Hydantoin-based molecular photoswitches

Article abstract of DOI:10.1021/acs.joc.5b00244

A new family of molecular photoswitches based on arylidenehydantoins is described together with their synthesis and photochemical and photophysical studies. A series of hydantoin derivatives have been prepared as single isomers using simple and versatile chemistry in good yields. Our studies show that the photostationary states of these compounds can be easily controlled by means of external factors, such as the light source or filters. Moreover, the detailed investigations proved that these switches are efficient (i.e., they make efficient use of the light energy, are high fatigue resistant, and are very photostable). In some cases, the switches can be completely turned on/off, a desirable feature for specific applications. A series of theoretical calculations have also been carried out to understand the photoisomerization mechanism at the molecular level.

Full text of DOI:10.1021/acs.joc.5b00244

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Article
Hydantoin-based Molecular Photoswitches
David Martínez-López, Meng-Long Yu, Cristina García-Iriepa, Pedro José Campos,
Luis Manuel Frutos, James A. Golen, Sivappa Rasapalli, and Diego Sampedro
J. Org. Chem., Just Accepted Manuscript • Publication Date (Web): 25 Mar 2015
Downloaded from http://pubs.acs.org on March 27, 2015
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Hydantoinꢀbased Molecular Photoswitches
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David Martínez-López,a Meng-Long Yu,b Cristina García-Iriepa,a,c Pedro J. Campos,a
Luis Manuel Frutos,c James A. Golen, b Sivappa Rasapallib* and Diego Sampedroa*
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a) Departamento de Química, Universidad de La Rioja,
Centro de Investigación en Síntesis Química (CISQ), Madre de Dios, 51, 26006 Logroño, Spain.
b) Department of Chemistry and Biochemistry, University of Massachusetts Dartmouth,
North Darmouth, USA
c) Departamento de Química Analítica, Química Física e Ingeniería Química, Universidad de
Alcalá, 28871 Alcalá de Henares, Madrid, Spain
diego.sampedro@unirioja.es
srasapalli@umassd.edu
RECEIVED DATE (to be automatically inserted after your manuscript is accepted if required
according to the journal that you are submitting your paper to)
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Abstract
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A new family of molecular photoswitches based on arylidenehydantoins is described together with their
synthesis, photochemical and photophysical studies. A series of hydantoin derivatives have been
prepared as single isomers using a simple and versatile chemistry in good yields. Our studies show that
the photostationary states of these compounds can be easily controlled by means of external factors such
as the light source or filters. Moreover, the detailed investigations proved that these switches are
efficient i.e., they make efficient use of the light energy, high fatigue resistant and very photostable. In
some cases, the switches can be completely turned on / off, a desirable feature for specific applications.
A series of theoretical calculations have been also carried out in order to understand the
photoisomerization mechanism at the molecular level.
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1. Introduction.
In recent years, molecular switches and motors1,2 have been used in very different applications
with great success. Specifically, photoactive molecular switches have allowed to control diverse
properties from the shape of crystals3 to protein engineering,4 activity of biological systems5 or
orientation in liquidꢀcrystalline polymers.6 Modifying the structure of the switch and, thereby, the
properties of complex systems by light as the external stimulus has some advantages:1 1) selected
wavelengths can be used to minimize the damaging effect of light in the system; 2) a high degree of
spatial and temporal resolution can be obtained; 3) no waste products are obtained after the photon
absorption. In spite of the increasing number of promising applications of photoswitches, most of the
compounds employed to photocontrol the properties of complex systems share a common set of basic
chemical properties. The available molecular photoswitches can be classified in two main classes
depending on the photochemical process that allows the switch between the two isomeric forms. Type 1:
Compounds that can photoisomerize between closed and open forms. Among this type of photoswitches
the dimethyldihydropyrene (DHP)/cyclophanediene (CPD) system,7ꢀ10 diarylethenes11ꢀ13 and
spiropyranes14ꢀ17 have been developed. Type II: Compounds that can undergo the isomerization of a
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double bond as in the E/Z photochemical switches. Of the several families of E/Z photochemical
switches, azobenzenes18ꢀ20 have been extensively used due to their stability, adequate photophysical
properties, and the ease of synthesis and derivatization. Besides, overcrowded alkenes21 have been
applied as photoswitches to control diverse properties, but also as frameworks in the preparation of
molecular motors.22 Several other switches have been reported23 including retinalꢀbased switches,24ꢀ29
green fluorescence protein (GFP) analogues30,31 and hemithioindigos.32ꢀ38 Although numerous efforts
have been made in order to generalise the concept of molecular photoswitch at the theoretical level,39
the preparation and photochemical study of new families of switches seems critical for enabling more
complex and specific applications. Towards this direction, we have been intrigued by the possibility of
developing a new family of photoswitches based on 5ꢀarylidenehydantoins.
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5ꢀArylidenehydantoins (5ꢀAHTs) belong to a novel family of (GFP) analogues, where the
imidazoline unit is replaced by an imidazolinone moiety.23 The first synthesis of 5ꢀAHT, from the
reaction of phenylpropiolic acid and urea, was reported in 1899.40 Some years later, Wheeler and
Hoffman described a more effective method to prepare 5ꢀAHT from the condensation of hydantoin and
benzaldehyde in acidic solution.41 The applications of 5ꢀAHTs are mainly focused on their biological
activities. To date, multiple functions such as anticancer, antibiotic, and antagonistic properties, have
been reported.42 However, there are only scant reports about the study of their photochemical properties.
In this respect, it was demonstrated43 the application of 5ꢀAHT in the cosmetics industry as an excellent
UV absorbent. Due to the structural similarity of 5ꢀAHTs with the GFPꢀlike molecular photoswitches
previously reported,44 the capability of Z/E photoisomerization45ꢀ47 and their superb thermal and
photochemical stability, we decided to explore their behavior as molecular photoswitches. Accordingly,
we have carried out detailed investigations, and herein we report the synthesis, photochemistry and
theoretical study of a library of molecular photoswitches based on the 5ꢀarylidenehydantoin structure.
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O
NH4OAc,
ArCHO
AcOH
O
HN
NH
1
HN
NH
O
2
O
115 o
12h
C
Ar
3
1ꢀ5
4
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9
Base, MeI
O
Base, MeI
O
NH4OAc,
ArCHO
AcOH
O
O
N
N
Et3N
H+, heat
N
N
H
N
N
H
N
N
O
O
115 o
12h
C
Ar
HO
OH
O
O
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6ꢀ11
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Ph
Ar =
1
2
3
Ph
pꢀNO2Ph
oꢀHOPh
Ar =
pꢀMeOPh
pꢀNO2Ph
oꢀHOPh
4
10
11
N
H
N
H
5
Br
Br
Br
Br
OMe
OMe
Scheme 1. Synthesis of compounds 1-11.
