Synthesis and properties of molecular switches based on the opening and closing of oxazine rings

Article abstract of DOI:10.1016/j.jphotochem.2011.11.008

We designed and synthesized a family of molecular switches each pairing an oxazine ring to a chromophoric fragment. Under the influence of either chemical or optical stimulations, the oxazine ring opens to bring the chromophoric appendage in conjugation w

Full text of DOI:10.1016/j.jphotochem.2011.11.008

Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 20–28
Contents lists available at SciVerse ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis and properties of molecular switches based on the opening and closing
of oxazine rings
Erhan Deniza, Janet Cusidoa, Subramani Swaminathana, Mutlu Battala, Stefania Impellizzeria,
Salvatore Sortinob,∗, Franc¸ isco M. Raymoa,∗
a Laboratory for Molecular Photonics, Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, FL 33146-0431, United States
b Laboratory of Photochemistry, Department of Drug Sciences, University of Catania, Viale Andrea Doria 6, I-95125 Catania, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
We designed and synthesized a family of molecular switches each pairing an oxazine ring to a chro-
mophoric fragment. Under the influence of either chemical or optical stimulations, the oxazine ring
opens to bring the chromophoric appendage in conjugation with either a 3H-indolium cation or a phe-
nolate anion. These structural transformations alter the electronic structure of the chromophore and, as
a result, its electrochemical and spectroscopic signatures. Specifically, we demonstrated that the absorp-
tion of triphenylamine and thiophene fragments, the fluorescence of a coumarin appendage and the
oxidation potential of a ferrocene center can all be switched with acid, base or ultraviolet inputs. Thus,
these operating principles and structural designs for switching properties at the molecular level with the
aid of external stimulations might eventually lead to a general strategy for the realization of chemo- and
photo-responsive materials.
Received 19 October 2011
Received in revised form
19 November 2011
Accepted 26 November 2011
Available online 8 December 2011
Keywords:
Fluorescence
Halochromism
Molecular switches
Oxazines
© 2011 Elsevier B.V. All rights reserved.
Photochromism
1. Introduction
stimulations [15]. Specifically, our molecules fuse 2H,3H-indole and
2H,4H-benzo[1,3]oxazine heterocycles in their molecular skele-
Organic molecules can be designed to undergo pronounced
structural changes in response to external stimulations [1]. In turn,
these switching events can be exploited to control motion at the
molecular level as well as to regulate the electrochemical and spec-
troscopic signatures of multicomponent assemblies [2]. Indeed,
molecular switches are becoming valuable building blocks for the
construction of a diversity of functional materials with controllable
properties. In particular, photochromic switches offer the opportu-
nity to implement such structural transformations reversibly under
the influence of light [3–8]. The photochemical events associated
with these compounds can be accompanied by significant changes
in absorption coefficients, dipole moments, fluorescence quantum
yields and redox potentials together with pronounced modifica-
tions in molecular shapes and dimensions. In fact, the ability to
photoregulate these parameters has already translated into the
realization of photoresponsive molecular and supramolecular con-
structs based on photochromic components [9–14].
tons. Upon ultraviolet illumination, the [C O] bond at the junction
of the two heterocycles cleaves on a subnanosecond timescale. The
photoinduced bond cleavage opens the oxazine ring and generates
a phenolate chromophore. As a result, this photochemical process
is accompanied by the appearance of an absorption band in the
visible region of the electromagnetic spectrum. The photogener-
ated isomer, however, reverts spontaneously to the original one
on a submicrosecond timescale. Indeed, these molecules can be
switched back and forth between their two interconvertible states
hundreds of times with no sign of degradation, even in the presence
of molecular oxygen.
In the course of investigating the photochemical and photophys-
ical properties of our photochromic compounds, we discovered that
their oxazine ring opens also under the influence of either acid or
base with the formation of either a 3H-indolium or a phenolate
chromophore respectively [15a,g]. As a result, these transfor-
mations are, once again, accompanied by significant changes in
absorption across the ultraviolet and visible regions of the elec-
tromagnetic spectrum. On the basis of these observations, we
envisaged the opportunity of exploiting the halochromic and pho-
tochromic character of our relatively simple switching subunit
to control the properties of an appended component. Specifi-
cally, we devised versatile synthetic strategies to attach covalently
an oxazine ring to a diversity of chromophoric groups with
the ultimate goal of being able to modulate their absorption,
In search of strategies to implement photochromic transforma-
tions with fast switching speeds, we developed a family of oxazines
able to open and close their heterocyclic core in response to optical
∗
Corresponding authors.
E-mail addresses: ssortino@unict.it (S. Sortino), fraymo@miami.edu
(F.M. Raymo).
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.11.008

E. Deniz et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 20–28
21
electrochemical and/or emission properties under the influence of
light, acid and/or base. In order to assess the generality of these
operating principles, we designed eight molecular switches all pair-
ing covalently an oxazine ring to an appropriate chromophore. In
this article, we report the synthesis of these molecules and of appro-
priate models together with the electrochemical, photochemical
and photophysical properties of these functional compounds.
2. Results and discussion
2.1. Design and synthesis
The photoinduced opening (from a to b in Fig. 1) of the oxazine
ring of our photochromic compounds generates a zwitterionic iso-
mer incorporating a phenolate anion and a 3H-indolium cation
Fig. 2. Synthesis of the molecular switches 1–6.
[15]. This transformation can be exploited to perturb the electronic
structure of a chromophoric appendage attached to either position
4 on the phenoxy fragment (R1 in Fig. 1) or the chiral center at the
junction of the two heterocycles (R2 in Fig. 1). Indeed, the devel-
oping charges on the phenolate and 3H-indolium fragments can
donate electrons to R1 and withdraw them from R2 respectively.
Thus, the photoinduced opening and thermal closing of the oxazine
ring can, in principle, be exploited to modulate the absorption,
emission and redox properties of a chromophore attached to either
one of these two positions. On the basis of these considerations and
the availability of appropriate precursors, we designed the molecu-
lar switches 1–8 (Fig. 1) and devised synthetic procedures for their
preparation in a single step from known starting materials with
yields ranging from 16% to 46%. In particular, we condensed the
preformed oxazine 9 with the aldehydes 10–15 (Fig. 2) in the pres-
ence of trifluoroacetic acid (TFA) to generate the target switches
1–6. Similarly, we condensed the preformed oxazine 16 (Fig. 3)
with the iodide salt of 17 and isolated the hexafluorophosphate
salt of the target switch 7, after counterion exchange. In the case
of 8, instead, we reacted the coumarin 18 (Fig. 4) with formalde-
hyde and hydrochloric acid first and then with the 3H-indole 19 to
produce the target switch.
