Synthesis, crystal structure, kamlet-taft and catalan solvatochromic analysis of novel imidazole derivatives

Article abstract of DOI:10.1007/s10895-011-0974-4

Novel imidazole derivatives were synthesized and its crystal structure has been studied by single crystal XRD analysis. The photophysical properties of these imidazole derivatives were studied in several solvents, which include a wide range of apolar, pol

Full text of DOI:10.1007/s10895-011-0974-4

J Fluoresc (2012) 22:409–417
DOI 10.1007/s10895-011-0974-4
ORIGINAL PAPER
Synthesis, Crystal Structure, Kamlet-Taft and Catalan
Solvatochromic Analysis of Novel Imidazole Derivatives
Jayaraman Jayabharathi & Venugopal Thanikachalam &
Marimuthu Venkatesh Perumal & Natesan Srinivasan
Received: 11 May 2011 /Accepted: 13 September 2011 /Published online: 23 September 2011
#
Springer Science+Business Media, LLC 2011
Abstract Novel imidazole derivatives were synthesized and
its crystal structure has been studied by single crystal XRD
analysis. The photophysical properties of these imidazole
derivatives were studied in several solvents, which include a
wide range of apolar, polar and protic media. The observed
lower fluorescence quantum yield may be due to an increase in
the non-radiative deactivation rate constant. This is attributed to
a loss of planarity in the excited state provided by the non co-
planarity of the aryl rings attached to C(2) and N(1) atoms of the
imidazole ring. Such a geometrical change in the excited state
leads to an important Stokes shift, reducing the reabsorption
and reemission effects in the detected emission in highly
concentrated solutions. The highest fluorescence quantum yield
of the imidazole derivatives are observed in polar media.
[10] environmental probes in bio-molecules [11], etc. by
utilizing their fluorescence and chemiluminescence properties.
Several factors contribute to the best laser performance of
pyrromethene with respect to rhodamine as laser dye [12]: (1)
low triplet-triplet absorption capacity at the lasing spectral
region, which reduces the losses in the resonator cavity; (2) a
poor tendency to self-aggregate in organic solvents, avoiding
the fluorescence quenching of the monomer emission by the
presence of aggregates in highly concentrated solutions, as
was observed in rhodamine dyes: (3) and their high photo-
stability [13–18], which improves the lifetime of the laser
action with respect to that of rhodamines [19]. Owing to
these properties, PM dyes have been successfully incorpo-
rated into different solid matrixes (polymers, silica, etc.,)
[20–22] to develop solid-state syntonizable dye lasers.
Photophysical properties and laser characteristics of com-
pounds in liquid solutions and in polymeric matrixes [20–22],
indicating that the laser behaviour is a consequence of the
photophysical properties. Indeed, the evolution of the
fluorescence wavelength and quantum yield with several
environmental factors was similar to that observed for the
laser band and efficiency, respectively. The Stokes shift
modifies the lasing performance [23] in highly concentrated
solutions because it affects the reabsorption and reemission
phenomena, which shifts the emission band to longer
wavelengths and reduces its efficiency [24]. In the present
paper, the photophysical properties of imidazole derivatives
1 and 2 were investigated in a wide variety of solvents,
including apolar, polar and protic solvents. The solvent
effects on the absorption and fluorescence bands are
analyzed by a multi-component linear regression in which
several solvent parameters are simultaneously analyzed. The
fluorescence quantum yield and the Stokes shift are analyzed
to look for the best conditions to improve the lasing
efficiencies of these derivatives.
.
.
.
.
Keywords ORTEP Imidazole Laser Catalan parameters
Aggregation
Introduction
Recently, heterocyclic imidazole derivatives have attracted
considerable attention because of their unique optical properties
[1–3]. These compounds play very important role in
chemistry as mediators for synthetic reactions, primarily for
preparing functionalized materials [4–7]. Imidazole nucleus
forms the main structure of some well-known components of
human organisms and also has significant analytical applica-
tions such as laser [8], polymer stabilizer [9], Raman filters
:
:
:
J. Jayabharathi (*) V. Thanikachalam M. Venkatesh Perumal
N. Srinivasan
Department of Chemistry, Annamalai University,
Annamalainagar 608 002 Tamilnadu, India
e-mail: jtchalam2005@yahoo.co.in

410
J Fluoresc (2012) 22:409–417
Experimental Section
thin layer chromatography for the completion of the reaction.
The reaction mixture was then extracted with dichloro-
methane and the resultant resinous material was purified by
column chromatography using benzene:ethyl acetate (9:1) as
the eluent.
Materials and Methods
1,10-Phenanthroline-5,6-dione was synthesized and purified
according to the reported literature procedure [25], benzalde-
hyde, 4-Fluorobenzaldehyde, aniline and all the other
reagents were purchased from S.D. fine chemicals and used
without further purification. The experimental procedure was
used as same as described in our recent papers [26–34].
NMR spectra were recorded on a Bruker 400 MHz NMR
instrument. UV–vis absorption and fluorescence spectra were
recorded on Perkin Elmer spectrophotometer Lambda35 and
Perkin Elmer LS55 spectrofluorimeter, respectively. Fluores-
cence spectra were corrected from the monochromator
wavelength dependence and the photomultiplier sensibility.
