Synthesis and spectroscopic study of several novel annulated azulene and azafluoranthene based derivatives

Article abstract of DOI:10.1007/s10895-010-0722-1

A series of cyclized five-membered annulated azafluoranthene (AAF) and seven-membered annulated azulene (AA) derivatives have been synthesized and characterized by spectroscopic methods. The optical absorption and fluorescence spectra have been recorded i

Full text of DOI:10.1007/s10895-010-0722-1

J Fluoresc (2011) 21:443–451
DOI 10.1007/s10895-010-0722-1
SHORT COMMUNICATION
Synthesis and Spectroscopic Study of Several Novel
Annulated Azulene and Azafluoranthene
Based Derivatives
P. Ga¸siorski · K. S. Danel · M. Matusiewicz ·
T. Uchacz · R. Vlokh · A. V. Kityk
Received: 6 June 2010 / Accepted: 8 September 2010 / Published online: 1 October 2010
© Springer Science+Business Media, LLC 2010
Abstract A series of cyclized five-membered annulated
azafluoranthene (AAF) and seven-membered annu-
lated azulene (AA) derivatives have been synthesized
and characterized by spectroscopic methods. The op-
tical absorption and fluorescence spectra have been
recorded in organic solvents of different polarity and
analyzed within the semiempirical quantum chemical
model PM3. In combination with the molecular dynam-
ics simulations it properly reproduces the overall shape
of the measured absorption spectra of both AA and
AAF dyes including the strongest band in the region of
250–300 nm and the broad first absorption band above
400 nm. While the solvent polarity rises all the dyes ex-
hibit the hypsochromic shift of the first absorption band
and the bathochromic shift of the fluorescence band.
Such opposite solvatochromic trends appear to be con-
sistent with the Lippert–Mataga solvatochromic model.
Compared to AA compounds, both AAF dyes reveal
much stronger solvatochromic shift and broadening of
the fluorescence band likewise the relative decrease in
quantum yield on rising solvent polarity what may be an
evidence for the intramolecular charge transfer mech-
anism being involved into the fluorescence emission.
Depending on solvent polarity AA and AAF dyes emit
light in the green–yellow range of the visible spectra
what may be of interest for potential luminescent or
electroluminescent applications.
Keywords Annulated azafluoranthenes ·
Annulated azulenes · Optical absorption spectra ·
Fluorescence spectra
Introduction
The interest to low weight organic dyes is attracted
much by their photo-physical properties in a con-
text of broad range of optoelectronic applications as
fluorescent dopants (emitters) of dye lasers [1, 2] or
electroluminescent devices [3, 4]. For this reason the
electronic states and transitions as well as their corre-
lation with structural, environmental and chemical sub-
stitution effects appear in a scope of numerous experi-
mental and theoretical studies where the optical meth-
ods are combined with quantum chemical modeling.
Our previous studies have been concentrated mainly on
pyrazoloquinoline (1H-Pyrazolo[3,4-b]quinoline, here-
after abbreviated as PQ) and its derivatives represent-
ing intensely fluorescing dyes in solutions, polymer ma-
trixes or solid state with the quantum yield approach-
ing the unity in many cases [5, 6]. However the color
gamut of PQ emitters corresponds mainly to the blue
range of the spectra [7–12] what considerably limits the
range of their potential applications. For this reason
P. Ga¸siorski · M. Matusiewicz · A. V. Kityk (
Faculty of Electrical Engineering,
)
B
Cze¸stochowa University of Technology,
Al. Armii Krajowej 17, 42-200, Cze¸stochowa, Poland
e-mail: kityk@ap.univie.ac.at
K. S. Danel
Department of Chemistry, University of Agriculture,
Balicka str. 122, 30-149 Kraków, Poland
T. Uchacz
Department of Chemistry, Jagiellonian University,
ul. R. Ingardena 3, 30-060, Kraków, Poland
R. Vlokh
Institute of Physical Optics,
Dragomanova str. 23, 79005, Lviv, Ukraine

444
J Fluoresc (2011) 21:443–451
we recently started a new program focused on the
synthesis of annulated analogues of azafluoranthene
and heteroazulene representing cyclized five- or seven-
membered regioisomeric products of the phenyl PQ
derivatives [13]. The spectral investigations and photo-
physical properties of a few such compounds have been
reported in several recent publications [14–16]. The cy-
clization of PQs into azafluoranthenes or heteroazulene
is found to be accompanied by a significant red shift
of the first optical absorption and fluorescence bands.
