Substituent effects on the ground-state properties of naphthalene-based analogues of salicylideneaniline in solution

Article abstract of DOI:10.1246/bcsj.79.305

Substituent effects on the ground-state properties of seven naphthalene analogues of salicylideneaniline (1-7) were studied by comparison of both steady-state and transient absorption spectra. The naphthalene-based salicylideneanilines 1-5 were prepared from 2-hydroxy-1-naphthaldehyde and corresponding amines. The UV absorption spectra of 1-5 were largely dependent on the substituent group on the amino group. Transient absorption studies revealed that the introduced naphthyl group in 1-5 prolonged the lifetime of ¹K_trans in non-polar solvent probably due to the formation of intramolecular hydrogen bonding between nitrogen and C-H hydrogen on the naphthyl group. In acetonitrile, the lifetimes of 1-6 are much longer than those in benzene, probably because the zwitterionic forms are stabilized in polar solvents, whereas the much shorter lifetime in ethanol suggests that ¹K_trans undergoes the solvent assisted re-enolizaiton reaction in ethanol.

Full text of DOI:10.1246/bcsj.79.305

ꢀ
2006 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 79, No. 2, 305–311 (2006)
305
Substituent Effects on the Ground-State Properties of Naphthalene-
Based Analogues of Salicylideneaniline in Solution
ꢀ
Asuka Ohshima, Atsuya Momotake, and Tatsuo Arai
Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571
Received July 29, 2005; E-mail: arai@chem.tsukuba.ac.jp
Substituent effects on the ground-state properties of seven naphthalene analogues of salicylideneaniline (1–7) were
studied by comparison of both steady-state and transient absorption spectra. The naphthalene-based salicylideneanilines
1–5 were prepared from 2-hydroxy-1-naphthaldehyde and corresponding amines. The UV absorption spectra of 1–5
were largely dependent on the substituent group on the amino group. Transient absorption studies revealed that the in-
1
troduced naphthyl group in 1–5 prolonged the lifetime of Ktrans in non-polar solvent probably due to the formation of
intramolecular hydrogen bonding between nitrogen and C–H hydrogen on the naphthyl group. In acetonitrile, the life-
times of 1–6 are much longer than those in benzene, probably because the zwitterionic forms are stabilized in polar sol-
1
vents, whereas the much shorter lifetime in ethanol suggests that Ktrans undergoes the solvent assisted re-enolizaiton
reaction in ethanol.
Salicylideneaniline (SA), reported by Cohen et al. in 1960s,
is one of the typical molecules exhibiting excited-state intra-
in non-polar solvents, probably due to the formation of intra-
molecular hydrogen bonding between nitrogen and C–H hy-
drogen on the naphthyl group (Fig. 4).
1–3
molecular proton (or hydrogen-atom) transfer (ESIPT).
The potential energy diagram of the ESIPT reaction of SAs
has been proposed as depicted in Fig. 1. SA and its analogues
1
usually exist in their enol form ( E) in the ground state. On
irradiation of SAs, ESIPT takes place from the excited-state
1
ꢀ
1
ꢀ
enol form ( E ) to the excited-state cis-keto tautomer ( Kcis ),
which deactivates by 1) non-radiative decay, 2) emission of
large Stokes-shifted fluorescence, 3) intersystem crossing to
3
ꢀ
the excited triplet species ( Kcis ), or 4) isomerization to pro-
duce the trans-isomer in the ground state ( Ktrans). The meta-
stable Ktrans reverts back to the stable E. These deactivation
processes on the excitation of SAs have been studied both in
their crystal form and in solution.
