Synthesis, characterization and optical properties of aryl and diaryl substituted phenanthroimidazoles

Article abstract of DOI:10.1016/j.jphotochem.2012.05.018

This article presents a facile synthesis of novel class of blue-green fluorescent phenanthroimidazoles and report on their optical, electrochemical, and thermal properties. Detailed photophysical and quantum chemical studies with a series of hydroxy-, met

Full text of DOI:10.1016/j.jphotochem.2012.05.018

Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis, characterization and optical properties of aryl and diaryl substituted
phenanthroimidazoles
Manoj Kumar Nayak∗
Department of Chemistry, Texas A&M University at Qatar, Qatar Foundation, PO Box 23874, Doha, Qatar
a r t i c l e i n f o
a b s t r a c t
Article history:
This article presents a facile synthesis of novel class of blue-green fluorescent phenanthroimidazoles and
report on their optical, electrochemical, and thermal properties. Detailed photophysical and quantum
chemical studies with a series of hydroxy-, methoxy-, nitro-, amino- and tosylaminophenyl substi-
tuted derivatives of 2-phenyl-1H-phenanthro[9,10-d]imidazole and 1,2-diphenyl-1H-phenanthro[9,10-
d]imidazole have been performed to elucidate the origin of the surprisingly divergent emission shifts.
Out of these, three dyes (HPhI, HPPhI, and TsPPhI) undergo excited state intramolecular proton trans-
fer (ESIPT) reaction leading a large Stokes’ shifted fluorescence emission from the phototautomer. The
results of quantum chemical investigations not only confirmed the intramolecular charge transfer char-
acteristics of the ESIPT tautomers but also provided a rational for the observed high fluorescence quantum
efficiency in the solid state. The high photoluminescence quantum yield (˚PL ∼ 39–68%) in the solid state
is ascribed to twisted chromophores due to phenyl substituents at 1,2-position of the phenanthroimida-
zole ring which restricted intramolecular motion, leading to an optically allowed lowest optical transition
without self quenching.
Received 14 January 2012
Received in revised form 8 May 2012
Accepted 18 May 2012
Available online 29 May 2012
Keywords:
Phenanthroimidazoles
Excited state intramolecular proton
transfer (ESIPT)
Photoluminescence
Restricted intramolecular rotation
Intramolecular charge transfer (ICT)
Density functional theory (DFT)
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
the labeling of proteins and DNA) and model base pairs [18] (for
studying the environments within the major and minor grooves of
Proton- and charge-transfer reactions are most fundamental
processes involved in chemical reactions as well as in living sys-
tems [1,2]. Among the various studies of proton transfer, organic
molecules exhibiting excited-state intramolecular proton transfer
(ESIPT) have attracted considerable research interest from the
viewpoint of the development of new functional materials for
optoelectronic applications such as UV photostabilizers [3], pho-
toswitches [4], fluorescent probes [5], and organic light-emitting
diodes (OLEDs) [6]. Photoinduced ESIPT is generally observed
for organic compounds featuring both a protic acid group (e.g.,
DNA duplexes).
The most stable form of the mentioned ESIPT molecules in
the ground state is in equilibrium between several different con-
formers arising from tautomerism and rotamerism (Fig. 1) [8].
The normal planar syn form (Nsyn) features an intramolecular
hydrogen bond between protic acid group (the acidic hydroxyl
or amine group) and the basic nitrogen atom of the imidazole
ring. The Nsyn conformer can undergo proton transfer to form
its tautomer (Tsyn). Upon excitation of the normal form to its
first excited singlet state (Nsyn*), undergoes an ultrafast excited-
state intramolecular proton transfer to yield the planer tautomer
OH, NH2, NHSO2R, etc.) and a basic site (
N
, ꢀC O, etc.)
with a suitable conformation, one that forms an intramolecular
Tsyn* accompanied with large Stokes’ shifted fluorescence emission.
˚
hydrogen bond within the distance of 2.0 A. For an efficient ESIPT,
The syn-conformers can rotamerize to form their non-planar anti
forms (Nanti and Tanti, respectively). In a protic solvent, the Nanti
conformer can form intermolecular hydrogen bonds with solvent
molecules, but it cannot undergo ESIPT. Therefore, dual fluores-
cence is frequently observed in polar protic solvents; in addition
to the fluorescence from the Tsyn* form, the Nanti* form can also
exhibit rather normal Stokes’ shifted weak fluorescence [16,19].
Moreover, the typical ESIPT process produces the tautomeric form
can be coupled with the intramolecular charge transfer (ICT) state
with a rotation around interannular bond to produce a non-planar
configuration (TICT*) between the two rings. The TICT* state can
be deactivated back to its ground state via radiationless relax-
ation.
the acidity and/or basicity of these groups must increase upon
photoexcitation [7]. Among such molecules, hydroxyphenylben-
zazoles (HBA) such as 2-(2ꢁ-hydroxyphenyl)benzimidazole
(HBI)
[10–12],
[8–10],
2-(2ꢁ-hydroxyphenyl)benzoxazole
(HBT)
(HBO)
[10,13],
2-(2ꢁ-hydroxyphenyl)benzthiazole
2-(2ꢁ-arylsulfonamidophenyl)benzimidazole [14,15] and 2-(2ꢁ-
arylsulfonamidophenyl)benzoxazole [16] have attracted particular
attention due to their potential for fluorescent probes [17] (for
∗
Tel.: +974 4423 0380; fax: +974 4423 0060.
E-mail addresses: manoj.nayak@qatar.tamu.edu, manoj17675@yahoo.com
1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jphotochem.2012.05.018

M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
27
Fig. 1. Structures of various isomers of phenanthroimidazole derivatives in the ESIPT and ICT processes. From the TD-DFT results, the o-quinoid structures are presented for
the tautomeric and ICT forms.
The ESIPT molecules exhibit rather low fluorescence quantum
efficiency in the solid state due to concentration quenching, deac-
tivation by various pre-existing isomers, and the ICT states. Aiming
at the high performance OLEDs as well as the solid-state lasers,
a high value of solid-state fluorescence quantum efficiency with
understanding and control of radiative and non-radiative decay
processes is basically in demand to contribute to the improvement
of their technological applications [20,21]. Recently, a number of
papers have been published reporting conjugated organic small
molecules that showed high quantum yield in the solid state with-
out concentration quenching problem [22–28]. In solution, the
conjugated backbone of these molecules is significantly twisted
by steric hindrance [23] and the radiative decay pathway of the
resulting twisted chromophores [24] is generally suppressed as
they dissipate energy for molecular rotation. In the solid state,
the twisted conjugated backbone, however, enhances the emis-
sive property of the molecules because the molecular rotation is
hindered by adjacent molecules upon aggregation [23–28].
also studied for comparison. The results gained from this study
should provide guidelines for tuning the emission properties of
this class of ESIPT fluorophores with potential applications in
chemistry (e.g., molecular materials, probes, sensors, and tracers)
and material science (e.g., optoelectronics).
2. Experimental
2.1. General
All solvents and reagents were purchased from commercial
sources and used as received. The column chromatography was
performed with 200–400 mesh silica gel as stationary phase. The
1H NMR and 13C NMR (proton-decoupled) spectra were measured
by using a 500 MHz instrument in CDCl3 containing 0.03 (v/v)%
tetramethylsilane (TMS) as the internal standard (1H NMR: TMS at
0.00 ppm, CDCl3 at 7.24 ppm, and 13C NMR: CDCl3 at 77.23 ppm).
