Ground- and excited-state properties of some 3,4-dihydro-1-(2-p-substituted benzylidene)naphthalenones: Substituent and environmental effects

Article abstract of DOI:10.1002/(SICI)1099-1395(199709)10:9<687::AID-POC932>3.0.CO;2-A

Electronic absorption and steady-state fluorescence emission of seven 3,4-dihydro-1-(2-p-substituted benzylidene)naphthalenones (1-7) show sizable solvent dependence. The charge-transfer (CT) absorption maxima of these compounds in various solvents show a

Full text of DOI:10.1002/(SICI)1099-1395(199709)10:9<687::AID-POC932>3.0.CO;2-A

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 687–696 (1997)
GROUND- AND EXCITED-STATE PROPERTIES OF SOME 3,4-DIHYDRO-
1-(2-p-SUBSTITUTED BENZYLIDENE)NAPHTHALENONES:
SUBSTITUENT AND ENVIRONMENTAL EFFECTS
IBRAHIM A. Z. AL-ANSARI
Department of Chemistry, Faculty of Science, University of Qatar, P.O. Box 2713, Doha, State of Qatar
Electronic absorption and steady-state fluorescence emission of seven 3,4-dihydro-1-(2-p-substituted benzylidene)na-
phthalenones (1–7) show sizable solvent dependence. The charge-transfer (CT) absorption maxima of these compounds
in various solvents show a red shift for the electron-donating substituted compounds (1–5), whereas a blue shift is
observed for compounds possessing electron-withdrawing substituents (6, 7). Excitation into the lowest energy
absorption gives emission from the locally excited state, which relaxes to the emitting intramolecular charge-transfer
state for compounds with strong electron-donating substituents (1–3) in hydroxylic solvents. However, in moderately
polar solvents, dual emissions are observed for these compounds. No CT emission is observed for the compounds with
moderate electron-donating substituents (3 and 4) or those with electron-withdrawing substituents (6 and 7). Compound
1
with an –N(CH ) substituent shows an excited-state dipole moment of 17 D. © 1997 John Wiley & Sons, Ltd.
3 2
J. Phys. Org. Chem. 10, 687–696 (1997) No. of Figures: 9 No. of Tables: 3 No. of References: 38
Keywords: absorption; fluorescence; solvatochromism; hydrogen bonding; dipole moment; naphthalenones
Received 20 January 1997; revised 24 February 1997; accepted 15 March 1997
29
INTRODUCTION
in molecular or bimolecular systems.
In this work, a series of seven structurally related
,4-dihydro-1-(2-p-substituted benzylidene)naphthalenones
In the past 30 years, there has been increasing interest in
donor- and acceptor-containing compounds that are capable
of initiating intramolecular charge-transfer (ICT) processes.
Earlier studies focused on systems where conformational
folding brings the donor and acceptor moieties into a
parallel plane within the through-space interaction dis-
3
(
Scheme 1, 1–7) were synthesized and characterized by IR
1
and H NMR spectroscopic methods. The photophysical
behaviour of these compounds in various non-polar and
polar solvents at room temperature, was investigated. The
influence of the dielectric properties and hydrogen bonding
of both the substituents and solvents on the photophysics of
these compounds was closely examined.
1
–3
tance. Investigations were also focused on systems in
which donor and acceptor groups are directly connected
4–6
through a single bond.
The dual fluorescence of
dimethylaminobenzonitrile (DMABN) in polar solvents,
7
discovered by Lippert et al., which was attributed to a
8
twisted intramolecular charge transfer (TICT) state, led to
extensive research to explore this phenomenon in other
9
–16
molecules.
Systems possessing electron donor–acceptor
fragments of the type D–B–A are widely used as candidates
for studying photoinduced electron transfer, where B is a
17–21
rigid or a flexible bridge.