1
1
1
4
–6
7–9
Substituent effects on the photochemistry of SA have been
studied in solution.10–16 Among many SAs, interesting features
of naphthalene-based analogues of SA derivatives have been
reported. When naphthalene is introduced in the A ring of SA
(Fig. 2), the compounds exhibit solvatochromism, thermochro-
mism, and photochromism, while naphthalene introduced in
17
the B ring of SA exhibits only photochromism. Despite their
interesting features, there are few reports on the structure–
property relationship of naphthalene-based SAs based on spec-
troscopic study in solution. In this paper, we explore the struc-
ture–property relationship of naphthalene-based SA analogues
and hydrogen-atom transfer in the ground state. In order to
investigate systematically, compounds 1–7 (Fig. 3) were pre-
pared. In 1–5, the naphthyl group is introduced in the A ring
of SA, whereas the naphthyl group is introduced in the B ring
in 6. In addition, intramolecular hydrogen bonding is blocked
in 7. Briefly, the introduction of substituents in the B ring con-
siderably influenced their absorption spectra in various sol-
vents, when naphthalene was substituted in the A ring. Tran-
sient absorption studies revealed that the introduction of naph-
Fig. 1. The schematic diagram of the excited-state intramo-
lecular hydrogen transfer (ESIPT) reaction of salicylidene-
aniline (SA) and its analogues: excitation of the enol pri-
1
1
Ã
mary structure ( E and E ) give excited keto tautomer
(1
Ã
Kcis ) by ESIPT, which undergoes intersystem crossing
3
Ã
to the excited triplet species ( Kcis ), photoisomerization
1
to the ground-state trans-isomer ( Ktrans), or decays to the
ground state ( Kcis) with fluorescence emission or non-
1
1
radiatively. The meta-stable K
1
trans
undergoes thermal iso-
1
cis
merization to K , which immediately reverts to E by
1
thyl groups in the A ring prolonged the lifetime of Ktrans
proton-transfer reaction in the ground state.
Published on the web February 10, 2006; DOI 10.1246/bcsj.79.305

3
06 Bull. Chem. Soc. Jpn. Vol. 79, No. 2 (2006)
Substituent Effects on Salicylideneanilines
Fig. 2. Structures of naphthyl analogues of SAs. Naphthalene is introduced in the A ring of SA 1, and in the B ring 2. Compound 1
exhibits photochromism, solvatochromism, and thermochromism, whereas 2 shows photochromism only.
Fig. 3. Chemical structures of naphthyl analogues of SAs 1–7.
Experimental
Materials. Compounds 1,17 2,17 4,18 and 519 have been report-
ed previously; however, no NMR data were shown in Ref. 18 for
Fig. 4. Hydrogen bonding between nitrogen and hydrogen
1
on naphthyl ring in Ktrans of 1–5.
4
and Ref. 19 for 5, respectively.
Typical Procedure: A mixture of a corresponding amine and
(1H, d, J ¼ 9:2 Hz, ArH), 7.63 (1H, d, J ¼ 8:4 Hz, ArH), 7.44–
7.21 (7H, m, ArH), 6.96 (1H, d, J ¼ 9:2 Hz, ArH), 4.83 (2H, s,
CH2Ar). Elemental Analysis. Anal. Calcd for C18H15NO: C,
82.73; H, 5.79; N, 5.36%. Found: C, 82.34; H, 5.78; N, 5.20%.
1-Butyliminomethylnaphthalen-2-ol (5) was prepared from
2-hydroxy-1-naphthaldehyde and n-butylamine in 82% yield.
aldehyde in ethanol was refluxed for 3 h. After evaporation, a
crude mixture was purified by recrystallization twice from ethanol
to give a pure product.
1-(Anthracen-2-yliminomethyl)naphthalen-2-ol (3) was prepar-
ed from 2-hydroxy-1-naphthaldehyde and 2-aminoanthracene in
1
1
7
2% yield: H NMR (CDCl3, 400 MHz, Me4Si) ꢀ 15.68 (1H, d,
H NMR (CDCl3, 400 MHz, Me4Si) ꢀ 14.42 (1H, s, OH), 8.67
J ¼ 4:4 Hz, OH), 9.53 (1H, d, J ¼ 4:0 Hz, CH=N), 8.46 (2H, d,
(1H, d, J ¼ 8:0 Hz, CH=N), 7.83 (1H, d, J ¼ 8:2 Hz, ArH), 7.67
(1H, d, J ¼ 9:2 Hz, ArH), 7.59 (1H, dd, J ¼ 6:8 and 1.2 Hz, ArH),
7.42 (1H, td, J ¼ 6:8 and 1.2 Hz, ArH), 7.22 (1H, t, J ¼ 8:2 Hz,
ArH), 6.90 (1H, d, J ¼ 9:2 Hz, ArH), 3.61 (2H, t, J ¼ 6:8 Hz,
CH2), 1.75 (2H, m, CH2), 1.47 (2H, m, CH2), 0.99 (3H, t, J ¼
7:6 Hz, CH3). Elemental Analysis. Anal. Calcd for C15H17NO:
C, 79.26; H, 7.54; N, 6.10%. Found: C, 79.22; H, 7.53; N, 6.16%.