Proton coupling patterns were described as singlet (s), doublet (d),
triplet (t), quartet (q), multiplet (m) and broad (br). Coupling con-
stants (J) are quoted in Hertz (Hz). Mass Spectra were recorded
on a mass spectrometer by the electron impact (EI) ionization
technique. The thermal properties of the materials were exam-
ined by differential scanning calorimetry (DSC) at heating rate of
10 ◦C min−1 under N2. UV–visible absorption and corrected flu-
orescence spectra (with excitation wavelengths of ꢀexc = 325 or
355 nm) were recorded in argon-bubbled CHCl3, CH3CN and 1-
propanol, respectively at room temperature with a fixed emission
and excitation slit width of 2 nm each. The fluorescence quan-
tum yields (˚PL) were determined in different solvents at 298 K
against the 9,10-diphenylanthracene standard (˚PL = 0.84) in ben-
zene [29]. On the other hand, ˚PL of PMMA film doped with
5 wt% of aryl phenanthroimidazole dyes on the wedged quartz
plate were measured using a 6-in. integrating sphere equipped
with a 325-nm CW He-Cd laser and a PMT detector attached to a
monochromator. The detailed analytical procedure to obtain solid-
state ˚PL has been described elsewhere [30]. Cyclic voltammetric
experiments were carried out using three-electrode cell assem-
blies including a glassy-carbon working electrode, a platinum wire
counter electrode, and a Ag/Ag+ reference electrode. After argon
saturation in one compartment cell, measurements were carried
A rational design to maximize the ESIPT signal demands
new strategies to minimize the content of Nanti and Tanti
,
while preserving the valuable optical characteristics of HBA.
With this intention, described herein is the synthesis of o-
substituted
2-phenyl-1H-phenanthro[9,10-d]imidazole
(PhI)
and 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole
(PPhI)
derivatives and the substituent effects on their ESIPT pro-
cesses, rotational processes, and optical properties. To
elucidate the higher fluorescence quantum efficiencies
of
2-(1-phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenol
(HPPhI) and 4-methyl-N-(2-(1-phenyl-1H-phenanthro[9,10-
2-(1H-phenanthro[9,10-d]imidazol-2-yl)phenol
(HPhI),
d]imidazol-2-yl)phenyl)benzenesulfonamide (TsPPhI) in the
solid state than those in solution, structural analyses and
time-dependent density functional theory (TD-DFT) cal-
culations were carried out, which was correlated with
their photophysical properties. The model compounds 2-
(2-methoxyphenyl)-1H-phenanthro[9,10-d]imidazole
(MPhI),
2-(2-methoxyphenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole
(MPPhI),
d]imidazole
2-(2-nitrophenyl)-1-phenyl-1H-phenanthro[9,10-
(NPPhI), 2-(1-phenyl-1H-phenanthro[9,10-
d]imidazol-2-yl)aniline (APPhI), unable to undergo ESIPT, were

28
M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
out in dichloromethane (DCM) solution with tetrabutylammonium
tetrafluoroborate (Bu4NBF4) as a supporting electrolyte at a scan
rate of 100 mV s−1. Each potential was calibrated with ferrocene as
a reference [31].
was added to complete the precipitation. The crude product was
collected by filtration, washed with water, dried by suction. The
resultant solid was dissolved in the minimum volume of DCM
and purified by flash chromatography on a silica gel column using
eluent 20–25 (v/v)% of ethyl acetate in n-hexane to give 2.28 g
(5.90 mmol, 60.0% yield) of pure product HPPhI as an off white
solid. m.p. 184 ◦C; 1H NMR (CDCl3, 500 MHz), ı = 13.83 (s, 1H), 8.75
(d, J = 8.38 Hz, 1H), 8.70 (d, J = 4.23, 1H), 8.68 (d, J = 4.54 Hz, 1H),
7.76 (m, 4H), 7.67 (t, J = 7.71, 7.39 Hz, 1H), 7.61 (d, J = 7.52 Hz, 2H),
7.51 (t, J = 8.04, 7.64 Hz, 1H), 7.24 (t, J = 7.79, 7.30 Hz, 1H), 7.21 (t,
J = 7.53 Hz, 1H), 7.12 (d, J = 7.99 Hz, 1H), 7.03 (d, J = 8.36 Hz, 1H),
6.72 (d, J = 8.18 Hz, 1H), 6.49 ppm (t, J = 7.62, 7.35 Hz, 1H); 13C NMR
(CDCl3, 125 MHz) ı = 159.46, 148.68, 139.31, 134.67, 131.11 (2C),
130.94, 130.85, 129.72, 129.32 (2C), 128.67, 127.73, 127.27, 126.75,
126.29 (2C), 126.04, 125.48, 124.41, 123.43, 122.85, 122.82, 121.10,
118.28, 118.26, 113.34 ppm; MS (EI) m/z 386 (M+, 100.0), 369 (6.3),
357 (3.1), 281 (2.2), 266 (7.0), 239 (1.7), 193 (2.2), 165 (4.1), 149
(1.2), 97 (1.7); HRMS calcd for C27H18N2O: 386.14, found: 386.14;
Element analysis calcd (%) for C27H18N2O: C 83.92, H 4.69, N 7.25,
and O 4.14; found: C 83.46, H 4.88, N 7.19, and O 4.33.
2.2. Synthesis of phenanthroimidazoles
2.2.1. 2-(1H-Phenanthro[9,10-d]imidazol-2-yl)phenol (HPhI)
A solution of phenanthrene-9,10-dione (2.04 g, 9.82 mmol), sal-
icylaldehyde (1.2 g, 9.82 mmol) and ammonium acetate (5.67 g,
73.55 mmol) in glacial acetic acid (40 mL) was refluxed under
nitrogen for 4 h, during which time a brownish yellow precipi-
tate formed. An excess of de-ionized water (30 mL) was added to
complete the precipitation. The crude product was collected by fil-
tration, washed with water, dried by suction. The resultant solid
was dissolved in the minimum volume of DCM and purified by flash
chromatography on a silica gel column using eluent 25–30 (v/v)%
of ethyl acetate in n-hexane to give 1.62 g (5.22 mmol, 53% yield)
of pure product HPhI as a white solid. m.p. 260 ◦C; 1H NMR (CDCl3,
500 MHz), ı = 11.53 (s, 1H), 8.74 (t, J = 9.51, 8.95 Hz, 2H), 8.51 (d,
J = 7.37, 1H) 8.34 (d, J = 7.27 Hz, 1H), 8.16 (d, J = 7.83 Hz, 1H), 7.73 (m,
4H), 7.44 (t, J = 7.98, 7.47 Hz, 1H), 7.17 (d, J = 8.39 Hz, 1H), 7.06 ppm
(t, J = 7.53 Hz, 1H); 13C NMR (CDCl3, 125 MHz) ı = 162.08, 158.31,
143.63, 133.68, 133.24, 129.63, 129.27, 127.84, 127.71, 126.97,
126.88, 126.73, 125.35, 124.03, 123.75, 123.08, 121.10, 120.91,
119.89, 117.68, 111.33 ppm; MS (EI) m/z 310 (M+, 100.0), 282 (7.7),
254 (6.4), 221 (3.3), 164 (14.2), 127 (5.0), 74 (1.6); HRMS calcd for
2.2.4.