While the solvent polarity rises all the dyes exhibit the
hypsochromic (blue) shift of the first absorption band
and the bathochromic (red) shift of the fluorescence
band. These trends have been reproduced within the
semiempirical calculations and may be interpreted by
a specific orientation of the dipole moments in ground
and excited states of molecules.
referenced hereafter as compounds AAF-1 and AAF-
2, correspondingly. By means of the optical absorption
and fluorescence measurements we characterize here
the electronic structure of these dyes and especially its
modification due to solute-solvent interaction. For this
reason the optical absorption and fluorescence spectra
have been recorded in solvents of different polarity
and consequently subjected to the quantum chemical
analysis performed within the semiempirical method
PM3.
Synthesis
Both AAs and AAFs may be easily obtained from the
PQs according to the procedure described in [13], see
also scheme below. To synthesize the seven-membered
(AA) compounds the PQ starter (R3 = Ph, X = Cl,
Br) has been heated to reflux in (izo)quinoline in a
presence of powdered KOH until digestion of the sub-
strate. The palladium annulation reaction of PQs (X =
Br) leads to five-membered (AAF) analogues in milder
reaction conditions.
In this paper we report the spectroscopic study and
photophysical properties of several newly synthesized
annulated azulene (AA) and azafluoranthene (AAF)
based derivatives. They may be given by the following
chemical structures:
10-phenoxy-6-phenyl-6H-5,6,7-triazadibenzo[f,h]
naphtho[3,2,1-cd]azulene (AA-1). Orange crystals; Mp =
◦
1
73–75 C; H NMR (CDCl3, 300 MHz) δ 7.05 (d, 2H,
J = 8.4 Hz), 7.10 (t, 1H, J = 7.2 Hz), 7.22–7.44 (m, 7H),
7.51 (dd, 1H, J = 9.3, 2.7 Hz), 7.55 (t, 2H, J = 7.2 Hz),
7.65 (dd, 1H, J = 8.1, 1.5 Hz), 7.72–7.76 (m, 1H), 7.77
(dd, 1H, J = 7.8, 1.5 Hz), 7.93 (d, 1H, J = 2.4 Hz), 8.14
(d, 1H, J = 9.3 Hz), 8.27-8.30 (m, 1H), 8.57 (d, 2H, J =
8.7 Hz); Anal. Calcd. for C34H21N3O; C, 83.76; H, 4.34;
N, 8.62. Found: C, 83.65; H, 4.81; N, 8.46.
8,10-dimethyl-6-phenyl-6H-5,6,7-triazadibenzo[f,h]
naphtho[3,2,1-cd]azulene (AA-2). Orange powder; Mp =
◦
1
225–227 C; H NMR (CDCl3, 300 MHz) δ 2.48 (d,
3H, J = 0.3 Hz, Me), 2.86 (s, 3H, Me), 7.27 (t, 1H, J =
7.5 Hz), 7.36–7.50 (m, 5H), 7.56 (t, 2H, J = 7.2 Hz), 7.71
(dd, 1H, J = 7.8, 1.5 Hz), 7.77–7.82 (m, 1H), 7.86 (dd,
1H, J = 7.8, 1.5 Hz), 8.06 (s, 1H), 8.27–8.36 (m, 1H),
8.71 (d, 2H, J = 8.9 Hz); Anal. Calcd. for C30H21N3;
C, 85.08; H, 5.00; N, 9.92. Found: C, 84.87; H, 5.09; N,
10.01.
