Synthesis, photophysical and computational studies of two lophine derivatives with electron-rich substituents in the 2-position

Article abstract of DOI:10.1016/j.molstruc.2016.10.050

An exploration of the photophysical properties of two lophine derivatives with electron-donating groups in the 2-position began with the preparation of compounds 1 and 2 via one-pot reactions in good yields, 83% and 74%, respectively. The absorption spect

Full text of DOI:10.1016/j.molstruc.2016.10.050

Journal of Molecular Structure 1130 (2017) 284e290
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Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
Synthesis, photophysical and computational studies of two lophine
derivatives with electron-rich substituents in the 2-position
Terianne Hamada, Tammy Le, Matthew J. Voegtle, Bryan Doyle, Jarred Rimby,
Ralph Isovitsch*
Department of Chemistry, Whittier College, Whittier, CA 90608, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
An exploration of the photophysical properties of two lophine derivatives with electron-donating groups
in the 2-position began with the preparation of compounds 1 and 2 via one-pot reactions in good yields,
83% and 74%, respectively. The absorption spectra of 1 and 2 had bands at approximately 300 nm
Received 13 July 2016
Received in revised form
19 September 2016
Accepted 14 October 2016
Available online 15 October 2016
(ε
z
25,000e34,000 Mꢀ1 cmꢀ1), while that of
2
had an additional band at 348 nm
* transitions. Excitation into the ab-
(ε ¼ 35,600 Mꢀ1 cmꢀ1). These absorptions were assigned to
p/p
sorption band of 1 at approximately 300 nm produced emission at 387 nm, while excitation into either of
the absorption bands of 2 produced emission at 406 nm. Of the two compounds, 2 had the higher
quantum yield. The emission spectra of compounds 1 and 2 were slightly blue-shifted at 77 K. Excited
state lifetimes for 1 and 2 were short (indicating that the observed emission was fluorescence) at room
temperature and 77 K, ranging from 1.1 to 1.8 ns. Computational studies of both compounds 1 and 2 were
performed to better understand how their structures relate to their photophysical properties.
© 2016 Elsevier B.V. All rights reserved.
Keywords:
Lophines
Synthesis
Absorption
Emission
Excited states
Computational
1. Introduction
of 2,4,5-triphenyl-1H-imidazole) have been extensively studied
since Radziszewski first observed light being emitted from a solu-
The synthesis and characterization of molecules with unique
photophysical properties is of interest to a number of research areas
[1,2]. Toward that end, many different strategies exist to prepare
luminescent molecules with exploitable properties. One strategy
utilized is the incorporation of a polycyclic aromatic hydrocarbon
moiety [3,4]. A recent example of this strategy in the literature
involves the preparation and photophysical investigation of eight
perylenetetracarboxy-3,4:9,10-diimide derivatives with potential
use in a wide range of applications, such as organic electronics and
environmental analysis. All eight of the perylenediimide de-
rivatives produced have large molar extinction coefficients and
fluorescence quantum yields, key requirements for the application
of luminescent molecules in devices [5]. Another strategy involves
the use of (E)-stilbene (a well-studied and easily prepared lumi-
nescent compound) as a building block for potentially useful
luminescent molecules [6e9]. One of the many areas of research
into the application of derivatives of (E)-stilbene involves the cre-
ation of solid-state molecular switches [10]. Lophines (derivatives
tion of lophine and sodium hydroxide in the presence of air over
125 years ago [11,12].
Current interest in lophines seeks to take advantage of their
novel luminescent properties to construct novel materials as well
as to create new lophines with photophysically interesting and
useful properties. Lophines have been prepared with thio-
phenevinyl moieties that have resulted in a class of molecules with
high thermal stability and notable molecular nonlinearity [13].
Thermochromic host-guest materials have been constructed using
nitrolophine derivatives and guest molecules that possess
hydrogen donor or hydrogen acceptor groups [14]. Lophines
designed to take advantage of excited-state intramolecular proton
transfer were observed to be highly luminescent over a wide range
of the visible spectrum, and were thus incorporated into a molec-
ular pixel system [15,16]. 1-Phenylnaphthalene bridged lophine
dimers have been prepared that possess the ability to engage in
two-photon absorption processes. Such bridged lophine dimers are
viable candidates for optical materials [17].