2-(2-Methoxyphenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole
(MPPhI)
Compound HPPhI (0.25 g, 0.65 mmol) was dissolved in DMF
(5 mL) at room temperature. K2CO3 (0.10 g, 0.73 mmol) and MeI
(0.14 g, 0.98 mmol) were added to this solution. The reaction mix-
ture was stirred under dark conditions for 20 h. Water (20 mL) was
added to the reaction mixture and extracted using ethyl acetate
(60 mL × 2). The organic layer was washed with water (50 mL),
brine (50 mL), and dried over MgSO4. The solvent was evaporated
under reduced pressure and the product obtained was purified on
a silica gel column using eluent 10–15 (v/v)% of ethyl acetate in n-
hexane to give 0.21 g (0.52 mmol, 81% yield) of pure product MPPhI
as a white solid. m.p. 211 ◦C; 1H NMR (CDCl3, 500 MHz), ı = 8.84 (d,
J = 8.13 Hz, 1H), 8.76 (d, J = 8.36 Hz, 1H), 8.70 (d, J = 8.35 Hz, 1H), 7.70
(t, J = 7.01, 7.79 Hz, 1H), 7.61 (t, J = 7.73 Hz, 1H), 7.49 (m, 2H), 7.40
(m, 5H), 7.31 (t, J = 7.87 Hz, 1H), 7.25 (m, 2H), 6.96 (t, J = 7.45 Hz, 1H),
6.72 (d, J = 8.38 Hz, 1H), 3.53 ppm (s, 3H); 13C NMR (CDCl3, 125 MHz)
ı = 157.90, 150.20, 138.59, 137.78, 132.77, 131.34, 129.41, 129.17
(3C), 128.80 (2C), 128.40, 127.69, 127.64, 127.39, 126.33, 125.57,
124.96, 124.28, 123.33, 123.27, 123.12, 121.23, 120.58, 110.75,
55.15 ppm; MS (EI) m/z 400 (M+, 100.0), 382 (57.9), 369 (31.3), 355
(4.8), 323 (3.6), 295 (11.6), 267 (4.3), 239 (2.1), 219 (7.6), 200 (10.0),
183 (8.9), 165 (6.7), 132 (1.2), 77 (1.1); HRMS calcd for C28H20N2O:
400.16, found: 400.16; Element analysis calcd (%) for C28H20N2O:
C 83.98, H 5.03, N 7.00, and O 4.00; found: C 83.44, H 5.08, N 6.89,
and O 4.16.
C21H14N2O: 310.11, found: 310.09; Element analysis calcd (%) for
C21H14N2O: C 81.27, H 4.55, N 9.03, and O 5.16; found: C 80.76, H
4.49, N 8.81, and O 5.32.
2.2.2. 2-(2-Methoxyphenyl)-1H-phenanthro[9,10-d]imidazole
(MPhI)
Compound HPhI (0.25 g, 0.80 mmol) was dissolved in DMF
(5 mL) at room temperature. K2CO3 (0.12 g, 0.86 mmol) and MeI
(0.17 g, 1.19 mmol) was added dropwise to this solution. The reac-
tion mixture was stirred under dark conditions for 20 h. Water
(20 mL) was added to the reaction mixture and extracted using
ethyl acetate (50 mL × 2). The organic layer was washed with
water (20 mL), brine (20 mL), and dried over MgSO4. The solvent
was evaporated under reduced pressure and the product obtained
was purified on a silica gel column using eluent 10–15 (v/v)% of
ethyl acetate in n-hexane to give 0.25 g (0.77 mmol, 94% yield)
of pure product MPhI as a white solid. m.p. = 158 ◦C; 1H NMR
(CDCl3, 500 MHz) ı = 8.75 (d, J = 8.43 Hz, 1H), 8.73 (d, J = 8.45 Hz,
1H), 8.67 (dd, J = 8.03 1H), 8.34 (dd, J = 7.42, 7.80 Hz, 1H), 8.26 (dd,
J = 7.71 Hz, 1H), 7.69 (m, 4H), 7.50 (t, J = 7.38, 8.42 Hz, 1H), 7.15 (t,
J = 7.57 Hz, 1H), 7.12 (d, J = 8.40 Hz, 1H), 4.06 ppm (s, 3H); 13C NMR
(CDCl3, 125 MHz) ı = 161.08, 158.41, 144.90, 135.56, 132.52, 131.34,
129.51, 129.08, 127.55, 127.41, 126.54, 126.50, 126.23, 123.95,
123.60, 123.39, 121.39, 121.25, 121.05, 116.95, 112.43, 56.45 ppm;
MS (EI) m/z 324 (M+, 100.0), 296 (22.0), 282 (10.0), 253 (3.4), 194
(5.0), 179 (5.7), 163 (15.8), 133 (4.1), 118 (25.9), 91 (1.6); HRMS cal-
culated for C22H16N2O: 324.13, found: 324.11; Elemental analysis
calcd (%) for C22H16N2O: C 81.46, H 4.97, N 8.64, and O 4.93; Found:
C 81.12, H 4.97, N 8.55, and O 5.18.
2.2.5.
2-(2-Nitrophenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole
(NPPhI)
A mixture of 2-nitro-benzaldehyde (2.0 g, 13.23 mmol) and
phenanthrene-9,10-dione (2.75 g, 13.20 mmol) were dissolved in
glacial acetic acid (60 mL) at room temperature. Aniline (1.85 g,
19.86 mmol) was added dropwise to this solution; subsequently
ammonium acetate (5.09 g, 66.03 mmol) was added to it. The mix-
ture was heated to reflux at 110 ◦C for 24 h, during which time a
yellow precipitate formed. An excess of de-ionized water (50 mL)
was added to complete the precipitation. The crude product was
collected by filtration, washed with water, dried by suction. The
resultant solid was dissolved in the minimum volume of DCM
and purified by flash chromatography on a silica gel column using
eluent 30–35 (v/v)% of ethyl acetate in n-hexane to give 5.20 g
(12.51 mmol, 95% yield) of pure product NPPhI as a white solid.
1H NMR (CDCl3, 500 MHz), ı = 8.77 (dd, J = 8.48 Hz, 1H), 8.75 (dd,
2.2.3. 2-(1-Phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)phenol
(HPPhI)
A
mixture of salicylaldehyde (1.2 g, 9.82 mmol) and
phenanthrene-9,10-dione (2.04 g, 9.82 mmol) were dissolved
in glacial acetic acid (40 mL) at room temperature. Aniline (1.37 g,
14.7 mmol) was added dropwise to the above solution; subse-
quently ammonium acetate (3.9 g, 49 mmol) was added to it. The
mixture was heated to reflux at 110 ◦C for 20 h, during which time
a yellow precipitate formed. An excess of de-ionised water (30 mL)

M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
29
J = 7.54 Hz, 1H), 8.71 (dd, J = 8.62 Hz, 1H), 8.0 (d, J = 7.52 Hz, 1H), 7.7
2.3. Computational details
(t, J = 7.72 Hz, 1H), 7.63 (m, 3H), 7.53 (m, 2H), 7.46 (m, 3H), 7.41
(m, 2H), 7.27 (t, J = 7.51 Hz, 1H), 7.20 ppm (d, J = 7.47 Hz, 1H); 13C
NMR (CDCl3, 125 MHz) ı = 149.18, 147.77, 137.88, 137.46, 133.71,
133.12, 130.67, 130.08 (2C), 129.86, 129.64, 128.84 (2C), 128.55,
127.89, 127.56, 127.38, 126.87, 126.54, 125.92, 125.40, 124.84,
124.36, 123.35, 123.04, 122.97, 121.14 ppm; MS (EI) m/z 415 (M+,
53.6), 385 (49.3), 281 (100.0), 256 (19.3), 213 (7.4), 185 (11.4), 149
(12.8), 129 (19.2), 97(19.2), 81 (33.3); HRMS calcd for C27H17N3O2:
415.44, found: 415.13; Element analysis calcd (%) for C27H17N3O2:
C 78.06, H 4.12, N 10.11, and O 7.70; found: C 77.90, H 4.11, N 10.05,
and O 7.94.