Fluorescence quantum yields (ϕ) were determined by means
of the corrected fluorescence spectra of a dilute solution of
imidazole derivatives in dichloromethane, using coumarin 47
in ethanol as a reference and by taking into account the solvent
refractive index. Radiative decay curves were recorded by
means of a time-correlated single-photon counting technique.
Mass spectrum was recorded using Agilant 1100 Mass
spectrometer.
1,2-Diphenyl-1H-imidazo[4,5-f][1,10]phenanthroline (1)
Yield: 55%. mp>306 °C, Anal. calcd. for C25H16N4: C,
80.63; H, 4.33; N, 15.04. Found: C, 79.95; H, 4.08; N,
14.98. 1H NMR (500 MHz, CDCl3): δ 7.22–7.38 (m, 5 H),
7.53–7.72 (m, 5H), 9.01 (d, 2 H, H-aryl, J=4.28 Hz), 9.09
(d, 2H, J=7.68 Hz), 9.17 (d, 2 H, J=2.16 Hz). 13C
(100 MHz, CDCl3): δ 119.62, 121.85, 123.30, 123.88,
126.65, 127.65, 128.20, 128.75, 129.11, 129.16, 129.75,
130.07, 130.31, 130.42, 136.05, 137.99, 144.20, 144.75,
147.67, 148.77, 151.88. MS: MS: m/z. 373 [M+1].
2-(4-Fluorophenyl)-1-phenyl-1H-imidazo[4,5-f][1,10]-
phenanthroline (2)
Yield: 55%. mp. 294 °C, Anal. calcd. for C25H15N4F: C,
76.91; H, 3.87; N, 14.35. Found: C, 76.59; H, 3.68; N,
14.34. 1H NMR (500 MHz, CDCl3): δ 7.05 (t, 2H), 7.27 (q,
1H), 7.34 (m, 1H), 7.59 (m, 2H), 7.69 (m, 3H), 9.05 (d, 2H,
H-aryl, J=4.4 Hz), 9.08 (d, 2H, J=8.0 Hz), 9.21 (d, 2H, J=
2.8 Hz). 13C (100 MHz, CDCl3): δ 115.55, 115.76, 119.77,
122.06, 123.54, 124.05, 126.87, 127.75, 129.02, 130.46,
130.55, 130.69, 131.31, 131.39, 136.27, 138.25, 144.55,
145.10, 147.95, 149.10, 151.07, 162.10, 164.60. MS: m/z.
390.9 [M+1], 391.9 [M+2].
The photophysical properties of the imidazole derivatives
were recorded in dilute concentrations (1×10−5 M). The
fluorescence decay curves were analyzed as mono exponen-
2
tials (
), and the fluorescence decay time (C) was
< 1.2
obtained from the slope. The rate constant for the radiative (kr)
and non-radiative (knr) deactivation pathways were calculated
to be: kr ¼ Φp=t; knr ¼ 1=t ꢀ Φp=t and t ¼ ðkr þ knrÞꢀ1
.
Computational Details
General Procedure for the Synthesis of the Imidazole
Derivatives
Quantum mechanical calculations were used to carry out
the optimized geometry and HOMO-LUMO energies with
Guassian-03 program using the Becke3-Lee-Yang-Parr
(B3LYP) functional supplemented with the standard 6–
31G(d,p) basis set [35].
The imidazole derivatives were synthesized from an unusual
four components assembling of 1,10-Phenanthroline-5,6-
dione, ammonium acetate, aniline and the corresponding
benzaldehyde (Scheme 1). The reaction was monitored by
Scheme 1 Synthesis of
imidazole derivatives (1) and (2)

J Fluoresc (2012) 22:409–417
411
Table 1 Photophysical data of the imidazole derivatives 1 and 2 in different solvents
Imidazole derivative (1)
solvent
λab ( 0.1 nm) λfl ( 0.4 nm) Δυss (cm−1
)
Φ ( 0.05 nm) τ ( 0.01 ns) εmax (104 M−1cm−1
)
Kr (108 s−1
)
knr (108 s−1
)
Hexane
270.5
272.0
268.1
264.2
269.1
265.5
263.0
265.0
263.8
267.0
268.1
268.1
267.3
389.0
392.4
394.5
395.0
408.0
393.0
413.0
415.0
397.0
409.0
396.0
406.0
409.0
11261
11280
11950
12533
12651
12219
13809
13639
12718
13003
12046
12668
12961
0.35
0.36
0.41
0.45
0.48
0.45
0.49
0.50
0.44
0.41
0.40
0.42
0.39
4.13
4.25
4.26
4.30
4.43
4.35
4.08
4.2
3.58
3.41
8.99
6.49
7.36
7.21
5.40
7.50
5.39
7.69
4.12
4.21
7.13
0.85
0.85
0.96
1.05
1.08
1.03
1.20
1.19
1.03
0.87
0.93
1.00
0.93
1.57
1.51
1.38
1.28
1.17
1.26
1.25
1.19
1.31
1.25
1.40
1.37
1.45
Cyclohexane
dioxane
ethyl acetate
Butanol
Acetone
ethanol
methanol
acetonitrile
DMF
4.28
4.71
4.28
4.22
4.20
ether
THF
1-propanol
Imidazole derivative (2)
Φ ( 0.05 nm) τ ( 0.01 ns) εmax (104 M−1cm−1
solvent
λab ( 0.1 nm) λfl ( 0.4 nm) Δυss (cm−1
)
)
Kr (108 s−1
)
knr (108 s−1
)
Hexane
267.5
267.5
268.5
268.8
269.4
268.6