It is clear from the literature that lophine derivatives possess
diverse and exploitable photophysical properties that are appli-
cable to numerous research areas. The overarching goal of our
research is to gain a better understanding of the photophysical
* Corresponding author.
E-mail address: risovits@whittier.edu (R. Isovitsch).
http://dx.doi.org/10.1016/j.molstruc.2016.10.050
0022-2860/© 2016 Elsevier B.V. All rights reserved.

T. Hamada et al. / Journal of Molecular Structure 1130 (2017) 284e290
285
characteristics of lophines, toward their application to a research
area new to the use of lophines, specifically light harvesting de-
vices. Compounds 1 and 2 have been prepared and their structures
incompletely characterized and their photophysics only given a
cursory investigation [18,19]. As we progress toward our research
goal, we present syntheses (Scheme 1) and complete structural
characterizations of compounds 1 and 2, a thorough investigation
of their steady-state and time-resolved photophysical characteris-
tics at room temperature and 77 K, as well as a computational study
to aid in understanding how their structures relate to their excited
state properties.
(
lex ¼ 310 nm and 330 nm respectively) were measured from
deoxygenated, room temperature acetonitrile solutions by com-
parison with 2,4,5-triphenyl-1H-imidazole (lophine) that had been
triply recrystallized from absolute ethanol (lmax,em ¼ 386 nm;
Ffl ¼ 0.27 in deoxygenated acetonitrile at room temperature)
[21,22]. Absorbances of solutions of 1 and 2 and the standard
lophine solutions were kept below 0.1 to avoid inner filter effects
and were optically matched to within 3e8%. The reported values
are an average of three trials.
2.4. Computational methods
2. Experimental
The Gaussian09 suite of programs, with a tighten self-consistent
field convergence threshold of 10ꢀ8 atomic units, was used in the
computational modeling of compounds 1 and 2[23]. Modeling
tasks were broken down into two steps: (i) optimization of ge-
ometries and bond vibrational frequencies (ii) quantification of
vertical transitions energies by TD-DFT methods [24]. Absorption
spectra were obtained after convolution with 0.100 eV half width at
full maximum Gaussian.
The modeling of compounds 1 and 2 was accomplished using
Becke's three parameter hybrid functional, B3LYP. The optimization
and frequency calculation of the ground state of 1 was done using
the 6-311Gþ(d,p) atomic basis set, and the ground state geometry
of 2 was calculated with 6-311Gþþ(2d,2p), due to the failure of
smaller basis sets to properly account for aromaticity present in the
imidazole core. Excited state information for both compounds was
extrapolated using 6-311Gþ(d,p) [25]. In order to properly model
the absorption spectra of 1 and 2, their solvation in acetonitrile was
considered using polarizable continuum model (IEFPCM), which
creates a solvent cavity from overlapping spheres of the solvent
molecules [26]. The processes employed in the computational
studies of compounds 1 and 2 are common methods used in the
study of the photophysics of organic molecules [27,28].
2.1. General synthetic methods
Standard procedures were used to synthesize compounds 1 and
2. Solvents and reagents were purchased from Sigma-Aldrich and
used as received. Melting points were measured in open capillaries
and are uncorrected. All 1H and 13C NMR spectra were obtained
from DMSO-d6 solutions of 1 and 2 using the residual solvent peak
as the internal standard on a JEOL ECX 300 MHz spectrometer. IR
spectra were recorded from KBr disks using a Perkin-Elmer Spec-
trum Two FT-IR spectrometer. M-H-W Laboratories of Tucson,
Arizona did the elemental analysis.
