Excited charge transfer states in donor-acceptor fluorescent phenanthroimidazole derivatives

Article abstract of DOI:10.1007/s10895-013-1309-4

Solvent-dependent electronic structure of the selected donor (D) acceptor (A) derivatives of phenanthroimidazole derivatives containing fluoro substituent as an electron acceptor fragment in the fluorescent charge transfer (CT) states has been investigated. The mechanism of the radiative charge recombination CT∈→∈S₀ is discussed in terms of the Mulliken-Murrell model of the CT complexes and the Marcus theory of photoinduced electron transfer (ET). Solvatochromic effects on the spectral position and profile of the stationary fluorescence spectra clearly indicate the CT character of the emitting singlet states of all of the compounds studied both in a polar and a non polar environment. An analysis of the CT fluorescence leads to the quantities relevant for the electron transfer in the Marcus inverted region. The values of the fluorescence rate constants (k _r ) and corresponding transition dipole moments (M) and their solvent polarity dependence indicate that the electronic coupling between the emitting ¹CT state and the ground state is a governing factor of the radiative transitions. The relatively large values of M indicate a nonorthogonal geometry of the donor and acceptor subunits in the fluorescent states. It is shown that Marcus theory can be applied for the quantitative description of the radiationless charge recombination processes in the cases when an intersystem crossing to the excited triplet states can be neglected.

Full text of DOI:10.1007/s10895-013-1309-4

J Fluoresc
DOI 10.1007/s10895-013-1309-4
ORIGINAL PAPER
Excited Charge Transfer States in Donor–Acceptor
Fluorescent Phenanthroimidazole Derivatives
J. Jayabharathi & V. Thanikachalam & R. Sathishkumar
Received: 19 July 2013 /Accepted: 27 September 2013
#
Springer Science+Business Media New York 2013
Abstract Solvent-dependent electronic structure of the select-
ed donor (D) acceptor (A) derivatives of phenanthroimidazole
derivatives containing fluoro substituent as an electron acceptor
fragment in the fluorescent charge transfer (CT) states has been
investigated. The mechanism of the radiative charge recombi-
nation CT→S0 is discussed in terms of the Mulliken–Murrell
model of the CT complexes and the Marcus theory of photoin-
duced electron transfer (ET). Solvatochromic effects on the
spectral position and profile of the stationary fluorescence
spectra clearly indicate the CT character of the emitting singlet
states of all of the compounds studied both in a polar and a non
polar environment. An analysis of the CT fluorescence leads to
the quantities relevant for the electron transfer in the Marcus
inverted region. The values of the fluorescence rate constants
(kr) and corresponding transition dipole moments (M) and their
solvent polarity dependence indicate that the electronic cou-
pling between the emitting 1CT state and the ground state is a
governing factor of the radiative transitions. The relatively large
values of M indicate a nonorthogonal geometry of the donor
and acceptor subunits in the fluorescent states. It is shown that
Marcus theory can be applied for the quantitative description of
the radiationless charge recombination processes in the cases
when an intersystem crossing to the excited triplet states can be
neglected.
Introduction
The wide investigations of the intramolecular electron transfer
(ET) in donor (D)-acceptor (A) compounds such as carbazol-
9-yl derivatives of aromatic nitriles [1] and aryl or hetero aryl
derivatives of aromatic amines [2] suggest a possibility to
predict the photophysical behaviour of a particular D–A mol-
ecule from the properties of the individual chromophores as
far as the electronic interactions between the lowest excited
charge transfer state CT and the ground state S0 (V0) and/or
the locally excited states LE, localized either in the acceptor
(V1,3A) or in the donor (V1,3D) respectively, are taken into
account. A similar approach can be applied to describe the
properties of the singlet CT states e.g., the transition dipole
moments for the CT absorption (1CT←S0) and the fluores-
cence (1CT→S0) as well as the characteristics of the triplet
3CT states [3] (e.g., the zero-field splitting parameters).
The study of the intramolecular ET reactions is to identify
structural elements that promote electronic coupling between
an electron donor and an acceptor is a challenging one. The
application of different models [4–6] used to determine the
electronic matrix elements from the solvent polarity effects on
the electronic transition dipole moments of the CT emission
(Mflu) [7, 8] and the CTabsorption (Mabs) [2]. The appropriate
values of the electronic coupling elements are mainly deter-
mined by the interactions between the atoms forming the bond
A–D can be theoretically predicted following the formalism
proposed by Dogonadze et al. [9, 10]. Neglecting contribu-
tions from the σ orbitals, one can obtain for π-electronic
systems
.
.
.
Keywords Phenanthroimidazole Charge transfer Donor
.
Acceptor Lifetime
V0 ¼ CALUMOCHd OMOβADcosðθAꢀꢀDÞ þ const:;
V1A;3 ¼ CAHOMOCHDOMOβADcosðθAꢀꢀDÞ þ const:;
V1D;3 ¼ CALUMOCLDUMOβADcosðθAꢀꢀDÞ þ const:;
ð1Þ
ð2Þ
ð3Þ
:
:
J. Jayabharathi (*) V. Thanikachalam R. Sathishkumar
Department of Chemistry, Annamalai University,
Annamalainagar 608 002, Tamilnadu, India
e-mail: jtchalam2005@yahoo.co.in

