Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline state: Probing the role of a metastable dimer

Article abstract of DOI:10.1016/j.jphotochem.2015.07.017

Cis-trans isomerization is one of the most common and well-investigated photoreaction of olefins in solution. This occurs via 180° rotation of one of the substituents on C[dbnd]C bond requiring large space around the double bond. One would expect that in crystals the neighboring molecules would prohibit such a process. In spite of this in early 1960s Schmidt and co-workers reported occurrence of such a process in crystals via 2?+?2 ‘meta cyclobutane’ formation. In this study by examining the photochemistry of four cis-stilbazolium salts we show that the photoconversion to the corresponding trans isomers does not occur via 2?+?2 addition. Large separation that ensures considerable space between neighboring olefins favors cis-trans isomerization in crystals. An aspect that is puzzling is that the product trans isomer phase separates and recrystallizes within the parent cis crystals and photodimerizes to the cyclobutane dimer. Details of this phenomenon are to be understood.

Full text of DOI:10.1016/j.jphotochem.2015.07.017

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Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Volume demanding geometric isomerization of cis-4-stilbazole. HCl
salts in the crystalline state: Probing the role of a metastable dimer
*
*
Barnali Mondal, Burjor Captain , V. Ramamurthy
Department of Chemistry, University of Miami, Coral Gables, FL, 33146, USA
A R T I C L E I N F O
A B S T R A C T
Article history:
Cis-trans isomerization is one of the most common and well-investigated photoreaction of olefins in
solution. This occurs via 180ꢀ rotation of one of the substituents on C
C bond requiring large space
Received 8 June 2015
Received in revised form 27 July 2015
Accepted 28 July 2015
Available online xxx
¼
around the double bond. One would expect that in crystals the neighboring molecules would prohibit
such a process. In spite of this in early 1960s Schmidt and co-workers reported occurrence of such a
process in crystals via 2 + 2 ‘meta cyclobutane’ formation. In this study by examining the photochemistry
of four cis-stilbazolium salts we show that the photoconversion to the corresponding trans isomers does
not occur via 2 + 2 addition. Large separation that ensures considerable space between neighboring
olefins favors cis-trans isomerization in crystals. An aspect that is puzzling is that the product trans
isomer phase separates and recrystallizes within the parent cis crystals and photodimerizes to the
cyclobutane dimer. Details of this phenomenon are to be understood.
Keywords:
Solid state photochemistry
cis-trans isomerization
Metastable dimer
Free space
ã 2015 Elsevier B.V. All rights reserved.
Stilbazolium salts
1. Introduction
motion at least at one end of the C C bond occurs in the crystalline
¼
state. As indicated above cis-trans isomerization in crystals was a
serendipitous discovery. The dimers from excitation of cis-
cinnamic acids examined by Schmidt and co-workers could only
be understood by dimerization of two trans-cinnamic acids and not
two cis-cinnamic acids. To accommodate this anomalous phenom-
The sporadic interest for over a century on the photochemistry
of organic molecules in the solid state and as host-guest assemblies
in solution have intensified during the last five decades [1]. Early
studies on light induced reactions performed in the solid state
focused mostly on synthesis of new molecules. Recent availability
of X-ray diffractometers and powerful computing programs for
structure solution has enabled chemists to connect a molecule’s
crystal packing with their reactivity and probe molecular
transformation mechanisms in solids. The topochemical postulate
formulated by Schmidt for solid-state photodimerization of trans-
cinnamic acids, having stood the test of time, has served as the
foundation for the development of solid-state photochemistry [2];
the few exceptions to the postulate have helped fine-tune it [3].
Olefins in general undergo two main reactions in solution:
unimolecular geometric isomerization and bimolecular dimeriza-
tion [4]. Schmidt’s attempt to extend the topochemical postulate to
cis-cinnamic acids led to the observation of geometric isomeriza-
tion in crystals [5]. Unlike photodimerization, Schmidt’s views on
light induced geometric isomerization of cinnamic acids in crystals
have not been widely accepted with no clear understanding till
date of how the cis-trans isomerization requiring large molecular
enon Schmidt proposed
a cis- to trans isomer conversion,
prompted by interaction of an excited cis isomer with an adjacent
ground state molecule within a distance of 4.2 Å, to precede
dimerization [5]. The ‘metastable dimer’ thus formed is presumed
to be the intermediate that facilitates cis to trans isomerization.
This proposal implied the requirement of <4.2 Å separation of
adjacent molecules for cis-trans isomerization. The proposed
mechanism by Schmidt and co-workers illustrated in Scheme 1
involves several steps: the initial formation of a metastable dimer
leading to the trans-isomer as a solid solution within the cis lattice
followed by the separation of the trans-isomers in a thermally
controlled recrystallization step into their own lattice. In the final
step the excited trans isomer yields the lattice controlled stable
dimer.
After almost four decades following the early (1902 and 1923)
reports on solid-state photoisomerization of olefins from the
laboratories of Paal, Schulze and Rice on dibenzoylethylene and
benzoylacrylic acid esters [6,7], Schmidt and co-workers reported
their results on the geometric isomerization of cis-cinnamic acids
[5,8]. This report prompted several studies focusing on the
mechanism of cis-trans isomerization in crystals [9–16]. Currently
four mechanisms exist, including the original suggestion by
* Corresponding authors.
E-mail addresses: captain@miami.edu (B. Captain), murthy1@miami.edu
(V. Ramamurthy).
http://dx.doi.org/10.1016/j.jphotochem.2015.07.017
1010-6030/ã 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: B. Mondal, et al., Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline
state: Probing the role of a metastable dimer, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.07.017