In addition to the molecular switches 1–8, we also synthesized
the model compounds 20–24, 26 and 27 (Fig. 5). Specifically, we iso-
lated the hexafluorophosphate salts of these 3H-indolium cations in
yields ranging from 24% to 85%, after the condensation of the iodide
salt of 17 to the corresponding aldehydes and counterion exchange.
Compounds 20–24 have essentially the same chromophoric frag-
ment of the ring-open isomer (b in Fig. 1) of the molecular switches
1–5. Similarly, the model 26 and the conjugate base of the phenol
Fig. 1. The molecular switches 2–8 adopt preferentially a ring-closed form (a) in
solution, while 1 is exclusively in the ring-open state (b) under the same conditions.
The oxazines 2a, 3a and 5a switch reversibly to the corresponding zwitterionic iso-
mers 2b, 3b and 8b upon ultraviolet illumination. Instead, 4a and 6a–8a do not
undergo this photochemical transformation.

22
E. Deniz et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 20–28
Fig. 3. Synthesis of the molecular switch 7.
Fig. 6. Steady-state absorption spectra (50 ␮M, MeCN, 20 ◦C) of 1 before (a) and
after (b) the addition of TFA (5 equiv.) and of 20 (c).
27 resemble the chromophores incorporated within the ring-closed
and -open isomers (a and b in Fig. 1) of 7.
2.2. Steady-state spectroscopy
The steady-state absorption spectrum of 1 (a in Fig. 6), recorded
in acetonitrile at 20 ◦C, indicates that its ring-open isomer (b in
Fig. 1) is the predominant species in solution. Specifically, the spec-
trum shows the characteristic absorbance of the 4-nitrophenolate
chromophore [15a] of this isomer at 408 nm. This band shifts to
307 nm (b in Fig. 6) after the addition of acid and the formation
of a 4-nitrophenol chromophore (c in Fig. 7) [15g]. In addition,
both spectra show also an intense absorption at ca. 530 nm (ꢀAb
in Table 1). This band resembles that of the model compound 20
and corresponds to the 3H-indolium chromophore incorporated
within both the ring-open isomer (b in Fig. 1) and its protonated
form (c in Fig. 7). These observations demonstrate that the pyrrole
fragment of 1 locks this particular system in the ring-open form.
Presumably, the extended conjugation of the 3H-indolium cation,
possible only within the ring-open isomer, is mostly responsible
for the preferential population of this species over the ring-closed
one.
Fig. 4. Synthesis of the molecular switch 8.
In contrast to the behavior of 1, the spectra of 2–6 (a in Figs. 8 and
S1–S4) indicate that the ring-closed isomer (a in Fig. 1) is, instead,
the predominant species in solution, under otherwise identical
conditions. The corresponding spectra reveal bands for the chro-
mophore (R2 in Fig. 1) appended to the chiral center of the oxazine
ring at wavelengths ranging from 288 to 337 nm (Table 1). Upon
addition of acid, the oxazine ring opens and brings R2 in conjugation
with the adjacent 3H-indolium cation in the resulting protonated
species (c in Fig. 7). This transformation shifts bathochromically the
absorption band of R2 into the visible region (b in Figs. 8 and S1–S4).
In fact, the resulting absorptions resemble those of the model
3H-indolium cations 21–24 (c in Figs. 8 and S1–S3), which are cen-
tered at wavelengths ranging from 400 to 529 nm (Table 1). Thus,
the acid-induced opening of the oxazine rings of 2–6 alters the elec-
tronic structure of their chromophoric appendages and shifts their
Fig. 5. Model compounds 18 and 20–28.
Fig. 7. Opening of the oxazine ring with the addition of either acid or base.

E. Deniz et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 20–28
23
Table 1
Photochemical and photophysical parameters [a] of the ring-open isomer of 1, ring-closed isomers of 2–8 and their models 18 and 20–27.
ꢀAb [b] (nm)
ꢁP [c]
ꢂP [d] (␮s)
ꢀAb [b] (nm)
1b
2a
3a
4a
5a
6a
7a
8a
530
326
288
337
316
316
428
320
–
–
20
21
22
23
24
25
27 [e]
18 [e]
510
529
400
493
382
275
538
388
0.03
0.01
–
0.01
–
0.2
32
–
50
–
–
–
–
–
[a] All parameters were measured in MeCN at 20 ◦C. [b] The absorption wavelength (ꢀAb) was estimated from the spectra in Figs. 6, 8, 9 and S1–S5. [c] The quantum yield (ꢁP)
for the photochromic transformation was determined with a benzophenone standard, following a literature protocol (Ref. [15k]). [d] The lifetime (ꢂP) of the photogenerated
isomer was determined from the temporal absorbance evolutions in Figs. 10, S6 and S8. [e] The values of ꢀAb reported for 27 and 18 were measured in the presence of Et3N
(5 equiv.) and Bu4NOH (100 equiv.) respectively.
absorptions into the visible region. In the case of 2, 3 and 6, a similar
band, albeit significantly less intense, is also observed in the spectra
(a in Figs. 8, S1 and S4) recorded before the addition of acid. This
absorption indicates the co-existence in solution of a small fraction
of ring-open isomer, together with the predominant ring-closed
species, for these particular three switches.
appears at 470 nm (f in Fig. 9) upon excitation at 405 nm. Indeed,
the spectra of the model coumarin 28 (g in Fig. 9) and of the con-
jugate base of the model phenol 18 (h in Fig. 9) reveal that, while
the former is virtually not emissive under these excitation con-
ditions, the latter emits with a quantum yield of 0.99. Thus, the
base-induced opening of the oxazine ring of 8 alters the electronic
structure of the appended coumarin chromophore and switches its
fluorescence from off to on.
The absorption spectra of 7 and 8 (a in Figs. 9 and S5) are also
indicative of the predominant presence of the ring-closed isomer
in solution. In fact, they reveal bands at 428 and 320 nm respec-
tively (Table 1), which resemble those of the model compounds 26
and 28 (c in Fig. 9 and b in Fig. S5). Instead, there are no bands
in the range of wavelengths where the phenolate chromophores
of the ring-open isomers are expected to absorb, according to the
spectra of the conjugate bases of the model phenols 27 and 18 (d
in Fig. 9 and c in Fig. S5). Nonetheless, a weak absorption for the
phenolate chromophore can be detected at ca. 390 nm (b in Fig. 9),
after the addition of tetrabutylammonium hydroxide to 8. Indeed,
the oxazine ring of our compounds is known to open under these
conditions with the formation of the corresponding hemiaminal
(d in Fig. 7) [15a, 16]. In the case of 7, the nucleophilic hydroxide
anion attacks also the 3H-indolium cation appended to the phenoxy
fragment, in addition to opening the oxazine ring, and prevents
the detection of the absorption band for the extended phenolate
chromophore.