2. Results and Discussion.
The new series of molecular photoswitches was synthesized according to Scheme 1. Thus, 5ꢀ
arylidenehydantoin derivatives 1ꢀ5 were synthesized from the condensation of hydantoin and different
aromatic aldehydes in acetic acid/ammonium acetate mixture in good yields (70ꢀ80%, see Supporting
Information for synthetic details), according to a previously reported method.48 The disubstitued 1,3ꢀ
dimethylꢀ5ꢀarylidenehydantoins 6ꢀ11 were obtained in two different ways. In the first method, via the 5ꢀ
arylidenehydatoins previously synthesized from aldehydes and hydantoin condensation as described
above, were subjected to bisꢀmethylation with excess of methyl iodide to produce the 1,3ꢀdimethylꢀ5ꢀ
arylidenehydandoin series. However, this method had limitations regarding to purification and the
presence of other potential nucleophilic functionalities. So, we resorted to an alternative method in order
to solve this problem by accessing the dialkylated precursor, i.e., 1,3ꢀdimethylhydantoin obtained either
via de novo synthesis or through bisꢀmethylation of hydantoin. Thus, treatment of glyoxal with N, N’ꢀ
dimethylurea, followed by pinacol rearrangement delivered clean formation of 1,3ꢀdimethylhydantoin.
The condensation of this compound with various aldehydes afforded the 1,3ꢀdimethylꢀ5ꢀ
arylidenehydandoins as final products. Under both reaction conditions, the E isomer was found to be the
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only isomer. In any case, the Z isomer was not detected by NMR. The configuration of the obtained
products was in turn confirmed by the Xꢀray diffraction data for 7 (Figure 1).
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Figure 1: Solid state structure of Eꢀ7 with thermal ellipsoids shown at 50% probability.
All the hydantoin based compounds synthesized share a typical UV absorption spectrum as
exemplified in Figure 2 by the spectrum of 7 in CH2Cl2 : MeOH (1:1) . In all the cases, a strong
absorption band between 320 and 370 nm was found for 1ꢀ11 (see Table 1). This is relevant as lowꢀ
energy wavelengths could be used to promote the photoisomerization which, in turn, confers a wider
applicability to these switches. Also, the solvent effect was investigated by recording the UV spectra in
acetonitrile, chloroform or DMSO. No significant changes were detected in the spectra for the tested
solvents. The effect of dimethylation of the hydantoin moiety has a minor effect in the absorption
spectra for these compounds. A small shift of 10ꢀ15 nm was found when comparing the 5ꢀ
arylidenehydantoins 1-5 with their equivalents 1,3ꢀdimethylꢀ5ꢀarylidenehydantoins 6ꢀ11. However, the
two series of compounds show different solubility features, as 1ꢀ5 are only soluble in CH2Cl2 : MeOH
mixtures and DMSO while 6ꢀ11 are very soluble in common organic solvents. Thus, different
experimental conditions are available for these switches while the absorption properties remain similar.
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1,0
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λ (nm
) ε
(cmꢀ1Mꢀ1)
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10115
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Wavelength (nm)
Figure 2. UVꢀVis spectrum of 7.
Irradiation of the compounds 1ꢀ11 was performed with a 400 W mediumꢀpressure Hg lamp
through a Pyrex filter (to avoid light with wavelength below 290 nm) in an immersion well or in a Pyrex
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NMR tube. The progress of the reaction was easily monitored by HꢀNMR and TLC as the Z and E
isomers for every compound show clearly different chemical shifts (specially the vinyl hydrogen) and Rf
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values. For instance, the vinyl hydrogen of 1 appears in HꢀNMR as two distinct singlets at 6.41 ppm
(E-1) and 6.32 ppm (Z-1, see Supporting Information). In the case of 5, these two signals appear at 5.59
ppm (E-5) and 6.33 ppm (Z-5). Thus, photoisomerization could be easily and quantitatively followed by
NMR. Irradiation was maintained until the photostationary state (PSS) was reached. Typically, this
required ca. 45 min in a Pyrex tube. The composition of the PSS under these conditions for compounds
1ꢀ11 is shown in Table 1.
Interestingly, no decomposition was observed by NMR in the course of the photoisomerization
even after several hours of continued irradiation once the PSS was reached. This is relevant as potential
applications of these switches that imply long periods of irradiation could be envisaged. Thus,
photostability is a valuable feature for any photoswitch. We have probed the stability of compounds 1ꢀ
11 further in two different ways. First, all the compounds were subjected to further irradiation for at
least 4 hours after the PSS was reached (ca. 45 min). In no case decomposition was observed by NMR.
It should be noted that during this time both isomers are continuously absorbing and reacting in a
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dynamic equilibrium. As a second measure of photostability and capability of performing several cycles
without decomposition, several photoisomerization cycles were performed for compounds 2 and 7 as
representative examples (Figure 3). For instance, a solution of E-7 was irradiated using light of 375 and
300 nm alternatively and the isomerization was followed by the absorbance at 333 nm (maximum
absorption wavelength for E-7). Both isomers could be interconverted for several cycles without
apparent decomposition. Therefore, those two experiments indicate the high photostability and fatigue
resistance of these switches which constitutes a requisite for practical applications.
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Table 1. Absorption maximum and photostationary state (PSS) for 1ꢀ11 at different irradiation
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3
wavelengths in CH2Cl2 : MeOH (1:1).
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[a]
Compound
PSS (E:Z)[b]
PSS(E:Z)[c]
λmax
317
353
340
367
321
309
333
339
326
373
316
1
2
100:0[d]
64:36
86:14
89:11
0:100
100:0[d]
5:95
95:5
53:47
48:52
22:78
19:81
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[a] Value in nm for E isomer [b] Pyrexꢀfiltered mediumꢀpressure Hg lamp. [c] Photoreactor (350 nm).
[d] Photoisomerization takes place in quartz, see text.
Figure 3. Photoisomerization cycles for a) 2 and b) 7.
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In addition, the photoisomerization kinetics was studied for E-7 using 1HꢀNMR to determine the
1
2
relative concentrations at short irradiation time intervals (Figure 4). From the first points, when the
composition of the mixture is mainly the E isomer and the absorption of the Z isomer is minimized, a
kinetic constant of 2.57 sꢀ1 was obtained. Thus we concluded that, under these conditions, a fast
photoisomerization takes place.
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Figure 4. Composition of the mixture during irradiation of E-7 at different time intervals.