2.2. Photophysical measurements
Emission, excitation and absorption spectra were recorded at
room temperature and 77 K from 5e6 ꢁ 10ꢀ6 M deoxygenated
solutions of 1 and 2 prepared with spectrophotometric grade
acetonitrile and propanenitrile (99%þ) using a HoribaJobinYvon
FluoroMax-4 fluorometer and a Hewlett Packard 8453 diode array
spectrometer. Emission spectra were corrected for detector
response using a correction curve supplied by the fluorometer
manufacturer.
2.5. Preparation of 4,5-diphenyl-2-(3,4,5-trimethoxyphenyl)-1H-
imidazole (1)
Excited state lifetimes were measured from 5e6 ꢁ 10ꢀ6
M
deoxygenated solutions of compounds 1 and 2 in spectrophoto-
metric grade acetonitrile and propanenitrile (99%þ) utilizing a
HoribaJobinYvon TCSPC apparatus with excitation from a pulsed
LED laser at 330 nm. A multiexponential decay analysis program
provided by the instrument manufacturer was used to analyze the
data. Three criteria were used to assess the quality of fitted decay
Benzil (0.50 g, 2.4 mmol), 3,4,5-trimethoxybenzaldehyde
(0.47 g, 2.4 mmol), and ammonium acetate (2.00 g, 25.9 mmol)
were dissolved in 20 mL of glacial acetic acid and refluxed for 2 h. At
the end of 2 h the reaction mixture turned a deep orange-red color.
The reaction mixture was cooled and poured into 300 mL of water
resulting in the formation of a cream colored precipitate. The cream
colored solid was collected by suction filtration, washed well with
H2O, and air-dried. 0.77 g (83% yield) of the pure 1 was obtained.
M.P. 245e246 ꢂC. Rf (silica, ethyl acetate) ¼ 0.86. IR (KBr): 3061,
2934, 1589, 1125 cmꢀ1. 1H NMR (300 MHz, DMSO-d6): 3.76 (d, 9 H);
7.36 (m, 12 H); 12.59 (br.s. 1H). 13C NMR (75 MHz, DMSO-d6): 56.5,
60.6, 102.8, 126.4, 127.0, 127.6, 128.4, 128.6, 128.7, 129.1, 129.3, 131.7,
135.6, 137.4, 138.1, 145.9, 153.6. Anal. Calc. for C24H22N2O3 (386.42):
C, 74.59; H, 5.74; N, 7.25; Found: C, 74.34; H, 5.82; N, 7.26. HR-FAB-
curves: chi-squared values (c
2) that were less than 1.2; plots of the
residuals that displayed the least oscillation and varied no more
than three standard deviations; and visual inspection of the
goodness-of- fit between the experimental and fitted decay curves
[20].
2.3. Quantum yield determination
Fluorescence quantum yields for compounds
1 and 2
Scheme 1. The syntheses of compounds 1 and 2.

286
T. Hamada et al. / Journal of Molecular Structure 1130 (2017) 284e290
MS: 387.1726 [MþH]þ: calc. 387.1709.
Table 1
Absorption and emission data for compounds 1 and 2.a
lmax,abs nm (ε)b
lmax,em nm (lexc
)
Ffl
2.6. Preparation of 4,5-diphenyl-2-(N,N-diphenylamino)-1H-
imidazole (2)
Compound 1
CH3CN solution
310 (30,700)
226 (33,900)
387 (310)
0.29
Benzil (0.50 g, 2.4 mmol), 4-(diphenylamino)benzaldehyde
(0.59 g, 2.4 mmol), and ammonium acetate (2.00 g, 25.9 mmol)
were added to 20 mL glacial acetic acid. The solution was refluxed
for 3.5 h, during which the reaction mixture turned a deep red
color. The reaction mixture was cooled to room temperature and
then poured into 200 mL of water, which resulted in the formation
of a yellow precipitate. The yellow solid was collected by suction
filtration, washed well with H2O, and air-dried. Recrystallization
from glacial acetic acid gave 0.83 g (74% yield) of pure 2 as yellow
crystals.
CH3CH2CN glass at 77 K
Solid state
373 (310)
567 (310)
469
Compound 2
CH3CN solution
348 (35,600)
298 (25,400)c
406 (348)
0.47
CH3CH2CN glass at 77 K
402 (349)
392
373 (348)
Solid state
a
Spectra were acquired at room temperature unless otherwise noted.
