was filtered, washed with cool water, and dried over a steam bath. The white
powder was purified by column chromatography that used hexane/ethyl acetate (10:1) as the
1
eluent to obtain 2 as white crystalline solid (81%). H NMR (CDCl3) δ (ppm): 1.05 (d, 12H),

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2.65 (m, 2H), 7.25 (d, J = 7.6 Hz, 2H), 7.42 (t, J = 7.8 Hz, 1H), 7.85 (t, J = 7.8 Hz, 1H), 8.05 (d,
J = 8.0 Hz, 1H), 8.40 (d, J = 8.0 Hz, 1H), 8.59 (d, J = 7.6 Hz, 1H), 8.66 (d, J = 7.2 Hz, 1H).
These NMR data are in accordance with the literature information [7].
General procedure of 4-aryloxy-N-(2’,6’-diisopropylphenyl)-1,8-naphthalimide (A-G)
To a mixture of 2 (0.11 g, 0.25 mmol), anhydrous K2CO3 (0.10 g, 0.75 mmol), and DMF (6 mL),
2 mole equivalent of a phenolic compound was added and the reaction was heated at 110 oC and
stirred for 12 h. The mixture was then poured into ice/water and the crude precipitate was
collected by vacuum filtration and washed with water. Target compound was obtained after
purification by column chromatography using hexane/ethyl acetate (10:1) as the eluent.
4-(4-Methoxy-phenoxy)-N-(2’,6’-diisopropylphenyl)-1,8-naphthalimide (A)
1
This compound was synthesized from 2 and 4-methoxyphenol in 81% yield. H NMR
(CDCl3) δ (ppm): 1.15 (m, 12H), 2.74 (m, 2H), 3.88 (s, 3H), 6.90 (d, J = 8.4 Hz, 1H), 7.00 (d, J
= 9.2 Hz, 2H), 7.15 (d, J = 9.2 Hz, 2H), 7.32 (d, J = 7.6 Hz, 2H), 7.45 (t, J = 7.6 Hz, 1H), 7.83 (t,
J = 8.4 Hz, 1H), 8.50 (d, J = 8.4 Hz, 1H), 8.70 (d, J = 8.4 Hz, 1H), 8.80 (d, J = 8.4 Hz, 1H). 13C
NMR (CDCl3) δ (ppm): 24.0, 29.1, 55.7, 109.8, 115.5, 116.2, 122.0, 122.7, 123.9, 126.5, 128.9,
129.4, 130.4, 131.1, 132.4, 133.4, 145.7, 147.9, 157.4, 161.0, 163.8, 164.4.
4-(4-tert-Butyl-phenoxy)-N-(2’,6’-diisopropylphenyl)-1,8-naphthalimide (B)
1
This compound was synthesized from 2 and 4-tert-butylphenol in 78% yield. H NMR
(CDCl3) δ (ppm): 1.16 (d, 12H), 1.39 (s, 9H), 2.75 (m, 2H), 7.00 (d, J = 8.4 Hz, 1H), 7.25 (d, J =
8.0 Hz, 2H), 7.32 (d, J = 8.0 Hz, 2H), 7.45–7.55 (m, 3H), 7.85 (t, J = 8.4 Hz, 1H), 8.52 (d, J =
8.4 Hz, 1H), 8.73 (d, J = 8.4 Hz, 1H), 8.80 (d, J = 8.4 Hz, 1H). 13C NMR (CDCl3) δ (ppm): 24.0,