268.5
268.3
268.3
268.5
268.4
268.0
269.2
394.0
395.0
397.0
398.0
411.0
400.0
416.0
414.0
396.0
412.0
398.0
409.0
412.0
12002
12066
12055
12076
12788
12230
13205
13117
12019
12972
12132
12863
12875
0.41
0.42
0.43
0.46
0.49
0.45
0.53
0.54
0.48
0.41
0.41
0.42
0.41
4.23
4.21
4.34
4.35
4.31
4.39
4.03
4.35
4.38
4.41
4.32
4.33
4.51
2.05
2.03
3.56
4.09
6.77
3.65
4.26
3.93
4.10
4.56
4.32
4.63
6.23
0.97
1.00
0.99
1.06
1.14
1.03
1.32
1.24
1.10
0.93
0.95
0.97
0.91
1.39
1.38
1.31
1.24
1.18
1.25
1.17
1.06
1.19
1.34
1.37
1.34
1.31
Cyclohexane
dioxane
ethyl acetate
Butanol
Acetone
ethanol
methanol
acetonitrile
DMF
ether
THF
1-propanol
Results and Discussion
UV–vis absorption and fluorescence spectra (Table 1) of
the imidazole derivatives 1 and 2 are shown in Fig. 1. In
apolar solvents, the main absorption band is centered
around 268 nm with a high molar absorption coefficient
(ε≈9×104 M−1cm−1), whereas the fluorescence spectrum is
centered around 390 nm with a fluorescence quantum yield
around 0.40. The fluorescence decay curve of 2, which can
be analyzed as a mono-exponential decay with a fluores-
cence lifetime of 4.2 ns.
The fluorescence spectrum of the imidazole derivative
2 is shifted to lower energies with respect to the parent
compound 1 in common solvents. These spectral shifts are
attributed to the higher inductive electron-acceptor char-
acter of the fluorine atom located at C(27) carbon atom. In
the case of alcoholic solvents, bathochromic shifts were
observed for both absorption and emission providing large
Fig. 1 Absorption and fluorescence spectra of 1 (in bold lines) and 2
(in dotted lines) in hexane and ethanol

412
J Fluoresc (2012) 22:409–417
Table 2 Selected Bond lengths, Bond angles and Torsional angles for the imidazole derivative (1)
Bond connectivity
Bond length (Å)a
Bond connectivity
Bond angle (º)
Bond connectivity
Torsional angle (º)
N1-C5
1.4020 (1.3840)
1.4152 (1.4402)
1.4222 (1.3752)
1.4653 (1.4782)
1.3966 (1.3789)
1.3570 (1.3168)
1.4292 (1.3770)
1.4277 (1.4330)
1.4270 (1.4321)
1.3701 (1.3506)
1.4102 (1.3840)
1.4116 (1.3745)
1.4018 (1.3860)
1.3999 (1.3890)
1.1000 (0.9500)
N1-C2-N3
111.6157 (112.63)
132.3707 (131.74)
105.9006 (105.14)
120.3840 (119.73)
120.2787 (118.93)
106.4092 (106.40)
121.0290 (121.16)
119.0361 (119.87)
105.7293 (104.52)
110.3424 (111.31)
124.5154 (124.59)
128.8832 (127.30)
121.7286 (123.04)
120.7742 (121.38)
127.4376 (128.80)
119.8997 (120.44)
119.7874 (120.30)
C4-C5-C6-C7
−179.3862 (−175.39)
0.4470 (0.8)
N1-C18
N1-C2
N1- C5-C6
N1- C5-C4
N1-C18-C19
N1-C18-C23
C2-N1-C5
N1-C5-C6-C7
N3-C4-C5-C6
179.4411 (176.28)
−0.7134 (−3.5)
−2.8872 (−2.3)
−179.3215 (−175.82)
95.5898 (82.65)
C2-C24
N3-C4
C17-C4-C5-C6
C2–N1- C5-C6
C2 -N1-C5-C6
C5-N1-C18-C19
C5-N1-C18-C23
C5-N1-C2-N3
N3-C2
C4-C5
C2-C24-C25
C2-C24-C29
C2- C3-C4
N3- C4-C5
N3- C2-C24
N3- C4-C17
C4-C5-C6
C4-C17
C5-C6
−84.7810 (−98.62)
−0.4790 (−0.73)
−179.6887 (−179.76)
−179.7216 (−179.93)
180.0063 (179.25)
179.5595 (177.39)
−179.7109 (−176.22)
−135.6627 (−125.55)
−176.9773 (−174.58)
44.5636 (55.54)
N10-C11
C18-C19
C18-C23
C24-C29
C24-C25
C27-H27
C5-N1-C2-C24
C2- N3-C4-C17
C4-N3-C2-C24
N1-C18-C19-C20
N1-C18-C23-C22
N1-C2-C24-C29
C18-N1-C2-N3
N3-C2-C24-C29
N1-C2-C24-C25
C2-C24-C29-C28
C2-N1-C18-C23
C7-C6-C11-N10
C9-N10-C11-C6
C14-N13- C12-C17
C5-C4-C17
C5-N1-C18
C24-C25-C26
C24-C29-C28
46.2322 (56.56)
−178.6538 (−177.29)
74.91 (73.81)
0.0300 (0.6)
−0.0252 (−0.1)
−0.0146 (−3.2)
a values given in the brackets are corresponding to XRD values
R[F2 >2σ(F2 )]=0.042; wR(F2 )=0.117; R indices (all data)=0.0458
Stokes shift. These results suggest an important geomet-
rical rearrangement in the S1 excited state and these
observations are in good agreement with the literature
report [36].