2.3. Time-resolved spectroscopy
The steady-state absorption spectra (a in Figs. 8, 9 and S1–S5)
of 2–8 show that these molecular switches adopt preferentially the
The steady-state emission spectra of 1–6 show negligible fluo-
rescence and do not change significantly after the addition of acid.
Similarly, also 7 and 8 (e in Fig. 9) are essentially not emissive. After
the addition of tetrabutylammonium hydroxide, however, a small
fraction of the ring-closed isomer of 8 is converted into the corre-
sponding hemiaminal (d in Fig. 7) and an intense emission band
Fig. 9. Steady-state absorption spectra (50 ␮M, MeCN, 20 ◦C) of 8 before (a) and
after (b) the addition of Bu4NOH (100 equiv.), of 28 (c) and of 18 (d) after the addi-
tion of Bu4NOH (100 equiv.). Steady-state emission spectra (50 ␮M, MeCN, 20 ◦C,
ꢀEx = 405 nm) of 8 before (e) and after (f) the addition of Bu4NOH (100 equiv.), of 28
(g) and of 18 (h) after the addition of Bu4NOH (100 equiv).
Fig. 8. Steady-state absorption spectra (50 ␮M, MeCN, 20 ◦C) of 2 before (a) and
after (b) the addition of TFA (5 equiv.) and of the hexafluorophosphate salt of 21 (c).

24
E. Deniz et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 20–28
Fig. 10. Time-resolved absorption spectrum (a) of a solution (18 ␮M, MeCN, 20 ◦C)
of 2, recorded 0.1 ␮s after pulsed illumination at 355 nm (15 mJ, 6 ns), and the subse-
quent absorbance evolution (b) at 550 nm with the corresponding monoexponential
fitting.
Fig. 11. Differential pulse voltammograms (1.5 mM, 0.1 M Bu4NPF6, MeCN, 20 ◦C, V
vs. Ag/AgCl, scan rate = 20 mV s−1, pulse amplitude = 50 mV) of 6 before (a) and after
(b) the addition of TFA (1 equiv.) and of 9 before (c) and after (d) the addition of TFA
(20 equiv).
crossing follows the excitation of 4 and prevents the opening
of its oxazine ring. By contrast, the spectral changes observed
upon irradiation of 6 are irreversible. Thus, this particular switch
appears to undergo significant degradation under illumination.
Presumably, intramolecular electron transfer pathways from the
ferrocene appendage to the excited 4-nitrophenoxy chromophore
are responsible for the photochemical decomposition of this com-
pound.
In contrast to the behavior of 2–5, the transient absorption spec-
tra of 7 and 8 do not reveal any significant change on a nanosecond
timescale upon illumination. The inability of their oxazine ring to
open upon excitation on this temporal scale is, presumably, a result
of the lack of a nitro group in position 4 of their phenoxy fragments.
Indeed, all the structural modifications explored at this particu-
lar position so far, besides the introduction of either a nitro or
a 4-nitrophenyl group, have translated in the suppression of the
photochemical character of the oxazine ring [15d,j].
ring-closed form (a in Fig. 1) in acetonitrile at ambient tempera-
ture. Upon illumination at 355 nm with a pulsed laser, however,
the oxazine ring of 2, 3 and 5 opens within the duration of the laser
pulse (6 ns) to generate the corresponding zwitterionic isomer (b
in Fig. 1) with a quantum yield (ꢁP in Table 1) ranging from 0.01
to 0.03. Consistently, the absorption spectra (a in Figs. 10, S6 and
S8), recorded 0.1 ␮s after excitation, reveal the appearance of bands
in the visible region. These bands resemble those observed in the
steady-state spectra (c in Figs. 8, S1 and S3) of the model compounds
21, 22 and 24 and correspond to ground-state absorptions of the
3H-indolium chromophores of the ring-open isomers of 2, 3 and 5.
The photogenerated isomers, however, revert spontaneously back
to the original species and, consistently, their transient absorptions
in the visible region decay monoexponentially (b in Figs. 10, S6
and S8). Curve fitting of the resulting temporal absorbance profiles
indicates the lifetime (ꢂP in Table 1) to range from 0.2 to 50 ␮s.
Interestingly, the lifetimes of the ring-open isomers of 3 and 5 are
one order of magnitude longer than that of 2. Presumably, their
thiophene appendages stabilize effectively the 3H-indolium cation
of the ring-open isomer, by extending its conjugation, and delay
the ring-closing kinetics.
The time-resolved absorption spectra (a in Figs. S7 and S9)
of 4 and 6, recorded under otherwise identical illumination con-
ditions, also reveal an absorbance increase in the visible region.
However, the resulting bands do not resemble those expected for
the 3H-indolium cations of the corresponding ring-open isomers
(b in Figs. S2 and S4). The one observed for 4 is, instead, similar to
the triplet–triplet absorption of bisthiophene [17] and its lifetime
shortens from 2.7 to 0.3 ␮s in the presence of molecular oxygen
(b and c in Fig. S7). These observations suggest that intersystem
2.4. Differential pulse voltammetry
The steady-state absorption spectrum of 6 (a in Fig. S4) reveals
intense bands for the ring-closed isomer and a weak absorption for
the ring-open species. In agreement with the co-existence of both
isomers in solution, the differential pulse voltammogram of 6 (a in
Fig. 11) shows two peaks of different intensities at +0.52 and +0.76 V
for the oxidation of the ferrocene fragments of the ring-closed and
-open species respectively. Consistently with this assignment, the
opening of the oxazine ring with acid (a and c in Fig. 7) results in
an increase in the intensity of peak at +0.76 V with a concomitant
decrease in that at +0.52 V (b in Fig. 11). Thus, the electrochemical

E. Deniz et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 20–28
25
response of the redox center appended to the switchable unit can,
indeed, be controlled by operating the latter with chemical inputs.