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29.1, 31.5, 34.6, 110.5, 116.4, 120.2, 122.7, 124.0, 124.2, 126.5, 127.3, 128.9, 129.4, 130.4,
131.0, 132.4, 133.4, 133.6, 145.7, 148.7, 152.4, 160.4, 163.8, 164.4.
4-Cumylphenoxy-N-(2’,6’-diisopropylphenyl)-1,8-naphthalimide (C)
1
This compound was synthesized from 2 and cumylphenol in 89% yield. H NMR
(CDCl3) δ (ppm): 1.08 (d, 12H), 1.67 (s, 6H), 2.67 (m, 2H), 6.80 - 7.30 (m, 12H), 7.39 (t, J = 7.8
Hz, 1H), 7.75 (t, J = 7.8 Hz, 1H), 8.45 (d, J = 8.4 Hz, 1H), 8.64 (d, J = 7.2 Hz, 1H), 8.70 (d, J =
8.4 Hz, 1H), 13C NMR (CDCl3) δ (ppm): 24.0, 29.2, 30.9, 42.9 110.7, 114.8, 120.2, 125.9, 126.8,
128.2, 128.9, 129.4, 132.4, 133.4, 145.8, 148.4, 150.3, 152.6, 160.3, 163.8, 164.5.
4-Phenoxy-N-(2’,6’-diisopropylphenyl)-1,8-naphthalimide (D)
1
This compound was synthesized from 2 and phenol in 79% yield. H NMR (CDCl3) δ
(ppm): 1.20 (d, 12H), 2.80 (m, 2H), 7.00 (d, J = 8.0 Hz, 1H), 7.25 (d, J = 7.6 Hz, 1H), 7.36 (d, J
= 7.6 Hz, 2H), 7.53 (dd, J = 6.4 Hz and 7.6 Hz, 2H), 7.85 (t, J = 7.6 Hz, 1H), 8.57 (d, J = 8.0 Hz,
1H), 8.77 (d, J = 7.4 Hz, 1H), 7.50 (t, J = 7.6 Hz, 1H), 8.61 (d, J = 8.0 Hz, 1H), 8.67 (d, J = 6.8
and 7.6 Hz, 1H), 8.83 (d, J = 8.0 Hz, 1H).13C NMR (CDCl3) δ (ppm): 14.2, 24.0, 29.3, 110.9,
115.4, 116.7, 120.8, 122.8, 124.3, 125.7, 126.7, 128.9, 129.5, 130.5, 131.1, 132.5, 133.4, 145.8,
155.0, 160.2, 163.8, 164.5.
4-(4-Chloro-phenoxy)-N-(2’,6’-diisopropylphenyl)-1,8-naphthalimide (E)
1
This compound was synthesized from 2 and 4-chlorophenol in 96% yield. H NMR
(CDCl3) δ (ppm): 1.13 (s, 12H), 2.72 (m, 2H), 6.95 (d, J = 8.0 Hz, 1H), 7.16 (d, J = 8.4 Hz, 2H),
7.32 (d, J = 8.4 Hz, 2H), 7.46 (d, J = 8.4 Hz, 3H), 7.84 (t, J = 8.0 Hz, 1H), 8.50 (d, J = 8.0 Hz,
1H), 8.65 – 8.75 (m, 2H). 13C NMR (CDCl3) δ (ppm): 24.0, 29.1, 111.0, 117.2, 121.9, 122.8,
124.0, 124.2, 126.8, 128.6, 129.4, 130.4, 130.5, 130.9, 132.5, 133.1, 145.7, 153.6, 159.6, 163.6,
164.3.

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4-(4-Carbomethoxy-phenoxy)-N-(2’,6’-diisopropylphenyl)-1,8-naphthalimide (F)
This compound was synthesized from 2 and methyl 4-hydroxybenzoate in 19% yield. 1H
NMR (CDCl3) δ (ppm): 1.15 (d, 12H), 2.74 (m, 2H), 3.95 (s, 3H), 7.10 (d, J = 8.4 Hz, 1H), 7.20
(d, J = 8.8 Hz, 2H), 7.33 (d, J = 7.6 Hz, 2H), 7.48 (t, J = 7.6 Hz, 1H), 7.85 (t, J = 8.4 Hz, 1H),
8.15 (d, J = 8.8 Hz, 2H), 8.57 (d, J = 8.4 Hz, 1H), 8.70 (d, J = 8.4 Hz, 1H), 8.74 (d, J = 8.4 Hz,
1H). 13C NMR (CDCl3) δ (ppm): 24.0, 29.1, 52.3, 112.8, 117.9, 119.5, 122.9, 124.0, 124.6,
127.0, 128.6, 129.5, 130.4, 130.8, 132.2, 132.6, 133.0, 145.7, 158.5, 159.5, 163.6, 164.2, 166.2.
2.3
Photophysical properties
Absorption
The stock solution of each compound was prepared in CH2Cl2 at 10-5–10-3M, then diluted to
more than 5 different concentrations. To obtain accurate results, all absorbance should not
exceed 1.0. The molar absorptivity (ε) was obtained from the slope of the calibration curve
between concentration and absorbance. The λmax was chosen from the wavelength that provides
the highest absorbance.
Emission
The emission spectra were obtained with the excitation wavelength at 364 nm using a slit width
of 5 nm and the PMT was controlled at 595 volts (for the compound with Φ < 0.50) and at 540
volt (for the compound with Φ ≥ 0.50).
Determination of quantum efficiency
The emission quantum efficiency was determined from the slopes of a plot between the
fluorescence integral and absorbance at λmax of each compound compared with that of quinine