X-ray data of 1 (Table 2) reveal that the fused ring
system is essentially planar and the R indices datas are
given in Table 2. The imidazole ring makes dihedral angles
of 77.41° and 56.26° with the phenyl ring attached to N(1)
nitrogen and phenyl ring attached to C(2) carbon, respec-
tively. The dihedral angle between the two phenyl rings is
65.50° found in the crystal structure (Fig. 2). In order to
eliminate the crystal packing effect in the solid state, the
molecular geometry of 1 and 2, was further examined by
DFT calculation [DFT/B3LYP16–31G(d,p)] [35] to make
necessary comparison on an equal state basis. The key twist
(α) is used to indicate the twist of imidazole ring from the
aromatic six-membered ring at C(2) (Fig. 3). It reveals that
α twist is correlated with fluorescent property, the larger α
twist, the more drops the fluorescence quantum yield. Such
a clear correlation indicates the importance of co-planarity
between the imidazole and the phenyl ring at C(2). This
correlation can be ascribed to the conjugation rigidity.
Fig. 2 ORTEP diagram of 1,2-Diphenyl-1H-imidazo[4,5-f][1,10]
phenanthroline (1)

J Fluoresc (2012) 22:409–417
413
molecule in the gaseous phase whereas the XRD results
were aimed at the molecule in the solid state.
The higher bathochromic shifts observed in the fluores-
cence band of the imidazole derivative 2 with respect to the
parent imidazole derivative 1 can be interpreted by the
Brunings-Corwin effect [37]. Because the distortion of the
geometry in the excited state implies a decrease in the
resonance energy, the fluorescence band is bathochromi-
cally shifted to a higher extent than the absorption band.
Moreover, the loss of planarity in the excited state of the
imidazole derivative could explain the lower fluorescence
quantum yield in apolar solvents owing to an increase in the
non-radiative processes.
Fig. 3 The key α twist of imidazole ring at C(2)
However, the shape of the absorption band is indepen-
dent of the imidazole concentration suggesting the poor
aggregation and it is a suitable behaviour in the perfor-
mance of active media of lasers. Indeed, high concentration
is necessary to bring about laser action, and the presence of
aggregates could drastically reduce the fluorescence quan-
tum yield owing to the efficient quenching of the monomer
fluorescence by the aggregates [17]. However, experimental
data indicate that the fluorescence band is shifted to lower
energies by increasing the concentration. This bathochro-
mic shift is attributed to reabsorption and reemission
phenomena [38] and this result corroborates the importance
of registering photophysical properties in dilute solutions.
Figure 4 shows the calculated contour maps for the
electronic distribution of the HOMO and LUMO states by
the B3LYP method. The electron density at position C(2)
and the aryl ring attached to the C(2) carbon is augmented
in the HOMO with respect to that in the LUMO state.
Taking into account the resonance structures of the
imidazole chromophore, we observed that the resonance
structure “b” has the largest charge separation along the
When the two adjacent aromatic species are in a coplanar
geometry, the p-orbitals from the C–C bond connecting the
two species will have maximal overlapping and the two
rings will have a rigid and delocalized conjugation, as the
result, the bond is no longer a pure single bond, as evident
from the X-ray data of 1. When the two rings are deviated
from each other, the p-orbital overlapping will be reduced.
The partial conjugation will lead to less rigid structure;
therefore, radiationless twist motion will deactivate the
emitting state, leading to the low quantum yields (Table 1).
Thus, it is suggested that fluorophores with the substitution
at C(2) with a minimum loss of fluorescence property. All
these XRD data are in good agreement with the theoretical
values (Table 2). However, from the theoretical values it
can be found that most of the optimized bond lengths, bond
angles and dihedral angles are slightly higher than that of
XRD values. These deviations can be attributed to the fact
that the theoretical calculations were aimed at the isolated
Fig. 4 HOMO-LUMO orbital
picture of (1) and (2)

414
J Fluoresc (2012) 22:409–417
Fig. 5 Correlation between the
experimental absorption and
fluorescence wavenumber with
the predicted values obtained by
a multicomponent linear regres-
sion using the π*, α and β-scale
(Taft) solvent parameters for (1)
and (2)
short molecule axis. Consequently, its contribution would
be more important in the S0 ground states than in the S1
excited states. Thus, the polar solvents would stabilize the
S0 state more extensively than the S1 state, thereby
increasing the energy gap between both states and explain-
ing the solvatochromic shifts [39].