J Fluoresc
where θA-D denotes the angle between the planes of the
acceptor and donor subunits and CHOMO and CLUMO are the
LCAO coefficients (as obtained for the individual chromo-
phores) of the 2pZ atomic orbitals (where z is the axis perpen-
dicular to the acceptor or donor rings) of the highest occupied
molecular orbital (HOMO) and of the lowest unoccupied
molecular orbital (LUMO) located on the atoms forming the
A–D bond. β is the resonance integral for these AD atoms and
const. is related to the electronic interactions between the
remaining pairs of atoms in the D–A molecule (this contribu-
tion is usually small and negligible) . Expressions 2 and 3
assume that the 1LE state is mainly described by a configura-
tion corresponding to the HOMO→LUMO excitation (e.g.,
1La state in Platt’s notation)
In this work, we used the Lippert solvent parameter (Δf),
the normalized ET (30) polarity scale, and the multi parameter
Kamlet-Taft and Catalan solvent scales to describe the solvent
effect on the fluorescence emission and stokes shift of
phenanthroimidazole derivatives. We have also addressed
the influence of solvents on the photophysical properties of
2-(4-fluorophenyl)-1-phenyl-1H-phenanthro [9,10-d]
imidazole (1)
Yield: 94 %. Anal. Calcd. for C27H17FN2: C, 83.49; H,
1
4.41; N, 7.21. Found: C, 83.61; H, 4.32; N, 7.30. H
NMR (400 MHz, d6-DMSO): δ 7.07 (d, J=8 Hz, 1H),
7.23 (t, J=7.2 Hz, 2H), 7.36 (t, J=7.2 Hz, 1H), 7.55 (t, J=
8 Hz, 1H), 7.59–7.63 (m, 2H), 7.67–7.72 (m, 6H), 7.78 (t,
J=6.8 Hz 1H), 8.69 (d, J=6.8 Hz 1H), 8.88 (d, J=8.4 Hz
1H), 8.93 (d, J=8 Hz 1H). 13C NMR (100 MHz, d6-
DMSO): δ 115.16, 115.37, 120.11, 121.96, 122.41,
123.65, 124.48, 125.22, 125.74, 126.63, 126.75,
126.78, 127.46, 127.64, 127.70, 128.47, 129.11,
130.26, 130.35, 131.34, 131.43, 136.34, 137.99,
149.69, 161.08, 163.54. MS: m/z. 389.27 [M+].
2-(4-fluorophenyl)-1-p-tolyl-1H-phenanthro [9,10-d]
imidazole (2)
Yield: 92 %. Anal. calcd. for C28H19FN2: C, 83.56; H,
1
4.76; N, 6.96. Found: C, 83.95; H, 4.65; N, 7.01. H
NMR (400 MHz, CDCl3): δ 2.48(s, 3H), 7.13 (d, J=
7.6 Hz, 1H), 7.21 (t, J=8.8 Hz, 2H), 7.35 (t, J=7.6 Hz,
1H), 7.46–7.48 (m, 2H), 7.53–7.63 (m, 5H), 7.68 (t, J=
6.8 Hz 1H), 7.77 (t, J=7.2 Hz, 1H), 8.68 (d, J=8.8 Hz,
the synthesized molecules in terms of hceυvabasc; hceυvac and
flu
ꢀ
ꢁ
hceυvaabcs−hceυvac with solvent polarity function. The reported
1H), 8.87 (d, J=8 Hz, 1H), 8.92 (d, J=8.4 Hz, 1H). 13
C
flu
NMR (100 MHz, d6-DMSO): δ 20.92, 115.16, 115.37,
120.13, 121.95, 122.48, 123.64, 124.44, 125.19, 125.71,
126.64, 126.82, 126.85, 127.43, 127.62, 127.73, 128.43,
128.77, 13.80, 131.33, 131.41, 135.36, 136.29, 139.79,
149.79, 161.06, 163.52. MS: m/z. 403.44 [M+].
2-(4-fluorophenyl)-1-(4-methoxyphenyl)-1H-phenanthro
[9,10-d] imidazole (3)
results are based on a study of the solvent effects on the
spectral position of absorption and fluorescence spectra as
well as on the CT emission quantum yields and excited state
depopulation kinetics.
Yield: 95 %. Anal. calcd. for C28H19FN2O: C, 80.37;
H, 4.58; N, 6.69. Found: C, 81.01; H, 4.25; N, 6.60. 1H
NMR (400 MHz, d6-DMSO): δ 3.89 (s, 3H), 7.15–7.25
(m, 5H), 7.38 (t, J=8 Hz, 1H), 7.595–7.67 (m, 6H), 7.76
(t, J=7.6Hz, 1H), 8.66 (d, J=12 Hz, 1H), 8.89 (dd, J=
12 Hz, 2H). 13C NMR (100 MHz, d6-DMSO): δ 60.86,
120.36, 120.65, 125.45, 127.21, 128.85, 129.66, 130.42,
130.92, 131.91, 131.99, 132.20, 132.64, 132.93, 133.18,
133.75, 135.46, 135.72, 136.56, 136.68, 141.55, 155.27,
165.34. MS: m/z. 419.39 [M+].
Experimental
Chemicals
4-fluorobenzaldehyde and Phenanthrene 9, 10-dione have
been supplied by Sigma Aldrich (St. Louis, USA). Aniline,
4-Methylaniline, 4-Methoxyaniline, and 3,5- Dimethylaniline
used were of analytical grade and received from S.D. Fine
(Mumbai, India).
2-(4-fluorophenyl )-1-(3,5-dimethylphenyl )-1H-
phenanthro [9,10-d] imidazole (4)
One Pot Synthesis of Phenanthrimidazoles
Yield: 97 %. Anal. calcd. for C29H21FN2: C, 83.63; H,
1
Phenanthrimidazole derivatives have been prepared by
four components condensation of phenanthrene-9, 10-dione
(40 mmol), ammonium acetate (30 mmol), 4-
fluorobenzaldehyde (30 mmol) and aniline (30 mmol)
refluxed in ethanol at 80 °C in the presence of Indium (III)
fluoride (InF3) as Lewis acid catalyst (Scheme 1) for 30 min.
The completion of the reaction was monitored by thin layer
chromatography. The reaction mixture was extracted with di-
chloromethane and purified by column chromatography using
benzene: ethyl acetate (9:1) as the eluent.
5.08; N, 6.73. Found: C, 83.21; H, 5.24; N, 6.38. H
NMR (400 MHz, d6-DMSO): δ 2.41 (s, 6H), 7.07 (t, J=
8Hz, 2H), 7.15–7.31 (m, 5H), 7.54 (t, J=7.6 Hz, 1H),
7.60–7.68 (m, 3H), 7.73 (t, J=8 Hz 1H), 8.74 (d, J=
7.6 Hz 2H), 8.80 (d, J=8 Hz 1H). 13C NMR (100 MHz,
d6-DMSO): δ 20.79, 114.75, 114.96, 120.43, 122.00,
122.35, 122.88, 123.64, 124.76, 125.32, 125.83,
126.20, 126.44, 126.91, 127.54, 127.57, 128.37,
130.83, 130.91, 131.36, 136.19, 137.50, 139.72,
149.26, 161.06. MS: m/z. 417.02 [M+].