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Ar
COOH
Ar
+
COOH
Solid solution
COOH
Ar
COOH
Ar
Ar
HOOC
COOH
COOH
Ar
HOOC
HOOC
Ar
HOOC
h
Ar
Ar
Ar
Ar
COOH
COOH
Ar
Ar
Ar
h
Ar
Ar
COOH
COOH
HOOC
Ar
COOH
Ar
HOOC
COOH
Formation of metastable dimers
Ar
HOOC
Ar
HOOC
Ar
COOH
Phase separation
Scheme 1. Proposed consolidated mechanism for the cis-trans isomerization of cis-cinnamic acids in crystals.
Schmidt, to rationalize the cis-trans isomerization in crystals: (a)
cis-stilbazole.HCl salts (1b–4b) shown in Scheme 2 whose results
isomerization via a metastable dimer intermediate, (b) conven-
are presented below.
tional one bond (C C) rotation,[17,18] (c) two bond rotation of
¼
adjacent single and double bonds (Hula twist) [19–22], and (d)
bicycle pedal mechanism involving concurrent isomerization of
more than one double bond [23–26].
3. Experimental
3.1. Synthesis of cis-stilbazole derivatives
A few examples of cis-trans isomerization where the adjacent
C C bonds are separated by more than 4.2 Å have also been
¼
The cis-stilbazoles were prepared by irradiation of the trans
reported since Schmidt and co-workers’ report casting doubt on
the role of a metastable dimer during cis-trans isomerization
[17,18,27–29]. This prompted us to ponder the importance of
metastable dimer and probe the generality of it as an intermediate
during cis-trans isomerization. In this context we have currently
examined the excited state behavior of four cis-stilbazole. HCl salts
compounds in a CHCl3 solution in Pyrex test tubes (
l
ꢁ 290 nm), to
afford a mixture of cis and trans and preparative TLC used to
separate the liquid cis compound. The purity of cis compound was
verified by 1H NMR and UV–vis spectrum (Fig. 1). The cis stilbazole.
HCl salt formed on addition of 3 equivalents HCl (37%) was air dried
overnight to give powder. These salts were vacuum dried for 12 h
prior to irradiation.
1b–4b (Scheme 2) where the adjacent C C bonds were separated
¼
by >5.5 Å (see discussion below). Such a long distance of separation
should prevent metastable dimer formation. To examine the
second part of the reaction namely the trans isomer, formed from
the cis isomer, yielding the dimer by recrystallizing in the lattice of
the cis-isomer we turned to our recent finding that excitation of
hydrated trans-stilbazole.HCl salts (5b–8b) resulted in [2 + 2]
cycloaddition to yield a anti-head-tail dimer [30,31]. Thus, cis-
stilbazole.HCl salts 1b-4b seemed ideal to test whether the
corresponding trans-stilbazole. HCl salts formed in the cis lattice
would dimerize. Finally, if metastable intermediate is not involved,
identification of a pre-requisite condition for cis-trans isomeriza-
tion in crystals is important. With these in mind we carried out a
combined X-ray crystallographic and photochemical study of four
3.2. Irradiation technique
About 8–10 mg of stilbazole. HCl salts were spread uniformly
between two Pyrex glass plates, sealed with parafilm and
irradiated. The plates were turned around every 1 h, the samples
thoroughly mixed and spread on the glass plate and irradiation
continued. Irradiations were performed using a 450 W medium
pressure mercury arc lamp placed in a water-cooled Pyrex
immersion jacket. The progress of the reaction was monitored
up to a maximum of 30 h. After a given duration of irradiation, a
small portion of the irradiated solid material was dissolved in H2O
and neutralized with 1N NaOH. This neutral solution was extracted
X
NH
Cl
Cl
NH
N
X
X
1a X = F
2a X = Cl
3a X = Br
4a X = I
1b X = F
2b X = Cl
3b X = Br
4b X = I
5b X = F
6b X = Cl
7b X = Br
8b X = I
Scheme 2. Structures of cis and trans-stilbazoles and their HCl salts discussed in this manuscript.
Please cite this article in press as: B. Mondal, et al., Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline
state: Probing the role of a metastable dimer, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.07.017