(2-Methoxynaphthalen-1-ylmethylene)phenylamine (7) was
prepared from 2-methoxy-1-naphthaldehyde and aniline in 74%
J ¼ 3:2 Hz, ArH), 8.17 (1H, d, J ¼ 8:3 Hz, ArH), 8.11 (1H, d, J ¼
9
:1 Hz, ArH), 8.03–8.01 (2H, m, ArH), 7.92 (1H, d, J ¼ 2:0 Hz,
ArH), 7.82 (1H, d, J ¼ 9:2 Hz, ArH), 7.74 (1H, d, J ¼ 8:0 Hz,
ArH), 7.60–7.54 (2H, m, ArH), 7.51–7.49 (2H, m, ArH), 7.36
(
(
1H, t, J ¼ 8:0 Hz, ArH) 7.13 (1H, d, J ¼ 9:2 Hz, ArH). 13C NMR
CDCl3, 125 MHz, Me4Si) ꢀ 170.7, 154.0, 142.0, 136.9, 133.3,
132.4, 131.8, 131.7, 130.3, 130.2, 129,4, 128.3, 128.1, 128.0,
127.3, 126.5, 126.2, 126.0, 125.6, 123.6, 122.4, 119.8, 118.9,
117.2, 109.1. Elemental Analysis. Anal. Calcd for C25H17NO: C,
86.43; H, 4.93; N, 4.03%. Found: C, 86.15; H, 5.19; N, 3.95%.
1
yield. H NMR (CDCl3, 400 MHz, Me4Si) ꢀ 9.52 (1H, d, J ¼ 8:3
Hz, ArH), 9.25 (1H, s, CH=N), 7.92 (1H, d, J ¼ 9:1 Hz, ArH),
7.77 (1H, d, J ¼ 8:1 Hz, ArH), 7.57 (1H, m, ArH), 7.44–7.37 (3H,
m, ArH), 7.31–7.21 (4H, m, ArH), 3.97 (3H, s, Me). 1 C NMR
(CDCl3, 125 MHz, Me4Si) ꢀ 159.4, 158.6, 153.4, 133.7, 131.9,
129.2, 129.1, 129.0, 128.4, 128.2, 125.8, 125.5, 124.2, 121.0,
1-(Benzyliminomethyl)naphthalen-2-ol (4) was prepared from
-hydroxy-1-naphthaldehyde and benzylamine in 28% yield.
H NMR (CDCl3, 400 MHz, Me4Si) ꢀ 14.78 (1H, s, OH), 8.88
1H, d, J ¼ 7:6 Hz, CH=N), 7.87 (1H, d, J ¼ 8:0 Hz, ArH), 7.70
3
2
1
(

A. Ohshima et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 2 (2006)
307
1
17.1, 115.0, 112.5, 56.5. Elemental Analysis. Anal. Calcd for
C18H15NO: C, 82.73; H, 5.79; N, 5.36%. Found: C, 82.73; H,
.81; N, 5.41%.
Measurements. The H and C NMR spectra were measured
with Bruker ARX-400 (400 MHz for 1H NMR) and Bruker
5
1
13
1
3
AVANCE 500 (125 MHz for C NMR) spectrometers respective-
ly, in solutions of CDCl3 with tetramethylsilane as an internal
standard. The UV absorption and fluorescence spectra were re-
corded on a Shimadzu UV-1600 UV–visible spectrophotometer
and on a Hitachi F-4500 fluorescence spectrometer, respectively.