Molar extinction coefficient units Mꢀ1 cmꢀ1
.
b
c
M.P. >260 ꢂC. Rf (silica, CH2Cl2) ¼ 0.51. IR (KBr): 3057, 1590,
1489 cmꢀ1. 1H NMR (300 MHz, DMSO-d6): 7.05 (m, 8 H); 7.39 (m,
14 H); 7.94 (m, 2 H); 12.53 (br.s, 1 H). 13C NMR (75 MHz, DMSO-
d6):123.2, 123.9, 124.8, 125.0, 127.0, 127.6, 128.2, 128.4, 128.9, 129.1,
130.1, 131.7, 135.7, 137.4, 146.0, 147.5, 147.8, 172.6. Anal. Calc. for
Shoulder.
Table 2
Excited state lifetimes with relative amplitudes. and chi-squared (
c
2) values for
,
b
compounds 1 and 2.a
C
33H25N3$C2H4O2 (523.62): C, 80.28; H, 5.58; N, 8.02; Found: C,
80.04; H, 5.51; N, 7.98. HR-FAB-MS: 464.2120 [MþH]þ: calc.
t
(ns)
c2
464.2127.
Compound 1
CH3CN solution
CH3CH2CN glass at 77 K
1.8 (100.00)
1.9 (94.28)
5.4 (5.72)
1.04
1.06
3. Results and discussion
Compound 2
CH3CN solution
3.1. Syntheses of 1 and 2
1.5 (100.00)
0.9 (0.62)
0.98
1.06
CH3CH2CN glass at 77 K
1.1 (99.38)
The preeminent method of lophine synthesis remains the
condensation route pioneered by Radziszewski and Japp, which
involves the reaction of a 1,2-dicarbonyl compound with an aro-
matic aldehyde and a source of ammonia [29,30]. This route is
attractive because it is easy to execute and permits a variety of
starting materials. In our hands, this synthetic method gave com-
pounds 1 and 2 pure (as evidenced by elemental analyses that gave
experimental values within 0.24 of the theoretical values), and in
good yields, 83 and 74%, respectively (Scheme 1).
a
Excited state lifetimes were acquired at room temperature unless otherwise
noted.
b
Relative amplitudes are presented as percentages in parentheses.
Compounds 1 and 2 exhibited spectral characteristics that
supported their identity. For example, the HR-FAB-MS of 1 had a
[MþH]þ that was observed at m/z ¼ 387.1726, while 2 had a
[MþH]þ that was observed at m/z ¼ 464.2120. Both of these
observed [MþH]þ peaks were less than 5.0 ppm from the calculated
values. Another key indicator of product formation was the 1H NMR
spectra of 1 and 2, which had broad singlets for their imidazole
hydrogen atoms at 12.59 and 12.53 ppm, respectively [31]. The only
anomaly was in the 13C NMR spectrum of 2, which exhibited one
less carbon resonance than expected. This is attributed to unre-
solved carbon atoms with very similar chemical shifts (see the 13C
NMR spectra of 2 presented in Figs. S1 and S2 in the Supplementary
Material).
3.2. Photophysical properties of compounds 1 and 2
Fig. 1. Electronic absorption and emission spectra of compound 1. Absorption spec-
trum in acetonitrile (ꢀꢃꢀ); room temperature emission spectrum in acetonitrile
Understanding the photophysical properties of lophines is
necessary for their inclusion in devices, such as molecular pixels
[16]. Steady-state and time-resolved photophysical data for 1 and 2
are presented in Tables 1 and 2, respectively. The absorption and
emission spectra of 1 (Fig. 1) and 2 (Fig. 2) are also presented.