J Fluoresc
InF3
C
H
O
R'
NH3
-H2O
InF3
O
C
R'
HN
O
O
H2N
H
R
InF3
NH4COOCH3
-H2O
N
R
InF3
InF3
O
H
O
H
N
N
R'
H
R'
H2O
N
-H2O
-InF3
N
R
R
N
H
N
N
R'
R'
N
R
R
Scheme 1 Possible mechanism of catalytic synthesis of phenanthroimidazole

J Fluoresc
Table 1 Effect of amount and catalytic activity of catalyst in the synthe-
sis of phenanthroimidazoles (1–4)
Spectral Measurements
The infrared spectra have been recorded with an Avatar 330-
Thermo Nicolet Fourier transform infrared spectrometer
(Thermo, America). The proton spectra at 400 MHz were
obtained at room temperature using a Bruker 400 MHz
NMR spectrometer (Bruker biospin, California, USA). Proton
decoupled 13C NMR spectra were also recorded at room
temperature employing a Bruker 400 MHz NMR spectrome-
ter operating at 100 MHz. The mass spectra of the samples
were obtained using a Thermo Fischer LC-Mass spectrometer
in fast atom bombardment (FAB) mode (Thermo, France).
Fluorescence lifetime measurements were carried out with a
nanosecond time correlated single photon counting (TCSPC)
spectrometer Horiba Fluorocube-01-NL lifetime system with
NanoLED (pulsed diode exCitation source) as the excitation
source and TBX-PS as detector. The slit width was 8 nm and
the laser excitation wavelength was 270 nm. The fluorescence
decay was analyzed using DAS6 software.
S. No
Compound
Time (minutes)
Catalyst (mol %)
Yield
1
1
1
1
1
2
2
2
2
3
3
3
3
4
4
4
4
> 400
30
No entry
Trace
65
2
10
3
60
10
79
4
60
15
94
5
> 400
30
No entry
Trace
61
6
10
7
60
10
74
8
60
15
92
9
> 400
30
No entry
Trace
63
10
11
12
13
14
15
16
10
60
10
76
60
15
95
> 400
30
No entry
10
Trace
69
60
10
77
60
15
97
Result and Discussion
correspond in Platt’s notation to the1Lb and 1La excited states.
Effect of the Catalytic Activity of Indium (III) Fluoride
1
1
The low and high energy transitions, Lb←S0 and La←S0
respectively, with a relatively high probability can be clearly
observed in the absorption spectra of all the compounds
studied [11–13]. Detailed inspection of the low-energy ab-
sorption region of the D-A phenanthroimidazole derivatives
containing fluoro substituents as an electron acceptor clearly
indicates the presence of additional charge transfer singlet
states. A long wave shoulder attributed to the 1CT←S0 tran-
sition is also observed. The magnitude of the shifts suggests
that the ground state of the molecule is polar.
The emission spectra of 1–4 displayed in Fig. 2, show a
red-shifted band in the region of 380–417 nm from hexane to
water. The fluorescence maximum is greatly affected by sol-
vent polarity where a red-shifted fluorescence is observed on
increasing the solvent polarity. The effect of polarity of the
medium on the fluorescence maximum is more intense than
that on the absorption maximum. This observation suggests
that the emitting state is more polar than the ground state
[14–16]. The emission yields are more prominent in polar
solvents compared to that of non-polar solvents. The linear
variation of the Stokes shift with ET (30) has been shown in
Fig. 3a, where a double linear correlation is obtained [17].
Polar protic solvents fall on a separate line indicating that the
mode of solvation of the emitting state is different from that in
the other polar aprotic solvents. For polar protic solvents,
gradual increment of Stokes shift is due to intermolecular
hydrogen bonding interactions. The studied compounds ex-
hibit overall increase of Stokes shift from non-polar to polar
aprotic solvents mainly due to combined effect of increasing
Initially, we have carried out the condensation reaction of
phenanthrene-9, 10-dione, p-fluorobenzaldehyde with
substituted anilines in ethanol for 24 h with stirring, in the
absence of catalyst and observed only trace amount of prod-
ucts. In order to improve the yield and shorten the reaction
time, we have carried out the reaction using different amounts
of the catalysts at different time intervals (Table 1). It was
observed that the amount of catalyst plays a significant role in
controlling the activity of the catalyst. Among the various
amounts of catalyst used, 15 mol% was found to be the best
in ethanol medium. A possible mechanism is that the reaction
proceeds as shown in Scheme 1. The present catalyst is novel
and efficient. The salient features of this protocol are high
product yields, shorter reaction time, low catalyst loading and
easy work-up procedure, which make this procedure quite
simple, more convenient and environmentally benign. Hope-
fully, our methodology could be a valid contribution to the
existing processes in the field of phenanthroimidazole
synthesis.
Solvent Modulated Absorption and Emission Behaviour
Room temperature absorption and the fluorescence maxima of
phenanthroimidazole derivatives (1–4) are shown in Figs. 1
and 2 respectively. The spectra show a superposition of the
bands corresponding to the donor and acceptor subunits which
seem to be only slightly perturbed by their interactions. The
two absorption bands have been assigned to the 1(π–π*) states