As shown in Table 1, the PSS ratio is strongly dependent on the aryl substituent as different E:Z
ratios were obtained. The composition of the PSS is affected by the relative absorption of the isomers,
their relative stability and the topology of the potential energy surfaces. In turn, all these factors could
be different for every compound in Table 1. Also, the PSS depends on the quantum yields of the
forward (E to Z) and backward reactions (Z to E). In turn, these quantum yields are affected by the
relative absorption of both isomers and the shape of the potential energy surfaces. Thus, providing a
general quantitative rationale of the PSS is a complex issue. However, some qualitative indications of
the relevance of those factors could be obtained. As the relative absorption of the E and the Z isomers is
clearly dependent upon the irradiation source, a different PSS could be obtained for the switches under
different irradiation conditions (see below). In this case, it is relevant to check as to which isomer
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absorbs light preferentially at a specific light source or wavelength. Also, the topology of the potential
energy surface is relevant in order to determine the composition of the PSS. For these compounds, the
decay to the ground state after excited state relaxation implies a geometry twisted ca. 90º (see the
computational paragraphs below). However, this torsion angle is expected to be affected by the
electronic nature of the substituents as it has been found for other E / Z photoswitches.23 A complete
exploration of the effect of substitution (in both the absorption spectrum and the potential energy
surface topology) is beyond the scope of this paper. However, some conclusions can be derived from the
analysis of Table 1. For instance, in most cases the thermally stable isomer E is also the main isomer in
the PSS when a Pyrex filter is used (see 2ꢀ4, 8ꢀ9, Table 1). This is probably due to the slightly stronger
absorption of the E isomers under these irradiation conditions (see Supporting Information). A different
situation was found for 1, 6 and 11 as these compounds do not photoisomerize under the same
conditions. However, for all three compounds, photoisomerization can take place by simply removing
the filter and using a quartzꢀfiltered light. In these cases, the relative absorption of both isomers changes
when the irradiation source is modified. Under these different conditions, the PSS is 79:21 for 1, 61:39
for 6 and 54:46 for 11. Isolation of the photoproduct 1-Z by column chromatography allowed for the
recording of the UV spectrum (see Figure 5).
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Z-1
1,25
1,00
0,75
0,50
0,25
0,00
317
pyrex
330
225
250
275
300
325
350
375
400
425
450
Wavelength (nm)
Figure 5. UVꢀVis spectrum of E-1 and Z-1.
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As it can be seen, the photoisomer features a tail at longer wavelengths causing a higher
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2
absorption when a Pyrex filter is used. Thus, although the photoisomerization can take place, the back
reaction from the Z isomer to the thermally stable E isomer is faster under these conditions. However,
when a quartzꢀfiltered light is used, the whole band of the E isomer is included in the absorption
window and the competitive backꢀreaction is not that effective, allowing for the observed PSS. We
checked this effect by following the conversion of a pure E-1 sample until the PSS was reached using
1HꢀNMR (Figure 6).
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Figure 6. Composition of the mixture under irradiation of 1 using a) quartz and b) Pyrex filters.
As it can be seen, using a quartz filter, the irradiation continuously induces the
photoisomerization of 1 until a mixture of isomers in a 79:21 ratio is reached (Figure 6, a). Subsequent
irradiation of the same solution through a Pyrex filter leads to the starting situation of pure Eꢀisomer
(Figure 6, b). Alternative irradiation of the same sample of 1 using Pyrex and quartz as filters during
several cycles allowed the formation of different mixtures with Pyrex and quartz with no observable
decomposition (Figure 7). We anticipate that this stability would allow to easily alter the PSS by an
external agent as is the use of different types of filters. In addition, this result proves the high fatigue
resistance of the hydantoin based switches under relatively long irradiation times even using a highꢀ
energy irradiation source. The same behaviour was also found for 6 and 11.
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1
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Compound 1
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Quartz filter
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Pyrex filter
600
2
0
0
100
200
300
400
500
Time (minutes)
Figure 7. Photoisomerization cycles for 1.
Some relevant results were also found for 5 and 7. In these two cases, the Z isomer was not only
the main product in the PSS, but it was obtained almost quantitatively. This implies that starting from
the pure E-5 obtained directly from synthesis, only the photoisomer Z-5 was detected by NMR after its
irradiation. This excellent feature allows for a complete change in the switch position, that is, the switch
can be completely changed from the off state to the on state. Although this would be highly desirable for
photoswitches, such a complete change is not common at all. In fact, most systems reported in the
literature only allow the modification of the PSS but not a complete change in the switch state.2
In order to generalize the effect of the light source on the process, we also tested the
photoisomerization using a photoreactor with emission centred at 350 nm (14 lamps x 8W/lamp). For
this, we chose compounds 2ꢀ4 as they showed neat absorptions in the 350 nm region (see Supporting
Information, Figure S1), together with 1, 5 and 7 to check the generality of the effect. Results are shown
in Table 1. For 2ꢀ4, the use of this light source causes a different PSS enriched in the Z isomer (see
Figures S2ꢀS4). Again, this fact attests additional practical flexibility of these systems as the PSS can
also be modified easily with an external agent.
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Furthermore, the effect of monochromatic light in the photoisomerization process was studied.
1
2
For this study we chose 5 and 7, as the Z isomer was found almost exclusively in the PSS. The
comparative UV spectra for the E and Z isomers of 5 and 7 are shown in Figures S5 and S6. Regarding
5, the thermal E isomer featured a stronger absorption along the complete range of wavelengths. Thus,
its irradiation would lead in any case to the Z isomer as the main isomer in the PSS. However, in the
case of 7, both isomers showed a slight displacement between their absorption maxima. Although small,
this difference could be enough to displace the PSS. We tested this fact by irradiating at three different
wavelengths, namely 300, 333 and 375 nm. Results are shown in Table 2.
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7
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Table 2. Irradiation of 7 at selected wavelengths.
E isomer (%)
Z isomer (%)
λ (nm)
300
78
55
25
22
45
75
333
375
Hence, it can be concluded that small variations in the irradiation wavelength lead to significant
changes in the PSS. Specifically for 7, changing the irradiation wavelength from 300 nm to 375 nm may
change the main isomer in the PSS from the E isomer to the Z isomer. This fact shows the enormous
versatility of this new family of molecular switches which can be interchanged between both states with
ease. In order to further clarify the effect of monochromatic light, we followed the irradiation of fresh
samples of pure E-7 using light of varied wavelengths i.e., 300, 333 and 375 nm (Figure 8). As can be
seen, the PSS is different for each wavelength, 22%, 45% and 75% of the Z isomer, respectively (Table
2). Also, using light of 375 nm the PSS (75% in Z isomer) is reached faster than using light of 300 or
333 nm. It should be noted that the absorption at 333 nm is higher for both isomers (see Figure S6).