Absorption spectra obtained from dilute acetonitrile solutions of
(
(
lex
¼
310 nm) (ꢀꢀꢀ); emission spectrum in propanenitrile glass at 77
K
lex ¼ 310 nm) (d).
manifested
an
additional
absorption
at
348
nm
(ε ¼ 35,600 Mꢀ1 cmꢀ1) that overlapped the one at approximately
300 nm, and was assigned to a
p/p* transition. Comparison of
compounds
1
and
2
both exhibited intense bands
(ε z 25,000e34,000 Mꢀ1 cmꢀ1) at approximately 300 nm. Intense
absorptions at this wavelength were observed in similar lophine
derivatives [32]. The magnitudes of these bands suggest that they
these results with ones obtained from the literature, but in different
solvents, led to the conclusion that the absorption spectra of 1 and 2
are somewhat insensitive to solvent polarity [18,19]. While the
above absorptions can be generally assigned to
p/p* transitions,
arise from a
p/p*. This absorption band was not reported in the
the computational section below will describe more thoroughly
literature spectrum of
2
in chloroform [18]. Compound
2

T. Hamada et al. / Journal of Molecular Structure 1130 (2017) 284e290
287
discussed in the following section.
Time correlated single photon counting (TCSPC) measurements
of dilute acetonitrile solutions of 1 and 2 at room temperature
manifested decay traces that exhibited mono-exponential behavior,
which yielded excited state lifetimes of 1.8 ns for 1 and 1.5 ns for 2.
Decay traces that exhibited bi-exponential behavior were observed
from both 1 and 2 in propanenitrile glasses at 77 K, resulting in
excited state lifetimes of 1.9 and 1.1 ns, respectively. The observed
lifetime of 1 at 77 K had a relative amplitude of 94.28%, while that of
2 was 99.38%. The decay traces of 1 and 2 at 77 K both exhibited
minor components of 5.4 and 0.9 ns, with the former attributed to
aggregation and the latter to scattered light [20]. The short duration
of the observed lifetimes of 1 and 2 at room temperature and at
77 K characterize their emission as fluorescence. The decay traces
with their fitted curves for compounds 1 and 2 at room tempera-
ture and at 77 K can be found in the Supplementary Material,
Figs. S3 and S4.
Overall, the observed photophysical properties of 1 and 2
compare well to other lophine derivatives [18,21,32]. Note that the
photophysical properties of compound 1 were found to be very
similar to lophine, while those of compound 2 were different [21].
In the following text we explain this and describe in more detail the
excited state transitions of 1 and 2 with the use of TD-DFT methods.
Fig. 2. Electronic absorption and emission spectra of compound 2. Absorption spec-
trum in acetonitrile (ꢀꢃꢀ); room temperature emission spectrum in acetonitrile
(
(
lex
¼
348 nm) (ꢀꢀꢀ); emission spectrum in propanenitrile glass at 77
K
lex ¼ 348 nm) (d).
how the structure of these molecules contribute to their photo-
physical properties.
3.3. Computational studies of 1 and 2
Excitation of a dilute acetonitrile solution of compound 1 at
310 nm produced emission at 387 nm, while excitation into either
of the absorption bands of 2 (298 and 348 nm) produced emission
at 406 nm, which suggests that 2 has one excited state LUMO
accessible from the HOMO and HOMO-1. This conclusion was
supported by an excitation spectrum of 2 (obtained while emission
was observed at 406 nm) that closely resembled its absorption
spectrum with a maximum at 352 nm and a shoulder at 298 nm.
The emission observed from acetonitrile solutions of 2 is within
5 nm of the emission reported from a chloroform solution of 2,
further supporting its lack of solvatochromism [18,19]. Room tem-
perature, solid state emission spectra of both 1 and 2 were red-
shifted and broadened, which is explained by increased aggrega-
tion that can occur in the solid state [33,34]. The emission maxima
of 1 and 2 at 77 K were both blue-shifted slightly, with 1 having a
maximum at 373 nm, and 2 manifesting two closely spaced max-
ima at 392 and 402 nm. The energy difference between the maxima
of 2 was 1100 cmꢀ1, which corresponds to a vibrational mode of an
aromatic CeH bond [7,40].