where the upper row of this scheme is represented
by the cyclized seven-membered AA derivatives, 10-
phenoxy-6-phenyl-6H-5,6,7-triazadibenzo[f,h]naphtho
[3,2,1-cd]azulene and 8,10-dimethyl-6-phenyl-6H-5,6,7-
triazadibenzo[f,h]naphtho[3,2,1-cd]azulene referenced
hereafter as compounds AA-1, AA-2, respectively. Its
second row presents two cyclized five-membered AAF
derivatives, 1,5,6-trimethyl-3-phenyl-3H-indeno[1,2,3-
de]pyrazolo[3,4-b]quinoline and 7-methoxy-3-phenyl-
1-propyl-3H-indeno[1,2,3-de]pyrazolo[3,4-b]quinoline,
1,5,6-trimethyl-3-phenyl-3H-indeno[1,2,3-de]pyra-
zolo[3,4-b]quinoline (AAF-1). Yellow powder; Mp =
220–222 ◦C; 1H NMR (CDCl3, 300 MHz) δ 2.46 (d, 3H,
J = 0.3 Hz, Me), 2.71 (s, 3H, Me), 2.98 (s, 3H, Me), 7.28
(td, 2H, J = 7.5, 1.2 Hz), 7.38 (td, 1H, J = 7.5, 0.9 Hz),
7.45 (s, 1H), 7.56 (t, 2H, J = 7.5 Hz), 7.71 (ddd, 1H,
J = 7.5, 0.9, 0.6), 8.08 (ddd, 1H, J = 7.5, 0.9, 0.6 Hz),
8.58 (d, 2H, J = 8.7 Hz); Anal. Calcd. for C25H19N3;
C, 83.08; H, 5.30; N, 11.63. Found: C, 82.95; H, 5.48; N,
11.45.

J Fluoresc (2011) 21:443–451
445
X
R3
N
R3
N
Pd2(dba)3, DBU,
P(t-Bu)3, DMF, 130oC
KOH, (izo)quinoline,
reflux
R6
R7
R6
R7
R6
N
N
N
N
N
N
N
R7
R8
R8
R8
AAF-1,2
PQ starters, X = Cl, Br; R3 = Me, n-Pr, Ph
AA-1,2
AA1: R6 = OPh, R7 = R8 = H
AA2: R6 = Me, R7 = H, R8 = Me
AAF-1: R3 = Me, R6 = H, R7 = R8 = Me
AAF-2: R3 = n-Pr, R6 = OMe, R7 = R8 = H
7-methoxy-3-phenyl-1-propyl-3H-indeno[1,2,3-de]py-
razolo[3,4-b]quinoline (AAF-2). Orange powder; Mp =
174–176 ◦C; 1H NMR (CDCl3, 300 MHz) δ 1.16 (t, 3H,
J = 7.5 Hz), 2.00 (sextet, 2H, J = 7.5 Hz), 3.29 (t, 2H,
J = 7.5 Hz), 4.04 (s, 3H, OMe), 7.25–7.35 (m, 3H), 7.40
(td, 1H, J = 7.2, 0.9 Hz), 7.56 (t, 2H, J = 7.5 Hz), 7.83
(d, 1H, J = 9.3 Hz), 7.98 (d, 2H, J = 7.5 Hz), 8.49 (d,
2H, J = 8.7 Hz); Anal. Calcd. for C26H21N3O; C, 79.77;
H, 5.41; N, 10.73. Found: C, 79.60; H, 5.48; N, 10.66.
ature in cyclohexane (CHX), tetrahydrofuran (THF)
and acetonitrile (ACN) solutions with the molar con-
centration of the AA or AAF dyes of about 10−5 M in
each case.