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sulfate in 0.1 M H2SO4 used as the standard with the quantum efficiency of 0.54. The calculation
was corrected by the square of refractive indices according to the reference method [8].
2.4
Electrochemical analysis
For cyclic voltammogram, an electrochemical cell composed of a Ag/ AgNO3 (0.01 M) reference
electrode, glassy carbon (working electrode), and Pt wire electrode counter electrode was
configured under nitrogen atmosphere. A 0.1 M solution of supporting electrolyte
(tetrabutylammonium hexafluorophosphate, TBAPF6) in acetonitrile was used as a solvent for
the preparation of 1 mM solution of tested compounds. All cyclic voltammograms were recorded
at a scan rate of 50 mV per second. The onset oxidation potentials were estimated from the cyclic
voltamograms with the error of ±0.005 eV. The HOMO energy levels were calculated by an
empirical formula, HOMO = -(4.44+Eonset) (eV). The HOMO-LUMO energy gaps (Eg) were
obtained from the UV-Vis absorption band edges. The HOMO levels were calculated from the
on-set oxidation potentials. The LUMO levels were calculated by summation of the HOMO
levels and Eg.
2.5 Quantum chemical calculations and charge transfer analysis
The calculation of the ground state and first singlet electronic excited state of compounds A-G
were performed, using DFT and TDDFT methods [9-10], respectively. Becke’s three–parameter
hybrid exchange function [11-12], Lee–Yang–Parr gradient-corrected correlation (B3LYP
function) [13] and 6-31G(d) basis set [14] were used in both the DFT and TDDFT methods. As
TDDFT method was used to study 1,8-naphthalimide [6], benzene [15], and formaldehyde [15]
of which excited-state geometries and fluorescence energies were found to be too little affected

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by the basis set size, TD/B3LYP/6-31G(d) was therefore employed in this work. The structures
of all relevant compounds were optimized in dichloromethane solution (dielectric constant =
8.93, at 25 ºC), the solvent effect using the conductor-like polarizable continuum model (CPCM)
[16-17] with UAKS cavity model [18]. All calculations were performed with the GAUSSIAN
03 program [19].
3. Results and Discussion
3.1
Synthesis of A-F
The synthesis of target naphthalimide A-F started by a condensation reaction between the
commercially available 4-bromo-1,8-naphthalic anhydride (1) and 2,6-diisopropylaniline in
glacial acetic acid to afford naphthalimide 2 in 81% yield (Scheme 1). Nucleophilic substitution
of the bromo group by the electron-rich phenol derivatives gave rise to 4-aryloxy-1,8-
naphthalimides in excellent yields (A-E), whereas the reaction of 2 with the electron-poor methyl
4-hydroxybenzoate afford only low yield of F along with unreacted 2. The yield of F was not
further optimized as the reaction provided sufficient amount of material for further
investigations.
Scheme 1.
3.2
Absorption and emission properties
The photophysical properties of all compounds were investigated in terms of maximum
absorption wavelengths (λmaxab), molar absorption coefficients (ε), maximum emission
wavelengths (λmaxem), and fluorescent quantum yield (
Φ
). As tabulated in Table 1, the absorption