In addition to the two peaks for ferrocene oxidation, the dif-
ferential pulse voltammogram of 6 shows also a third peak at
+1.27 V (a in Fig. 11). This peak resembles that observed for the
model oxazine 9 (c in Fig. 11) and corresponds to the oxidation
of the 2H,3H-indole heterocycle. After the addition of acid and
the opening of the oxazine ring, this particular fragment becomes
positively charged, preventing the oxidation of the heterocyclic
fragment, and the peak disappears for both compounds (b and d
in Fig. 11). Thus, the redox response of the switchable unit itself
can also be controlled by opening the oxazine ring with chemical
stimulations.
analyzer under Ar, using a glassy-carbon working electrode (3 mm),
a platinum counter electrode and a Ag/AgCl reference electrode.
4.2. Synthesis of 1
A solution of 9 (100 mg, 0.3 mmol), 10 (151 mg, 1 mmol) and TFA
(0.5 mL, 7 mmol) in MeCN (10 mL) was heated under reflux for 24 h.
After cooling down to ambient temperature, the solvent was dis-
tilled off under reduced pressure and the residue was dissolved in
CH2Cl2 (5 mL). The addition of Et2O (50 mL) caused the precipita-
tion of a solid, which was filtered off, dissolved in CH2Cl2 (20 mL)
and washed with H2O (20 mL). The organic phase was dried over
Na2SO4 and the solvent was distilled off under reduced pressure to
give 1 (50 mg, 35%) as a purple solid. ESIMS: m/z = 444.2287 [M+H]+
(m/z calcd. for C27H30N3O3 = 444.2282); 1H NMR (CDCl3): ı = 1.10
(3H, t, 8 Hz), 1.73 (6H, s), 2.25 (3H, s), 2.42–2.48 (5H, m), 5.24 (2H,
s), 6.44 (1H, d, 9 Hz), 7.25–7.29 (2H, m), 7.38–7.43 (2H, m), 7.64 (1H,
d, 15 Hz), 7.77 (1H, s), 7.84–7.91 (2H, m).
3. Conclusions
A switchable oxazine can be condensed to a variety of formy-
lated chromophores in a single synthetic step. Either addition
of acid or ultraviolet illumination opens the oxazine ring of the
resulting constructs and brings the chromophoric fragment in con-
jugation with a 3H-indolium cation. This pronounced structural
transformation extends the conjugation of the chromophore to
shift its main absorption from the ultraviolet to the visible region
and, in the case of a ferrocene center, also the oxidation potential in
the positive direction. Alternatively, chromophoric fragments can
be attached to the phenylene ring fused to the oxazine heterocycle.
In this instance, the addition of base opens the oxazine ring and
brings the chromophoric appendage in conjugation with a pheno-
late anion. Once again, this pronounced modification in electronic
structure alters the absorption properties of the chromophore and,
in the case of a coumarin fluorophore, leads also to a significant
enhancement in emission intensity. Thus, this particular structural
design to switch properties at the molecular level, coupled to the
generality of the synthetic strategies, can facilitate the realization of
functional materials able to respond to chemical inputs and optical
stimulations.
4.3. Synthesis of 2
A solution of 9 (155 mg, 0.5 mmol), 11 (191 mg, 0.7 mmol) and
TFA (0.5 mL, 7 mmol) in EtOH (10 mL) was heated under reflux for
24 h. After cooling down to ambient temperature, the solvent was
distilled off under reduced pressure and the residue was dissolved
in CH2Cl2 (5 mL). The addition of Et2O (50 mL) caused the precipi-
tation of a solid, which was filtered off, dissolved in CH2Cl2 (20 mL)
and washed with H2O (20 mL). The organic phase was dried over
Na2SO4 and the solvent was distilled off under reduced pressure
to give 2 (119 mg, 42%) as a purple solid. ESIMS: m/z = 566.2445
[M+H]+ (m/z calcd. for C37H32N3O3 = 566.2438); 1H NMR (CDCl3):
ı = 1.41 (6H, bs), 4.60 (2H, s), 6.24 (1H, d, 16 Hz), 6.63 (1H, d,
8 Hz), 6.75 (1H, d, 16 Hz), 6.86–6.89 (2H, m), 7.00–7.15 (10H, m),
7.24–7.29 (6H, m), 7.96–8.02 (2H, m); 13C NMR (CDCl3): ı = 24.8,
41.2, 50.5, 109.2, 118.1, 119.7, 120.4, 122.7, 123.5, 123.6, 123.8,
124.5, 125.1, 125.5, 126.4, 126.7, 128.1, 128.2, 129.8, 130.0, 130.1,
131.7, 138.7, 140.9, 146.8, 147.6, 190.9.
4. Experimental procedures
4.4. Synthesis of 3
4.1. Materials and methods
A solution of 9 (300 mg, 1.0 mmol), 12 (89 ␮L, 1.0 mmol) and
TFA (100 ␮L, 1.4 mmol) in EtOH (10 mL) was heated under reflux
for 24 h. After cooling down to ambient temperature, the solvent
was distilled off under reduced pressure and the residue was dis-
solved in CH2Cl2 (5 mL). The addition of Et2O (50 mL) caused the
precipitation of a solid, which was filtered off, dissolved in CH2Cl2
(20 mL) and washed with H2O (20 mL). The organic phase was dried
over Na2SO4 and the solvent was distilled off under reduced pres-
sure to give 3 (100 mg, 26%) as a yellow solid. ESIMS: m/z = 405.1267
[M+H]+ (m/z calcd. for C23H21N2O3S = 405.1267); 1H NMR (CDCl3):
ı = 1.28 (3H, bs), 1.59 (3H, bs), 4.59 (2H, s), 6.64 (1H, dd, 3 and 8 Hz),
6.87–6.92 (3H, m), 6.96–7.02 (2H, m), 7.10–7.15 (2H, m), 7.23 (1H,
d, 5 Hz), 7.28 (1H, s), 7.97–8.01 (1H, m), 8.03 (1H, d, 3 Hz); 13C NMR
(CDCl3): ı = 19.1, 23.1, 26.9, 41.1, 50.6, 103.9, 109.2, 118.1, 120.4,
121.2, 122.7, 123.7, 124.5, 126.0, 127.9, 128.1, 129.6, 138.5, 141.1,
146.7, 159.5.