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3
(a)
(b)
0.5
0.6
trans 4-Cl stilbazole
cis 4- Cl stilbazole
trans 4-Br stilbazole
0.5
0.4
0.3
0.2
0.4
0.3
cis 4-Br stilbazole
0.2
0.1
0.0
200
0.1
0.0
200
250
300
350
400
450
250
300
350
400
450
Wavelength (nm)
Wavelength (nm)
Fig. 1. Absorption spectra in methanol solution of (a) cis/trans 4-Br stilbazole, (b) cis/trans 4- Cl stilbazole.
with CHCl3 four times and the combined organic layer was dried
over anhydrous Na2SO4.
Crystal data, data collection parameters, and results of the analyses
are listed in Table 1. Data for HCl salt of 4a is already published.
Crystals for X-ray diffraction analyses, were obtained by
evaporation of solution in chloroform/ethyl acetate solvent
mixture at 25 ꢀC. All compounds crystallized in the monoclinic
crystal system.
After evaporation of the solvent the product was characterized
by 1H NMR in CDCl3.
3.3. Crystallographic analysis
With 1b the systematic absences in the intensity data were
consistent with the unique space groups P21/c.
Each data crystal was glued onto the end of a thin glass fiber. X-
ray intensity data were measured by using a Bruker SMART APEX2
With 2b the systematic absences in the intensity data were
consistent with the unique space group P21/c. With Z = 8, there are
two formula equivalents of the cis 4Cl-stilbazol. HCl present in the
asymmetric crystal unit.
With 3b the systematic absences in the intensity data were
consistent with either of the space groups P21/m or P21. The
structure could only be solved in the latter space group. With Z = 4,
there are two formula equivalents of the cis 4Br-stilbazol.HCl
present in the asymmetric crystal unit.
Crystal structure co-ordinates for the three new structures have
been deposited at the Cambridge Crystallographic Data Centre.
Their CCDC numbers are CCDC 1405334, CCDC 1405335 and CCDC
1405336 for 3b, 2b and 1b respectively.
CCD-based diffractometer using Mo K
a radiation (l= 0.71073 Å)
[32]. The raw data frames were integrated with the SAINT+
program by using a narrow-frame integration algorithm. Correc-
tions for Lorentz and polarization effects were also applied with
SAINT+ [32]. An empirical absorption correction based on the
multiple measurement of equivalent reflections was applied using
the program SADABS. All structures were solved by a combination
of direct methods and difference Fourier syntheses, and refined by
full-matrix least squares on F2, by using the SHELXTL software
package [33,34]. All non-hydrogen atoms were refined with
anisotropic displacement parameters. Hydrogen atoms were
placed in geometrically idealized positions and included as
standard riding atoms during the least-squares refinements.
Table 1
Crystallographic data for HCl salts of compounds 1b, 2b, and 3b.
cis 4-F stilbazole. HCl
cis 4-Cl stilbazole. HCl
cis 4-Br stilbazole. HCl
Empirical formula
Formula weight
Crystal system
Lattice parameters
a (Å)
C13H11NF
235.68
Monoclinic
ꢂ
Cl
C13H11NCl
252.13
Monoclinic
ꢂ
Cl
C13H11NBr
296.59
Monoclinic
ꢂ
Cl
9.2263(6)
13.4939(9)
10.3239(7)
105.584(1)
1238.06(14)
P21/c (# 14)
4
1.264
0.293
296
56.0
16.8596(7)
8.2441(3)
18.9795(8)
104.970(1)
2548.47(18)
P21/c (# 14)
8
1.314
0.481
296
55.0
6.1667(3)
27.7948(15)
7.7559(4)
101.651(1)
1.99(12)
P21 (# 4)
4
1.513
3.335
296
56.0
b (Å)
c (Å)
b
(deg)
V (Å3)
Space group
Z value
rcalc (g/cm3)
m
(Mo K
Temperature (K)
umax (ꢀ)
No. Obs. (I > 2
a
) (mmꢃ1
)
2
s(I))
2390
4693
5095
No. Parameters
146
289
290
Goodness of fit
1.028
1.029
1.085
Max. shift in cycle
0.002
0.001
0.001
Residuals*:R1; wR2
0.0363; 0.1011
Multi-scan
0.7461/0.6713
0.220
0.0397; 0.1038
Multi-scan
0.7461/0.6568
0.198
0.0343; 0.0809
Multi-scan
0.7457/0.5244
0.359
Absorption Correction,
Max/min
Largest peak in Final Diff. Map (eꢃ/Å3)
2
*R = Shkl(kFobsjꢃjFcalck)/ShkljFobsj; Rw = [Shklw(jFobsj-jFcalcj)2/ShklW(Fobs
]
1/2, w = 1/
s
2(Fobs); GOF = [Shklw(jFobsjꢃjFcalcj)2/(ndata ꢃ nvari)]1/2
.
Please cite this article in press as: B. Mondal, et al., Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline
state: Probing the role of a metastable dimer, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.07.017