J Fluoresc
Fig. 1 Absorption spectra of 1-4 in different solvents
the polarity of the medium and intramolecular charge transfer
(CT) state.
The behaviour of phenanthroimidazole derivatives toward
different solvent polarities may be interpreted in terms of the
difference in the ground state and the excited state dipole
moments. The spectral shift in fluorescence band may be
attributed to the interaction between the dipole moment of
the solute and the polarizability of the solvent. The extent of
charge separation on electronic excitation of 1–4 have been
determined by measuring the change in the dipole moment
(Δμ=μe–μg) utilizing the spectral shift between the absorp-
tion and emission maxima as a function of solvent polarity.
According to Lippert–Mataga equation [19].
In order to obtain an insight about specific solvent—
fluorophore interactions, the influence on fluorescence emission
of 1 in methanol–water mixtures of different composition has
been monitored and the spectra are illustrated in Fig. 3b. The
addition of water to methanol–water mixture remarkably en-
hances the fluorescence of 1 with red shift. This effect may be
described by the combined effect of hydrogen bonding and
polarity of the medium. Mixing of two closely lying lowest
singlet states (n, π*and π, π*) of phenanthroimidazole deriva-
tives favours the intersystem crossing [18]. As the polarity of the
medium is increased, intermolecular hydrogen bonding interac-
tion in the excited state stabilized the π, π* state and enhancing
the energy between the two states, which diminishes the mixing
between the states (Scheme 2). As a result intersystem crossing
from S1 to T1 decreases and an enhancement of fluorescence is
observed. The same trend has been observed for compound 2–4.
2
3
ꢀ
ꢁ
2
2 μe−μg
6
4
7
5
ν−ss ¼ ν− −ν− ¼ const þ
f ðD; nÞ
ð4Þ
ab
fl
hca3
where f(D,n)=(D–1)/(2D +1)–(n2–1)/(2n2+1), which indi-
cates the orientation polarizability and depicts polarity

J Fluoresc
Fig. 2 Emission spectra of 1-4 in different solvents
Fig. 3 a Stokes shift with ET (30); b Methanol- water diagram of 1

J Fluoresc
Scheme 2 Intersystem crossing from S1 to T1 state
parameter of the solvent, n is refractive index, D is dielectric
constant, μe and μg are dipole moments of the species in S1
and S0 states, respectively, h is Planck’s constant, c is velocity
of light and a is Onsager’s cavity radius. The Lippert-Mataga
plot is linear for the non-polar and polar/aprotic solvents as
shown in Fig. 4a; correlation is satisfactory. The geometrical
optimization of 1–4 was done by DFT method using
Gaussian-03 [20] to calculate the μg. Using μg value, 5.56D
(1), 5.72D (2), 5.83D (3) and 5.75D (4) obtained from the
DFT calculation and the slope of Lippert–Mataga plot, the
value of μe calculated is in the range, 14.0–.23.0 D for the
studied phenanthriimidazoles.
Multiple linear regression analysis is performed to identify
the different modes of solvation determining the absorption
and emission energies. Kamlet and Taft [21] put forward the
π*, α and β parameters to characterize the polarity/
polarizability, the acidity and the basicity of a solvent respec-
tively. Conversely, Catalan et al. [22] proposed an empirical
solvent scales for polarity/polarizability (SPP), acidity (SA)
and basicity (SB) to describe the respective properties of a
given solvent
y y0 þ aαα þ bββ þ cπ*π* ðKamletꢀꢀTaftÞ
ð5Þ
ð6Þ
¼
y y0 þ aSASA þ bSBSB þ cSPPSPP ðCatalanÞ
¼
The dominant coefficient affecting the absorption and
fluorescence band of phenanthroimidazole derivatives (1–
4) are displayed in Table 2. Negative values of solvent
dipolar interaction (π*) and hydrogen bond accepting prop-
erty (β) indicate these two parameters contribute to the
stabilization of both the ground and the excited states of
phenanthroimidazole derivatives. The calculated ratio of α
over π* [1.31(υabs) & 1.49 (υemi) (1), 2.19(υabs) &
2.75(υemi) (2), 1.89(υabs) & 2.08(υemi) (3) and 2.77 (υabs
)
& 4.06(υemi) (4)] reveal that interactions between
phenanthroimidazole derivatives and solvents with acidity
property (α) predominate in the excited state. A good linear
variation is also obtained between the fluorescence maxima
Fig. 4 a Lippert – Mataga diagram; b Fluorescence maxima and ET (30) values