To analyze the solvatochromic effects, we have checked
several methods [40–42]. Neither the absorption nor the
fluorescence wavenumbers linearly correlate with the
Lippert parameter Δf(ε,n2) [43], which considers the
solvent polarity/polarizability or with the Reichardt param-
eter ETN(30) [41, 44], which takes into account several
Table 3 Adjusted Coefficients ((υx)0, ca, cb and cc) and Correlation
Coefficients (r) for the Multilinear Regression Analysis of the
Absorption υab and Fluorescence υfl Wavenumbers and Stokes Shift
(Δυss) of the imidazole derivative 1 and 2 with the Solvent Polarity/
Polarizability, and the Acid and Base Capacity Using the Taft (π*, α
and β) and the Catalan (SPPN, SA and SB) Scales
(υx)
(υx)0cm−1
(π*)
c
c
r
α
β
Imidazole derivative (1)
λab
(3.69 0.022)×104
(4.46 2.516)×103
(1.58 1.856)×103
(2.87 3.524)×103
(8.72 7.266)×103
−(7.71 5.359)×103
(1.02 1.177)×103
−(5.13 5.300)×103
−(4.76 3.908)×103
−(3.75 7.421)×103
0.66
0.87
0.76
r
λfl
(2.56 0.016)×104
(1.13 0.031)×104
(υx)0cm−1
Δυss=υab−υfl
(υx)
N
cSPP
cSA
cSB
λabλfl
(3.73 0.015)×104
(2.53 0.013)×104
(1.21 0.022)×104
(5.96 9.832)×103
−(10.93 8.674)×103
(16.89 14.273)×103
(2.39 42.167)×103
(35.87 37.198)×103
−(59.78 61.210)×103
−(2.51 43.459) x 103
−(34.716 38.338)×103
−(59.839 6.386)×103
0.64
0.69
0.56
Δυss=υab−υfl
Imidazole derivative (2)
λab
(3.73 0.002)×104
−(1.22 0.261)×103
(3.14 0.786)×103
−(19.17 4.343)×103
(22.31 4.940)×103
−(2.38 0.593)×103
−(13.81 3.277)×103
−(16.19 3.727)×103
0.89
0.92
0.89
r
λfl
(2.53 0.0113)×104
(1.20 0.0129)×104
(υx)0cm−1
(5.00 1.443)×103
Δυss=υab−υfl
−(6.22 1.641)×103
N
(υx)
cSPP
cSA
cSB
λab
(3.73 0.002)×104
(2.51 0.013)×104
(1.22 0.013)×104
−(1.59 1.620)×103
−(5.47 8.842)×103
(3.88 8.759)×103
(4.75 6.950)×103
(11.87 37.919)×103
−(7.13 37.565)×103
−(3.59 7.163)×103
−(8.77 39.081)×103
−(5.21 38.716)×103
0.64
0.62
0.58
λfl
Δυss=υab−υfl

J Fluoresc (2012) 22:409–417
415
and correlation coefficients by the Taft and Catalan
parameters. The dominant coefficient affecting the absorption
and fluorescence bands of these imidazole derivatives 1 and 2
is that describing the polarity/polarizability of the solvent,
N
Cπ* or CSPP having a positive value, corroborating the
above-mentioned solvatochromic shifts with the solvent
polarity. The coefficient controlling the H-donor capacity or
acidity of the solvent, Cα or CSA, is the lowest coefficient
(Table 3), therefore, the solvent acidity does not play an
important role in absorption and fluorescence displacements.
The adjusted coefficient representing the electron releasing
ability or basicity of the solvent, Cβ or CSB has a negative
value, suggesting that the absorption and fluorescence bands
shift to lower energies with the increasing electron-donating
ability of the solvent. This effect can be interpreted in terms
of the stabilization of the resonance structures of the
chromophore (Fig. 6). Resonance structure “b” has the
positive charge located at the nitrogen atom and it will be
stabilized in basic solvents because this resonance structure
is predominant in the S1 state, as discussed above and the
stabilization of the S1 state with the solvent basicity would
be more important than that of the S0 state. Consequently,
the energy gap between the S1 and S0 states decreases and
the absorption and fluorescence wavelengths shift to longer
wavelengths with increasing solvent basicity.
However, the fluorescence quantum yield and lifetime
values of imidazole derivatives were observed in com-
mon solvents. The solvent also affects the fluorescence
quantum yield and lifetime of imidazole derivative and ϕ
and τ values increase in polar/protic solvents (Table 1),
although a general tendency in both parameters with
respect to the solvent viscosity is not observed. These
results indicate an extra non-radiative deactivation of
imidazole derivative, which is not governed by the solvent
viscosity. Figure 7 shows a correlation between the knr
value and the fluorescence wave numbers of imidazole
derivatives 1 and 2 by changing the nature of the solvent,
indicating similar solvent parameters affecting both photo-
physical characteristics, as was also proposed for other
aromatic systems [47–50].
Fig. 6 Resonance structures (a & b) of the imidazole chromophore (2)
solvent properties (polarity and H-bond donor capacity) in a
common parameter. For these reasons, a multi-parameter
correlation analysis is employed in which a physicochem-
ical property is linearly correlated with several solvent
parameters by means of Eq. 1:
ðXYZÞ ¼ ðXYZÞ0 þ CaA þ CbB þ CcC þ . . .
ð1Þ
where (XYZ)0 is the physicochemical property in an inert
solvent and Ca, Cb, Cc and so forth are the adjusted
coefficients that reflect the dependence of the physico-
chemical property (XYZ) on several solvent properties.