Laser flash photolysis was performed by using an excimer laser
(
Lambda Physik LPX-100, 308 nm, 20 ns fwhm) as the excita-
tion light source and a pulsed xenon arc (Ushio UXL-159) was
used as a monitoring light source. A photomultiplier (Hamamatsu
R-928) and a storage oscilloscope (Iwatsu TS-123) were used for
detection.
Results
Steady-State Absorption and Fluorescence Spectra.
Figure 5 shows the steady-state UV absorption spectra of the
series of compounds 1–7 in various solvents under argon at
room temperature. The similar absorption spectra of com-
pounds 1, 2, and 6 have been reported previously.17 Measure-
ments were performed using methylcyclohexane and benzene
as non-polar solvents, acetonitrile as a polar aprotic solvent,
and ethanol as a polar protic solvent. Compounds 1–5, where
naphthalene was introduced in the A ring, exhibited solvato-
chromism. Thus, the absorption maximum shifted to the longer
wavelength in the polar solvent and protic solvent. Compound
6
exhibited similar spectra to the parent compound SA in each
solvent (Fig. 5f). The absorption spectra of compound 7 are
almost the same in various solvents.
The fluorescence and fluorescence excitation (FLE) spectra
for 1–6 were observed at low temperature. Figure 6 shows the
fluorescence and fluorescence excitation spectra for 1–6 in
methylcyclohexane and in ethanol at 77 K. The similar fluores-
cence spectra of 1, 2, and 6 have been reported previously.17
The fluorescence excitation spectra for 1–5 in both solvents
were similar to the absorption spectra in ethanol at room tem-
perature (Figs. 6a–6e). The fluorescence and fluorescence
excitation spectra of 6 in both methylcyclohexane and in etha-
nol at 77 K (Fig. 6f) are similar to the absorption spectra at
room temperature.
Transient Absorption Spectra in Various Solvents. The
transient absorption spectra for 1 and 6 have already been
1
7
reported. Figure 7a shows the transient absorption spectra
ꢂ4
of 2 (1:0 ꢁ 10 M) in benzene under argon at room tempera-
ture. The decay curve of the transient species peaking at 460
nm was not affected by oxygen and therefore is ascribed to
1
Ktrans in the ground state. A weak peak at 550 nm was
Fig. 5. UV absorption spectra of 1–7 in various solvents at
room temperature under argon.
3
ꢀ
quenched by oxygen and was assigned to Kcis . In ethanol,
the transient absorption band for Ktrans of 2 could not be de-
1
tected (Fig. 7b). Compound 1 exhibited a similar solvent effect
to 2 on the transient absorption spectra. In the transient absorp-
under argon at room temperature. The band attributed to
1
Ktrans in the ground state was observed at 420 nm. The ab-
ꢂ4
1
tion spectra of 3 (1:0 ꢁ 10 M), only the shoulder of the tran-
sorption band for Ktrans of 4 in ethanol (Fig. 7e) appeared
in a different shape from that in benzene because 4 exists
1
sient absorption band for Ktrans was observed in benzene, ace-
tonitrile, and ethanol, because it overlapped with the band for
1
mainly as the K tautomer in ethanol and the absorption
cis
1
1
1
Kcis in each solvent (Fig. 7c). Therefore, the lifetimes of
Ktrans for 3 could not be determined. Figure 7d shows the
spectra of K is overlapped with that of Ktrans. The transient
cis
1
absorption spectra of 5 in each solvent were very similar to
those of 4, indicating that the effects of benzyl and the alkyl
transient absorption spectra of 4 (8:0 ꢁ 10ꢂ5 M) in benzene

3
08 Bull. Chem. Soc. Jpn. Vol. 79, No. 2 (2006)
Substituent Effects on Salicylideneanilines
5
, where naphthalene was introduced in the A ring, exhibited
solvatochromism. Thus, the solvatochromic property of naph-
thyl analogues of salicylideneaniline at the A ring was always
observed, regardless of the substituent at the B ring. The sol-
vatochromic behavior of those compounds is based on the
intramolecular hydrogen transfer in their ground states. Com-
1
pounds 1–5 mainly exist as their enol form ( E) in non-polar
1
solvents and cis-keto form ( Kcis) in polar solvents. A possible
reason for showing solvatochromism in 1–5 is that the de-
crease of aromaticity caused by tautomerization to Kcis is not
so large because the benzene ring is still present in the tauto-
mer form of the naphthalene ring in 1–5.10
Compound 6 exhibited similar absorption spectra to the
parent compound SA in each solvent (Fig. 5f). In this case, the
aromaticity is totally lost because a benzene ring instead of a
naphthalene ring is present at the hydrogen-bonding part. These
results give a clear indication that aromaticity in the A ring is
important to give solvatochromic behavior in SA derivatives.