The quantum chemical analysis has been performed
by means of the semiempirical method PM3 available
in the quantum chemical package HyperChem-8.0. The
geometrical optimization of the molecular structure
in the ground state, molecular dynamics (MD) sim-
ulations as well as the calculations of the transition
and state dipole moments have been performed us-
ing the same Hamiltonian, i.e. PM3 in each particular
case. The electron correlation was accounted via the
configuration interactions (CI) limited by a considera-
tion of 30 occupied and 30 unoccupied orbitals. The op-
tical absorption spectra have been calculated according
to the procedure described in [17–20]. The MD trajec-
tory has been generated in each particular case within
the standard MD procedure at the simulation tem-
perature T = 300 K and consisted of 200 consequent
instantaneous conformations separated by a temporal
interval of 0.5 ps. The optical absorption spectrum is
an average over the spectra obtained within the conse-
quent single point calculations. For each particular con-
formation of such MD trajectory the band shape of the
simulated optical absorption spectra was defined by the
Gaussian function, i.e. the molar extinction coefficient
Experimental and Calculation Procedures
The optical absorption spectra have been measured
by means of the scanning spectrophotometer UV-VIS
2101 (Shimadzu) in the range of 230–600 nm. The
steady state fluorescence was excited at the wavelength
λex = 365 nm and detected in a single photon counting
mode using a conventional spectrofluorimeter supplied
with the cooled photomultiplier EMI 955 8B. The spec-
tral resolution in optical absorption or fluorescence
meaurements was about 0.1 nm. The fluorescence spec-
tra were corrected on the spectral sensitivity of the
detecting system. The quantum yield was determined
in steady-state measurements using the quinine sul-
phate in 0.05M H2SO4 solution as an actinometer.
The fluorescence lifetime was determined by analyzing
the temporal decay of the fluorescence spectra being
excited by the picosecond diode laser from IBH-UK
(λ = 400 nm, τ = 70 ps pulse duration) and detected
in a time-resolved single photon counting mode. The
samples were prepared in the darkness and degassed
before each measurement using the freezing-pumping-
thawing technique. Both the optical absorption and
fluorescence spectra were measured at room temper-
2
(m.e.c.) is ∝ fi exp{2.773[(hν − ꢀEi)/ꢁ] }, where fi ∝
(t)
2
ꢀEi|μi | and ꢀEi are the oscillator strength and the
energy of the electronic transition from the singlet
ground state to ith excited singlet state, respectively,
μi(t) is the transition dipole moment of that electronic
transition, h is the Planck constant, ꢁ is the empirical
model parameter describing the Gaussian lineshape
broadening.

446
J Fluoresc (2011) 21:443–451
Experimental Results and Discussion
these derivatives are characterized by several broad
bands with the most strongest ones in the region of
250–270 nm. AA-1, AA-2 and AAF-1 dyes reveal the
first absorption band in the region of 440–460 nm being
slightly structured due to vibronic coupling. Similarly,
AAF-2 dye is characterized by a broad but more evi-
dently structured first absorption band with a maximum
in the region of 476–482 nm. While the solvent polar-
ity rises from CHX to ACN the absorption threshold
of both AA and AAF dyes undergoes a weak blue
shift what appears to be consistent with our recent
spectroscopic studies on several other AA and AAF
derivatives, see e.g. [14–16].
Figure 1a–d show the steady-state optical absorption
spectra of the AA and AAF dyes measured in CHX
(blue online color) and ACN (red online color) so-
lutions. In the range of 220–500 nm the spectra of
Figure 2a–d show the normalized steady-state
fluorescence spectra of the AA and AAF dyes mea-
sured in CHX (blue online color), THF (green online
color) and ACN (red online color) solutions. For all the
studied organic compounds the fluorescence bands are
unstructured in most cases. Exceptions constitute only
the fluorescence spectra of both AAF dyes measured in
a weakly polar CHX solution. In contrast to the optical
absorption the fluorescence exhibits a red shift while
the solvent polarity changes from CHX to ACN. More
evidently it is observed for the AAF derivatives where
corresponding solvatochromic shift is accompanied by
a significant broadening of the fluorescence bands in
highly polar solvents. Our recent spectral investigations
[14–16] show that stronger solvatochromism is typical
also for other AAF dyes.