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properties of all compounds are relatively similar which imply that the substituent at the 4-
position do not significantly influence the electronic states, the energy levels of the ground state
and Frank-Condon excited state, of the compounds (Absorption spectra are provided in the
supporting information). For the emission, the similar λmaxem within the range of 423 to 435 and
small stroke shift also indicated a similar radiative decay from their locally excited states.
Table 1 The photophysical and electronic properties of naphthalimide A-F.
Absorption properties
Emission properties
Electronic properties
Compound
abs
emis
λmax
(nm)a
364
364
365
364
360
359
log ε
(M-1 cm-1 )a
4.32
λmax
Eg
HOMO
(eV)e
LUMO
Φc
(nm)b
435
434
435
425
423
421
(eV)d
(eV)f
A
B
C
D
E
F
0.002
0.110
0.110
0.840
0.640
0.550
2.9952
3.0244
3.0244
3.0693
3.0617
3.0923
-5.8955
-6.1036
-6.1486
-6.1680
-6.1036
-6.2236
-2.9003
-3.0792
-3.1242
-3.0987
-3.0419
-3.1313
4.27
4.11
4.18
4.20
4.30
[a] Measured in diluted CH2Cl2 solution. [b] Excited at 364 nm. [c] Determined by using quinine sulfate in 0.1 M
H2SO4 as standard reference. [d] Estimated from the electronic absorption onset (Eg = 1240/λonset eV). [e] Calculated
from the onset oxidation potential on cyclic voltamograms. [f] Calculated from LUMO = Eg + HOMO.
Despite similarity in the electronic states and radiative decay process, the quantum
efficiencies of this set of compounds are drastically different, for instance, the quantum yield of
A is 0.002 while that of D is 0.84. We thus believe that the substituent at the 4-position is
responsible for these observed results by means of a photo-induced electron transfer (PET)
process. In case of D, the HOMO-LUMO level of fluorophore may lie between the large
HOMO-LUMO gap of the unactivated phenoxy group (π-π*) (Figure 1a). Therefore the PET
process could not occur to the excited fluorophore (Flu*). For the electron-rich aryloxyl
substituents such as p-methoxyphenoxyl in A, p-tert-butylphenoxyl in B and cumylphenoxyl in

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C, their HOMO level might lie in between the HOMO and LUMO level of the fluorophore
(Figure 1b). Upon the excitation of the fluorophore, the electron from the HOMO of the
substituent could transfer to the HOMO of the fluorophore. Subsequent transfer of electron from
the fluorophore LUMO to the substituent HOMO could occur and result in low quantum yield.
Figure 1
In contrast, the HOMO level of electron-poor aryloxy group such as p-chlorophenoxyl in E,
p-carbomethoxyphenoxyl in F might be lower than the HOMO level of the fluorophore, but the
LUMO level of the aryloxy group might lie between HOMO-LUMO gap (Figure 1c). The
excited fluorophore can transfer electron to the LUMO of the aryloxy group, then back to the
HOMO of the fluorophore.
To investigate the behavior of the relaxation process of the excited states of these compounds,
we measured the fluorescence decay times in CH2Cl2 solution. The lifetimes of all compounds
fitted the biexponential function (I/I0 = α1exp[t/τ1] + α2exp[t/τ2]) that allowed the calculation of
the lifetime components (τ1 and τ2) and their average (τave) (τave = a1τ1 + a2τ2). The radiative rate,
kr, and the non-radiative rate, knr, can be calculated using equation 1 and 2 [20].
kr =
Φ
/τave
(1)
(2)
knr = (1-Φ)/τave
The summarized data in Table 2 showed that compounds with electron-rich pendant, i.e. A,
B, and C, have high knr, while compounds with electron-poor pendant, i.e. E and F, have
moderate knr. As expected, compound D has lowest knr and highest kr. However, it should be
noted that knr represents the sum of several processes, for example, photo-induced electron