J Fluoresc
Table 2 Adjusted coefficients ((υx)0, ca, cb and cc) for the multilinear
regression Analysis of the Absorption υabs and fluorescence υemi
wavenumbers and stokes shift (Δυss) of 1–4 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
cmpd. (υx)
(υx)0cm−1
(π*)
c α
c β
1
λabs
λemi
(2.728 0.015)×104 −(3.386 2.639)×103
(7.998 8.496)×103
−(4.542 6.654)×103
(6.187 10.983)×103
(10.742 8.180)×103
(3.816 0.024)×104 (1.848 4.357)×103
−(6.896 14.025)×103
Δυss=υabs−υemi (1.088 0.092)×104 (5.242 3.245)×103
−(14.914 10.445)×103
N
(υx)
(υx)0cm−1
cSPP
cSA
cSB
λabs
λemi
(3.761 0.060)×104 (7.641 6.791)×103
−(21.620 17.990)×103
(8.536 12.040)×103
−(30.191 11.590)×103
−(17.131 13.474)×103
−(3.907 9.018)×103
(21.063 8.681)×103
(2.763 0.040)×104 −(4.510 4.545)×103
Δυss=υabs−υemi (0.099 0.038)×104 (12.166 4.375)×103
2
(υx)
(υx)0cm−1
(π*)
c α
c β
λabs
λemi
(3.774 0.017)×104 (1.982 4.026)×103
−(2.611 13.448)×103
(0.202 2.306)×103
−(2.812 10.747)×103
(0.898 10.701)×103
−(0.349 5.018)×103
(1.247 8.552)×103
(2.598 0.008)×104 (0.219 1.888)×103
Δυss=υabs−υemi (1.175 0.014)×104 (1.762 3.217)×103
(υx)0cm−1
cSPP
cSA
cSB
N
(υx)
λabs
λemi
(3.792 0.015)×104 −(1.415 6.070)×103
(10.316 26.878)×103
(19.101 11.061)×103
(8.693 21.438)×103
−(10.084 29.334)×103
−(20.627 12.071)×103
(10.544 23.396)×103
(2.607 0.006)×104 −(3.752 2.498)×103
Δυss=υabs−υemi (1.185 0.012)×104 (2.337 4.841)×103
3
(υx)
(υx)0cm−1
(π*)
c α
c β
λabs
λemi
(3.72 0.034)×104
(5.54 6.201)×103
−(3.74 8.591)×103
(9.26 12.542)×103
−(9.84 19.958)×103
(3.48 37.653)×103
−(13.29 40.369)×103
(2.75 15.630)×103
(0.72 21.656)×103
(1.83 31.615)×103
(2.52 0.047)×104
Δυss=υabs−υemi (1.20 0.069)×104
(υx)0cm−1
cSPP
cSA
cSB
N
(υx)
λabs
λemi
(3.68 0.011)×104
(4.18 11.392)×103
−(3.02 30.179)×103
−(4.34 22.604)×103
−(9.10 25.961)×103
(4.74 40.568)×103
(2.62 0.012)×104
−(12.13 13.084)×103 (20.77 34.661)×103
Δυss=υabs−υemi (1.06 0.018)×104
(16.29 20.446)×103
−(23.76 54.163)×103
c α
4
(υx)
(υx)0cm−1
(π*)
c β
λabs
λemi
(3.74 0.014)×104
(2.63 2.472)×103
−(6.58 7.739)×103
(9.22 5.914)×103
−(4.77 7.957)×103
(19.3 24.908)×103
−(24.06 19.036)×103
(2.35 6.232)×103
(2.23 0.042)×104
−(13.31 19.507)×103
(15.65 14.908)×103
Δυss=υabs−υemi (1.51 0.032)×104
(υx)0cm−1
cSPP
cSA
cSB
N
(υx)
λabs
λemi
(3.73 0.037)×104
(1.54 4.235)×103
−(91.75 11.220)×103 (40.86 32.685)×103 −(1.99 8.404)×103
(2.34 0.109)×104
−(15.67 12.338)×103 −(40.74 26.066)×103
−(28.81 24.481)×103
Δυss=υabs−υemi (1.38 0.087)×104
(17.21 9.839)×103
(26.79 19.523)×103
and ET (30) values [23] as shown by Fig. 4b. The free
energy change of solvation and reorganization energies of
1–4 in various solvents have been estimated (Table 3).
According to Marcus [24], E (A)=ΔGsolv+λ1 and E (F)=
ΔGsolv−λ0, where E(A) and E(F) are absorption and fluo-
rescence band maxima in cm−1, respectively, ΔGsolv is the
difference in free energy of the ground and excited states in
a given solvent and λ represents the reorganization energy .
The free energy change of solvation and reorganization
energies of 1–4 in various solvents has been estimated.
Under the condition that λ0≈λ1≈λ, we get, E (A)+E
free energy of solvation in hexane and different hydrogen
bonding solvents (i.e. ΔG solv) of 1–4 follow the order of the
hydrogen bond energy [25]. In the aprotic solvents the values
are small and interaction of phenanthroimidazole derivatives
with those solvents is purely due to dipolar interactions in the
excited state. The reorganization energy values of 1–4 have
also been determined in different solvents. The definite values
of reorganization energy confirmed the interaction between
low frequency motions such as reorientation of solvent cell
with low and medium frequency nuclear motion of the solute.
(F)=2ΔGsolv; E (A)−E (F)=2 λ. The ΔG
of 1–4 is
solv
Fluorescence Lifetime and Quantum Yield
maximum for hexane since it is purely non-polar and also α
and β values of hexane are zero. The ΔG solv is minimum in
water. The difference between these values (water and hexane)
should give the free energy change required for hydrogen
The time correlated single photon counting (TCSPC) results
fit to single exponentials decay
bond formation. The plot of Δ (ΔGsolv)=(ΔGhex−ΔG
versus ET(30) has been depicted in Fig. 5a. The difference in
)
water
f ðtÞ ¼ α1exp ð−t=τ1Þ
ð7Þ