All molecular structures were fully optimized using the hybrid
B3LYP functional method [32], in combination with 6-31G(d,p)
Gaussian basis set. For each optimized structure a frequency analy-
sis at the same level of theory was used to verify that it corresponds
to a stationary point in the potential energy surface. As the use
of diffuse functions is essential for accurate determination of the
energetics, particularly for ion radicals and excited states [33], to
perform a single-point calculation on the basis of B3LYP/6-31G(d,p)
optimized structures. The excited-state properties were calcu-
lated with the time-dependent density functional theory (TD-DFT)
[34–36] formalism, using the optimized ground state geometries.
TD-DFT in combination with the B3LYP hybrid functional and the
6-31G (d,p) basis set has previously been shown to provide accu-
rate energies for excited states within 0.2 eV (5 kcal mol−1) [37].
All calculations were performed with the Gaussian 03 package of
programs.
In addition to DFT calculations, semi-empirical methods offer
an attractive alternative for studying potential energy surfaces in
ground and excited states. Procedures such as the AM1 method
[38,39] allow examination of potential energy surfaces without
geometric assumptions. With inclusion of limited configuration
interaction, especially through the single and pair double excita-
tion (PECI) procedure [40], spectral properties of several conjugated
organic systems have been shown to be reliably reproduced [41,42].
Therefore, the spectral properties of ESIPT dyes have been simu-
lated using AM1/PECI = 8 calculations in order to guide the synthetic
efforts towards materials with enhanced performances and to help
with the interpretation of the experimental data.
2.2.6. 2-(1-Phenyl-1H-phenanthro[9,10-d]imidazol-2-yl)aniline
(APPhI)
A solution of NPPhI (4.5 g, 10.83 mmol) in THF and ethyl acetate
mixture (200 mL) was hydrogenated at room temperature under
ambient pressure in the presence of Pd on activated carbon (10 wt%,
0.75 g) as catalyst. Upon completion of the reaction (TLC), the
catalyst was filtered off through a pad of celite, and the fil-
trate was concentrated under reduced pressure providing 4.01 g
(10.4 mmol, 96% yield) of amine APPhI as a white solid. m.p. 227 ◦C;
1H NMR (CDCl3, 500 MHz), ı = 8.79 (d, J = 7.98 Hz, 1H), 8.76 (d,
J = 8.29 Hz, 1H), 8.70 (d, J = 8.26 Hz, 1H), 7.72 (t, J = 7.46 Hz, 1H), 7.64
(t, J = 8.21 Hz, 1H), 7.56 (m, 3H), 7.50 (m, 3H), 7.25 (t, J = 8.21 Hz, 1H),
7.20 (d, J = 8.31 Hz, 1H), 7.04 (t, J = 8.23 Hz, 1H), 6.83 (d, J = 7.74 Hz,
1H), 6.76 (d, J = 8.09 Hz, 1H), 6.40 (t, J = 7.59 Hz, 1H), 5.31 ppm (s,
2H); 13C NMR (CDCl3, 125 MHz) ı = 149.78, 147.57, 139.02, 137.07,
130.50, 130.17 (2C), 130.11, 129.76, 129.46, 129.34 (2C), 128.51,
127.44, 127.30, 126.50, 125.78, 125.06, 124.32, 123.37, 123.25,
122.86, 121.30, 116.90, 116.57, 114.10 ppm; MS (EI) m/z 385 (M+,
100.0), 369 (15.0), 266 (4.5), 192 (121), 165 (5.9), 129 (3.5), 112
(1.2), 57 (1.2); HRMS calcd for C27H19N3: 385.16, found: 385.16;
Element analysis calcd (%) for C27H19N3: C 84.13, H 4.97, and N
10.90; found: C 83.95, H 5.02, and N 10.84.
3. Results and discussion
3.1. Synthesis and characterization of phenanthroimidazoles
Scheme 1 outlines the three synthetic routes that were utilized
for synthesis of the phenanthroimidazoles. Key step in all the routes
is the formation of the phenanthroimidazole ring from the cor-
responding substituted aryl precursors using a slightly modified
procedure of the Steck and Day method [43]. The condensation
reaction of phenanthroquinone with salicyladehyde was conducted
in the presence of excess ammonium acetate as the source of
the imidazole nitrogen atoms in glacial acetic acid for a reaction
time of 4 h at 110 ◦C to afford HPhI in 53% isolated yield. Unlike
for 1,2-diphenyl-1,2-ethanedione, which forms only the imidazole
condensation product, phenanthroquinone or phenanthroline-5,6-
dione yields both imidazole and oxazole products competitively
[44,45]. The rigidity and co-planarity of the carbonyl groups in
the fused-ring starting materials enabled close proximity of het-
eroatoms towards competitive cyclization of NN diimine and NO
keto-imine intermediates. In order to alter the electronic envi-
ronments of the five-membered imidazole and six-membered
phenol rings of the 2-(1H-imidazol-2-yl)phenol nucleus, a rational
reaction of appropriate aryl aldehydes (salicyladehyde, 2-nitro-
benzaldehyde) with phenanthroquinone was considered for the
preparation of 1,2-diphenyl-phenanthro[9,10-d]imidazoles. Both
HPPhI and NPPhI were readily synthesized according to the
reported literature procedure of 2-(1,4,5-triphenyl-imidazol-2-yl)-
phenol [20] in good yields 60% and 95%, respectively. The methoxy
analogues MPhI and MPPhI in which the absence of free phenolic
hydroxyl group blocks the formation an intramolecular hydrogen
bond were obtained from corresponding hydroxyl phenanthroimi-
dazoles HPhI and HPPhI with equimolar amount of CH3I and K2CO3
in DMF in high yields 94% and 81%, respectively. Through the
palladium-catalyzed hydrogenation of the nitro group of NPPhI
2.2.7. 4-Methyl-N-(2-(1-phenyl-1H-phenanthro[9,10-
d]imidazol-2yl)phenyl)benzenesulfonamide
(TsPPhI)
To a solution of amine APPhI (1.0 g, 2.59 mmol) in anhydrous
pyridine (20 mL) was added p-toluenesulfonyl chloride (0.495 g,
2.59 mmol). After stirring at room temperature for 12 h, the reaction
mixture was poured into aqueous 1 M HCl (50 mL), and the product
was extracted with ethyl acetate (3 × 100 mL). The organic layer
was washed with brine (100 mL), dried with anhydrous MgSO4,
filtered, and concentrated under reduced pressure. The crude prod-
uct was purified on silica gel column using eluent 30–40 (v/v)%
of ethyl acetate in n-hexane to give 1.19 g (2.20 mmol, 85% yield)
of pure product TsPPhI as a white solid. m.p. 266 ◦C; 1H NMR
(CDCl3, 500 MHz), ı = 11.57 (s, 1H), 8.83 (d, J = 7.93 Hz, 1H), 8.80 (d,
J = 8.40 Hz, 1H), 8.73 (d, J = 8.36 Hz, 1H), 7.81 (t, J = 6.93, 7.75 Hz, 2H),
7.72 (t, J = 7.72, 7.07 Hz, 1H), 7.57 (m, 2H), 7.49 (t, J = 7.76 Hz, 2H),
7.36 (d, J = 8.26 Hz, 2H), 7.25 (t, J = 7.74 Hz, 2H), 7.02 (d, J = 8.27 Hz,
1H), 6.90 (d, J = 7.98 Hz, 2H), 6.75 (m, 3H), 6.67 (d, J = 8.78 Hz, 1H),
2.03 ppm (s, 3H); 13C NMR (CDCl3, 125 MHz) ı = 147.08, 143.29,
138.39, 137.29, 136.52, 130.39 (2C), 130.19, 130.03, 129.90, 129.28
(2C), 129.01, 128.90 (2C), 128.68, 128.16, 127.39, 127.29 (2C),
126.68, 126.63, 126.48, 125.65, 124.85, 124.53, 124.07, 123.41,
122.95, 122.81, 121.22, 120.32, 21.31 ppm; MS (EI) m/z 539 (M+,
100.0), 475 (7.6), 384 (87.2), 368 (12.7), 206 (2.0), 180 (4.0), 165
(7.4), 91 (2.4); HRMS calcd for C34H25N3O2S: 539.17, found: 539.17;
Element analysis calcd (%) for C34H25N3O2S: C 75.67, H 4.67, N 7.79,
O 5.93, and S 5.94; found: C 75.29, H 4.67, N 7.80, O 6.02, and S
5.78.