At low temperature, the fluorescence spectra of 1–6 were
observed (Fig. 6). The peaks of the emission bands of 1–5
were largely dependent on the substituent groups in the B ring.
On the other hand, the peaks of the emission bands in methyl-
cyclohexane and in ethanol were not very different in each
compound. These results indicate that the energy gap between
the ground state and excited singlet state strongly depend on
the substituent group in the B ring, whereas the solvent polar-
ity does not strongly affect the energy gap. Compounds 1–5
1
exist as Kcis in the ground state regardless of the solvents
(methylcyclohexane or ethanol), which was revealed by fluo-
rescence excitation spectra (Figs. 6a–6e), indicating that com-
pounds 1–5 show thermochromism regardless of the substitu-
ents in the B ring.
Effects of Substituent in the B Ring on the Absorption
Spectra. Here, we focus on the effect of each substituent
group in the B ring on the absorption spectra. The substituent
effect on the absorption spectra of naphthalene-based SAs was
significant. Although there is no conjugation between A and B
Fig. 6. Fluorescence (FL) and fluorescence excitation
(
(
FLE) spectra of compounds 1–6 in methylcyclohexane
dotted line) and in ethanol (solid line).
1
rings in the drawing of Kcis in Fig. 1, the absorption spectra
1
for Kcis in polar solvents greatly changed upon the substituent
groups in the B ring. With increasing the aromaticity of the
Table 1. The Absorption Maxima of the Transient Absorption
1
Bands and Lifetimes for Ktrans of Compounds 1, 2, 4–6, and
SA in Various Solvents at Room Temperature under Argon
aromatic ring from phenyl in 1 to the 2-anthryl in 3, the
1
1
bands for both E and Kcis shifted to the longer wavelength
Figs. 5a–5c), which indicates that the aromaticity is preserved
(
In Benzene
In Acetonitrile
In Ethanol
throughout the whole range of molecules (1–3) in solution. On
the other hand, a blue-shift of the absorption spectra was ob-
served in compounds 4 and 5, having benzyl or alkyl groups
in the B ring. It is noted that the introduction of the naphthyl
group only in the B ring of SA did not affect the absorption
spectra as shown in Fig. 5g. Thus, one can say that the chro-
mophore in the B ring largely affects the steady-state absorp-
tion spectra when naphthalene is introduced in the A ring of
SA. Compound 7 did not show solvatochromism (Figs. 5g
and 5h) as expected. The relevant absorption data of 1–7 in
each solvent are shown in Table 1.
ꢁ
max/nm ꢂ/ms
ꢁ
max/nm
ꢂ/ms
1400
—
1800
1900
630
ꢁ
max/nm ꢂ/ms
1
2
4
5
6
SA
430
460
420
420
460
480
220
2300
760
490
28
450
—
440
440
460
480
480
—
190
—
13
63
55
—
440
440
470
—
40
1500
group on the transient species are not significant. The lifetimes
1
of the transient species ( Ktrans) of 1–6 in each solvent are
summarized in Table 1.
Substituent Effects on the Transient Absorption Spectra
1
of Ktrans. In ethanol, only depletion of the absorption spectra
was observed. Compound 2 predominantly exists as a tautomer
Discussion
1
1
Effects of Naphthyl Group on the A Ring on Solvato-
chromic Behaviors. As shown in Figs. 5a–5e, compounds 1–
Kcis in the ground state and the absorption band of Kcis
reaches 500 nm. If we compare the absorption spectra (Fig. 5b)

A. Ohshima et al.
Bull. Chem. Soc. Jpn. Vol. 79, No. 2 (2006)
309
Fig. 7. The transient absorption spectra (ꢁex ¼ 308 nm) of 2 in benzene (a), 2 in ethanol (b), 3 in benzene (c), 4 in benzene (d), and
in ethanol (e).