These facts can be explained by the relative absorption of both isomers at each wavelength. As both the
forward (E → Z) and reverse reaction (Z → E) take place the kinetics and the PSS are affected by the
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difference in absorption of both isomers. In this specific case, the absorption difference is bigger at 375
nm which explains the reported results.
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3
4
5
6
7
80
8
9
Irradiated at 375 nm
Irradiated at 333 nm
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60
Irradiated at 300 nm
40
20
0
0
100
200
300
Time (minutes)
Figure 8. Composition of the mixture when irradiating E-7 using monochromatic light.
Having established the feasibility of the photoisomerization, in order to determine the
proficiency of these compounds as molecular photoswitches, their relative efficiency in relation with
alternative deactivation pathways needed to be evaluated. A common competitive pathway with the
photoisomerization is the fluorescence emission. Thus, we checked the luminescent properties of 2, 7
and 8 as representative examples (see Figure 9 and Figures S7 and S8).
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7
8
9
370
429
λemis= 429 nm
λexc= 370 nm
1,0
0,8
0,6
0,4
0,2
0,0
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350
400
450
500
550
Wavelength (nm)
Figure 9. Excitation and emission spectra for 7.
A similar emission pattern was found for 2, 7 and 8, in all cases, a single emission band was
found. For 7, the emission is located at 429 nm, while for 2 and 8 the band is centred at ca. 530 nm. A
slightly bigger Stokes shift was found in the case of 8 (150 nm). However, in all cases, the emission
quantum yield was below the apparatus threshold which, in our case, allowed to set an upper limit of
Φem < 0.1%.49 Thus, we proved that the radiative deactivation of the excited state is not a competitive
pathway. To quantify the efficiency of the photoisomerization, we measured the reaction quantum yield
for 3 and 7 as representative examples. Using trans–azobenzene as actinometer50 and measuring the
conversion of the E into the Z isomer at short times we obtained a Φ of 15%±1% for irradiation at 334
nm for 3 and 16%±1% for 7. Although these values are not as high as some of the recently reported ,44
they do confirm that this new family of compounds is quite efficient and promising.
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Finally, the thermal reverseꢀreaction of the Z to E isomer was also measured for different compounds as
this could be relevant for the potential applications. While thermal stability of the photogenerated
isomer is usually desirable, for some applications a fast thermal backꢀreaction is required.51 Thus, we
studied the thermal isomerization of the Z isomer for 4, 5 and 7. A slow backꢀreaction (1 week) was
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found for 4 at 70ºC but not for 5 or 7. Increasing the temperature to 100ºC lead to faster isomerization
for 4 and some degree of isomerization was found for 7 (22 days), while no backꢀreaction was found
for 5. In this case, slow thermal degradation was found by NMR upon heating at 120ºC (two days).
Thus, in general, these compounds proved to be quite stable to the thermal back reaction. The different
time scales found can be attributed to the electronic influence of the substituents on the aryl moiety that
affect the transition structure of the isomerization.
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In order to understand the photophysics and photochemistry of the reported photoswitches, a
computational study using the MSꢀCASSCF//CASPT2 methodology52,53 was performed on 7, as a
representative example (see Supporting Information for Computational Details). First, the structures of
the E and Z isomers were optimized in the ground state at the MP2/6ꢀ31G* level of theory (Figure 10).
While the E isomer minimum is planar, the Z isomer is slightly twisted due to the steric hindrance
between the methyl and phenyl group. Indeed, the phenyl group is much more twisted ca. 47 ° (dihedral
C5 C4 C3 C2) compared to the E isomer.
a)
b)
Figure 10. Structure of the ground state minima for the E (a) and the Z (b) isomers of compound 7.
(C1 C2 C3 C4 dihedral is defined as ϕ).
The absorption spectrum for both isomers and the photoisomerization mechanism of the E isomer
(thermally stable) were calculated with the MSꢀCASPT2//SAꢀCASSCF methodology using a 6ꢀ31G*
basis set. For the absorption spectra simulation we chose six π orbitals (all the π backbone), the
corresponding π* orbitals and two n orbitals (one of the nitrogen atom and other one of the carbonyl
moiety closer to the chromophore). Then, the active space was reduced by removing those orbitals with
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almost 2.0 or 0.0 occupation numbers to calculate the minimum energy path, including 5 π orbitals, the
corresponding π* orbitals and one n orbital of the carbonyl moiety (see Supporting Information for
complete details), obtaining the same energetics as in the large active space.
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5
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7
8
9
Both isomers exhibited similar absorption spectra, as experimentally observed. The first excited state
(S1) is an optically bright one (see Tables 3) whereas the second (S2) and the third (S3) excited states are
dark states (see Tables 3). The electronic transition nature of S1 is described mainly by the excitation of
one electron from the bonding π to the antibonding π* orbital of the central C2=C3 bond, selective of
photoisomerization (see Figure S9). S2 and S4 are also described by an excitation (single and double,
respectively) from one bonding π to one antibonding π* orbital, whereas the S0→S3 electronic transition
is due to the monoꢀexcitation from an n orbital to an antibonding π* orbital. The effect of the solvent
was modelled by applying the polarizable continuum model (PCM) obtaining similar results as those
found for the gas phase (Table 3).
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Table 3. Experimental (chloroform), CASPT2 vertical transitions energies (gas phase and in
chloroform), electronic nature and oscillator strengths (f) for each transition of 7.
1
2
3
4
5
6
7
8
9
ꢇꢈꢉꢊꢊꢋ
ꢄꢁꢌ
Band
ꢀ
ꢀ
ꢁꢂꢃꢄꢅꢆ
ꢁꢂꢃꢄꢅꢆ
Isomer
State
S1
Transition
(π,π*)
f
Relative f
0.5609
eV (nm)
eV (nm) eV (nm)
3.72
(333)
4.64
3.69
0.4982
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(336)
4.65
S2
S3
S4
S1
S2
S3
S4
(π,π*)
(n,π*)
0.0009
0.0006
0.3885
0.4910
0.0209
0.0078
0.3995
0.0010
0.0007
0.4374
0.5342
0.0227
0.0085
0.4346
(267)
4.78
(266)
4.86
E
3.72 (333)
(259)
5.81
(255)
5.75
(π,π*)2, (π,π*)
(π,π*)
(213)
3.59
(215)
3.56
(345)
4.53
(347)
4.54
(π,π*)
(274)
4.74
(273)
4.84
Z
3.83 (324)
(n,π*)
(262)
6.00
(256)
5.94
(π,π*)2, (π,π*)
(207)
(209)
Finally, the photochemistry of the thermal stable E isomer was studied as detailed below. As per our
calculations, after excitation to the optically bright state, S1, from the Franck–Condon structure, the
system relaxes along the stretching coordinate in a first step, leading to a bond length alternation of the
single and double bonds (d1 from 1.347 Å to 1.432 Å and d2 from 1.459 Å to 1.402 Å, see Figure 10 for
d1 and d2 definition). Following the minimum energy path, the system relaxes on the first excited state
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rotating along the formal double bond ca. 90 degrees. At this point, the energy gap between S1 and S0 is
ca. 20 kcal/mol at the CASPT2 level of theory and the coupling between both states is large (Figure 11).