Quantum yields for 1 and 2 were obtained from room temper-
ature acetonitrile solutions, with the former being 0.29 and the
latter being 0.47. Increased molecular rigidity of 2, arising from
resonance interaction between its imidazole core and triphenyla-
mino moiety (Scheme 2), would decrease the rate constants for
other excited state deactivation processes and increase the fluo-
rescence rate constant and thus its quantum yield [36,37].
Computational results supporting resonance interaction between
the imidazole core of 2 and it's triphenylamino substituent are
Assignment of observed absorption and emission properties can
be done in a general way in terms of a certain spectral feature
arising from a particular type of molecular orbital transition, (for
example, a
p/p* transition) using molar extinction coefficient
magnitudes. Computational methods allow these assignments to
be made more specifically, allowing insight into the composition of
a particular molecular orbital that a transition originates from, and
the eventual rationale tailoring of the structure of a molecule to
obtain a desired photophysical property. The first step in the
computational study of 1 and 2 was the optimization of their
ground state geometries, which are shown in Fig. 3. These struc-
tures were validated through a comparison of the calculated bond
lengths of the imidazole cores of compounds 1 and 2, and those
obtained from the x-ray crystal structures of three lophine de-
rivatives with electron-donating groups in the 2-position (see
Table S1 in the Supplementary Material) [32,38]. A comparison of
only the imidazole cores was done, as this was the main structural
feature shared by compounds 1 and 2 and the literature x-ray
crystal structures used. Discrepancies between theoretical and
experimental bond lengths ranged from 0.002 to 0.014 Å, with a
mean absolute error of 0.007 Å for both compounds 1 and 2.
Considering that the experimental and calculated and experi-
mental data are done in different environments, these deviations
are small and compare well with a similar study, thus validating the
use of the B3LYP functional [27].
A foundation for the resonance structures of 2 that are shown in
Scheme 2 was provided by the optimized geometry of compound 2
(Fig. 5). The length of C(3)eC(29), which connects a phenyl ring to
the imidazole core, was 1.4607 Å, which is slightly shorter than the
average length of 1.476 Å for a similar bond reported by Allen and
coworkers [41]. This calculated CeC bond length was then
compared with, and found to be quite similar to corresponding
bond lengths in lophine derivatives with the electron-donating
groups 4-hydroxyphenyl and with 2-methoxyphenyl in the 2-
position, which were 1.465 and 1.462 Å, respectively [32e38].
This comparison is appropriate as resonance structures showing
the interaction of the substituents in the 2-position with their
imidazole cores can be drawn for these two literature examples.
Additional evidence for resonance interaction in compound 2, is the
Scheme 2. Resonance interaction in compound 2.

288
T. Hamada et al. / Journal of Molecular Structure 1130 (2017) 284e290
Fig. 3. Ground state structures of compounds 1 (top) and 2 (bottom) calculated using the 6-311Gþ(d,p) and 6-311Gþþ(2d,2p) basis sets, respectively.
1.4117 Å length of the C(36)eN(39) bond that is shorter than the
reported 1.426 Å average length for such a bond [41]. The two other
CeN bonds, C(40)eN(39) and C(51)eN(39), in the triphenylamino
moiety of 2 were longer than C(36)eN(39) at 1.4225 and 1.4224 Å,
respectively. The shortening of the bonds discussed above support
the idea that the resonance interaction depicted in Scheme 2 occurs
to some extent, and the described effect on the photophysical
properties of compound 2.
The average calculated length of the CareO, (C32eO39,
C35eO38, and C34eO37) bonds in 1 (Fig. 5) was 1.3740 Å, which is
close to the reported average CareO bond length of 1.370 Å for alkyl
aromatic ethers, indicating no significant resonance interaction
between the methoxy groups of compound 1 and its imidazole core
[41].
Theoretical absorption spectra of compounds 1 and 2 were
calculated from their optimized structures via TD-DFT (see Fig. S5
in the Supplementary Material), with 1 having a maximum at 251
and 326 nm and 2 having a shoulder at 311 and a maximum at
364 nm. The discrepancy between the theoretically modeled
spectra and the experimental ones corresponds with related
studies [27,39,40]. The similarity between the theoretical and
experimental absorption spectra further validates the optimized
structures of compounds 1 and 2 (Fig. 5).