ˆ
ˆ

30
M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
Scheme 1. Synthetic routes for preparation of the phenanthroimidazole derivatives.
at ambient pressure gave APPhI in 96% yield. Subsequently, the
condensation reaction of APPhI with p-toluenesulfonyl chloride in
anhydrous pyridine at room temperature provided TsPPhI in 85%
yield. The easy synthetic procedures and relatively high yields of
the presented 1,2-diphenyl-phenanthroimidazole derivatives are,
therefore, promising for large-scale production. All synthesized
phenanthroimidazole derivatives were carefully characterized by
1H NMR, 13C NMR, MS-EI/HRMS, and elemental analysis.
structure, whereas mono-aryl substituted phenanthroimidazoles
are coplanar. The torsion angles between the imidazolyl group
and the neighboring 2-phenylene group in HPhI, HPPhI, MPPhI,
and TsPPhI molecules were estimated as (ϕ2) 0, 9.4, 96.8, and 43.6
whereas the torsion angles between the imidazolyl group and
the neighboring 1-phenylene group in HPPhI, MPPhI, and TsPPhI
molecules were estimated as (ϕ1) 85.0, 71.9, and 99.0, respec-
tively. The calculated bond lengths, angles, and hydrogen-bond
parameters are provided in Table 1.
The molecular structures of phenanthroimidazoles, o-
substituted with electron-donating
(
OH, OMe, NH2, and
The most striking feature of the TsPPhI structure is the parallel
orientation of the tosyl and phenanthroimidazole rings, leading to
significant intramolecular ␲-stacking interactions. The two planes
deviate only by 12.7◦ from a parallel orientation with a distance
NHSO2R) or electron attracting ( NO2) groups in the 2-phenyl
ring were designed on the frame of an imidazole ring with
phenyl groups attached to it. The optimized geometry of these
phenanthroimidazoles in the ground state was investigated
with quantum chemical calculations. Ground state geometry
optimizations were obtained by density functional theory (DFT)
calculation using Becke’s three-parameter function combined
with Lee, Yang, and Parr’s correlation function (B3LYP) [32]. From
the calculated molecular conformations of mono- and di-aryl
phenanthroimidazoles shown in Fig. S1, it is noted that the diaryl
phenanthroimidazoles are non-planar and very twist molecular
˚
of approximately 4.69 A. The unusual geometry is best revealed
with a projection along the interannular bond axis, which shows
a dihedral angle of 43.6◦ between the phenanthroimidazole ring
and the attached phenyl ring. The substantial out-of-plane twist of
the aryl–phenanthroimidazole unit is presumably a result of the
␲-stacking interaction, which is most effective with a planar orien-
tation of the aryl rings. This assumption is further supported by the
geometry reported for 2-(2ꢁ-tosylaminophenyl)benzimidazole, in

M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
31
Table 1
Energetic characterization of enol–keto and amine–imine (Nsyn–Tsyn) tautomers in ground (S0) and excited (S1) states in parenthesis.
Conformer
HPhI enol (Nsyn
)
HPhI keto (Tsyn
)
HPPhI enol (Nsyn
)
HPPhI keto (Tsyn
)
TsPPhI amine (Nsyn
)
TsPPhI imine (Tsyn)
Energy/hartree
−993.4820
0 (2.7)
0.995
–
−993.4639
11.37 (0)
1.584
1.046
–
−1224.5265
0(3.53)
0.998
–
−1224.5098
10.47 (0)
1.512
1.045
–
−2023.6001
0(6.31)
–
−2023.5802
12.51 (0)
–
Rel. energy/kcal mol−1 S0 (S1)
˚
Bond length/A O
H
N
H
N· · ·H
O
1.027
2.036
–
2.878
1.474
99.0
1.618
–
–
2.499
1.444
92.8
1.724
2.621
–
1.455
–
1.661
2.564
–
1.467
85.0
N
N
C
2.483
–
1.419
–
0
0
2.437
–
1.433
89.6
0
0.2
N
˚
C (interannular bond length/A)
ϕ1 (1ꢁ-ph)/deg
ϕ2 (2ꢁ-ph)/deg
ϕ3 (IHB)/deg
0
0
9.4
3.0
43.6
14.0
3.2
5.9
ꢁ/D
3.62
342.1
0.357
6.32
411.4
0.148
4.93
343.6
0.312
6.80
413.0
0.147
2.09
330.3
0.242
11.77
422.2
0.240
Excitation energy/nm
Oscillator strength (f)
which the two aryl rings are essentially coplanar with a small dihe-
dral angle of ∼3.3◦ [14]. Additional geometric constraints induced
by the ␲-stacking interaction can be observed along the sulfon-
of dyes in CHCl3 and 1-propanol were compared, the peaks hardly
showed a response to solvent polarities (i.e. 0–3 nm shifts). Polar-
ities of the solvents might not be important in affecting electronic
properties of the compounds, and this observation is in agreement
with the prominence of a ␲–␲* nature of transition to the excited
states rather than n–␲*. In the solid thin films, the chromophores
maintained similar absorption spectral positions to those of the
solution.
amide backbone. Whereas the bond angle tosyl C
S
N of 109.8◦ is
C angle of
in the typical range for aromatic sulfonamides, the S
N
114.7◦ is considerably more acute than 124–125◦, which is typically
found for similar compounds [46].