Much of the work on charge transfer (CT) in organic
systems has been chiefly motivated by the search for
organic materials with potential applications as fluorescent
2
2–24
25
and laser dyes,
optical recording media, molecular
26, 27
28
switching devices
and non-linear optical materials, or
by the desire to elucidate the mechanism of electron transfer
*
Correspondence to: I. A. Z. Al-Ansari.
1997 John Wiley & Sons, Ltd.
Scheme 1
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CCC 0894–3230/97/090687–10 $17.50

6
88
I. A. Z. AL-ANSARI
3,4-Dihydro-1-(2-p-methoxybenzylidene)naphthalenone
EXPERIMENTAL
1
(3): yellow crystalline solid, m.p. 10–109 °C; H NMR
Materials and methods of characterization of com-
(DMSO-d ), (ppm) 2·503–2·934 (m, 2H, CH ),
␦
6
2
pounds 1–7. Compounds 1–7 were prepared by regular
condensation, ␣-tetralone being treated with NaOH solution
and then reacted with the corresponding para-substituted
aldehydes under reflux in ethanol for 2 h. The products were
collected in each case and purified by recrystallization twice
from a minimum volume of methanol–water (1:1). The
yields were 80–90%.
2·972–3·086 (m, 2H, CH ), 3·816 (s, 3H, OCH ),
6·986–7·997 (aromatic).
2
3
3,4-Dihydro-1-(2-p-methylbenzylidene)naphthalenone
1
(4). yellow crystalline solid, m.p. 119–120 °C; H NMR
(DMSO-d ), ␦ (ppm) 2·377 (s, 1H, CH ), 2·914–2·974 (m,
6
3
2H, CH ), 3·068–3·118 (m, 2H, CH ), 7·174–8·169 (aro-
2
2
matic).
3
,4-Dihydro-1-(2-p-N,N-dimethylaminobenzylidene)na-
3,4-Dihydro-1-(2-benzylidene)naphthalenone (5): yellow
1
phthalenone (1): orange crystalline solid, m.p. 145–147 °C;
H NMR (DMSO-d ), ␦ (ppm) 2·507–2·789 (m, 2H, CH ),
crystalline solid, m.p. 103–105 °C; H NMR (DMSO-d ), ␦
6
1
(ppm) 2·502–2·533 (m, 2H, CH ), 2·996–3·039 (m, 2H,
6
2
2
2
·979 [s, 6H, N(CH ) ], 7·315–7·965 (aromatic).
CH ), 7·326–8·020 (aromatic).
3
2
2
3
,4-Dihydro-1-(2-p-hydroxybenzylidene)naphthalenone
3,4-Dihydro-1-(2-p-bromobenzylidene)naphthalenone
(6): yellow crystalline solid, m.p. 153–154 °C; H NMR
1
1
(
2): dark-brown crystalline solid, m.p. 203–205 °C;
H
NMR (DMSO-d ), ␦ (ppm) 2·498–2·532 (m, 2H, CH ),
(DMSO-d ),
␦ (ppm) 2·496–2·529 (m, 2H, CH ),
6
2
6
2
2
·936–3·039 (m, 2H, CH ), 7·324–8·071 (aromatic), 9·963
2·982–3·352 (m, 2H, CH ), 7·326–8·004 (aromatic).
2
2
(s, 1H, OH).
3,4-Dihydro-1-(2-p-nitrobenzylidene)naphthalenone (7):
Figure 1. Plot of carbonyl vibrational frequencies (␯C=O ) of compounds 1–7 as a function of Hammett substituent constants (␴ ). The solid
p
line is the best fit through all frequencies except for compound 1 (filled circle)
©
1997 John Wiley & Sons, Ltd.
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GROUND- AND EXCITED-STATE PROPERTIES OF SOME NAPHTHALENONES
689
1
Table 1. Ground-state properties of compounds 1–7
dark-pink crystalline solid, m.p. 188–190 °C; H NMR
(
2
DMSO-d ), (ppm) 2·090–2·507 (m, 2H, CH ),
␦
6
2
Ionization
potential
(eV)
·923–3·044 (m, 2H, CH ), 7·358–8·337 (aromatic).