4
and the transient absorption spectra (Fig. 7b), we can estimate
1
that the absorption maximum for Ktrans of 2 comes around
1
4
50 nm. The absorption band for Ktrans comes at a shorter
1
wavelength than that for Kcis in 2.
On the other hand, the transient spectra peaking at 440 nm
was observed for 4, indicating that the absorption maximum
1
for Ktrans of 4 exists at around 440 nm. These results indicate
1
that the absorption spectra of Ktrans are not strongly affected
by substituents in the B ring of SA, whereas those of both
1
Fig. 8. Formation of zwitterion of Ktrans in acetonitrile.
1
1
E and Kcis are strongly influenced in the naphthalene-based
SAs 1–5.
Effects of Naphthyl Group in the A Ring on the Lifetime
nitrile or ethanol, which may lead to the similar lifetimes of
1
Ktrans among 1–6 and SA in these solvents. In acetonitrile,
1
1
of Transient Ktrans. As depicted in Fig. 1, the ground-state
the lifetimes of Ktrans for 1–6 were much longer than in ben-
zene. The keto forms Ktrans should be more polar than enol
1
1
1
Ktrans species is produced by excitation of E. The following
1
ꢀ
ESIPT takes place because in the excited singlet state, Kcis is
more stable than E . The photoisomerization of Kcis gives
the meta-stable Ktrans. The lifetimes of Ktrans for 1–6 and SA
in different solvents are summarized in Table 1. The lifetimes
for 1, 2, and 6 have been reported previously.17 The results
suggest the existence of hydrogen bonding between nitrogen
forms, which may result in longer lifetimes in polar solvents
than in non-polar solvents. In addition, the contribution of
20
the zwitterionic form is suggested in acetonitrile (Fig. 8).
On the other hand, the lifetimes in ethanol are much shorter
than in acetonitrile, or even in benzene for 1–5. The results
indicate that the re-enolization process is considerably promot-
ed by the intramolecular proton transfer assisted by the sol-
1
ꢀ
1
ꢀ
1
1
1
and C–H hydrogen atoms on the naphthalene ring in Ktrans
of 1–5 (Fig. 4). In benzene, the lifetimes of Ktrans for 1–5
1
21
1
0
vent to give E , which may immediately rotate back to the
1
were much longer than those of 6 and SA, which may be ex-
plained by the effect of the intramolecular hydrogen bonding
between nitrogen and CH hydrogen in the naphthalene ring.
This intramolecular hydrogen bonding is not present in aceto-
stable conformation of E (Fig. 9). Thus, the introduction of
a naphthyl group in the A ring of SA not only changed the
ground-state properties, but also influenced the properties of
transient species produced by photoinduced reaction.

3
10 Bull. Chem. Soc. Jpn. Vol. 79, No. 2 (2006)
Substituent Effects on Salicylideneanilines
Fig. 9. Possible mechanism of solvent assisted re-enolization reaction in ethanol.
Fig. 10. Comparison of the chemical properties of naphthyl analogues of SAs.
Conclusion
This work was supported by a Grant-in-Aid for Scientific
We described the substituent effects on the ground-state
Research on Priority Areas (417), a Grant-in-Aid for Scientific
Research (No. 16350005) and the 21st Century COE Program
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of the Japanese Government, by the Uni-
versity of Tsukuba Research Projects, Asahi Glass Foundation
and JSR Corporation.
properties of naphthalene analogues of salicylideneaniline by
comparison of both steady-state and transient absorption spec-
tra of the series of compounds 1–6 (Fig. 10). The generality of
the solvatochromic property of naphthyl analogues of SA has
been shown by the steady-state absorption spectra. The intro-
duction of substituents in the B ring considerably influenced
their absorption spectra in various solvents, when naphthalene
was substituted in the A ring. Transient absorption studies re-
vealed that the introduction of naphthyl groups in the A ring
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