Chemicals were purchased from commercial sources and used
as received with the exception of MeCN, which was distilled over
CaH2. Compounds 9, 10, 11, 16–19 and 28 were prepared according
to literature procedures [15a,g, 18–22]. Reactions were monitored
by thin-layer chromatography, using aluminum sheets coated with
silica. Electrospray ionization mass spectra (ESIMS) were recorded
with a Bruker micrOTO-Q II spectrometer. Fast atom bombardment
mass spectra (FABMS) were recorded with a VG Mass Lab Trio-2
spectrometer in a 3-nitrobenzyl alcohol matrix. Nuclear magnetic
resonance (NMR) spectra were recorded with a Bruker Avance
400 spectrometer. Steady-state absorption spectra were recorded
with a Varian Cary 100 Bio spectrometer, using quartz cells with
a path length of 0.5 cm for 1–7 and their models and of 1.0 cm
for 8 and its models. Steady-state emission spectra were recorded
with a Varian Cary Eclipse spectrometer in aerated solutions.
Fluorescence quantum yields were determined with fluorescein
and rhodamine B standards, following a literature protocol [23].
Time-resolved absorption spectra were recorded with a Luzchem
Research mLFP-111 spectrometer in aerated solutions by illumi-
nating orthogonally the sample with a Continuum Surelite II-10
Nd:YAG pulsed laser. The quantum yields for the photochromic
transformations were determined with a benzophenone standard,
following a literature protocol [15k]. Differential pulse voltammo-
grams were recorded with a CH Instruments 660 electrochemical
4.5. Synthesis of 4
A solution of 9 (250 mg, 0.8 mmol), 13 (156 mg, 0.8 mmol) and
TFA (60 ␮L, 0.8 mmol) in EtOH (10 mL) was heated under reflux for
24 h. After cooling down to ambient temperature, the solvent was
distilled off under reduced pressure and the residue was dissolved
in CH2Cl2 (5 mL). The addition of Et2O (50 mL) caused the precipi-
tation of a solid, which was filtered off, dissolved in CH2Cl2 (20 mL)
and washed with H2O (20 mL). The organic phase was dried over

26
E. Deniz et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 20–28
Na2SO4 and the solvent was distilled off under reduced pressure
to give 4 (100 mg, 26%) as a purple solid. ESIMS: m/z = 487.1144
[M+H]+ (m/z calcd. for C27H23N2O3S2 = 487.1145); 1H NMR (CDCl3):
ı = 1.57 (6H, bs), 4.63 (2H, s), 6.19 (1H, d, 5 Hz), 6.63 (1H, d, 4 Hz),
6.88 (4H, t, 7 Hz), 7.02–7.05 (2H, m), 7.10–7.18 (3H, m), 7.22 (1H,
d, 4 Hz), 7.99 (1H, dd, 2 and 9 Hz), 8.03 (1H, s); 13C NMR (CDCl3):
ı = 18.9, 25.8, 52.3, 54.1, 110.9, 115.1, 116.9, 120.9, 123.3, 125.1,
126.4, 127.7, 129.8, 135.9, 139.2, 140.7, 141.9, 143.4, 146.8, 148.5,
164.0.
128.5, 128.9, 129.3, 129.6, 129.7, 130.9, 131.0, 136.6, 138.0, 142.6,
143.9, 147.7, 154.4, 159.2, 182.7, 205.2, 205.7, 206.1.
4.9. Synthesis of 8
An aqueous solution of formaldehyde (37%, 0.81 mL, 10 mmol)
was added dropwise to a suspension of 18 (880 mg, 5 mmol) in
HClconc (8 mL). The mixture was stirred at 40 ◦C for 4 h and, after
cooling down to ambient temperature, diluted with H2O (20 mL).
The resulting precipitate was filtered off, washed with H2O (20 mL)
dissolved in MeCN (20 mL) together with 19 (721 mg, 3 mmol). The
mixture was heated for 24 h under reflux. After cooling down to
ambient temperature, the solvent was distilled off under reduced
pressure and the residue was dissolved in CH2Cl2 (20 mL). The
resulting solution was washed with H2O (2 × 20 mL). The solvent
of the organic phase was distilled off under reduced pressure and
the residue was purified by column chromatography [SiO2: CH2Cl2]
to afford 8 (650 mg, 16%) as a white solid. ESIMS: m/z = 410.1751
[M+H]+ (m/z calcd. for C27H24NO3 = 410.1747); 1H NMR (CDCl3):
ı = 0.85 (3H, s), 1.61 (3H, s), 2.28 (3H, s), 4.51 (1H, d, 16 Hz), 4.99
(1H, d, 16 Hz), 6.05 (1H, s), 6.79 (2H, d, 8 Hz), 6.89 (1H, t, 8 Hz),
7.14–7.18 (2H, m), 7.26 (1H, d, 8 Hz), 7.32–7.41 (3H, m), 7.67 (2H,
bs); 13C NMR (CDCl3): ı = 18.8, 18.9, 19.1, 37.2, 104.8, 108.5, 109.1,
109.8, 111.8, 111.9, 113.4, 114.3, 120.5, 128.5, 128.5, 128.9, 129.1,
129.2, 136.5, 137.9, 147.7, 151.9, 153.4, 157.0, 161.4.
4.6. Synthesis of 5
A solution of 9 (180 mg, 0.6 mmol), 14 (65 mg, 0.6 mmol) and
TFA (0.29 mL, 6.5 mmol) in EtOH (10 mL) was heated under reflux
for 24 h. After cooling down to ambient temperature, the solvent
was distilled off under reduced pressure and the residue was dis-
solved in CH2Cl2 (5 mL). The addition of hexane (50 mL) caused
the precipitation of a solid, which was filtered off, dissolved in
CH2Cl2 (20 mL) and washed with H2O (20 mL). The organic phase
was dried over Na2SO4 and the solvent was distilled off under
reduced pressure. The residue was purified by preparative thin-
layer chromatography [SiO2: AcOEt/hexanes (3:7, v/v)] to give 5
(100 mg, 42%) as a yellow oil. ESIMS: m/z = 405.1266 [M+H]+ (m/z
calcd. for C23H
20N2O3S = 405.1267); 1H NMR (CDCl3): ı = 1.57 (6H,
s), 4.59 (2H, s), 5.31 (1H, s), 6.18 (1H, d, 16 Hz), 6.62 (1H, d, 7 Hz),
6.79–6.89 (3H, m), 7.12–7.14 (1H, t, 4 Hz), 7.23 (1H, bs), 7.25–7.27
(1H, bs), 7.30 (1H, d, 3 Hz), 7.98–8.02 (2H, m).