2
a
b
Ϫ1
Compound
␴p
␯C=O (cm
)
Solvents. Spectroscopy-grade n-hexane, diethyl ether,
1
2
3
4
5
6
7
Ϫ0·32
Ϫ0·40
Ϫ0·27
Ϫ0·14
0·00
7·12
8·51
8·21
8·82
9·24
8·98
9·92
1654
1661
1663
1664
1665
1666
1670
dichloromethane, acetonitrile dimethylformamide (DMF),
ethanol, methanol, n-butanol, n-propanol and tetrahy-
drofuran (THF) were purchased from Fluka and used as
received.
0·26
0·71
Ϫ4
Sample preparation. Stock solutions (10
M) of the
compounds were prepared by dissolving an accurately
weighed amount of these compounds in n-hexane, with-
drawing 1 ml and adding it to a 10 ml volumetric flask. This
was then evaporated with nitrogen bubbling to leave a thin
a
Ref. 30.
b
Ionization potential of substituted phenyl ring, taken from Ref. 32.
Figure 2. Absorption spectra in (A) n-hexane and (B) in ethanol of compounds 1 (- - -), 2 (᭹), 3 (. . .), 4 (– · – · –), 5 (——), 6 (ϫϫϫ)
and 7 (###)
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I. A. Z. AL-ANSARI
Table 2. Intramolecular charge-transfer absorption maxima (␯a ) of compounds
a
1
–7 in various solvents
Compound
Solvent
1
2
3
4
5
6
7
n-Hexane
Diethyl ether 25·06
THF
25·10
30·62
29·07
29·10
29·10
28·90
29·41
28·17
28·73
28·61
28·94
43·5
30·07 31·64
29·85 31·40
29·58 31·00
29·58 31·00
29·24 31·54
29·54 30·96
28·94 30·44
29·07 30·72
29·07 30·60
29·20 31·00
32·46
33·67
32·15
32·15
33·90
33·16
32·36
32·00
32·46
32·84
0
32·00
33·16
31·74
31·74
33·80
31·90
31·84
31·74
32·15
32·05
32·73
32·46
31·84
31·84
32·10
31·64
32·15
31·84
32·15
32·10
24·70
24·10
24·01
24·70
23·75
23·78
23·80
23·86
45·5
CH Cl2
2
DMF
CH CN
3
Butan-1-ol
Propan-1-ol
Ethanol
Methanol
b
(⌬␯a )
87
91
1000
133
a
b
Ϫ3
Ϫ1
Values in 10 cm
.
Ϫ1
Ϫ4
Values in 10 cm , ⌬␯
a
=␯
a
(ethanol)Ϫ␯
a
(n-hexane).
Figure 3. Correlation between the charge-transfer absorption maxima (␯ ) vs ionization potential of the substituted phenyl ring (I.P)
a
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GROUND- AND EXCITED-STATE PROPERTIES OF SOME NAPHTHALENONES
691
film. The desired solvent was added and the solution was
electronic effect of a substituent R in an aryl system on the
reaction centre para to the reaction site:
Ϫ5
diluted to volume to give a 10
M
solution.
log k=␳␴ +log k0
(1)
R
Optical measurements. Spectra were recorded with the
following spectrometers: Bruker ESP300 (260 MHz,
H NMR), Shimadzu UV-160 (UV–Vis) and Shimadzu RF/
where k is the rate constant of the reaction being studied
[carbonyl group vibrational frequency (␯C=O ) in this study]
1
5
00 (fluorescence and excitation spectra). Chemical shifts
with a substituent R present and k is the corresponding rate
0
(
␦) are in parts per million relative to (CH ) Si. Emission
constant of the unsubstituted compound (R=H).