Owing to their rigid and non-coplanar structures diaryl phenan-
throimidazoles are expected to decrease the intermolecular
stacking efficiency due the steric crowding. Therefore, it is expected
to exhibit good solubility without loss of their excellent thermal
properties. The diaryl substituted phenanthroimidazoles exhibited
high melting points (Tm) around 184–266 ◦C. In addition, distinct
glass transition temperature (Tg) could be observed for HPPhI,
MPPhI, and TsPPhI at 68.0, 78.3, and 99.0 ◦C, respectively whereas
the Tg of HPhI and MPhI was not detectable (Tables 2 and S1).
Among them, HPPhI shows a relatively lower Tg and Tm at 68.0 and
184 ◦C, respectively. This is probably because of the lower rota-
tional barrier of the central phenylene group in HPPhI compared
with other diaryl substituted phenanthroimidazoles like TsPPhI.
All of the diaryl phenanthroimidazoles showed no crystallization
or phase transition behavior upon heating beyond Tg. This suggests
that the diaryl phenanthroimidazoles have amorphous nature in
condensed state, which may greatly benefit the formation homo-
geneous and amorphous films through thermal evaporation.
All the dyes exhibit the wavelength absorption peaks around
260 5, 330 10, 360 5 nm in their solutions (2 × 10−6 to
5 × 10−6 M) of chloroform, acetonitrile, 1-propanol, and methanol
at room temperature, which can be considered to be absorptions
belonging to ␲–␲* transitions on the basis of extinction coefficients
(εꢀmax , Tables 2 and S1) and oscillator strengths (f) from molecular
orbital (MOs) calculation (Table 1). The molar absorption coeffi-
cients range between 12,580 and 38,100 cm−1 M−1, which is in
the typical range for allowed ␲–␲* transitions. Regardless of the
solvent polarity, all spectra show a distinct vibrational structure,
which is typical for 9,10-phenanthroimidazole [47] and 2-phenyl-
9,10-phenanthroimidazole (PhI) [48] rigid molecular framework.
Considering the 9,10-phenanthroimidazole or PhI nucleus as the
fundamental skeleton of the chromophores, absorption data shows
that conjugation of the ␲ system is extended by the electronic
substituents on the 1,2 positions of the imidazole ring, leading to
bathochromic and hyperchromic shifts. The close similarity of all
spectra in different solvents indicates that the ground state struc-
ture of these phenanthroimidazoles must be very similar in all
solvents. Even in protic and very polar solvents such as 1-poropanol
or methanol, no significant change that might imply the presence of
an additional ground state conformation or rotamer was observed.
In general, absorption peaks were slightly blue shifted in methanol
compared to those in chloroform. When the absorption positions
Tables 2 and S1 list absorption maxima and optical bandgaps
of all synthesized phenanthroimidazoles. This data indicates that
the optical bandgap (Eg) of aryl and diaryl phenanthroimidazole
derivatives is lower than that of parent PhI (Eg = 3.522 eV in CH3CN)
[47,48]. As shown in Tables 2 and S1: HOMO energy levels of these
dyes were found to be around 5.06–5.61 eV. This difference can be
attributed to the electron donating ability from different charge
transporting substituents on the phenanthroimidazole moiety. For
example with strong electron donating arylamine, APPhI possesses
the highest HOMO energy level among these dyes. Similarly, MPPhI
has second small oxidation potential and the second high HOMO
energy level next to that of APPhI. This is because the anisole moi-
ety is less electron rich than that of aniline. On the other hand, it
is reasonable to find NPPhI with highest oxidation potential and
the lowest HOMO energy level among phenanthroimidazoles dye
due to its built-in electron withdrawing ArNO2 moiety. The ion-
ization potential (IP) of electron donating substituted derivatives is
smaller than those of electron withdrawing groups. It suggests that
the electron donating groups favors the decrease in the ionization
potential (IP).
Fluorescence data and comparative overlays of emission spec-
tra of all synthesized compounds are presented in Tables 2, S1,
and Figs. 2B, D and S2–S3, respectively. The excitation spec-
tra are insensitive toward alteration of the emission wavelength
at which the spectra were acquired (data are not shown). The
peak excitation wavelengths are also identical with the mea-
sured maxima of the UV spectra. This again is consistent with a
single ground-state rotamer, which is responsible for the abnor-
mally large Stokes’ shifted emission band. It could be noticed that
non-ESIPT chromophores (MPhI, MPPhI, APPhI, and NPPhI) gave
normal Stokes’ shifted emissions in the deep blue region of the
visible spectrum (366–418 nm) while intramolecularly hydrogen-
bonded chromophores HPhI, HPPhI and TsPPhI produced mainly
large Stokes’ shifted tautomer emissions in the blue-green region
(464–490 nm) as a result of ESIPT process. The emission spectra
of the phenanthroimidazoles vary with the increase of the polarity
and hydrogen-bonding capability of the solvents; the band maxima
of the tautomer emissions get blue shifts, whereas the band max-
ima of the normal emissions get red shifts. In comparison with the
solution PL spectra, the emission of respective phenanthroimida-
zoles in the solid state is slightly blue-shifted with a reduced FWHM

32
M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
(full-width at half-maximum) of ∼3–4 nm; this reduced FWHM
implies that there would be weak aggregation involved in their
solid state [49].
It is interesting to note that the emission maximum of TsPPhI
appears at a longer wavelength with a shift of 20 nm compared
to HPPhI. This indeed would be consistent with an excited-
state interaction of the phenanthroimidazole chromophore with
the arenesulfonamide group, which results in a more stabilized
excited state and therefore a lower emission energy (Supporting
information Fig. S1). Because the pKa value of aryl-substituted sul-
fonamides are comparable to their phenol analogues, the ESIPT
process in TsPPhI is expected to be similarly efficient as for HPPhI.
In all solvents, an intense large Stokes’ shifted emission band was
observed, which can presumably be attributed to the sulfonimino
tautomer Tsyn* (Fig. 1). Initial measurements of ESIPT dyes showed
a weak emission band at higher energy in most solvents, which we
interpreted with the presence of the trans-rotamer Nanti (Fig. 2B,
D). Nevertheless, as quantum chemical calculations suggested, the
ground-state equilibrium should be exclusively dominated by the
cis-rotamer Nsyn, and therefore the normal emission band was a
surprising observation.
The solid state fluorescence quantum efficiency of HPhI, HPPhI,
and TsPPhI reaches to the very high value of 29.10%, 38.50%, and
67.85%, respectively, increasing up to 2–6 folds compared to their
solutions (Fig. 3 and Tables 2, S1). Also an interesting fact is that,
unlike for any other compound in the studied series, compound
TsPPhI displayed a very brilliant solid film photoluminescence in
the blue-green region of the visible spectrum at room temper-
ature. Besides electronic effects, there is probably an interesting
steric effect of the multiple phenyl substituents on molecular
arrangements in solid state, and is most probably interacts in
an “offset face-to-face” fashion with another imidazolyl ring of a
contiguous moiety in TsPPhI compared to HPPhI. Possibly, the rel-
ative small intermolecular ␲–␲ overlapping areas of these diaryl
phenanthroimidazoles compared with that of mono-aryl phenan-
throimidazoles is probably a key factor that accounts for their
high fluorescence quantum yields in the solid state and in the thin
film. The corresponding intramolecular motions and relative free-
rotation of end phenyl groups in solution may be the origin of weak
fluorescence (vide infra). In the solid state, the free-rotation occur-
ring in both compounds in solution will be depressed due to the
intermolecular interaction, which is beneficial to enhanced fluo-
rescent efficiency.