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4. Results
Although most cis-stilbazoles 1a–4a were liquid, their HCl salts
1b–4b were solids. Results of X-ray structural investigation are
presented first followed by those of photochemical studies. All
structures were solved satisfactorily yielding final refined R factors
less than 4%. Crystallographic data for 1b–3b are presented in
Table 1. The crystal data for 4b has been published previously. The
packing arrangements of cis-stilbazole. HCl salts 1b-4b are
presented in Figs. 2 and 3. We have previously reported the X-
ray crystal structures of hydrated 5b–8b and dry 6b–8b, and
packing arrangements of these are reproduced in Figs. 4–6. First,
one should note that while trans-stilbazole. HCl salts 5b–8b could
be crystallized with and without water molecules, cis-stilbazole.
HCl salts 1b–4b could only be crystallized without water
molecules. In two of these crystals the solvent of crystallization
methanol was included in the lattice. Analyses of the packing
arrangements provided in Figs. 2–6 reveal the following: (a) The
distance between adjacent C
HCl salts 6b-8b was less than 4 Å (Figs. 4 and 5). (b) The distance
between adjacent C C bonds in dry trans-stilbazole.HCl salts 6b-
8b was more than 4.7 Å (Fig. 6). (c) Most importantly, the distance
between adjacent C C bonds in cis-stilbazole. HCl salts 1b–4b was
¼
C bonds in hydrated trans-stilbazole.
¼
¼
more than 6.0 Å (Figs. 2 and 3). Clearly of the three types of crystals,
the trans isomers in hydrated crystals were most tightly packed
(<4.2 Å) and would be expected to dimerize to yield anti-head-tail
dimers. Consistent with this reasoning we had reported earlier that
hydrated crystals 5b–8b upon exposure to light dimerized while
the ‘dry’ crystals were photostable.
Inspection of the packing in Figs. 2 and 3 reveals loose packing
with considerable free space in crystals of 1b–4b. To estimate the
available free space surrounding the olefin molecule we generated
a cavity plot using the PLATON program [35]. From Figs. 7 and 8
Fig. 3. Packing arrangement of (a) cis-4-Br stilbazole.HCl (3b) (b) cis-4-I stilbazole.
HCl (4b). Clꢃ is represented as green sphere.
showing the packing of the four olefins with the cavity generated
using radius (r) varying between 1.00 and 1.11 Å we conclude that
the three cavities in 1b have volumes of 5.73, 5.58, and 4.19 Å3; 2b
has two cavities with volumes 5.27, and 4.71 Å3; and 3b and 4b
have single cavity each with volumes of 4.99 Å3 and 5.13 Å3,
respectively. It is important to note that the free space is adjacent
to the C
¼
C bond and the aryl substituent in the crystals of 1b–4b.
C separation >6 Å) and
Based on the loose packing (distance of C
¼
the location of free space in the crystals of 1b–4b one would
predict no dimerization but likely isomerization to corresponding
trans isomer of these molecules upon excitation.
With the X-ray structures of 1b–4b on hand photochemical
studies of these crystals were performed. The products were
identified to be the corresponding trans isomer and the anti-head-
tail dimer of the trans-isomer. The cis and trans isomer products
present at shorter times of irradiation (ꢄ2 h) were accompanied by
anti-head-tail dimer at longer times. 1H NMR spectra taken at
various time intervals are presented in Figures S1-S4 in the
Supporting Information section (SI) and the yields of the various
products of irradiation estimated from the 1H NMR spectra
tabulated in Tables S1–S4 in SI are presented in a graphical format
in Fig. 9. The product dimer isolated in each case upon complete
conversion was identified to have anti-head-tail stereochemistry
by comparing with authentic sample prepared previously by
irradiating trans-5b-9b. 1H NMR spectra of the product and the X-
ray structure of 4-cholorostilbazole and 4-bromostilbazole are in
Figs. S5 and S6 in SI. Examination of Fig. 9, Tables S1–S4 (SI) and
Fig. 2. Packing arrangement of (a) cis-4-F stilbazole.HCl (1b) (b) cis-4-Cl stilbazole.
HCl (2b). Clꢃ is represented as green sphere.
Please cite this article in press as: B. Mondal, et al., Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline
state: Probing the role of a metastable dimer, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.07.017