3
4
spectra were obtained using small angle (22·5°) front-
surface excitation geometry. The excitation and emission
Thus, a plot of ␯
vs Hammett substituent constant
C=O
2
31
(␴para
) gave an excellent correlation (R =0·970), exclud-
slits were both 0·5 mm. The excitation wavelengths (␭exc
)
ing the value for compound 1 since it shows a very low
energy for the vibrational frequency of the carbonyl group.
This can be understood on the basis that this compound
possesses a considerable degree of charge-transfer character
originating from the N,N-dimethylamino lone pair and
terminating at the carbonyl group, which loses its double
bond character in this case. This would account for the
considerable red shift in absorption in this compound with
respect to the others. The experimental data and best-fit line
are shown in Figure 1, and Table 1 gives the carbonyl
frequencies. Our results show that from this correlation we
were 398, 326, 332, 316, 308, 312 and 305 nm for
compounds 1–7, respectively. The experiments were carried
out at room temperature (24 °C) using 1 cm matched quartz
cells. Fresh solutions were used for all measurements.
Emissions spectra were not corrected for the spectral
response of the instrument.
RESULTS AND DISCUSSION
Correlation of the carbonyl frequencies with the
structure
obtain log k =1665, which is in good agreement with the
0
experimentally measured vibrational frequency of the
carbonyl in the unsubstituted compound 5 (R=H). Further,
the slope of the best-fit line gave a value of 7·35, which
It is widely known that the Hammett equation and its
30
modifications [equation (1)] have been used to assess the
Scheme 2. Molecular structure and resonance forms in hydroxylic solvents
1997 John Wiley & Sons, Ltd. JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, VOL. 10, 687–696 (1997)
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6
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I. A. Z. AL-ANSARI
Assignment of the wavelength of the absorption bands in
each of these compounds to CT transition is evident from
the bathochromic shift in the absorption from the non-polar
to the polar solvents (see Table 2). Figure 2(A) and 2(B)
show a displacement of the absorption maxima towards
longer wavelength from the non-polar n-hexane to the protic
polar ethanol. This red shift follows the order
2
>1>3>4>5. Compounds 6 and 7 show a hypsochromic
shift due to the opposite direction of the charge. This order
of the displacement of the CT band suggests an electronic
interaction of the substituent with the electron acceptor
fragment of the molecules. The order in which the
wavelength of the CT absorption is affected is thus related
32
to the ionization energy of the substituted phenyl ring.
Figure 3 shows this relationship. The frequency shift is
greater for the –N(CH ) substituent, in which the lone-pair
Figure 4. Absorption spectra of compound 2 in the solvents
methanol (- - -), ethanol (——), propanol (– · – · –), butanol (. . .)
3
2
participation is enhanced by the low nuclear charge on
and 0·1
M NaOH (ϫ)
nitrogen; the shift is least for the –NO substituent since it
2
has a great electron-attracting capability, which is reflected
in its high ionization potential.
corresponds to the reaction constant ␳ in equation (1). This
positive value indicates that the carbonyl frequencies are
very sensitive to the electron-donating/withdrawing influ-
ence of the substituent attached to the phenyl moiety.
Compounds with electron-donating substituents, 1–4,
show absorptions in polar protic solvents (alcohols) at
longer wavelength (lower energy) than in polar aprotic
solvents, e.g. acetonitrile (Table 2 and Figure 1). On the
other hand, compounds 6 and 7, with electron-withdrawing
groups, show absorption maxima at shorter wavelength
(higher energy) in hydroxylic solvents compared with
aprotic solvents.
It seems that hydroxylic solvents tend to solvate the CT
structure b (Scheme 2) which is dominant in compounds
1–3 through specific hydrogen-bonding interactions better
than polar aprotic solvents. This will lower the energy of the
CT state owing to additional stabilization of this state.