Table 1 lists the optical absorption and fluorescence
maxima obtained in solvents of different polarity to-
gether with other photophysical characteristics. The
quantum yield ꢂ f , fluorescence lifetime τ f , radiative
(k f ) and radiationless (kn) rate constants as well as the
fluorescence moment Mf have been evaluated within a
simple kinetic model of an irreversible excited charge
transfer state [21]. One can see that the fluorescence
efficiency decreases for all the dyes in strongly polar
solvents, however the relative changes in ꢂ are con-
siderably larger for the AAF dyes. On the other hand,
the strongest quantum yield (ꢂ f =94 %) is found for
AA-1 in CHX solution. All the derivatives demonstrate
ꢀFig. 1 The steady state optical absorption spectra of the AA
and AAF dyes measured in CHX (blue online color) and ACN
(red online color) solutions. Vertical lines (black online color) are
the electronic spectra calculated by PM3 method for the EMC
(PM3+EMC) in vacuo (T = 0 K, ꢁ = 0). The continuous dot
lines (black online color) are the optical absorption spectra in
the gas phase being calculated by PM3 method in combination
with the MD simulations (PM3+MD) at T =300 K (ꢁ = 0.3 eV).
a AA1 dye; b AA2 dye; c AAF1 dye; d AAF2 dye. The inserts in
the sections a–d show the region of the first absorption band in
details

J Fluoresc (2011) 21:443–451
447
ing equilibrium states differ from each other mainly
by an angular orientation of the phenyl ring defined
via the bond torsion angle N-N-C-C. For the AAFs
the phenyl moiety appears in a symmetric double-well
potential with the equilibrium torsion angles 20.9 deg
and 22.1 deg as for AAF-1 and AAF-2, respectively.
Contrary to this, the phenyl ring of the AAs is char-
acterized by an essentially asymmetric potential with
the equilibrium torsion angles 46.3 and −15.1 deg
(AA-1) or 40.7 and −12.7 deg (AA-2), respectively.
Such asymmetry is obviously related with a specific
geometry of phenyl-azulene moiety being substantially
bent in the ground state as one can see in Fig. 3.
For comparison, the phenyl-azafluoranthene moiety of
AAFs is perfectly flat. However, it is more important
that the equilibrium angular states in both AAs and
AAFs are found to be separated by relatively small
energy barriers being comparable or much less than
the thermal energy kBT at T = 300 K. This leads to
quite large libration amplitudes of the rotating phenyl
ring, even up to about 40 deg. For this reason the
semiempirical calculations of the electronic spectra are
combined in the current study with the MD simulations.
The vertical lines (black online color) in Fig. 1a–d
represent the electronic spectra calculated by PM3
method for the equilibrium molecular conformation
(EMC) in vacuo (referred hereafter as PM3+EMC) at
T = 0 K and ꢁ = 0. The continuous dot lines (black
online color) are the optical absorption spectra in the
gas phase being calculated by PM3 method in com-
bination with the MD simulations (referred hereafter
as PM3+MD) at T = 300 K (ꢁ = 0.3 eV). One can
realize that quantum chemical calculations in combina-
tion with MD simulations quite properly reproduce the
overall shape of the absorption spectra including the
strongest band(s) in the region of 250–300 nm and
the broad first absorption band which occurs above
400 nm. However, more detailed comparison of the
measured and calculated spectra indeed reveals a
difference in the spectral position of the first absorption
band. For all the dyes the semiempirical calculations
give the absorption threshold position somewhat blue
shifted comparing to the measured ones. However, for
most dyes this difference appears to be less than 0.1 eV.
Only for AAF-2 the evaluated absorption threshold
position is characterized by a larger error, ∼0.2 eV. The
observed discrepancies should not be attributed to the
precision of the semiempirical calculations only but are
also related with an ignoring of solvent effects during
the calculation.
Fig. 2 The steady state normalized photoluminescence spectra
of the AA and AAF dyes measured in CHX (blue online color),
THF (green online color) and ACN (red online color) solutions.
a AA1 dye; b AA2 dye; c AAF1 dye; d AAF2 dye
rather similar values of the fluorescence momentum in
different solvents. The magnitude of Mf decreases here
less than 20% while the solvent polarity changes from
CHX to ACN.