Solvent properties that mainly affect the photophysical
properties of aromatic compounds are polarity, H-bond
donor capacity and electron donor ability. Different scales
for such parameters can be found in the literature, Taft et al.
[45] propose the π*, α and β scales, whereas more recently
Catalan et al. [46] suggest the SPPN, SA and SB scales to
describe the polarity/polarizability, the acidity and basicity
of the solvents respectively.
Figure 5 shows the obtained correlation between the
absorption and fluorescence wavenumbers calculated by
the multi-component linear regression employing the Taft-
proposed solvent parameters and the experimental values
are listed in Table 3. Table 3 lists the obtained adjustment
Fig. 7 Correlation between the
rate constant of non-radiative
deactivation (log knr) with the
fluorescence wavenumbers of
(1) and (2) in several solvents

416
J Fluoresc (2012) 22:409–417
It is well known that the flexibility/rigidity of the
References
imidazole derivatives can control the mechanics of internal
conversion. The flexibility/rigidity of the aromatic com-
pounds can be analyzed in terms of the planarity of the π-
electron system [51, 52]. This loss of planarity, especially
in the excited state implies a less rigid structure and the
excitation energy is more easily converted to vibrational
energy and dissipated as heat, enhancing the internal
conversion processes. Drexhage et al. [51] pointed out that
the structure loosens up in some compounds upon
excitation, enhancing the non-radiative deactivation pro-
cesses. In the imidazole derivatives 1 and 2, the steric
hindrance between the aryl ring at N(1) of the imidazole
ring and the aryl ring at C(2) of the imidazole, which makes
the molecule in distorted way. Such a distortion from
planarity leads to an augmentation of the non-radiative
deactivation. Pavlopoulos et al. [13] also referred to the
non-planarity of the chromophore provided by bulky
substituents to explain the low fluorescence quantum yield.
The present photophysical characteristics of these imid-
azole derivatives would suggest a lower lasing efficiency
because of its lower fluorescence quantum yield. However,
high optical densities are required to produce laser signals
and under these conditions, the losses at the resonator
cavity by the reabsorption and reemission effects can be
important. These imidazole derivatives are characterized by
a higher Stokes shift which would reduce the losses at the
resonator cavity by reabsorption and reemission effects,
governed by the overlap between the absorption and
emission spectra [30]. Therefore, the low fluorescence
quantum yield to some extent can be compensated by its
high Stokes shift and high laser efficiencies for imidazole
derivatives [53–56]. Because the higher Stokes shift and
highest fluorescence quantum yield of the imidazole
derivative are recommended to achieve the highest laser
efficiencies in liquid media.
1. Santos J, Mintz EA, Zehnder O, Bosshard C, Bu XR, Gunter P
(2001) New class of imidazoles incorporated with thiophenevinyl
conjugation pathway for robust nonlinear optical chromophores.
Tetrahedron Lett 42:805–808
2. Hung WS, Lin JT, Chien CH, Tao YT, Sun SS, Wen YS (2004)
Highly phosphorescent bis-cyclometallated iridium complexes con-
taining benzoimidazole-based ligands. Chem Mater 16:2480–2488
3. Chen CH, Shi J (1998) Metal chelates as emitting materials for
organic electroluminescence. Coord Chem Rev 171:161–174
4. Kamidate T, Yamaguchi K, Segawa T (1989) Lophine chemilu-
minescence for determination of chromium(VI) by continuous
flow method. Anal Sci 5:429–433
5. Nakashima K, Yamasaki H, Kuroda N, Akiyama S (1995)
Evaluation of lophine derivatives as chemiluminogens by a
flow-injection method. Anal Chim Acta 303:103–107
6. MacDonald A, Chain KW, Nieman TA (1979) Lophine chemilumi-
nescence for metal ion determinations. Anal Chem 51:2077–2082
7. Marino DF, Ingle JD Jr (1981) Determination of chromium (VI) in
water by lophine chemiluminescence. Anal Chem 53:294–298
8. Chou PT, McMorrow D, Aartsma TJ, Kasha M (1984) The
proton-transfer laser. Gain spectrum and amplification of sponta-
neous emission of 3-hydroxyflavone. J Phys Chem 88:4596–4599
9. Keck J, Roessler M, Schroeder C, Stueber GJ, Waiblinger F, Stein M,
Legourrierec D, Kramer HEA, Hoier H, Henkel S, Fischer P, Port H,
Hirsch T, Tytz G, Hayoz P (1998) Ultraviolet absorbers of the 2-(2-
Hydroxyaryl)-1,3,5-triazine class and their methoxy derivatives:
fluorescence spectroscopy and x-ray structure analysis. J Phys Chem