For compounds 6 and 7, polar aprotic solvents stabilize
structure d through dipole–dipole interactions, thus lower-
ing the energy of the CT state.
Absorption spectra
The absorption spectra of compounds 1–7 in n-hexane and
ethanol are shown in Figure 2. Each of the spectra in these
solvents and other polar protic and non-protic solvents
exhibits two major broad bands.
The first band (band I) is slightly structured and located in
the range 220–300 nm (Table 1), and shows moderate
sensitivity both to the polarity on the solvent in each
compound and to the variation of the substituent on the
phenyl ring when all compounds are compared.
The second band (band II) is found in the 300–400 nm
region (Table 1) and is sensitive to both solvent polarity and
the substituent. This broad long-wavelength absorption
band is attributed to ICT from the high-energy, occupied
orbital of the substituted-phenyl donor to the low-energy,
unoccupied orbital of the ketone acceptor moiety (e.g.
␲Ph→␲*C=O ).
Polarity effects on the CT band of compounds 1–7
depend strongly on the hydrogen bonding ability of the
solvents. We noticed that the plot of the energy of the
absorption maxima of compounds 1–7 against the solvent
33
polarity parameter (E (30) gave good correlations only in
T
protic solvents. We also attempted to plot the long-
a
Table 3. Fluorescence data (␯ ) for compounds 1–7 in various solvents
f
Compound
Solvent
1
2
3
4
5
6
7
n-Hexame
Diethyl ether
THF
22·22
27·70
26·45
26·31
25·77 (sh)
25·91 (sh)
25·64 (sh)
27·17
27·03
26·88
28·41
26·46
27·17
28·10
26·46
27·25
28·10
26·45
27·17
28·10
26·46
17·62
(21·21)
sh (20·16)
sh (19·61)
22·12 (18·87)
(18·65)
CH Cl2
25·91 (sh) 26·45 (21·55) 26·74 (21·83) 26·31 (21·10) 27·32 (sh)
25·91 (sh) 27·17 (20·57)
25·91 (sh) 27·17 (20·57)
2
DMF
27·17
27·17
27·78
27·47
27·47
27·80
27·17
27·17
27·93
CH CN
3
Ethanol
sh (19·38) 27·67 (22·83)
26·88 (22·73)
27·62
a
Ϫ3
Ϫ1
Values in 10 cm . sh=Shoulder. The values in parentheses are from the charge-transfer state
©
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GROUND- AND EXCITED-STATE PROPERTIES OF SOME NAPHTHALENONES
693
wavelength absorption maxima against the solvatochromic
ethanol, propanol and butanol, respectively. This suggests
the occurrence of a substantial increase in delocalization of
charge with ionization of the hydroxyl group caused by the
increase in the solvent’s basicity. Thus, in hydroxylic
solvents the presence of a contribution of charge from the
ionized hydroxyl (conjugate base) causes extra charge
transfer [compare ␭abs (ethanol)=349·5 nm vs ␭abs (acetoni-
trile)=340 nm, Table 2].
34
parameters ␲*, ␣ and ␤. We found good correlations only
for compound 1, 3 and 7 in protic and aprotic solvents with
the ␲* scale, which is an index of solvent dipolarity/
polarizability. These findings suggest that the energy of the
CT absorption bands in these compounds does not vary
strongly with solvent polarity, because these compounds are
polarized in the ground state.
On the other hand, the absorption maxima correlate
excellently with the ␣-scale for compounds 1–4 in protic
solvents, but moderately in aprotic solvents. The CT
absorption energies, however, correlate well with the ␤-
scale for compounds 1–3, 5 and 7 in protic solvents, but the
correlation is poor for compounds 4 and 6. In aprotic
solvents a good correlation is obtained for compound 2,
which explains the extra bathochromic shift in the absorp-
tion found for this compound in many solvents, specifically
hydroxylic ones (see Table 2 and the next section).