3.2. Quantum chemical calculations and photophysical properties
To study further about the relationship between their molec-
ular structures and optical properties, the geometrical parameters
of Nsyn–Tsyn tautomers and their energy-minimized, preferred con-
formations were calculated by DFT using B3LYP/6-31G(d,p) [32]
and semi-empirical using AM1 calculations [38,39]. The geome-
try optimization in the S0 and S1 of Nsyn, Tsyn and transition state
(TS) of the phenanthroimidazoles was performed and respective
parameters are listed in Table 1. As shown in Table 1, in the S0
and S1 for Nsyn form, the distance of N· · ·H in HPhI, HPPhhI, and
˚
TsPPhI was about 1.66–2.03 A. It is well known that H-bond is
short distance force in essence; hence the H-bond force between
H and N should be operating in the Nsyn form of these dyes. On
the other hand, the distance of N· · ·H was too short in the S0 and
˚
S1 for the Tsyn form, even up to about 1.51–1.62 A. Generally, the
Nsyn → Tsyn tautomerization brought a variation of the chemical
bond length of phenol or sulfonamide ring. For instance, the maxi-
mum of the difference between the bond lengths of phenolic O
H
˚
in the Nsyn form of HPhI, HPPhI was <0.01 A, while the value in the
˚
T
syn form increased to 0.14 A, implying that aromaticity of this ring
in the Tsyn form was lost because of the internal proton transfer.

M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
33
Fig. 2. (A) Normalized UV–vis absorption and (B) photoluminescence spectra of HPhI, green; HPPhI, blue; MPPhI, black; TsPPhI, red in CHCl3; (C) normalized UV–vis absorption
and (D) photoluminescence spectra of HPhI, green; HPPhI, blue; MPPhI, black; TsPPhI, red in a polymethylmethacrylate (PPMA) solid film doped with 5 wt% of each dye. (For
interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
Interestingly, during the ESIPT process (Nsyn* → Tsyn*) the observed
essentially in the same plane. In a system with a great delocaliza-
tion of ␲-electron, the torsion angle decided whether the molecule
could be well-conjugated. Consequently, the small twisted config-
uration in the excited state allowed the extension of the conjugated
␲-electron in the entire molecular framework, and the energy bar-
rier in the ESIPT process caused by the ground state twisting was
reduced. It was favorable to undergo ESIPT for HPPhI and TsPPhI.
The plots of the potential curves of internal proton trans-
fer in S0 and S1 states of HPPhI as a function of O H distance
were performed using AM1/PECI = 8 and presented in Fig. 4,
change in the bond lengths of sulfonamide N H in TsPPhI was large
˚
˚
(ca. 0.42 A) as compared to HPhI and HPPhI (ca. 0.14 A) which in turn
more polar in nature (ꢁ = 11.77 D) and manifest with decrease in
rate of ESIPT reactions. Again the stable ground state geometry of
Nsyn of HPPhI and TsPPhI had twisted bond ϕ2 (2ꢁ-ph) by ca. 9.4◦
and 43.6◦ at B3LYP/6-31G(d,p) level respectively. Dihedral angle
was decreased to ca.0◦ and 3.2◦ respectively during Nsyn → Tsyn tau-
tomerization, reflecting that the extent of molecular distortion was
reduced. While in the excited state of HPhI, the tautomers remained
Fig. 3. PL images of PMMA film doped with 5 wt% dye on the wedged quartz plate and solution and their absolute photoluminescence quantum yield (˚PL).

34
M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
phototautomer Tsyn* reverts to the original Nsyn via reverse pro-
ton transfer barrier (ꢂEr#) of 8.97 kcal mol−1. This implied that
although the occurrence of GSIPT from Nsyn → Tsyn should be very
difficult, the reverse proton transfer process from Tsyn → Nsyn could
take place easily, which was much favorable for ESIPT occurrence.
Finally, the protons transfer from Tsyn to the starting Nsyn in thermal
processes in S0 states to finish the cyclic four-level photophysi-
1
1
1
1
cal scheme (1Nsyn → Nsyn* → Tsyn* → Tsyn → Nsyn), immediately
after photoexcitation of the intramolecular H-bonded molecules.
Therefore, an abnormally large Stokes shift without self-absorption
is detected, providing an ideal scheme for UV-photostabilizers or
proton transfer lasers.
The electron distribution of the frontier orbitals reflects the elec-
tron transition characteristics. Seen from Fig. 5, the HOMO and
LUMO of substituted phenanthroimidazoles exhibited ␲-type sym-
metry. From HOMO to LUMO, the electron density distribution
displayed the transfer from phenolic ring to phenanthroimida-
zole moiety. The calculated long wavelength absorption peak
of HPhI (ca. 342.1 nm, oscillator strength (f) = 0.357), HPPhI (ca.
343.6 nm, oscillator strength (f) = 0.312), TsPPhI (ca. 330.3 nm, oscil-
lator strength (f) = 0.242) is a mixture of different transitions, e.g.,
HOMO → LUMO, HOMO → LUMO + 1, HOMO → LUMO + 2, etc. The
shapes of the HOMOs look very similar to each other for these
molecules. However, the LUMO + 1 of HPhI, HPPhI and LUMO + 2 of
TsPPhI are delocalized through the acceptors and ␲-bridges. This
change in the electron density acts as a driving force for the very
fast intramolecular proton transfer in these compounds upon exci-
tation to the S1 state. Thus, the phenolic ring had smaller electron
density in the excited state, which was driving force for the occur-
rence of proton transfer for HPPhI in the excited state. Hence, the
HOMO → LUMO transition could be ascribed to ␲–␲* excitation
with internal charge transfer character [50].
Fig. 4. The internal proton-transfer reaction potential energy curves of HPPhI
relaxed along the O H distance for the ground and excited state using AM1/PECI = 8
calculation.
which shows clearly that the Nsyn form is most stable in the
S0 state whereas the Tsyn form is most stable in the S1 state.
ꢂE (energy difference between Nsyn and Tsyn) again reveals that
Tsyn form has a higher energy than Nsyn form in S0 state as
the aromatic ring is broken by proton transfer. The activation
barrier for Nsyn → Tsyn in S0 electronic state is 18.55 kcal mol−1
,
large enough to make the ground state intramolecular pro-
ton transfer (GSIPT) unviable under thermal conditions, whereas
upon photoexcitation, much smaller interconversion barrier
(Nsyn* → Tsyn*) of 3.32 kcal mol−1 in S1 state preferably allows
ESIPT to give the large Stokes’ shifted fluorescence emission (ca.
104.9 nm and 6605 cm−1). After decaying to the ground state, the
The Mulliken net atomic charge population of the key atoms
(O H, N H, and N O) of HPPhI and TsPPhI are shown in Fig. 6.
Fig. 5. Calculated contour plots of frontier orbitals of enol (Nsyn) and keto (Tsyn) form of ESIPT chromophores (TD-DFT method at the B3LYP/6-31G(d,p) level).

M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
35
Fig. 6. The calculated Mulliken net atomic charge population of the key atoms of HPPhI (at the top) and TsPPhI (at the bottom) involving tautomerization (Nsyn–TS–Tsyn
)
using B3LYP/6-31G(d,p) method.