4.10. Synthesis of the hexafluorophosphate salt of 20
4.7. Synthesis of 6
A solution of 10 (200 mg, 0.9 mmol) and the iodide salt of 17
(240 mg, 0.8 mmol) in EtOH (10 mL) was heated under reflux for
24 h. After cooling down to ambient temperature, the solvent was
distilled off under reduced pressure and the residue was dissolved
in CH2Cl2 (5 mL). The addition of Et2O (30 mL) caused the precipita-
tion of a solid, which was filtered off and dissolved in Me2CO (5 mL).
After the addition of a saturated aqueous solution of NH4PF6 (5 mL),
the solution was concentrated under reduced pressure to half of
its original volume and the resulting precipitate was filtered off to
give the hexafluorophosphate salt of 20 (235 mg, 65%) as a purple
solid. FABMS: m/z = 307 [M−PF6]+ (m/z calcd. for C21H27N2 = 307);
1H NMR (CDCl3): ı = 1.08 (3H, t, 7 Hz), 1.71 (6H, s), 2.45 (3H, s), 2.42
(2H, q, 7 Hz), 2.64 (3H, s), 4.00 (3H, s), 7.17 (1H, d, 8 Hz), 7.30 (1H,
d, 9 Hz), 7.37–7.45 (2H, m), 7.50 (1H, d, 15 Hz), 7.66 (1H, d, 15 Hz),
12.47 (1H, bs); 13C NMR (CDCl3): ı = 10.2, 12.3, 15.0, 17.7, 28.6, 32.3,
50.0, 98.7, 11.8, 122.5, 126.7, 129.4, 129.8, 130.7, 135.1, 38.6, 141.3,
142.8, 150.1, 176.9.
A solution of 9 (150 mg, 0.5 mmol), 15 (104 mg, 0.5 mmol)
and TFA (0.24 mL, 6.5 mmol) in EtOH (10 mL) was heated under
reflux and Ar for 24 h. After cooling down to ambient temper-
ature, the solvent was distilled off under reduced pressure and
the residue was dissolved in CH2Cl2 (5 mL). The addition of hex-
anes (50 mL) caused the precipitation of a solid, which was filtered
off, dissolved in CH2Cl2 (20 mL) and washed with H2O (20 mL).
The organic phase was dried over Na2SO4 and the solvent was
distilled off under reduced pressure. The residue was dissolved
in EtOAc (10 mL) and filtered through a SiO2 plug. The plug was
washed with EtOAc (100 mL) and the solvent of the filtrate was
distilled off under reduced pressure to give 6 (100 mg, 40%) as
a dark-green solid. ESIMS: m/z = 507.1387 [M+H]+ (m/z calcd. for
C
29H26FeN2O3 = 507.1366); 1H NMR (CD3CN): ı = 1.38 (6H, bs), 3.95
(5H, s), 4.42 (2H, s), 4.46 (2H, s), 4.71 (2H, s), 5.46 (1H, s), 6.05 (1H,
d, 16 Hz), 6.70–7.20 (5H, m), 8.01 (1H, bs), 8.13 (1H, bs).
4.11. Synthesis of the hexafluorophosphate salt of 21
4.8. Synthesis of the hexafluorophosphate salt of 7
A solution of 11 (300 mg, 1 mmol) and the iodide salt of 17
(300 mg, 1 mmol) in EtOH (10 mL) was heated under reflux for 24 h.
After cooling down to ambient temperature, the solvent was dis-
tilled off under reduced pressure and the residue was dissolved in
CH2Cl2 (5 mL). The addition of Et2O (30 mL) caused the precipita-
tion of a solid, which was filtered off and dissolved in Me2CO (5 mL).
After the addition of a saturated aqueous solution of NH4PF6 (5 mL),
the solution was concentrated under reduced pressure to half of
its original volume and the resulting precipitate was filtered off to
give the hexafluorophosphate salt of 21 (184 mg, 32%) as a pur-
ple solid. ESIMS: m/z = 429 [M]+ (m/z calcd. for C31H29N2 = 429);
1H NMR (CDCl3): ı = 1.82 (6H, s), 4.35 (3H, s), 2.45 (3H, s), 7.02
(2H, d, 9 Hz), 7.20 (2H, d, 8 Hz), 7.24 (2H, t, 7 Hz), 7.38 (5H, t, 8 Hz),
7.46–7.56 (5H, m), 7.61 (1H, d, 16 Hz), 8.02 (2H, d, 9 Hz), 8.08 (1H,
d, 16 Hz); 13C NMR (CDCl3): ı = 27.3, 33.9, 52.2, 107.5, 114.1, 119.7,
122.8, 126.5, 126.7, 127.0, 129.2, 129.8, 130.3, 133.6, 142.0, 142.9,
145.6, 154.2, 154.7, 181.1.
A mixture of 16 (142 mg, 0.4 mmol) and the iodide salt of 17
(100 mg, 0.3 mmol) in EtOH (20 mL) was heated under reflux for
24 h. After cooling down to ambient temperature, the solvent was
distilled off under reduced pressure and the residue was dissolved
in CH2Cl2 (5 mL). The addition of Et2O (50 mL) caused the precip-
itation of a solid, which was filtered off, and dissolved in Me2CO
(5 mL). After the addition of a saturated aqueous solution of NH4PF6
(5 mL), the solution was concentrated under reduced pressure to
half of its original volume and the resulting precipitate was fil-
tered off to give the hexafluorophosphate salt of 7 (100 mg, 46%) as
an orange powder. ESIMS: m/z = 511.2760 [M−PF6]+ (m/z calcd. for
C
36H35N2O = 511.2744); 1H NMR (CDCl3): ı = 0.78 (3H, s), 1.56 (3H,
s), 1.71 (6H, s), 4.22 (3H, s), 4.53 (1H, d, 18 Hz), 5.02 (1H, d, 18 Hz),
6.72–6.82 (2H, m), 6.90 (1H, d, 8 Hz), 7.07 (2H, q, 8 Hz), 7.25–7.40
(3H, m), 7.44 (4H, s), 7.55–7.64 (5H, m), 8.06 (1H, d, 16 Hz), 8.50
(1H, s); 13C NMR (CDCl3): ı = 18.2, 25.7, 27.4, 34.2, 40.6, 49.8, 52.7,
105.5, 109.7, 110.6, 115.1, 119.0, 121.0, 122.7, 123.1, 127.6, 128.0,

E. Deniz et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 20–28
27
4.12. Synthesis of the hexafluorophosphate salt of 22
4.45 (3H, s), 7.07 (2H, d, 8 Hz), 7.54 (2H, s), 7.57 (2H, s), 7.78 (1H,
d, 16 Hz), 8.18 (1H, d, 16 Hz), 8.23 (2H, d, 8 Hz); 13C NMR (CDCl3):
ı = 25.8, 34.2, 52.8, 55.8, 110.5, 115.1, 115.5, 115.7, 123.2, 127.8,
129.6, 129.7, 133.4, 142.6, 143.9, 154.5, 165.1, 182.8.