Fluorescence spectra
The fluorescence emission characteristics of compounds
1
–7 were examined in various solvents of different polarity.
Table 3 displays the fluorescence emission maxima (␯f )
observed upon excitation at energies corresponding to the
CT absorption bands (band II), and Figure 5(A)–(G) show
fluorescence spectra for compounds 1–7 in n-hexane and
ethanol as examples.
Based on these results, compound 1 gives a broad,
slightly structured band corresponding to emission from the
locally excited state (LE) in non-polar solvents such as n-
hexane, but dual bands corresponding to emission from the
LE and a second lower energy, highly solvatochromic broad
band in moderately polar protic and aprotic solvents [Figure
Absorption characteristics of compound 2
In compound 2, both band I and band II (CT band) shift to
35
longer wavelength with increase in the basicity of the
solvent (Figure 4). For instance, absorption maxima of band
II are seen at 345·5, 349·5, 350 and 355 nm in methanol,
Figure 5. Fluorescence spectra in n-hexane (——) and ethanol (- - -) of compounds 1 (A), 2 (B), 3 (C), 4 (D), 5 (E), 6 (F) and 7 (G)
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6
94
I. A. Z. AL-ANSARI
5
(A)]. However, in highly polar solvents such as acetoni-
Dipole moment of the excited (CT) state of compound 1
trile, only emission from the second long-wavelength band
is observed. The second band is assigned as originating
from the ICT.
Compounds 2 and 3, on the other hand, show emissions
from the LE state, with vibronic structure in most low and
moderately polar solvents. However, in highly polar
solvents, the broad CT band appears as a shoulder. The
situation is diffeent in hydroxylic solvents, where dual
emission is observed for these compounds, as shown in
Figure 5(B) and (C).
Since compound 1 bears the strongest electron-donating
substituent among the series of compounds studied, emis-
sions from the CT state are observed in both polar protic and
aprotic solvents. The validity of the assumption that this
state has a charge-separated nature comes from the fact that
both the spectral position and the shape of the band were
sensitive to changes in the polarity of the solvents. In this
regard, we noticed a bathochromic shift and a gradual
disappearance of the LE state band with increasing solvent
polarity. This red shift is a manifestation of charge
separation stabilized by the solvent, and thus an increase in
the dipole moment of the compound upon excitation. Figure
No CT emissions were observed for compounds 4–7 in
most of the solvents studied, except in dichloromethane
[Figure 5(D)–(G)]. In dichloromethane, compounds 4–6
6
shows the effect of the solvents on the emission spectra of
show dual emissions but compounds 2 and 3 give emissions
from the LE state and a shoulder in the region of the CT
band. Compound 7, however, gives the LE state emission
along with a weak emission in the CT domain. For
compound 1 only the CT band is observed in dichloro-
methane (Table 3).
Our measurements show that the LE state emission of
compounds 4–7 in non-polar and polar solvents are
structureless with a slight red shift from non-polar, e.g. n-
hexane, to polar solvents, e.g. ethanol or acetonitrile, while
a considerable red shift for compounds 1–3 and a slight red
shift for compounds 4–7 are observed. The LE state bands,
however, resemble those of the absorption spectra for the
compounds studied.
compound 1, where it can be seen that the CT state is
completely stabilized in acetonitrile. In contrast, CT is not
feasible in n-hexane since this non-polar solvent cannot
stabilize the CT state.
In order to establish the magnitude of the excited-state
37
dipole moment, we attempted to use equation (2) to
correlate the energy of the intramolecular charge transfer
(CT) state with the solvent’s polarity parameter ⌬f. The CT
energy shift increases linearly with ⌬f in solvents of
different polarities (Figure 7).
2
2
␮ ⌬f
e
␯¯ = ␯¯ (0)Ϫ
(2)
f
f
3
hca
We observed dual emission for compound 3 in acetoni-
trile (Table 3). The locally excited state is structured, while
the CT emission is seen as broad band situated at 486 nm.