A 102:6975–6985
10. Martinej ML, Cooper WC, Chou PT (1992) A novel excited-state
intramolecular proton transfer molecule, 10-hydroxybenzo[h]
quinoline. Chem Phys Lett 193:151–154
11. Wu Y, Peng X, Fan J, Gao S, Tian M, Zhao J, Sun S (2007)
Fluorescence sensing of anions based on inhibition of excited-
state intramolecular proton transfer. J Org Chem 72:62–70
12. Shah M, Thangaraj K, Soong ML, Wolford LT, Boyer JH, Politzer
IR, Pavlopoulos TG (1990) Pyrromethene-BF2 complexes as laser
dyes. Part 1. Heteroat Chem 1:389–399
13. Paviopoulos TG, Boyer JH, Thangaraj K, Sathyamoorthi G, Shah
MP, Soong ML (1992) Laser dye spectroscopy of some
pyrromethene-BF2 complexes. Appl Opt 31:7089–7094
14. Costela A, Garcia-Moreno I, Gomez C, Sastre R, Amat-Guerri F,
Liras M, Lopez Arbeloa F, Banuelos Prieto J, Lopez Arbeloa I
(2002) Photophysical and lasing properties of new analogs of the
boron−dipyrromethene laser dye PM567 in liquid solution. J Phys
Chem A 106:7736–7342
15. Lopez Arbeloa F, Ruiz Ojeda P, Lopez Arbeloa I (1988) On the
aggregation of rhodamine B in ethanol. Chem Phys Lett 148:253–
258
16. Rahn MD, King TA, Gorman AA, Hamblett I (1997) Photostability
enhancement of pyrromethene 567 and perylene orange in oxygen-
free liquid and solid dye lasers. Appl Opt 36:5862–5871
17. Assor Y, Burshtein Z, Rosenwaks S (1998) Spectroscopy and
laser characteristics of copper-vapor-laser pumped pyrromethene-
556 and pyrromethene-567 dye solutions. Appl Opt 37:4914–
4920
Conclusions
The presence of aryl rings at C(2) in the imidazole
chromophore core originates a distortion from planarity in
the imidazole units, mainly in the excited state, which leads
to an increase in the rate constant of non-radiative
deactivation and in the Stokes shift. Both photophysical
factors have an opposite effect on the lasing efficiency.
Thus, the increase in the loss of the resonator cavity due to
the augmentation in the non-radiative processes could be
compensated to some extent by a reduction in the
reabsorption and reemission losses, owing to the high
Stokes shift. From this photophysical studies, polar solvents
are recommended to obtain the highest laser efficiencies in
liquid media.
18. Costela A, Garcia-Moreno I, Sastre R (2001) In: Nalwa HS (ed)
Handbook of advanced electronic and photonic materials and
devices. Academic, San Diego, p 7, 161
19. O’Neil MP (1993) Synchronously pumped visible laser dye with
twice the efficiency of Rhodamine 6 G. Opt Lett 18:37–38
20. Rhan MD, King TA (1995) Comparison of laser performance of
dye molecules in sol-gel, polycom, ormosil, and poly(methyl
methacrylate) host media. Appl Opt 34:8260–8271

J Fluoresc (2012) 22:409–417
417
21. Dubois A, Canva M, Brun A, Chaput F, Boilot JP (1996)
Photostability of dye molecules trapped in solid matrices. Appl
Opt 35:3193–3199
22. Yariv E, Schultheiss S, Saraidarov T, Reisfeld R (2001) Efficiency
and photostability of dye doped solid state lasers in different
hosts. Opt Mater (Amsterdam) 16:29–38
38. Lopez Arbeloa I (1980) Fluorescence quantum yield evaluation.
Reabsorption and reemission corrections. J Photochem 14:97–105
39. Bergstrom F, Mikhalyov I, Hagglof P, Wortmann R, Ny T (2002)
Dimers of dipyrrometheneboron difluoride (BODIPY) with light
spectroscopic applications in chemistry and biology. J Am Chem
Soc 124:196–204
23. Lopez Arbeloa F, Lopez Arbeloa I, Lopez Arbeloa T (2001) In:
Nalwa HS (ed) Handbook of advanced electronic and photonic
materials and devices. Academic, San Diego, p 7, 209
40. Marcus Y (1993) The properties of organic liquids that are
relevant to their use as solvating solvents. Chem Soc Rev 22:409–
416
24. Jayabharathi J, Thanikachalam V, Srinivasan N, Saravanan K
(2010) Synthesis, structure, luminescent and Intramolecular
proton transfer in some imidazole derivatives. J Fluoresc.
doi:10.1007/s10895-010-0747-5
25. Yamada M, Tanaka Y, Yoshimoto Y, Kuroda S, Shimao I (1992)
Synthesis and properties of diamino-substituted dipyrido [3,2-a:
2′,3′-c]phenazine. Bull Chem Soc Jpn 65:1006–1011
41. Reichardt C (1994) Solvatochromic dyes as solvent polarity
indicators. Chem Rev 94:2319–2358
42. Catalan J (1997) On the E(T) (30), pi, P(y), S’, and SPP empirical
scales as descriptors of nonspecific solvent effects. J Org Chem
62:8231–8234
43. Lippert E (1975) In: Birks JB (ed) Organic molecular photo-
physics. Wiley-Interscience, Bristol, p 2, 1
26. Jayabharathi J, Thanikachalam V, Saravanan K, Srinivasan N
(2010) Iridium(III) complexes with orthometalated imidazole
ligands subtle turning of emission to the saturated green colour.