In the following we analyze the measured optical
absorption spectra and solvatochromic shifts within
the semiempirical quantum chemical model PM3. The
geometry optimization leads to the equilibrium confor-
mations as shown in Fig. 3. Both types of derivatives are
characterized by several equilibrium molecular geome-
tries with nearly the same total energy. Correspond-
A challenging question remains an opposite sol-
vatochromic behavior of the first absorption and
fluorescence bands. We interpret this within the

448
J Fluoresc (2011) 21:443–451
Table 1 The photophysical
constants of AA and AAF
dyes in different solutions
Compound
AA-1
Solvent
λma
[nm]
λmf
ꢂf
τf
[ns]
kn
k f
Mf
[D]
[nm]
[107, s−1
]
[107, s−1
]
CHX
THF
ACN
CHX
THF
ACN
CHX
THF
ACN
CHX
THF
ACN
460.0
458.5
457.0
454.5
453.0
452.0
445.5
441.0
439.0
482.0
480.0
476.0
517.0
533.0
538.5
515.0
526.0
531.0
498.0
530.0
547.0
499.0
524.5
541.0
0.940
0.722
0.713
0.51
0.53
0.41
17.8
18.8
19.5
16.3
17.2
18.6
8.3
7.3
4.8
9.4
9.5
0.34
1.5
1.5
3.0
2.7
5.3
3.8
3.7
3.1
3.1
2.2
1.9
1.0
1.1
4.3
2.4
2.0
2.83
2.59
2.73
2.17
2.27
2.08
1.62
1.34
1.51
2.41
2.0
AA-2
λam is the absorption maxima,
λmf is the fluorescence
3.2
maxima, ꢂf is the
AAF-1
AAF-2
0.16
10.1
12.7
19.8
6.4
8.1
18.0
fluorescence quantum yield,
τf is the fluorescence lifetime,
kn is the radiationless rate
constant, kf is the radiative
rate constant and Mf is the
fluorescence moment
0.076
0.051
0.4
0.228
0.098
5.0
2.02
Lippert–Mataga dielectric polarization model [22, 23]
describing the solvent modified wavenumbers of the
optical absorption (νa = 1/λam) or fluorescence (ν f =
ν0a and ν0f are the spectral positions of the absorption
and fluorescence maxima in the gas phase, respec-
tively, c is the light speed, a0 is the Onsager (cavity)
radius, μg and μe are the ground and excited state
dipole moments, respectively. Accordingly, the type
of solvatochromism (hypsochromic or bathochromic)
likewise its strength are in fact defined by the signs and
magnitudes of the solvatochromic factors μg(μe − μg)
and μe(μe − μg). Table 2 presents their magnitudes
determined from the measured spectra and evaluated
within the semiempirical model via the calculated state
dipole moments μg and μe. The quantum chemical
calculations result for all the dyes to a proper signs
of the solvatochromic shift what in fact appears to
be a sequence of a specific spatial orientation of the
dipole moments μg and μe. Comparing the calculated
and measured solvatochromic slopes of the first optical
absorption band, being defined by the factor μg(μe −
μg)/a30, one must also pay attention on rather good
quantitative agreement. However, this is not the case
in respect to other factor, μe(μe − μg)/a30, which defines
the solvatochromic shift of the fluorescence band. The
quantitative discrepancy between the experiment and
theory is here about one order of magnitude. The
experiment reveals much stronger solvatochromic red
shift indicating thus on considerably larger dipole mo-
ment |μe| of the excited state being involved into a
fluorescence emission. In regard to this several possible
reasons should be mentioned:
1/λmf ) maxima as:
ꢀ
ꢁ
ꢀ
ꢁ
νa = ν0a − 2μg μe − μg F(ε, n)/ hca30
(1)
(2)
ꢀ
ꢁ
ꢀ
ꢁ
ν f = ν f − 2μe μe − μg F(ε, n)/ hca30
0
where F(ε, n) = (ε − 1)/(2ε + 1) − (n2 − 1)/(4n2 + 2)
is the polarity function of the solvent being defined by
its refractive index n and static dielectric constant ε,
i) The dipole moments have been calculated in the
gas phase, i.e. as for the ground state equilib-
rium molecular geometry in vacuo ignoring any
solvation effects. To treat the problem properly,
a program which includes the solvation energy in
the Hamiltonian should be used.