A solution of 12 (77 ␮L, 0.8 mmol) and the iodide salt of 17
(250 mg, 0.8 mmol) in EtOH (10 mL) was heated under reflux for
24 h. After cooling down to ambient temperature, the solvent was
distilled off under reduced pressure and the residue was dissolved
in CH2Cl2 (5 mL). The addition of Et2O (30 mL) caused the precipita-
tion of a solid, which was filtered off and dissolved in Me2CO (5 mL).
After the addition of a saturated aqueous solution of NH4PF6 (5 mL),
the solution was concentrated under reduced pressure to half of
its original volume and the resulting precipitate was filtered off to
give the hexafluorophosphate salt of 22 (80 mg, 24%) as a brown
solid. FABMS: m/z = 268 [M−PF6]+ (m/z calcd. for C17H18NS = 268);
1H NMR (CDCl3): ı = 1.87 (6H, s), 4.39 (3H, s), 7.29 (1H, s), 7.40 (1H,
d, 16 Hz), 7.55–7.61 (4H, m), 7.82 (1H, d, 5 Hz), 8.34 (1H, s), 8.51
(1H, d, 16 Hz).
4.16. Synthesis of the hexafluorophosphate salt of 27
A solution of 4-hydroxybenzaldehyde (61 mg, 0.5 mmol) and the
iodide salt of 17 (100 mg, 0.3 mmol) in EtOH (20 mL) was heated
under reflux for 24 h. After cooling down to ambient temperature,
the solvent was distilled off under reduced pressure and the residue
was dissolved in CH2Cl2 (5 mL). The addition of Et2O (30 mL) caused
the precipitation of a solid, which was filtered off and dissolved in
Me2CO (5 mL). After the addition of a saturated aqueous solution
of NH4PF6 (5 mL), the solution was concentrated under reduced
pressure to half of its original volume and the resulting precipitate
was filtered off to give the hexafluorophosphate salt of 27 (110 mg,
78%) as a yellowish-orange solid. ESIMS: m/z = 278 [M−PF6]+ (m/z
calcd. for C19H20NO = 278); 1H NMR [(CD3)2CO]: ı = 1.85 (6H, s),
4.12 (3H, s), 6.98 (2H, d, 9 Hz), 7.44 (1H, d, 9 Hz), 7.61–7.66 (2H, m),
7.43–7.81 (2H, m), 8.00 (2H, d, 9 Hz), 8.39 (1H, d, 16 Hz); 13C NMR
[(CD3)2CO]: ı = 25.9, 34.1, 52.7, 109.7, 114.9, 117.0, 123.1, 126.9,
129.6, 133.8, 142.6, 143.8, 154.9, 163.7, 182.7, 205.5.
4.13. Synthesis of the hexafluorophosphate salt of 23
A solution of 13 (161 mg, 0.8 mmol) and the iodide salt of 17
(250 mg, 0.8 mmol) in EtOH (10 mL) was heated under reflux for
24 h. After cooling down to ambient temperature, the solvent was
distilled off under reduced pressure and the residue was dissolved
in CH2Cl2 (5 mL). The addition of Et2O (30 mL) caused the precipita-
tion of a solid, which was filtered off and dissolved in Me2CO (5 mL).
After the addition of a saturated aqueous solution of NH4PF6 (5 mL),
the solution was concentrated under reduced pressure to half of
its original volume and the resulting precipitate was filtered off to
give the hexafluorophosphate salt of 23 (100 mg, 25%) as a brown
solid. FABMS: m/z = 351 [M−PF6]+ (m/z calcd. for C21H20NS2 = 350);
1H NMR (CDCl3): ı = 1.86 (6H, s), 4.32 (3H, s), 7.10–7.13 (1H, m),
7.20–7.24 (1H, m), 7.34 (1H, d, 4 Hz), 7.42–7.46 (2H, m), 7.53–7.56
(4H, m), 8.37 (1H, d, 4 Hz), 8.53 (1H, d, 15 Hz).
Acknowledgments
FMR thanks the National Science Foundation (CAREER Award
CHE-0237578, CHE-0749840 and CHE-1049860) for supporting his
research program and for providing funds to purchase a mass
spectrometer (CHE-0946858). SS thanks the MIUR (PRIN 2008) for
financial support and the Royal Society of Chemistry for a travel
grant.
Appendix A. Supplementary data
4.14. Synthesis of the hexafluorophosphate salt of 24
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2011.11.008.
A solution of 14 (116 mg, 1.0 mmol) and the iodide salt of 17
(180 mg, 1.0 mmol) in EtOH (10 mL) was heated under reflux for
24 h. After cooling down to ambient temperature, the solvent was
distilled off under reduced pressure and the residue was dissolved
in CH2Cl2 (5 mL). The addition of Et2O (30 mL) caused the precip-
itation of a solid, which was filtered off and dissolved in Me2CO
(5 mL). After the addition of a saturated aqueous solution of NH4PF6
(5 mL), the solution was concentrated under reduced pressure to
half of its original volume and the resulting precipitate was filtered
off to give the hexafluorophosphate salt of 24 (100 mg, 36%) as a
brown solid. ESIMS: m/z = 268 [M]+ (m/z calcd. for C17H18NS = 268);
1H NMR (CDCl3): ı = 1.79 (6H, s), 4.35 (3H, s), 7.47–7.63 (6H, m),
7.88 (1H, s), 8.23 (1H, d, 17 Hz), 8.36 (1H, s); 13C NMR [(CD3)2CO]:
ı = 25.2, 34.1, 52.6, 112.4, 114.9, 122.8, 125.8, 128.6, 129.3, 129.6,
136.5, 138.8, 142.2, 143.6, 147.4, 183.0.
References
[1] B.L. Feringa, W.R. Browne (Eds.), Molecular Switches, Wiley-VCH, Weinheim,
2011.
[2] V. Balzani, M. Venturi, A. Credi, Molecular Devices and Machines: Concepts and
Perspectives for the Nanoworld, Wiley-VCH, Weinheim, 2008.
[3] G.H. Dorion, A.F. Wiebe, Photochromism, Focal Press, New York, 1970.
[4] G.H. Brown (Ed.), Photochromism, Wiley, New York, 1971.