Remarkably, as per expectation, hydrogen atom had high positive
charge, which indicated that it was a “real proton” transfer if ESIPT
or GSIPT took place. Interestingly, the positive charge of hydrogen
experienced increasing and then decreasing during intramolecu-
lar proton transfer both in ground and excited states, reaching the
maximum at TS because of largest hydrogen-bond strength. While
Nsyn was excited to Nsyn*, the part of negative charge was trans-
ferred from the hydroxyl to the imino group, which resulted in
the change of the “force balance” of hydrogen bond. Consequently
the negative charge of the proton donor decreased (more acidic),
and the negative charge of the proton acceptor increased (more
basic), which enhanced the change of geometry in the excited state
and prompted the ESIPT occurrence for photoreactive hydrogen-
bonded chromophore. Table S2 shows that the dipole moments
centralized on the Y-dimension and the electronic transition led
to the changes in the charge distribution, which was accompanied
by the increase of the molecular dipole moments in the excited
state. As compared with HPPhI, owing to the great changes of the
dipole moment of on the X- and Y-dimension in both the ground and
the excited state, the dipole moment of TsPPhI of normal form was
lowered, but the dipole moment of tautomer form was enhanced
greatly, thus the dipole moment difference of tautomers of TsPPhI
exhibited much large changes in both the ground and excited state
than HPPhI (Table S2 and Fig. S5). This meant that large change of
charge geometry occurred due to internal proton transfer, which
in turn indicated a bit difficult to undergo GSIPT or ESIPT as man-
ifested by relatively larger intensity ratio of normal to tautomer
emission in TsPPhI than that in HPPhI.
favor the ICT from the proton donor to the proton acceptor moiety
will increase the nonradiative decay and, consequently, decrease
the fluorescence quantum yield.
Restricted intramolecular/intermolecular motion [20–28] has
been suggested as one of the most significant mechanisms of
aggregation-induced emission enhancement (AIEE) phenomenon.
In dilute solutions of molecular dispersed phenanthroimidazole
dyes, two end-substituted phenyl units of the molecule could rotate
freely around the single bonds and the radiative decay would be
effectively restricted by this kind of intramolecular torsion. While
in the aggregate state, the intramolecular rotation and torsion were
greatly impeded and therefore the non-radiative decay channel
was effectively restricted, which in turn populated the irradiative
state of the excited molecules and resulted in a great increase of
fluorescence. Hence, it was easy to understand why the molec-
ularly dispersed dilute solutions of phenanthroimidazoles were
so weakly luminescent while their solid particles and aggregates
were highly emissive. All these results in agreement with the pro-
posed mechanism for the intramolecular H-bonded compounds
HPhI, HPPhI, and TsPPhI in dilute solution undergo an excited state
charge transfer coupled proton transfer depicted in Fig. 1. Upon
excitation of Nsyn an ESIPT process takes place to give Tsyn* flu-
orescent species, after which the excited tautomer undergoes a
large-amplitude conformational change associated with a charge
migration from deprotonated phenol or sulfonamide moiety to the
protonated imidazole moiety, yielding the non-fluorescent charge-
transfer intermediate TICT*, which probably deactivates very fast
[10]. The electronic and geometric structure of TICT* in Fig. 1 is hypo-
thetical, not only about the nature of the conformational change
experienced by Tsyn* but also in relation to the charge distribu-
tion. Accordingly, the fluorescence enhancement in the solid state
is due to the prevented TICT by kinetic constraint which blocks
large amplitude twisting motion about the interannular C C bond
(torsion angle, ϕ2) along the ESIPT reaction coordinate.
Recent quantum chemical modeling studies on salicylanilides
[51], salicylideneaniline (SA) [52] show that ESIPT occurs from a
planar Nsyn* which gives a planar Tsyn* (cis-keto*). This planar Tsyn
*
is not stable and undergoes out-of-plane torsion which is the pre-
cursor of syn-anti (cis–trans) isomerization to the Tanti (trans-keto)
photoproduct. Thus one hypothesis for the two fluorescent Tsyn
*
(cis-keto*) is a planar and a nonplanar isomer or different nonpla-
nar isomers. The bulky phenyl or tosyl group can stabilize them
and favor the existence of two fluorescent Tsyn* species in com-
parison with HPhI. The difference in fluorescence intensity among
HPhI, HPPhI, and TsPPhI in solid state can be well explained by their
different classes: as mentioned above, the Tsyn* in TsPPhI does not
lead to the Tanti (trans-imine) photoproduct which relaxes by non-
radiative decays (internal conversion) but fluoresce to Tsyn ground
state. In contrast, relatively fast formation of the Tanti photoproduct
in mono-aryl phenanthroimidazole HPhI reduces the fluorescence
of the Tsyn*. In solution, however, both mono- and di-aryl phenan-
throimidazoles are relatively stabilized an intramolecular charge
transfer (ICT) process in Tsyn* might induce the interannular bond
rotation, leading to a non-emissive, twisted excited state TICT* after
the ESIPT, which explains the weak fluorescence of the Tsyn*. Thus,
it can be stated that the substituents in the ESIPT molecules that
4. Conclusion
Seven phenanthroimidazoles derivatives (HPhI, MPhI, HPPhI,
MPPhI, NPPhI, APPhI and TsPPhI) have been synthesized and
detailed studies of their electronic structures, photophysical and
electrochemical properties are presented. The branched 1,2-
diphenyl-phenanthro[9,10-d]imidazoles are more emissive than
their linear 2-phenyl-1H-phenanthro[9,10-d]imidazole analogs,
both in solution and when aggregated, presumably because of
increased ␲-electron contributions to the electronic structure
of the molecule, as well as restricted nonradiative deactivation
resulting from less facile rotational deactivation via the interan-
nular C C bond (torsion angle, ϕ2). The quantum chemical studies
reveal that the formation of non-fluorescent isomers (TICT*) of
1,2-diphenyl phenanthroimidazoles was effectively suppressed

36
M.K. Nayak / Journal of Photochemistry and Photobiology A: Chemistry 241 (2012) 26–37
in the solid state due to the bulky substituents preventing a
large-amplitude conformational change in the excited-state. The
ESIPT dyes containing branched 1,2-diphenyl-phenanthro[9,10-
d]imidazole groups showed excellent thermal properties with high
Tg (around 68–99 ◦C) and an efficient large Stokes’ shifted emis-
sion at ca. 464–490 nm with very high fluorescence quantum yield
∼39–68% in the neat solid films without suffering from concentra-
tion self-quenching. The molecular design concept established in
this study should provide guidelines for fine-tuning the emission
properties of this class of ESIPT fluorophores, which is beneficial for
developing a new class of advanced optoelectronic applications.
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Supplementary information
Electronic supplementary information (ESI) available: 1H and
13C NMR spectra of pure phenanthroimidazoles, DSC traces, cyclic
voltammetry, UV–vis absorption and photoluminescence, opti-
mized geometries of keto–enol (HPhI and HPPhI) and amine–imine
(TsPPhI) tautomers, atom coordinates and absolute energies of
the computed structures, HOMO–LUMO orbital’s of ESIPT, and
non-ESIPT phenanthroimidazoles MPhI, MPPhI, NPPhI, APPhI upon
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The author is gratefully acknowledged to Professor Soo Young
Park, Department of Materials Science and Engineering, Seoul
National University, South Korea, for useful comments and assis-
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yields of phenanthroimidazole dyes in solid state. I also thank
Professor Hassan S. Bazzi, Department of Chemistry, Texas A&M
University at Qatar, for giving me technical knowledge in several
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