Examination of the molecular orbitals of compounds 1 and 2
(depicted in Figs. 3 and 4, respectively) allowed a detailed orbital
description of their excited states. In compound 1, the band at
310 nm arises from a HOMO (p)/LUMO (p *) transition, with the
HOMO being composed of orbitals based on the imidazole core and
it's phenyl rings and the LUMO being constructed of orbitals located
on the imidazole core and the phenyl rings in the 2- and 5-
positions. The calculated HOMO (p)/LUMO (p *) energy differ-
ence for 1 is ꢀ4.18 eV or 297 nm, which corresponds well to the
experimental value of 310 nm and confirms the orbital assignment
for this transition.
The close similarity of the photophysical properties of com-
pound 1 and lophine can be rationalized by its HOMO and LUMO,
which have little contributions from the methoxy oxygen atoms
(Fig. 4).
The HOMO (p)/LUMO (p *) transition in 2 is assigned to the
experimental band observed at 348 nm. In this case, the HOMO is
composed of orbitals on the imidiazole core and the phenyl ring at
the 2-position while the LUMO composition is similar to that of

T. Hamada et al. / Journal of Molecular Structure 1130 (2017) 284e290
289
Fig. 4. HOMO (bottom) and LUMO (top) of compound 1 computed at the B3YLP/6-
311Gþ(d,p) level of theory in acetonitrile (IEFPCM). The figure uses a 0.02 a.u. cutoff
for the contour of the molecular orbitals.
compound 1. The calculated energy difference for this transition
is ꢀ3.74 eV or 331 nm. While this value is 17 nm lower than the
experimental value, it is closer than the value reported by Tian et al.
for a similar computational study that yielded a HOMO (
*) energy difference of ꢀ3.17 eV or 391 nm, which was 36 nm
higher than the observed value [18]. The shoulder observed in 2 at
)/LUMO transition ( *).
p)/LUMO
(p
298 nm is attributed to the HOMOꢀ1 (
p
p
The HOMOꢀ1 is similar in construction to the HOMO of compound
1, with the main difference being clear resonance interaction be-
tween the imidazole core and the phenyl ring at the 2-position. The
Fig. 5. HOMO-1 (bottom), HOMO (middle) and LUMO (top) of compound 2 computed
at the B3YLP/6-311Gþþ(2d,2p) level of theory in acetonitrile (IEFPCM). The figure uses
a 0.02 a.u. cutoff for the contour of the molecular orbitals.
composition of the LUMO is as described above for the HOMO (
LUMO (
278 nm, which is 20 nm lower than the experimentally observed
p)/
p
*). The energy difference for this transition is ꢀ4.48 eV or
at room temperature and 77 K (1.1e1.8 ns) indicated that their
emission arises from fluorescence decay. Computational studies of
compounds 1 and 2 aided in the assignment of their excited state
transitions, and explain the differing photophysical properties of
compounds 1 and 2.
value, but still helpful in confirming assignment of this transition.
4. Conclusions
Operationally simple syntheses were done to give compounds 1
and 2 in good yield. Absorption spectra of both compounds had
bands that were assigned to
p/p* transitions. Irradiation into
these absorption bands produced emission that ranged from 380 to
410 nm, which was slightly blue-shifted at 77 K. The fluorescence
quantum yields of compound 2 was larger than that of 1 (0.47
versus 0.29), which is attributed to it's greater conjugation and
planar nature. The short excited state lifetimes of both compounds
Acknowledgements
This research is funded via the Chester and Olive McCloskey
Endowed Chair at Whittier College. Dr. Simon Jones is thanked for
providing HR-FAB-MS of compounds 1 and 2.

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T. Hamada et al. / Journal of Molecular Structure 1130 (2017) 284e290
G. Zheng, J.L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
Appendix A. Supplementary data
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven,
J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers,
K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari,
A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam,
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