[5] A.V. El’tsov (Ed.), Organic Photochromes, Consultants Bureau, New York, 1990.
[6] H. Bouas-Laurent, H. Dürr (Eds.), Photochromism: Molecules and Systems, Else-
vier, Amsterdam, 1990.
[7] J.C. Crano, R. Guglielmetti (Eds.), Organic Photochromic and Thermochromic
Compounds, Plenum Press, New York, 1999.
[8] M. Irie (Ed.), Chem. Rev. 100 (2000) 1683–1890.
[9] K. Matsuda, M.J. Irie, Photochem. Photobiol. C 5 (2004) 169–182.
[10] H. Tian, S.J. Yang, Chem. Soc. Rev. 33 (2004) 85–97.
[11] D. Gust, T.A. Moore, A.L. Moore, Chem. Commun. (2006) 1169–1178.
[12] H.D. Samachetty, N.R. Branda, Pure Appl. Chem. 78 (2006) 2351–2359.
[13] M.-M. Russew, S. Hecht, Adv. Mater. 22 (2010) 3348–3360.
[14] (a) F.M. Raymo, Angew. Chem. Int. Ed. 45 (2006) 5249–5251;
(b) F.M. Raymo, M. Tomasulo, Chem. Eur. J. 12 (2006) 3186–3193.
[15] (a) M. Tomasulo, S. Sortino, A.J.P. White, F.M. Raymo, J. Org. Chem. 70 (2005)
8180–8189;
4.15. Synthesis of the hexafluorophosphate salt of 26
A solution of 4-methoxybenzaldehyde (68 mg, 0.5 mmol) and
the iodide salt of 17 (100 mg, 0.3 mmol) in EtOH (20 mL) was heated
under reflux for 24 h. After cooling down to ambient temperature,
the solvent was distilled off under reduced pressure and the residue
was dissolved in CH2Cl2 (5 mL). The addition of Et2O (30 mL) caused
the precipitation of a solid, which was filtered off and dissolved in
Me2CO (5 mL). After the addition of a saturated aqueous solution of
NH4PF6 (5 mL), the solution was concentrated under reduced pres-
sure to half of its original volume and the resulting precipitate was
filtered off to give the hexafluorophosphate salt of 26 (120 mg, 83%)
as a yellow-orange solid. ESIMS: m/z = 292 [M−PF6]+ (m/z calcd. for
C20H22NO = 292); 1H NMR [(CD3)2CO]: ı = 1.85 (6H, s), 3.92 (3H, s),
(b) M. Tomasulo, S. Sortino, F.M. Raymo, Org. Lett. 7 (2005) 1109–1112;
(c) M. Tomasulo, F.M. Raymo, Org. Lett. 7 (2005) 4633–4636;
(d) M. Tomasulo, S. Sortino, A.J.P. White, F.M. Raymo, J. Org. Chem. 71 (2006)
744–753;
(e) M. Tomasulo, S. Sortino, F.M. Raymo, Asian Chem. Lett. 11 (2007) 219–222;
(f) M. Tomasulo, S. Sortino, F.M. Raymo, Adv. Mater. 20 (2008) 832–835;
(g) M. Tomasulo, S. Sortino, F.M. Raymo, J. Org. Chem. 73 (2008) 118–126;
(h) M. Tomasulo, S. Sortino, F.M. Raymo, J. Photochem. Photobiol. A 200 (2008)
44–49;
(i) M. Tomasulo, E. Deniz, T. Benelli, S. Sortino, F.M. Raymo, Adv. Funct. Mater.
19 (2009) 3956–3961;
(j) M. Åxman Petersen, E. Deniz, M. Brøndsted Nielsen, S. Sortino, F.M. Raymo,
Eur. J. Org. Chem. (2009) 4333–4339;

28
E. Deniz et al. / Journal of Photochemistry and Photobiology A: Chemistry 229 (2012) 20–28
(k) E. Deniz, M. Tomasulo, S. Sortino, F.M. Raymo, J. Phys. Chem. C 113 (2009)
8491–8497;
(l) M. Tomasulo, E. Deniz, S. Sortino, F.M. Raymo, Photochem. Photobiol. Sci. 9
(2010) 136–140;
[18] The synthesis of 9 was originally reported in:
(a) A.A. Shachkus, J. Degutis, A. Jezerskaite, in: J. Kovac, P. Zalupsky (Eds.),
Chemistry of Heterocyclic Compounds, vol. 35, Elsevier, Amsterdam, 1987, pp.
518–520;
(m) E. Deniz, S. Ray, M. Tomasulo, S. Impellizzeri, S. Sortino, F.M. Raymo, J. Phys.
Chem. A 114 (2010) 11567–11575;
(b) A.A. Shachkus, J. Degutis, A.G. Urbonavichyus, Khim. Geterotskil. Soed. 5
(1989) 672–676;
(n) E. Deniz, S. Sortino, F.M. Raymo, J. Phys. Chem. Lett. 1 (2010) 1690–1693;
(o) E. Deniz, S. Sortino, F.M. Raymo, J. Phys. Chem. Lett. 1 (2010) 3506–3509;
(p) E. Deniz, S. Impellizzeri, S. Sortino, F.M. Raymo, Can. J. Chem. 89 (2011)
110–116;
(q) E. Deniz, M. Tomasulo, J. Cusido, S. Sortino, F.M. Raymo, Langmuir 27 (2011)
11773–11783.
A.A. Shachkus, J. Degutis, A.G. Urbonavichyus, Chem. Heterocycl. Compd. (1989)
562–565.
[19] S.E. Bioadjiev, D.A. Lightner, Tetrahedron 55 (1999) 10871–10886.
[20] H. Tian, X. Yang, R. Chen, R. Zhang, A. Hagfeldt, L. Sun, J. Phys. Chem. C 112
(2008) 11023–11033.
[21] E. Hejchman, D. Maciejewska, I. Wolska, Monatsh. Chem. 139 (2008)
1337–1348.
[22] S. Cavar, F. Kovac, M. Maksimovic, Food Chem. 117 (2009) 135–142.
[23] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, Springer, New York,
2006.
[16] The absorbance at 390 nm (b in Fig. 9) together with the molar extinction coeffi-
cient of the model phenolate (d in Fig. 9) indicate that the addition of 100 equiv.
of tetrabutylammonium hydroxide converts ca. 3% of 8 into the corresponding
hemiaminal (d in Fig. 7).
[17] M. Montalti, A. Credi, L. Prodi, M.T. Gandolfi, Handbook of Photochemistry, CRC
Press, Boca Raton, 2006.