Fig. 3 The ground state EMC structure of the AA and AAF
dyes in vacuo (gas phase) as obtained within the optimization
procedure by the quantum chemical method PM3
ii) The quantum chemical analysis ignores the
Franck-Condon relaxation of molecular structure

J Fluoresc (2011) 21:443–451
449
Table 2 Solvatochromic factors for AA and AAF dyes as deter-
mined from optical absorption and fluorescence measurements
and calculated within the semiempirical model PM3
decrease if the solvent polarity rises from CHX to
ACN. Following [24] such behavior is typical for many
fluorescent materials. In the case of AAF dyes it may
be explained by a stabilization of the ICT state at
higher polar solvents, and emission from the LE state
in lower-polarity solvents. Accordingly, a decrease in ꢂ
is possible because the ICT emission is from a different
electronic state than the LE emission. In other words,
in highly polar solvents due to a considerable mixing
of the LE and ICT states the lowest excited state of
AAF dyes appears to be strongly modified and gets
features mostly of ICT state, and vice versa. Lowering
of ICT states in polar solvents activates radiationless
transitions of AAF dyes (see Table 1) what is the
reason for a considerable decrease in their fluorescence
efficiency. Here basically two competitive mechanisms
may be involved into the nonradiative depopulation
of the lowest excited state: i) the intersystem crossing
providing the population of the triplet states and ii) a
direct radiationless charge recombination to the singlet
ground (S0) state [25]. However, the available data
are not sufficient to make a conclusion which of these
mechanisms is dominant. One should be emphasized,
that a reduction in the quantum yield vs the rising
solvent polarity is substantially stronger for AAF-1 or
AAF-2 compared to the AA dyes. This finding quite
well correlates with the larger solvatochromic red shift
and broadening of the fluorescence band observed for
both AAF dyes in the experiment (see Fig. 2). Thereby,
it may be suggested that the red-shifted transition in
AAF-1 and AAF-2 compounds is preferably due to
ICT which is known to be affected by a solvent polarity.
In contrast to this, both AA-1 and AA-2 dyes exhibit
the excited fluorescent state which even in highly polar
solvent environment still keeps characteristic features
of the LE state.
The discussion presented above is relevant with the
quantum chemical analysis. In particular, it shows that
for the ground state geometry the next excited singlet
state (S2) of AAF dyes is indeed characterized by a
better charge separation upon the excitation comparing
to the lowest excited state S1. This provides therefore
a lager dipole moment |μ2| for this state, 4.4 D and
7.0 D as for AAF-1 and AAF-2, respectively. The state
S2 may be considered therefore as moderately dipolar
ICT state. Rotation of the phenyl ring towards planarity
gives further rise of these dipole moments to 4.7 D
and 7.5 D, correspondingly. Also the dipole moment of
the lowest excited state S1 exhibits lager magnitudes
for the planar conformations. This finding suggests that
in the polar solvents the AAF molecules after the exci-
tation will tend to more planar geometries with simul-
taneous lowering of their state energies. Corresponding
Compound
μg(μe − μg)/a3o
μe(μe − μg)/a3o
[eV]
[eV]
AA-1
Exp.
PM3+EMC
(Calc.)
−0.029
−0.032
0.164
0.026
PM3+MD
−0.035
–
(Calc.)
AA-2
Exp.
PM3+EMC
(Calc.)
−0.0248
−0.046
0.124
0.012
PM3+MD
−0.055
–
(Calc.)
AAF-1
AAF-2
Exp.
PM3+EMC
(Calc.)
−0.071
−0.067
0.377
0.041
PM3+MD
−0.05
–
(Calc.)
Exp.
PM3+EMC
(Calc.)
−0.055
−0.062
0.320
0.036
PM3+MD
−0.063
–
(Calc.)