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Figure 11. MSꢀCASPT2 single point energy corrections along the relaxation path on S1 computed at
the CASSCF level of theory, as a function of the central double bond torsion coordinate (ϕ) for
compound 7. The structure of the final point is depicted.
From this point, the system can evolve in the ground state after radiative decay, recovering the E isomer
or completing the photoisomerization reaching the Z isomer (Figure 12).
Figure 12. MSꢀCASPT2 singleꢀpoint energy corrections along the minimum energy path on S0 from
S1 minimum energy structure for 7 computed at the CASSCF level of theory.
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1
2
3
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5
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7
8
9
Hence, the systems described herein comprise a novel family of efficient photoactive molecular
switches as it has been verified computationally for one of its derivatives, 7. Regarding the
photophysical properties, the bright state is the first excited state (S1). Thus, crossings with other excited
states that could deactivate the photoisomerization mechanism are avoided. Also, as no minimum was
found in the excited state, no emission in the UVꢀVis region is expected, confirming the experimental
findings. Furthermore, these studies also confirm that the solvent effect in the absorption spectra of
these systems is not large.
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3. Conclusions
The synthesis and detailed photochemical studies of a new family of hydantoin based molecular
photoswitches are presented. Although many applications of molecular switches have appeared in
literature, most of them employ azobenzenes as the photoactive core. This family has shown an
impressive potential once incorporated in complex systems, but the design of novel uses of switches
requires the availability of diverse molecules with different properties. In this contribution we have
established arylidineimidazolidineꢀ2,4ꢀdiones as a new family of photoswitches: i) these compounds
are very photostable, ii) in some cases, the switch can change completely between the on /off states, iii)
the PSS can be controlled by external factors as the irradiation source or the filters used, iv) the light
energy is used with high efficiency for the photoisomerization as no alternative pathways are available
and v) easy to synthesize and amenable to scale up . In accordance, the mechanistic details obtained
from the theoretical calculations allow to explain the experimental findings. Some of the future studies
would be associated with modification of these compounds for specific applications. For example,
biological applications of these photoswitches limited due to the absorption maxima in the UV region
could be mitigated by affecting the absorption band through substitution, increasing the conjugation,
including charges in the structure or through sensitized irradiation following already successful
strategies for other switches.1,2,23 Also, optical applications could be facilitated by affecting the gap
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The Journal of Organic Chemistry
between the absorption bands for E and Z isomers and altering the PSS by careful choice of the
wavelength of irradiation. In conclusion, arylidineimidazolidineꢀ2,4ꢀdiones family consists of a
significant extension of the molecular photoswitch concept. Further studies to include these molecules
into complex systems are currently underway along with the design of new systems including 2ꢀ
aminoimidazolones and pyrazolones.
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4. Experimental Section
General Information. All reactions were carried out in flameꢀdried glassware. Anhydrous solvents
were purchased and used directly. Thinꢀlayer chromatography (TLC) was carried out on precoated glass
1
13
silica gel plates which were visualized with both ultraviolet light and stained with PMA. H and C
NMR spectra were recorded on 400 MHz spectrometer. Chemical shifts are reported in ppm with
reference to tetramethylsilane [1HꢀNMR: CDCl3 (0.00 δ)] or solvent signals [1HꢀNMR:CDCl3 (7.26 δ);
13CꢀNMR: CDCl3 (77.27 δ)]. Signal patterns are indicated as br (broad peak); s (singlet); d (doublet); t
(triplet); q (quartet); m (multiplet). Infrared spectral data were obtained using a spectrometer with
diamond ATR accessory as thin film. HRMS (TOF) data was obtained using ESI mode.
Synthesis of arylideneimidazolidine-2,4-diones: A series of arylideneimidazolidineꢀ2,4ꢀdiones were
prepared by following one of the two general methods described below.
Method 1:
Step 1: A solution of imidazolidineꢀ2,4ꢀdione (2.16 g, 20 mmol) and ammonium acetate (276 mg, 3.6
mmol) in acetic acid (5 mL) was added to aldehyde (20 mmol) and the resulting mixture was stirred at
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120 °C for 16 hours. After cooling, the reaction mixture was concentrated in vacuum and purified by
column chromatography on silica gel with EtOAcꢀhexane to afford the 5ꢀarylideneimidazolidineꢀ2,4ꢀ
diones (with yields ranging from 70 to 90%).
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7
8
9
Step 2: To a solution of 5ꢀarylideneimidazolidineꢀ2,4ꢀdione (20 mmol, 2.0 equiv) in dry DMF (50 mL)
was added K2CO3 (8.3 g, 60 mmol, 3.0 equiv) and methyl iodide (8.51 g, 60 mmol, 3 equiv). The
resulting mixture was stirred for 12 h at 60 °C. The reaction was quenched by the addition of water. The
resulting mixture was diluted with EtOAc (250 mL), and washed with water (50 ml x 3) and brine (100
ml). The organic layer was dried over anhydrous Na2SO4, filtered and concentrated. The resulting crude
residue was purified by column chromatography on silica gel to give 5ꢀarylidine 1,3ꢀ
dimethylimidazolidineꢀ2,4ꢀdiones in 50 to 60% yields.
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Method 2:
Step 1:
A 1000 ml glass roundꢀbottom flask equipped with a stirrer, thermometer and addition funnel was
charged with 290 g (2.0 mol) of 40 wt % aqueous glyoxal solution and triethylamine ( pH of the
reaction mixture represented 9). Then, while maintaining the liquid temperature to 25 to 35 °C, a
solution 1,3ꢀdimethylurea (176 g (2.0 mol in 176 ml of water) was gradually added to the mixture. After
addition was completed, the mixture was allowed to stir at the same temperature for 3 hours. After the
completion of the reaction as monitored by TLC, the solvent was removed under reduced pressure on
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rotavap to obtain 297 g of crude 4,5ꢀdihydroxyꢀ1,3ꢀdimethylꢀ2ꢀimidazolidinone as a colorless oil which
was taken further for step 2 without further purification.