J Fluoresc
Table 3 Absorption maxima (λabs max, nm), fluorescence emission maxima (λab max, nm), Stokes shift (νss, cm−1), fluorescence quantum yield (Φf ),
life time (τ, ns), radiative rate constant (kr, s−1), nonradiative rate constant (knr, s−1) and log(kr/knr) of 4 in various solvents
Solvents
ΔG (kcal/mol)
Δ(ΔGhex−ΔGsol
)
λ (kcal/mol)
Ф
τ
kr
Knr
log(kr/knr) (108)
νss
Hexane
4877.89
91.34
89.49
89.59
89.81
90.03
90.35
90.49
90.65
90.64
90.52
91.53
90.65
91.79
90.18
91.24
91.27
91.29
90.86
90.89
90.93
0.00
1.85
1.75
1.53
1.31
0.99
0.85
0.69
0.70
0.82
0.82
0.69
0.55
0.16
0.10
0.07
0.05
0.48
0.45
0.41
15.80
16.38
16.67
17.25
17.84
18.34
19.03
19.72
20.58
21.14
22.46
23.23
24.01
24.55
24.86
25.40
25.79
26.77
27.23
27.68
0.78
0.77
0.76
0.75
0.73
0.71
0.73
0.7
1.3
1.3
1.3
1.3
1.3
1.4
1.4
1.4
1.4
1.4
1.4
1.6
1.6
1.5
1.6
1.6
1.6
1.6
1.5
1.4
0.6
0.16
0.17
0.18
0.19
0.20
0.20
0.19
0.21
0.19
0.2
5.5
5.2
5.0
4.8
4.3
3.9
4.3
3.7
4.3
4.1
3.5
3.1
2.9
2.7
2.3
1.9
3.7
1.8
4.1
1.4
Cyclohexane
Dioxane
4850.092
4795.205
4835.053
5264.584
5332.48
0.59
0.58
0.57
0.56
0.50
0.52
0.5
Carbon tetrachloride
Benzene
Ether
Chloroform
Ethyl acetate
THF
4474.109
4694.971
4789.888
5116.057
6174.461
6396.919
5621.021
6069.699
5906.309
6513.597
5745.556
6366.135
6037.91
0.73
0.72
0.69
0.67
0.66
0.65
0.63
0.61
0.7
0.52
0.51
0.49
0.42
0.41
0.43
0.39
0.38
0.43
0.37
0.48
0.41
Dichloromethane
Propanol
0.22
0.20
0.21
0.23
0.23
0.24
0.18
0.25
0.18
0.3
Butanol
2-propanol
Acetone
Ethanol
Methanol
Acetonitrile
Ethylene glycol
DMSO
0.6
0.72
0.58
Water
5420.585
where α1 and τ1 are respectively, the pre-exponential factor
and lifetime of the various excited states involved. DAS6
software was used for the fit and the χ2 values are always less
than 1.2. The fluorescence lifetime measurements of all
phenanthroimidazole derivatives were made in ethanol with
laser excitation set at 270 nm and the fluorescence signal was
measured at the emission wavelength of individual com-
pound. The absolute PL quantum yields were measured by
comparing fluorescence intensities (integrated areas) of a
standard sample (Coumarin 46) and the unknown sample
using the formula
ꢂ
ꢃꢂ
ꢃꢂ
ꢃ
2
Iunk
Istd
Astd
ηunk
ηstd
Φunk ¼ Φstd
ð8Þ
Aunk
where, Фunk is the fluorescence quantum yield of the sample,
Фstd is the fluorescence quantum yield of the standard; Iunk
and Istd are the integrated emission intensities of the sample
Fig. 5 a Δ (ΔGsolv) versus ET(30); b φf with solvent polarity parameter ET (30)

J Fluoresc
and the standard, respectively. Aunk, and Astd are the absor-
bances of the sample and the standard at the excitation wave-
length, respectively. ηunk and ηstd are the indexes of refraction
of the sample and standard solutions. Fluorescence quantum
yield (φf) was measured in solvents of different polarity and
presented in Table 3. The radiative and non-radiative decay of
the excited state of 1–4 have been obtained using the quantum
yields and lifetimes. The formula employed to calculate the
radiative (kr) and non-radiative (knr) rate constants is kr=Φp/τ;
knr=(1/τ)−(Φp/τ);τ =(kr+knr)−1, where kr and knr are the
radiative and non-radiative deactivation, τf is the lifetime of
the S1 excited state. A typical set of kr and knr values are
tabulated (Table 3). Perusal of the radiative and non-radiative
rate constants shows that in most of the cases the radiative
emission is predominant over non-radiative transitions.
The variation of φf with solvent polarity parameter ET (30)
is depicted in Fig. 5b. The result shows that the φf values in
various solvents are sensitive towards solvent polarity. The
remarkable increase of φf in polar protic medium is compared
with that in polar aprotic medium. This may be due to differ-
ential contribution of CT and hydrogen bonding interactions.
Radiative (kr) and nonradiative (knr) rate constants are calcu-
lated from fluorescence quantum yields and lifetime values
(Fig. 6a) in different solvents to understand the effect of
solvation on the dynamics of the excited state. The logarithm
of (kr/knr) is plotted against the solvent polarity parameter ET
(30) which is shown in Fig. 6b. Two different straight lines are
obtained, one for aprotic solvents and the other for protic
solvents. In both the cases, upon increasing the polarity the
logarithm ratio of radiative to nonradiative rate decreases but a
steeper slope is obtained in the case of protic solvents. It
indicates that the radiative and nonradiative rates are more
sensitive toward protic solvents. It may be that the hydrogen
bonding interaction in polar protic environment enhances the
stabilization of the S1 state as a result the nonradiative relax-
ation rate increases [26].
An interesting result is provided by the blue shift of the CT
absorption bands with increasing solvent polarity (Fig. 7a)
[13, 27]. With the assumption that point dipole is at the center
of the spherical cavity and the mean solute polarizability (α)
to be insignificant, it follows,
ꢀ
ꢁ
hceυabs≈hceυvac−2μg μe−μg =αo3½ðε−1=2 ε þ 1Þ−1 n2−1=2n2 þ 1 ꢁ
ꢄ ꢅ
ꢆ
abs
2
ð9Þ
where μg and μe are the dipole moments of the solute in the
ground and excited state, correspondingly, νabs and eυvabasc are
the spectral positions of a solvent-equilibrated absorption
maxima and the value extrapolated to the gas-phase, respec-
tively, ao is the effective radius of the Onsager cavity, [28] and
ε and n are the static dielectric constant and the refractive
index of the solvent, respectively. In the case of the well-
separated CT absorption bands, Eq. 9 is used to determine
3
the values of μg(μe−μg)/αo and eυvabasc . In the excited state, the
negative and positive ends of the electric dipole are localized
nearly in the centres of the donor and acceptor fragments
respectively. A considerable red shift of their spectral position
and the increase of the Stokes shift and enlargement of the
emission bandwidth with increasing solvent polarity in fluo-
rescence spectra point to the CT character of the fluorescent
states and clearly indicate that the absolute values of μe are
much higher than those of μg. The excited state dipole mo-
ments μe can be estimated by the fluorescence solvatochromic
shift method due to the fact that the excited states live suffi-
ciently long with respect to the orientation relaxation time of
the solvent [29–31]. Under the same assumptions as used for
expression 4, it follows that
ꢀ
ꢁ
hceυflu≈hceυvac−2μe μe−μg =αo3½ðε−1=2 ε þ 1Þ −1 n2−1=2n2 þ 1 ꢁ
ꢄ ꢅ
ꢆ
flu
2
ð10Þ
Fig. 6 a Lifetime spectra of 1-4; b log (kr/knr) Vs ET(30)