J Fluoresc. doi:10.1007/s10895-010-0737-7
27. Saravanan K, Srinivasan N, Thanikachalam V, Jayabharathi J (2010)
Synthesis and photophysics of some novel imidazole derivatives
used as sensitive fluorescent chemisensors. J Fluoresc 21:65–80
28. Gayathri P, Jayabharathi J, Srinivasan N, Thiruvalluvar A,
Butcher RJ (2010) 2-(4-Fluorophenyl)-4,5-dimethyl-1-(4-methyl-
phenyl)-1H-imidazole. Acta Crystallogr E 66:o1703
29. Jayabharathi J, Thanikachalam V, Venkatesh Perumal M, Saravanan K
(2011) Displacement reaction using ibuprofen in a mixture of bioactive
imidazole derivative and bovine serum albumin – a fluorescence
quenching study. J Fluoresc. doi:10.1007/s10895-011-0878-3
30. Jayabharathi J, Thanikachalam V, Venkatesh Perumal M, Srinivasan
N (2011) Fluorescence resonance energy transfer from a bio-active
imidazole derivative 2-(1-phenyl-1H-imidazo[4,5-f][1, 10]phenan-
throlin-2-yl)phenol to a bioactive indoloquino lizine system. Spec-
trochim Acta A 79:236–244
31. Jayabharathi J, Thanikachalam V, Saravanan K, Srinivasan N,
Venkatesh Perumal M (2010) Physiochemical properties of
organic nonlinear optical crystal from a combined experimental
and theoretical study. Spectrochim Acta A 78:794–802
32. Jayabharathi J, Thanikachalam V, Jayamoorthy K, Venkatesh
Perumal M (2011) Physiochemical studies of excited state
intramolecular proton transfer process Luminescent chemosensor
by spectroscopic investigation supported by ab initio calculations.
Spectrochim Acta A 79:6–16
44. Dimroth K, Reichardt C (1969) Of pyridinium N-phenol betaines
and their use for characterizing the polarity of solvents, V
Expansion of the solvent polarity scale by using alkyl-
substituted pyridinium N-phenol betaines. Liebigs Ann Chem
727:93–105
45. Kamlet MJ, Taft RW (1976) The solvatochromic comparison
method. I. The β-scale of solvent hydrogen-bond acceptor (HBA)
basicities. J Am Chem Soc 98:377–383
46. Catalan J, Lopez V, Perez P (1996) Use of the SPP scale for the
analysis of molecular systems with dual emissions resulting from
the solvent polarity. J Fluoresc 6:15–22
47. Dunsbach R, Schmidt R (1994) Photophysical properties of some
polycyclic conjugated hydrocarbons containing five membered
rings. J Photochem Photobiol A 83:7–13
48. Biczok L, Berces T, Marta F (1993) Substituent, solvent, and
temperature effects on radiative and nonradiative processes of singlet
excited fluorenone derivatives. J Phys Chem 97:8895–8899
49. Andersson PO, Bachilo SM, Chen RL, Gillbro T (1995) Solvent
and temperature effects on dual fluorescence in a series of
carotenes. Energy gap dependence of the internal conversion rate.
J Phys Chem 99:16199–16209
50. Carvalho CEM, Brinn IM, Pinto AV, Pinto MCFR (2000)
Fluorescent symmetric phenazines from naphthoquinones. 3.
Steady state spectroscopy and solvent effect of seven phenazine
derivatives: structure-photophysics correlations. J Photochem
Photobiol A 136:25–33
51. Drexhage KH (1990) Dye lasers topics. In: Schafer FP (ed) Topics
in applied physics, 1. Springer, Berlin, p 187
33. Gayathri P, Jayabharathi J, Srinivasan N, Thiruvalluvar A,
Butcher RJ (2010) 4,5-Dimethyl-2-phenyl-1-(p-tolyl)-1H-imidaz-
ole. Acta Crystallogr E 66:o2826
34. Jayabharathi J, Thanikachalam V, Saravanan K (2009) Effect of
substituents on the photoluminescence performance of Ir(III)
complexes: Synthesis, electrochemistry and photophysical prop-
erties. J Photochem Photobiol A: Chem 208:13–20
35. Guassian 03 program, (Gaussian Inc., Wallingford CT) (2004)
36. Banuelos Prieto J, Lopez Arbeloa F, Virginia Martinez Martinez T,
Lopez A, Lopez Arbeloa I (2004) Photophysical properties of the
pyrromethene 597 dye: solvent effect. J Phys Chem A 108:5503–5508
37. Hofer JE, Grabenstetter RJ, Wiig EO (1950) The fluorescence of
cyanine and related dyes in the monomeric state. J Am Chem Soc
72:203–209
52. Birks JB (1970) Photophysics of aromatic molecules. Wiley-
Interscience, London
53. Allik T, Chnadra S, Robinson TR, Hutchinson JA, Sathyamoorthi
G, Boyer JH (1994) Laser performance and material properties of
a high temperature plastic doped with pyrromethene-BF2 dyes.
Mater Res Soc 329:291–296
54. He GS, Prasad PN (1998) Phase-conjugation property of one-
photon pumped backward stimulated emission from a lasing
medium. J Quant Electron 34:473–481
55. Yariv E, Risfeld R (1999) Laser properties of pyrromethene dyes
in sol gel glasses. J Opt Mater 13:49–54
56. Finlayson AJ, Peters N, Kolinsky PV, Venner RW (1999)
Flashlamp pumped solid-state dye laser incorporating pyrrome-
thene 597. Appl Phys Lett 75:457–459