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7
8
Step 2
9
A 2000 ml glass roundꢀbottomflask equipped with a stirrer, thermometer and a reflux condenser was
loaded with 297 g of crude 4,5ꢀdihydroxyꢀ1,3ꢀdimethylꢀ2ꢀimidazolidinone prepared from the previous
step, H2O (600 ml) and 98% sulfuric acid (54 g , 0.54 mol) of and the resulting mixture was stirred for 6
hours at 95°C to 100 °C. After completion of the reaction, the reaction mixture was cooled with an ice
bath, and 90 g of sodium bicarbonate was added slowly. The crude residue was diluted with EtOAc/H2O
(500 ml/200 ml). The aqueous layer was separated and extracted with EtOAc (100 ml x 3). The organic
layers were combined and washed with brine (200 ml), dried over anhydrous Na2SO4, filtered and
concentrated. The crude residue was purified by column chromatography on silica gel with EtOAcꢀ
hexane (20/80 to 35/65) to obtain 1,3ꢀdimethylimidazolidinꢀ2,4ꢀdione as a colorless oil (161 g, 63%
yield for two steps).
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13
1H-NMR (400 MHz): 3.76 (2H, s), 2.87 (3H, s), 2.87 (3H, s); C-NMR (100 MHz): 170.0, 157.0,
51.6, 29.5, 24.7; IR: 3444, 2949, 2358, 1760, 1699, 1493, 1419, 1388, 1272, 1248, 1110, 1077, 10005,
756, 599; HR-MS (ESI): calcd for C5H9N2O2 [M+H]+ 129.0664, found 129.0689.
Step 3: A solution of 1,3ꢀdimethylimidazolidinꢀ2,4ꢀdione (2.56 g, 20 mmol) and ammonium acetate
(276 mg, 3.6 mmol) in acetic acid (5 mL) was added to aldehyde (20 mmol) and the resulting mixture
was stirred at 115 °C for 16 hours. After cooling, the mixture was concentrated in vacuum and purified
either by recrystallization or by column chromatography on silica gel with EtOAcꢀhexane to afford the
5ꢀarylideneimidazolidineꢀ2,4ꢀdiones (with yields ranging from 70 to 90%).
Spectral data for various (E)ꢀ5ꢀArylideneꢀ1,3ꢀdimethylimidazolidineꢀ2,4ꢀdiones obtained via the above
methods are given below.
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(E)-5-(benzylidene)imidazolidine-2,4-dione (E-1):
1
2
3
1
Yield = 2.63 g, 70 % H-NMR (300 MHz, DMSOꢀd6) δ 7.61 (d, J = 7.5 Hz, 2H), 7.40 (t, J = 7.4 Hz,
4
5
6
13
2H), 7.35 – 7.29 (m, 1H), 6.41 (s, 1H). C-NMR (300 MHz, DMSO) δ 165.6 (s), 155.7 (s), 133.0 (s),
7
8
9
129.4 (s), 128.8 (s), 128.4 (s), 128.0 (s), 108.3 (s). UV-Vis (CH2Cl2 : MeOH): λ (nm) = 316 (ε=20471
Mꢀ1cmꢀ1), 331 (ε= 16123 Mꢀ1cmꢀ1). EM-ES (+): calcd for C10H8N2O2 [M+ H]+ 189.0664, found
189.0671. Observations: White solid.
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(Z)-5-(benzylidene)imidazolidine-2,4-dione (Z-1):
1H-NMR (300 MHz, DMSOꢀd6) ) δ ppm 7.89 (d, J = 7.3 Hz, 2H), 7.38 – 7.28 (m, 3H), 6.32 (s, 1H).
13C-NMR (300 MHz, CDCl3) δ 163.6 (s), 153.9 (s), 133.1 (s), 129.9 (s), 128.1 (m), 115.6 (s). UV-Vis
(CH2Cl2 : MeOH): λ (nm) = 330 (ε=13309 Mꢀ1cmꢀ1). EM-ES (+): calcd for C10H8N2O2 [M+ H]+
189.0664, found 189.0671. Observations: White solid.
(E)-5-(4-nitrobenzylidene)imidazolidine-2,4-dione (E-2):
1
Yield = 3.63 g, 78 %; H-NMR: (400 MHz, DMSOꢀd6) δ ppm 8.19 (d, J = 8.4 Hz, 2H), 7.84 (d, J = 8.6
Hz, 2H), 6.49 (s, 1H). 13C-NMR: (100 MHz, DMSOꢀd6) δ ppm 165.3 (s), 155.8 (s), 146.2 (s), 140.0 (s),
130.9 (s), 130.1 (s), 123.7 (s), 105.1 (s). UV-Vis (CH2Cl2 : MeOH (1:1)): λ (nm) = 236 (ε=8188 Mꢀ1cmꢀ
1), 353 (ε= 15854 Mꢀ1cmꢀ1) EM-ES (+): calcd for C10H8N3O4 [M+ H]+ 234.0515, found 234.0515.
Observations: Brown solid.
(E)-5-(2-hydroxybenzylidene)imidazolidine-2,4-dione (E-3):
Yield = 3.06 g, 75 % ;1H-NMR: (400 MHz, DMSOꢀd6) δ ppm 7.53 (d, J = 7.7 Hz, 1H), 7.15 (t, J = 7.9
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Hz, 1H), 6.90 – 6.77 (m, 2H), 6.67 (s, 1H). C-NMR: (100 MHz, DMSOꢀd6) δ ppm 165.7 (s), 156.0
(s), 155.5 (s), 130.0 (s), 129.5 (s), 127.2 (s), 120.1 (s), 119.4 (s), 115.6 (s), 104.0 (s). UV-Vis (CH2Cl2 :
MeOH (1:1)): λ (nm) = 235 (ε=9800 Mꢀ1cmꢀ1), 312 (ε= 13200 Mꢀ1cmꢀ1), 340 (ε = 15200 Mꢀ1cmꢀ1). EM-
ES (+): calcd for C10H8N2O3 [M+ H]+ 205.0613, found 205.0615. Observations: White solid.