J Fluoresc
vac
where eυflu and eυ
are the spectral positions of the solvent
energy gap between the lowest internal charge transfer (ICT)
states and the states excited locally in the nonpolar solvents,
which leads to increase of the contribution of the (π, π *)
character to the wave function of the CT states. It leads to a
lowering of energy with respect to a pure CTstate because of a
stabilizing character of such interactions and red shift obtained
in the fluorescence spectra.
Under the assumption that the CT fluorescence corre-
sponds to the state reached directly upon excitation, the quantity
(μe−μg)2/αo3 can be evaluated from the solvation effects on the
Stokes shift,
flu
equilibrated fluorescence maxima and the value extrapolated
to the gas-phase, respectively. The compounds studied show a
satisfying linear correlation between the energy hceυflu and the
solvent polarity function in a polar environment and also in all
3
the solvents (Fig. 7b) [32]. The values of μe(μe−μg)/αo
extracted from the data measured in polar media are somewhat
larger than those resulting from the analysis of the data ob-
tained for the whole range of the solvents. This finding can be
explained only by the dependence of the electronic structure
of the fluorescent states on solvation. Due to a relatively small
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
vac
flu
hc eυabs−eυflu ¼ hc hceυvabasc−hceυ
þ 2 μe−μg =α3o ðε−1=2 ε þ 1Þ −1
n2−1=2n2 þ 1
ð11Þ
2
ꢇ
ꢄ
ꢅ
ꢆꢈ
2
The compounds studied show a satisfying linear correlation
between the energy hceυabs−hceυflu and the solvent polarity
function in a polar environment and also in all the solvents
studied; the values of (μe−μg)/αo3 are 0.62 eV (1), 0.66 eV
(2), 0.72 eV (3) and 1.23 eV (4). The equations 9–11 relate the
measured quantities to the excited state dipole moments μe.
Under the assumption that μe>>μg and with the effective
spherical radius of the molecules a0, 5.94 Aº (1), 5.91 Aº
(2), 5.98 Aº (3) and 5.99 Aº (4), as estimated from the
molecular dimensions of the compounds calculated by molec-
ular mechanics, equations 10 and 11 yield very similar values
of μe being in the range of 14.40 D (1), 13.29 D (2), 18.68 D
(3) and 22.52 D (4) for the studied molecules. The large value
of Δμ=μe−μg≈8.06 D (1), 9.64 D (2), 13.99 D (3) ad 17.7 D
(4) corresponds to a charge separation of about 0.3 nm,
0.4 nm, 0.5 nm and 0.4 nm which roughly agrees with the
centre-to-centre distance between the donor and acceptor moi-
eties of the compound.
This conclusion is in agreement with a linear relationship
found between the CT fluorescence energies and the differences
in the redox potentials corresponding to the oxidation of the
donor subunit Eoxi (D) and the reduction of the acceptor moiety
Ered (A) in the D-A molecules. The correlation is shown in
Fig. 8. In this correlation the values of Eoxi (D)−Ered (A) are
taken from the electrochemical data obtained for the given D-A
molecule in ACN containing 0.1 M TBAPF6 . These values are
very similar to those expected from the electrochemical prop-
erties of the donor and acceptor alone. The small shift of Ered
(A) to more negative potentials can be explained by the electron
donating properties of substituted phenanthroimidazole frag-
ment bonded to the acceptor subunit. Correspondingly, the
small shift of Eoxi (D) to more positive potentials arises from
the electron withdrawing character of the acceptor moiety. The
Ered (A) and Eoxi (D) values indicate also (in agreement with the
absorption spectra) that both subunits of all the D-A molecules
studied interact very weakly.
Fig. 7 a CT Absorption maxima Vs solvent polarity function; b CT Fluorescence maxima Vs solvent polarity function