The Onsager radius a0 scales according to the cube root of
molecular volume and taken equal to 4.95, 4.77, 4.56 or 4.66 Å as
for AA-1, AA-2, AAF-1 or AAF-2, respectively
in the excited state toward a new (equilibrium)
conformation which may result to a substantial
modification of the electronic transitions likewise
the dipole moments of the excited states including
the lowest one.
iii) In addition to the eventual solute relaxation, as
mentioned in (ii), the solute-solvent interaction
(solute in Onsager reaction field) lowers the en-
ergy of the excited states. The effect is consid-
erably stronger for those excited states that are
characterized by large state dipole moments, such
as e.g. intramolecular charge transfer (ICT) states.
A considerable lowering of their energies in a po-
lar solvent environment may lead basically to the
two cases. Either one of the strongly polar excited
ICT states of the solute becomes in solvent the
lowest excited singlet fluorescent state (so-called
inversion of states) or the lowest locally excited
(LE) weakly dipolar state appears to be strongly
modified due to its mixing with strongly dipolar
ICT states that just approach the LE state in polar
solvent.
We favor the last case, especially for AAF dyes,
taking into account that the fluorescence quantum
yield (see Table 1) of these compounds reveals strong

450
J Fluoresc (2011) 21:443–451
energy shift ꢀE for the state characterizing by the
dipole moment |μi| can be evaluated within the polar-
ization continuous model as for the point charge dipole
to ACN all the dyes exhibit the hypsochromic shift of
the first absorption band and the bathochromic shift
of the fluorescence band. These findings appear to
be consistent with the Lippert–Mataga solvatochromic
model. The different solvatochromic trends in a spec-
tral position of the optical absorbtion and fluorescent
bands are explained by specific orientations of the
ground and excited state dipole moments. However,
the semiempirical calculations give quite reasonable
magnitude of the solvatochromic shift concerning the
first absorption band only. Regarding the fluorescence
spectra the experiment indicates on a considerably
larger dipole moment of the excited state(s) involved
into a fluorescence emission. As possible reasons for
such discrepancy may be geometrical relaxation of the
molecules in the excited state likewise a mixing of
the weakly dipolar LE state with the strongly dipolar
ICT states energy of which lowers in polar solvent
environment. Corresponding effects appear to be much
evident for the AAF dyes which are characterized in
highly polar solvents by a stronger relative changes in
quantum yield correlating with a larger solvatochromic
red shift of the emission band likewise its evident
broadening. Depending on solvent polarity the AA and
AAF dyes emit light in the green–yellow range of the
visible spectra what may be of interest for lumines-
cent or electroluminescent applications. The quantum
yield of these dyes considerably rises in a weakly polar
environment.
in the reaction Onsager field, ꢀE = −(|μi| /a30)(ε −
2
1)/(2ε + 1). Accordingly, due to a much stronger lower-
ing of the state S2 compared to the state S1, the energy
gap between them appears to be considerably reduced.
In the ACN solution (ε = 37.5) our evaluation give
its value even less than 0.17 or 0.09 eV, as for AAF-
1 or AAF-2, respectively. For this reason the mixing
of these states seems to be very likely in the highly
polar solvents. Assuming that the ICT state S2 would
be the emission one it would be characterized also by
a larger magnitude of the fluorescence solvatochromic
shift [μe(μe − μg)/a30] being equal to about 0.07 eV
(AAF-1) and 0.21 eV (AAF-2), i.e. in a better agree-
ment with the experiment. One should be emphasized
that the Onsager radius a0 represents here a crude
parameter of the model thus the evaluation given above
should be rather considered as a rough estimation
only. Nevertheless, the model presented above explains
the most possible reason for the essential modification
of the lowest excited state of AAF dyes in a polar
environment. For comparison, the quantum chemical
calculations of both AA dyes give the magnitudes of
the dipole moment for the excited state S2 less than
1.8 D. Thereby particularly this state, even in highly
polar solvents, should not influence the lowest excited
state S1. A weak fluorescence solvatochromism demon-
strating by AA-1 and AA-2 dyes remains to be rele-
vant with a weakly dipolar character of their emission
state S1.
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