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transfer, and internal charge-transfer. For compound A, the low quantum yield is more likely due
to the lower kr, probably via a greater degree of ICT process which is also supported by the
largest Stoke shift observed (Table 1).
Table 2. Fluorescence lifetimes (Biexponential) of naphthalimide A-F in CH2Cl2 solution.
Φa
τ1 (ns)
1.65
0.99
1.15
2.88
1.89
1.82
τ2 (ns)
5.30
3.10
4.53
6.21
4.05
6.51
τave (ns) kr (ns-1)
knr (ns-1)
0.2037
0.7946
0.7417
0.0343
0.0911
0.1335
Compound
a1
a2
0.002
0.110
0.110
0.840
0.640
0.550
0.11
0.94
0.98
0.46
0.05
0.67
0.89
0.06
0.02
0.54
0.95
0.33
4.90
1.12
1.20
4.67
3.95
3.37
0.0004
0.0982
0.0917
0.1799
0.1620
0.1632
A
B
C
D
E
F
[a] Data taken from Table 1.
3.3 Molecular orbital calculations
The optimization of structures of compounds A-F in dichloromethane solution obtained by
the CPCM(UAKS)–TDDFT/B3LYP/6–31G(d) computations and Mulliken partial charges for
their R' groups are shown in Figure 2. As compound D emits with highest quantum yield, it was
used as the reference compound for determination of charge transfer for other compounds. In
order to prove the reliability of our calculation method, an additional compound (G) [21] was
synthesized and its quantum yield was experimentally measured. This value will later be
compared with the calculated value.
Figure 2

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The quantum yield for compounds A-F, Mulliken partial charges for their R' groups and
charge transfer to their R' groups are listed in Table 3. It was found that the quantum yields for
compounds A-F vary with their charge transfers and plot of their relationship is shown in Figure
3. It shows fitted lines of four (A, D, E and F) and six (all) points of which the fitted equations
are Φ = 0.91(±0.08)+2.01(±0.33)∆q and Φ = 0.71(±0.21)+1.74(±0.92)∆q, respectively. The
above fitted line shows the high reliability of which the correlation coefficient approachs unity,
(r2=0.9211); the second fitted line has very low reliability of which the correlation coefficient is
very low, r2=0.3390. Nevertheless, x intercept (x value at y = 0 or ∆q at Φ=0) for the first and
second equations are –0.45(±0.11) and –0.41(±0.34) e, respectively. As the first equation is
reliable enough for prediction, it could be used to estimate quantum yield of test compound G. It
can be point that charge transfer to R' group of compound which is less than –0.455 e, its
quantum yield approachs to zero; the cutoff value of charge transfer for equation Φ =
0.91(±0.08)+2.01(±0.33)∆q) is ∆q = –0.455 e.
Due to prediction for quantum yield of compound G (compound E substituted its Cl atom by
nitro group), Mulliken partial charges of all atoms of compound G obtimized at B3LYP/6–
31G(d) level were obtained using CPCM(UAKS)–TDDFT/B3LYP/6–31G(d) method. The
B3LYP/6–31G(d)-optimized structure of compound G is shown in Figure 4 and its computed
partial charges (q = –0.444 e) and charge transfer (∆q = –0.593 e, see Table 2) to NO2 group
were obtained. Due to partial charges and charge transfer for the compound G, ∆q = –0.593 e, its
predicted quantum yield is therefore zero. If (∆q = –0.593 e) charge transfer value for the
compound G was directedly substitued in equation Φ = 0.91(±0.08)+2.01(±0.33) ∆q, its quantum
yield (Φ) is –0.28 which is zero. The predicted quantum yield for the compound G of zero was
found as shown in Figure 3.