J Fluoresc
constants, kr and knr, and the resulting electronic tran-
sition dipole moments Mflu are given by [34, 35]:
ꢂ
ꢃ
neυflu 3jMfluj
ð13Þ
vCT
64π4
kf ¼
3h
The electronic transition dipole moments Mflu can be
expressed by the following relation [1, 2, 7]
ꢀ
ꢁ
X
vCT
Mflu ¼ CET V0 μe−μg =hceυ
þ
iCiMi
ð14Þ
flu
The first term in the above equation corresponds the Mul-
liken two-state model [4, 5] (i.e., the interactions between the
solvent equilibrated fluorescent CTstate and the Franck–Con-
don ground state), the second one represents the Murrell
‘borrowing’ mechanism [6] (i.e., the contributions from the
locally excited states i of energy Ei) . The electronic transition
dipole moments Mi correspond radiative transitions (1LE)i→
S0 (states i in the studied systems to the 1La and 1Lb states of
the donor moieties, cf. Eq. 12).
Fig. 8 ED-EAVs SPF
This finding points to a different electronic structure of the
emitting singlet state CT of 1–4. The spectroscopic CT state
can be regarded as a linear combination of the zero-order ET
state (1ET) with: (i) various locally excited 1(π, π*) states and
(ii) with the ground state, S0 [33] :
The fluorescence analysis of the studied compounds do not
allow us to determine the electronic coupling elements between
the fluorescent CT state and the ground state (V0) or the locally
excited states (V1D) . The situation seems to be considerably
simplified for 1–4 because of the relatively small contribution of
the locally excited (π, π*) configurations to the wave function
ψ1CT≅CETϕ1ET þ Caϕa1 þ Cbϕb1 þ C0ϕ10
ð12Þ
1
ψCT of the fluorescent state (Eqs. 3 and 12). For these com-
pounds in highly polar media, similarly as for carbazol-9-yl
derivatives of aromatic nitriles [1] and aryl derivatives of aro-
matic amines [2], due to the large energy gap between the 1CT
1
1
1
where ϕ0 ,ϕET1,ϕa and ϕb represent the closed-shell con-
figuration of the ground state, the zero-order wave functions
of the pure 1ET state (which is described by a full ET from the
occupied orbital HOMO of the donor to the vacant LUMO
orbital of the acceptor) and the La1 and Lb1 states of the donor
moieties, respectively. The values of Δμ suggest that the wave
1
fluorescent state and LE states as well as the lowering of the
CT
flu
transition energy hceυ , the Mflu values can be approximated
assuming the dominant electronic coupling between the fluores-
cent state and the ground state. Neglecting the second term in
Eq. 14 and assuming that the cET value is close to unity, the V0,
0.18 eV (1), 0.11 eV(2), 0.19 eV(3) and 0.12 eV(4) value
calculated with Δμ=μe−μg≈8.06D (1), 9.64 D(2), 13.99 D(3)
and 17.7 D(4). This quantity is very close to the values calculated
from Eq. 1.
functions ψCT1 of the 1CT states are in the order 4>3>2 >1.
vac
abs
This finding is in agreement with the value of hceυ
mixing
of the lowest pure 1ETstate with the 1(π, π*) excitations leads
to the lowering of the spectroscopic 1CT state energy due to a
stabilising character of such interactions. It is also in agree-
ment with the magnitude of the electronic coupling elements
The reorganisation energy λ0 is related to the low-frequency
motions such as reorientation of the solvent shell λs as well as
any other low-frequency and medium frequency nuclear motions
of the solute (δλ0). The inner reorganization energy λi corre-
sponds to the high-frequency motions associated with the
changes in the solute bond lengths and angles. The values of
the low-frequency reorganization energy λ0 depend on the sol-
vent polarity Fig. 9a, as expected. Within the continuum dielec-
tric model of solvation, an analysis of the solvatochromic effects
on λ0 is possible according to the following expression [1, 2] :
D
V1 estimated from Eq. 12; one can expect that the contribu-
tion of the 1(π, π*) character to the wave function ψCT1 should
decrease in the order: 4>3>2 >1.
Electronic Coupling Elements
The electronic coupling elements between the lowest
excited 1CT state and the ground state (V0) or the
locally excited state lying most closely in energy (V1)
can be estimated from the CT absorption and fluores-
cence investigations. Applying a simple kinetic model
of an irreversible excited CT state Formation (with
100 % efficiency), the radiative and non-radiative rate
ꢀ
ꢁ
2
ꢄ
ꢅ
ꢆ
1
λ0 ≈δλ0 λ0 ≈δλ0
μe−μg =α3o ½ðε−1=2 ε þ 1Þ −
n2−1=2n2 þ 1 ꢁ
þ
þ
2
ð15Þ

J Fluoresc
(1), 0.65 eV (2), 0.74 eV (3) and 1.20 eV (4) obtained
from the mean slope of the plots corresponding to Eq.15
agree with those collected in Table 4. This finding sup-
1
ports again the hypothesis that the wave functions ψCT of
1
the CT states of the order 4>3>2>1.
The electrochemical properties of the phenanthroimidazole
derivatives (1–4) have been examined by cyclic voltammetry
and the redox potentials have been measured from the plot
potential versus current. The energies of the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) have been calculated using the relation,
EHOMO ¼ 4:8 þ E
1
oxi; ELUMO ¼ EHOMO–1239=λonset and
=
2
the calculated values are given in Table 4. The 3D plot of
HOMO−LUMO orbital picture is shown in (Fig. 9b). The
LUMO energies have been deduced from the HOMO energies
and the lowest-energy absorption edges of the UV–vis absorp-
tion spectra. The calculated energy gap (Eg=EHOMO−ELUMO
)
of 1–4 are 2.82, 3.94, 2.68 and 3.05 eV. Therefore, the HOMO
stability and the emission energy gap are controlled by the
nature and substituent present in the phenanthroimidazole
moiety.
Conclusions
Solvent-induced transformation of the electronic structure of
1
the CT states of selected D–A phenanthroimidazole deriv-
atives containing fluorine as an electron acceptor has been
analysed in terms of a model combining the Mulliken–
Murrell theory of the CT complexes and the Marcus theory
1
of the radiative charge recombination CT→S0. Analysis of
the electronic coupling elements show that the differences in
the photophysical properties of the studied molecules can be
interpreted in terms of the different electronic interactions
between the CT state and the ground state (V0) and/or the
1(π, π*) excited states most probably localized in the donor
moiety V1D. This approach can be used to explain the
solvatochromic effects on the spectral position of the CT
fluorescence spectra as well as on the Mflu values. The
conformation of the investigated D–A systems in the fluo-
rescent CT states seems to be the similar to that in the
ground state.
Fig. 9 a λo depend on the solvent polarity function; b HOMO-LUMO
maps of 1-4
This relation is suitable for the direct comparison with
the results of the investigations of the solvatochromic
effects on the spectral position of the CT fluorescence
3
maxima (Fig. 7b). The values of (μe−μg)2/αo (0.58 eV
Table 4 Slopes and intercepts of
3
compound
μgD
μe(μe−μg)/αo
θ
DHOMO
DLUMO
V0
V1D
vac
abs
the solvatochromic plots of the
CT Fluorescence of the imidaz-
oles (1–4)
hceυ , eV
1
2
3
4
4.40
3.65
4.69
4.82
0.75
0.66
0.72
1.78
3.76
4.02
4.25
4.81
0.75
0.66
0.72
1.78
1.32
1.30
1.10
2.01
14.67
16.23
25.77
16.99
1.11
2.21
2.50
2.95
0.08
0.18
0.12
0.10

J Fluoresc
Acknowledgments One of the authors Prof. J. Jayabharathi is thankful
to DST [No. SR/S1/IC-73/2010], DRDO (NRB-213/MAT/10-11) and
CSIR (NO 3732/NS-EMRII) for providing funds to this research study.
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