However, the CT emission is only detected as a shoulder in
this highly polar solvent for compounds 2 and 3, contrary to
what one would expect.
Based on these observations, it appears that compounds
1
–3, bearing strong electron-donating substituents, give CT
emissions in protic solvents, due to the specific interaction
through intermolecular hydrogen bonding between the
hydrogen of the solvents and the charge on the carbonyl’s
oxygen in the excited state. This solute–solvent interaction
36
is expected to stabilize the charge-separated state (CT).
The systematic shift in the emission maxima as a result of
the substituent on these molecules can be seen from the
variation of ␯ with the ionization potential of the substi-
f
tuted-phenyl moieties. As shown in Figure 5(A)–(C),
emissions from the CT state in protic solvents are found at
5
16, 440 and 438 nm for compounds 1, 3 and 2, respec-
tively. This trend reflects the increase in the ionization
potential (see Table 1) of the substituted-phenyl moieties.
Fluorescence excitation spectra were measured for each
of the compounds and were essentially identical with the
absorption spectra, implying that the emitting forms are
structurally close to the stable forms in the ground state.
Fluorescence excitation spectra were measured for each
of the compounds and were essentially identical with the
absorption spectra, implying that the emitting forms are
structurally close to the stable forms in the ground state.
Figure 6. Fluorescence spectra of compound 1 (␭exc =398 nm) in
the solvents (A) n-hexane, (B) THF, (C) ethanol, (D) DMF and (E)
acetonitrile
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1997 John Wiley & Sons, Ltd.
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GROUND- AND EXCITED-STATE PROPERTIES OF SOME NAPHTHALENONES
695
Figure 7. Plot of charge-transfer emission frequency (␯ ) of compound 1 vs the solvents parameter (⌬f). The solid line is the best fit through
f
all frequencies except for the frequency in h-hexane (filled circle)
2
leads to an increase in polarity and, consequently,
intensification of the donor–acceptor interaction.
␧
Ϫ1 n Ϫ1
⌬
f=
Ϫ
(3)
2
2
␧Ϫ1 4n +2
CONCLUSIONS
In equation (2), ␯ and ␯ (0) are the fluorescence maxima
f
f
Ϫ1
A comparison of the electronic absorption and emission
properties of seven compounds has indicated that intra-
molecular charge-transfer (ICT) absorption and emission
are possible in the series in which the substituents are strong
electron donors. CT absorption and emission are dependent
on the solvent polarity, hydrogen bonding and the ionization
energy of the substituted-phenyl moieties. In general, CT
absorption and emission are enhanced and red shifted as the
solvent polarity and hydrogen-bonding abilities increase for
compounds possessing electron-donating substituents,
while the reverse is true for those with electron-withdrawing
substituents. The photoinduced charge separation stabilized
by solvent polarity gives a large excited-state dipole
moment for compound 1.
(
in cm ) in solution and the gas phase, respectively, ␮ is
e
the excited state’s dipole moment, a is the solvent cavity
radius and h and c have their usual meanings. The plot of ␯f
against ⌬f for compound 1 in solvents of different polarities
is shown in Figure 7, where the fluorescence maximum in n-
hexane is shown for comparison but not included in the
fitting. The magnitude of ␮ is obtained from the slope
e
Ϫ1
(
=18,182 cm ) of the best line fit through the data except
for n-hexane, and based on estimation of the value of a,
which was calculated using the three-dimensional molecular
modelling program CHChe (CAChe Scientific, Beaverton,
OR, USA). The program was used to calculate the inter-
atomic distances, from which the value of
a was
approximated as 40% of the long axis of the ellipsoid
38
around the molecule. The ␮e value determined for
compound 1 is 17 D. This large excited-state dipole moment
suggests that considerable charge transfer occurs from the
amino group donor to the carbonyl acceptor moieties, which
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