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(E)-5-((1H-indol-3-yl)methylene)imidazolidine-2,4-dione (E-4):
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Yield = 3.35 g, 74 % ; H-NMR: (400 MHz, DMSOꢀd6) δ ppm 8.13 (s, 1H), 7.75 (d, J = 7.7 Hz, 1H),
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7.42 (d, J = 7.7 Hz, 1H), 7.21ꢀ7.09 (m, 2H), 6.75 (s, 1H). C-NMR: (100 MHz, DMSO) δ ppm 165.7
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(s), 155.7 (s), 136.0 (s), 127.0 (s), 126.8(s), 123.8 (s), 122.6 (s), 120.4 (s), 118.2 (s), 112.1 (s), 108.6 (s),
102.1 (s). UV-Vis (CH2Cl2 : MeOH (1:1)): λ (nm) =275 (ε=3602 Mꢀ1cmꢀ1), 367 (ε=11835 Mꢀ1cmꢀ1)
EM-ES (+): calcd for C12H9N3O2 [M+ H]+ 228. 0773, found 228.0779. Observations: Orange solid.
(E)-5-(3,5-dibromo-4-methoxybenzylidene)imidazolidine-2,4-dione (E-5):
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Yield = 5.71 g, 77 % H-NMR: (300 MHz, DMSOꢀd6) δ ppm 8.09 (s, 2H), 5.59 (s, 1H), 3.76 (s, 3H).
13C-NMR: (100 MHz, DMSOꢀd6) δ ppm 165.4 (s), 155.9 (s), 152.9 (s), 133.0 (s), 132.4 (s), 129.3 (s),
117.9 (s), 104.6 (s), 60.5 (s). UV-Vis (CH2Cl2 : MeOH (1:1)): λ (nm) =321 (ε= 22605 Mꢀ1cmꢀ1). EM-ES
(+): calcd for C11H8Br2N2O3 [M+ H]+ 374.8980, found 374.8974. Observations: Brown solid.
(Z)-5-(3,5-dibromo-4-methoxybenzylidene)imidazolidine-2,4-dione (Z-5):
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1H-NMR: (400 MHz, DMSOꢀd6) δ ppm 7.91 (s, 2H), 6.33 (s, 1H), 3.81 (s, 3H). C-NMR: (100 MHz,
DMSOꢀd6) δ ppm 165.20 (s), 155.7 (s), 153.0 (s), 133.0 (s), 132.1 (s) 129.1 (s), 117.9 (s), 104.5 (s), 60.5
(s). UV-Vis (CH2Cl2 : MeOH (1:1))): λ (nm) =328 (ε= 14942 Mꢀ1cmꢀ1), 342 (ε= 5747 Mꢀ1cmꢀ1). EM-ES
(+): calcd for C11H8Br2N2O3 [M+ H]+ 374.8980, found 374.8974. Observations: Brown solid.
(E)-5-(benzylidene)-1,3-dimethylimidazolidine-2,4-dione (E-6):
Yield = 3.51 g, 81 % H NMR (300 MHz, CDCl3) δ ppm 7.41ꢀ7.28 (m, 5H), 6.95 (s, 1H), 3.14 (s, 3H),
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2.95 (s, 3H). C NMR (300 MHz, CDCl3) δ 163.7 (s), 155.8 (s), 132.6 (s), 129.6 (s), 129.4 (s), 128.3
(s), 128.2 (s), 112.2 (s), 30.5 (s), 24,9 (s). UV-Vis (CHCl3): λ (nm) =240 (ε= 7904 Mꢀ1cmꢀ1), 309 (ε=
12316 Mꢀ1cmꢀ1). EM-ES (+): calcd for C12H12N2O2 [M+ H]+ 217.0977, found 217.0972. Observations:
Yellow oil.
(E)-5-(4-methoxybenzylidene)-1,3-dimethylimidazolidine-2,4-dione (E-7):
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Yield = 3.95 g, 80 %; H-NMR: (400 MHz, CD3CN) δ 7.96 (d, J = 8.9 Hz, 2H), 6.92 (d, J = 8.9 Hz,
2H), 6.28 (s, 1H), 3.81 (s, 3H), 3.12 (s, 3H), 2.98 (s, 3H). 13C-NMR: (100 MHz, CD3CN) δ (ppm) 163.1
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(s), 161.1 (s), 154.6 (s), 133.0 (s), 129.1 (s), 126.8 (s), 117.4 (s), 114.5 (s), 56.0 (s), 26.8 (s), 24.9 (s).
UV-Vis (CHCl3): λ (nm) =333 (ε= 16346 Mꢀ1cmꢀ1), 243 (ε= 10115 Mꢀ1cmꢀ1). ES-MS (+): calcd for
C13H14N2O3 [M+ H]+ 247.1083, found 247.1065. Observations: Yellow solid. CCDCꢀ922962contains
the crystallographic data for this compound.
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(Z)-5-(4-methoxybenzylidene)-1,3-dimethylimidazolidine-2,4-dione (Z-7):
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1H-NMR: (400 MHz, CDCl3) δ ppm 7.24 (d, J = 8.7 Hz, 2H), 6.93 (s, 1H), 6.90 (d, J = 3.8 Hz, 2H),
3.84 (s, 3H), 3.13 (s, 3H), 3.00 (s, 3H). 13C-NMR: (100 MHz, CDCl3) δ ppm 164.0 (s), 159.9 (s), 156.2
(s), 131.1 (s), 129.0 (s), 124.9 (s), 113.9 (s), 112.7 (s), 55.4 (s), 30.6 (s), 25.1 (s). UV-Vis (CHCl3): λ
(nm) =325 (ε= 14808 Mꢀ1cmꢀ1). ES-MS (+): calcd for C13H14N2O3 [M+ H]+ 247.1083, found 247.1065.
Observations: Yellow solid.
(E)-5-(4-nitrobenzylidene)-1,3-dimethylimidazolidine-2,4-dione (E-8):
Yield = 4.71g, 90 % ; 1H-NMR (300 MHz, CDCl3) δ ppm 8.27 (d, J = 8.4 Hz, 2H), 7.47 (d, J = 8.5 Hz,
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2H), 6.89 (s, 1H), 3.17 (s, 3H), 2.96 (s, 3H). C NMR (400 MHz, CDCl3) δ 163.3 (s), 155.7 (s), 147.5
(s), 139.8 (s), 131.7 (s), 130.4 (s), 123.6 (s), 108.7 (s), 30.7 (s), 25.4 (s). UV-Vis (CHCl3): λ (nm) =339
(ε= 11816 Mꢀ1cmꢀ1). EM-ES (+): calcd for C12H11N3O4 [M+ H]+ 262.0828, found 262.0822.
Observations: Orange solid.