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Table 3 The experimental quantum yields, computed partial charges, charge transfers and
calculated quantum yields of naphthalimide A-G.
Compound
Exp. Φa
q b,c,d
∆q d,e
Cal. Φf
A
B
C
D
E
F
0.002
0.110
0.110
0.840
0.640
0.550
0.002g
-0.274
-0.006
-0.026
0.149
-0.023
-0.070
-0.444
-0.423
-0.155
-0.175
0.000
-0.172
-0.219
-0.593
0.06
0.60
0.56
0.91
0.56
0.47
G
0.00h
[a] Taken from Table 1. [b] Partial charge of their substituents, R', in e. [c] Mulliken charges obtained from the
CPCM(UAKS)–TDDFT/B3LYP/6–31G(d) computations, in e. [d] In e. [e] Charge transfer defined as difference
between partial charge of their substituents and partial charge of the compound D, in e. [f] Computed using equation
Φ = 0.91(±0.08)+2.01(±0.33)∆q. [g] see ref. [20], [h] Its charge transfer is less than a cutoff value (∆q = –0.455 e).
Figure 3
Figure 4
4. Conclusion
Seven derivatives of N-(2,6-diisopropylphenyl)-1,8-naphthalimides with an aryloxy
substituent at the 4-position were synthesized from the commercially available 4-bromo-1,8-
naphthalic anhydride and subjected to a comparative evaluation of the photophysical properties.
The results revealed that their quantum efficiencies were exclusively depended on nature of
substituents on the aryloxy groups. The compound with a phenoxy group (D) exhibited the
highest quantum yield, while those compounds with electron-rich or electron-poor aryloxy
groups possessed lower quantum efficiencies. It was proposed that quantum yields of these
compounds depend the magnitude of electron transfer from fluorophores (1,8-naphthalimide) to

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their substituents. By CPCM(UAKS)–TDDFT/B3LYP/6–31G(d) computations, the quantum
yield (Φ) of these compounds can be theoretically predicted from their charge transfers (∆q)
using linear regression equation, Φ = 0.91(±0.08)+2.01(±0.33) ∆q, of which negative quantum
yield means zero value. The prediction was later proven as the experimentally determined
quantum yield of 4-(4’nitrophenoxyl)-N-(2,6-diisopropylphenyl)-1,8-naphthalimides agreed well
with the calculated value.
Acknowledgements
This work is supported by the Higher Education Research Promotion and National Research
University Project of Thailand, Office of the Higher Education Commission (AM1006A-56), the
Ratchadapiseksomphot Endowment Fund of Chulalongkorn University (RES560530125-AM),
and the Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology,
Thailand, through its program on Center of Excellence Network. This work is a part of the
Project for Establishment of Comprehensive Center for Innavative Food, Health Products and
Agriculture supported by the Thai Government Stimulus Package 2 (TKK2555, SP2).
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[21] This compound was synthesized from 2 and 4-nitrophenol in 8% yield. 1H NMR (CDCl3) δ
(ppm): 1.10 (d, 12H), 2.68 (m, 2H), 7.10 (d, J = 8.0 Hz, 1H), 7.27 (d, J = 7.6 Hz, 2H), 7.40
(t, J = 8.0 Hz, 1H), 8.20-8.30 (m, 4H), 8.52 (d, J = 8.0 Hz, 1H), 8.56 (d, J = 8.0 Hz, 1H),
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Scheme 1 Synthesis of naphthalimide A-F

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Figure 1. Proposed mechanisms for contribution of substituents’ energy levels on PET

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Figure 2. The optimization of structures of compounds A-F, computed at the CPCM(UAKS)–
TDDFT/B3LYP/6–31G(d) level of theory. Compound D, R'=H whose partial charge is 0.149 e
and which emitts with highest quantum yield, is used as reference compound for charge transfer
determination. Mulliken partial charges, in e, for their R' groups are surrounded by dotted lines.

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Figure 3. Plot of relationship between quantum yeilds and charge transfers for compounds A-F.
Above (red line) and bottom (green line) lines are fitted from four (A, D, E and F) and six (A-F)
points data, respectively. The above line (Φ = 0.91(±0.08)+2.01(±0.33)∆q), its cutoff value for
charge transfer is ∆q =
compound (G).
–0.455 e. The red outline square bullet is the predicted plot for tested

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Figure 4. The CPCM(UAKS)–TDDFT/B3LYP/6–31G(d)–optimized structure of compound G
and Mulliken partial charge (q= –0.444 e) of its NO2 group.

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Research Highlights
•
•
•
The substituent effect on quantum yields of 4-aryloxy-N-(2’,6’-diisopropylphenyl)-1,8-
naphthalimides was studied.
The photo-induced electron transfer process was hypothesized and justified by a
computational calculation.
The experimentally determined quantum yield of an additional compound agreed with the
calculated value.