(E)-5-(2-methoxybenzylidene)-1,3-dimethylimidazolidine-2,4-dione (E-9):
Yield = 3.91 g, 79% ; 1H NMR (300 MHz, CDCl3) δ ppm 7.34 (t, J = 7.4 Hz, 1H), 7.16 (d, J = 7.4 Hz,
1H), 7.02 – 6.88 (m, 3H), 3.85 (s, 3H), 3.13 (s, 3H), 2.94 (s, 3H). 13C NMR (300 MHz, CDCl3) δ 163.6
(s), 157.6 (s), 155.8 (s), 130.7 (s), 130.0 (s), 129.9 (s), 121.5 (s), 119.9 (s), 110.4 (s), 108.8 (s), 55.4 (s),
29.8 (s), 4.9 (s). UV-Vis (CHCl3): λ (nm) = 242 (ε=10870 Mꢀ1cmꢀ1), 326 (ε= 12077 Mꢀ1cmꢀ1). EM-ES
(+): calcd for C13H14N2O3 [M+ H]+ 247.1083, found 247.1077. Observations: Brown solid.
(E)-5-((1methyl-indol-3-yl)methylene))-1,3-dimethylimidazolidine-2,4-dione (E-10):
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Yield = 4.42 g, 82%; H NMR (300 MHz, CDCl3) δ ppm 7.68 (d, J = 8.0 Hz, 1H), 7.34 (t, J = 8.3 Hz,
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2H), 7.24ꢀ7.19 (m, 1H), 7.15 (s, 1H), 7.10 (s, 1H), 3.85 (s, 3H), 3.17 (s, 3H), 3.15 (s, 3H). C NMR
(400 MHz, CDCl3) δ 164.2 (s), 156.4 (s), 136.8 (s), 129.8 (s), 128.4 (s), 127.5 (s), 123.0 (s), 120.8 (s),
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119.7 (s), 109.93 (m), 107.7 (s), 106.0 (s), 33.3 (d, J = 15.0 Hz), 30.2 (s), 25.1 (s). UV-Vis (CHCl3): λ
(nm) =373 (ε=13610 Mꢀ1cmꢀ1). EM-ES (+): calcd for C15H15N3O2 [M+ H]+ 270.1243, found 270.1242.
Observations: Brown solid.
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(E)-5-(3,5-dibromo-4-methoxybenzylidene)-1,3-dimethylimidazolidine-2,4-dione (E-11):
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Yield = 6.75 g, 84%; H NMR (400 MHz, CDCl3) δ ppm 7.44 (s, 2H), 6.72 (s, 1H), 3.92 (s, 3H), 3.14
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(s, 3H), 3.00 (s, 3H). 13C NMR (300 MHz, CDCl3) δ 163.4 (s), 155.8 (s), 154.3 (s), 133.5 (s), 131.4 (s),
131.0 (s), 118.2 (s), 108.2 (s), 61.0 (s), 30.8 (s), 25.3 (s). UV-Vis (CHCl3): λ (nm) =316 (ε=8710 Mꢀ
1cmꢀ1). EM-ES (+): calcd for C13H12Br2N2O3 [M+ H]+ 402.9293, found 402.9282. Observations:
Yellow solid.
Typical procedure of irradiation: The solutions of different photoswitches 1ꢀ5 in DMSO (0.01M) and
7ꢀ11 in CDCl3 (0.01M ) were prepared and irradiated in separate NMR tubes using a 400 W mediumꢀ
pressure Hg lamp until the PSS was reached. The reaction was followed by 1HꢀNMR. A similar
procedure was followed with a light source centered at 350 nm. The solutions were placed in a merryꢀ
goꢀround appliance in a photoreactor and irradiated using lamps with emission wavelength centered at
350 nm (14 lamps x 8ꢀW/ lamps) until the PSS was reached.
Selective irradiation of the two isomers (Z and E) of 7: The two isomers (Z and E) of compound 7
were irradiated separately with monochromatic light at different irradiation wavelengths: 300 nm, 330
nm and 375 nm. All the experiments were carried out under the same conditions with the same intensity
of the monochromatic light. 5.36 x 10ꢀ5M solutions in dry acetonitrile were prepared. 3 ml of these
solutions were irradiated at the selected wavelengths in a quartz cuvette (1.0 cm path length) using a
monochromator until the PSS was reached. The photochemical reaction was followed by GC/MS.
Isomerization quantum yield: The isomerization quantum yield (photoswitch 4 or 7) was calculated
following the procedure described in the literature, using transꢀazobenzene as actinometer. First, a
solution of transꢀazobenzene in acetonitrile was prepared. The absorbance of this solution at 358 nm
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was set to 1. Then, the actinometer solution was irradiated in a quartz cuvette at 334 nm using a
monochromator. After a short period of time, the change in absorbance at 358 nm of was ca. 0.02. A
solution in acetonitrile (for 7) or CH2Cl2:MeOH 1:1 (for 4) was prepared. The absorbance at 334 nm
was adjusted to match the value of the actinometer. This solution was irradiated at 334 nm until a
conversion of 20%. The isomerization process is followed by GC/MS. The number of photons absorbed
by the sample was calculated by using the following formula:
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Ep (mol of photons x cmꢀ2x sꢀ1) = F(λ) x ꢁA (358nm) / t(s)
where ꢁA(358nm) is the change in the absorbance at 358 nm of the transꢀazobenzene solution when
irradiating at 334 nm, and t(s) is the irradiation time responsible for that change. The F factor, which
ꢀ2
depends on the wavelength, has a value of 3.6 ꢍ 10ꢎꢏ einstein x cm at 334 nm. Thus, the number of
photons absorbed corresponds to Ep multiplied by the irradiation time (in seconds). The photochemical
isomer formed after irradiating is quantified by GC/MS. The value for the isomerization quantum yield
is obtained from the following equation:
number of formed or destroyed molecules
ɸ =
number of photons absorbed by the system
Finally, the actinometer solution was measured again to ensure that the light intensity is constant. All
the solutions were kept in the dark during the whole experiment.
Acknowledgments. This research was supported by the Spanish Ministerio de Ciencia e Innovación
(MICINN)/Fondos Europeos para el Desarrollo Regional (FEDER) (CTQ2011ꢀ24800 and CTQ2012ꢀ
36966), and Project CCG2013/EXPꢀ089 for the University of Alcalá (UAH). C.G.ꢀI. thanks the Spanish
MEC for her grant. S.R. acknowledges the support of National Science Foundation, USA (Grants no.
CHE 1229339, and CHE1429086 for funds to purchase the 400 MHz NMR Spectrometer, and Xꢀray
Diffractometer respectively).
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Supporting Information Available. UV and luminescence data, NMR spectra, computational details
and Cartesian coordinates for computed compounds. This material is available free of charge via the
Internet at http://pubs.acs.org.
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