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5
Fig. 4. Packing arrangement of (a) trans-4-F stilbazole.HCl (5b.3H2O) (b) trans-4-Cl stilbazole.HCl (6b.2H2O). Oxygen of H2O represented as red sphere and Clꢃ as green
sphere.
Fig. 5. Packing arrangement of (a) trans-4-Br stilbazole.HCl (7b.3H2O) (b) trans-4-I stilbazole.HCl (8b.2H2O). Oxygen of H2O represented as red sphere and Clꢃ as green sphere.
Fig. 6. Packing arrangement of anhydrous (a) trans-4-Cl stilbazole.HCl (6b.MeOH) (b) trans-4-Br stilbazole.HCl (7b.MeOH) (c)) trans-4-I stilbazole.HCl (8b). Oxygen of
methanol represented as red sphere and Clꢃ as green sphere.
Figs. S1–S4 (SI) clearly shows that irradiation of 1b-4b as crystals
leads to the corresponding trans isomer as the primary product is
clear from examination of Fig. 9, Figs. S1–S4 (SI) and Tables S1–S4
(SI). Interestingly, no dimer is generated till at least 25% of the trans
isomer accumulates in the cis isomer lattice.
Please cite this article in press as: B. Mondal, et al., Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline
state: Probing the role of a metastable dimer, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.07.017

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Fig. 7. Cavity plot of (a) cis-4-F stilbazole. HCl (1b) (b) cis-4-Cl stilbazole. HCl (2b) using PLATON program. Clꢃ is represented as green sphere. The silvery sphere represents the
available free space near C C. Note that there is more than one sphere with different volumes.
¼
Fig. 8. Cavity plot of (a) cis-4-Br stilbazole. HCl (3b) (b) cis-4-I stilbazole. HCl (4b) using PLATON program. Clꢃ is represented as green sphere. The silvery sphere represents the
available free space near C C.
¼
Please cite this article in press as: B. Mondal, et al., Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline
state: Probing the role of a metastable dimer, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.07.017

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7
Fig. 9. Graphical presentation of % conversion vs time, upon irradiation of (a) cis-4-F stilbazole. HCl (1b), (b) cis-4-Cl stilbazole. HCl (2b), (c) cis-4-Br stilbazole. HCl (3b), (d) cis-
4-I stilbazole. HCl (4b). In the figures, the red line represents formation of trans, blue line represents depletion of cis and green line represents formation of dimer.
5. Discussion
believed that conventional one bond rotation is the likely pathway.
The conventional one bond rotation pathway requires no steric
compression between the rotating group and the surroundings
necessitating ample free space in the lattice around the rotating
group. Using PLATON program [35] we have estimated the free
space within the crystal lattice of 1b–4b represented in the form of
spherical cavity with their location as provided in Figs. 7 and 8. In
all four examples the free space present near the rotating aryl
group were estimated to be at least 5 Å3 and favorable for cis-trans
isomerization. Using similar data, such arguments have been made
earlier for diaryl olefins by one of our groups [37,17]. A similar
approach was utilized by the groups of Moorthy [18] and Saltiel
[24] later to understand cis-trans isomerization of styrylcoumarins
and 1,4-diaryl-1,3-butadienes. Kinbara and Saigo who investigated
From the results presented above it is clear that cis-stilbazole.
HCl salts 1b–4b upon excitation resulted in the corresponding
trans isomer as the primary product. The metastable dimer can be
ruled out as an intermediate during the conversion of these salts
1b–4b to the corresponding trans isomer where adjacent C C
¼
bonds are separated by more than 6 Å as deduced from their crystal
packing. These examples do not conform to Schmidt's original
proposal of lattice controlled cis-trans isomerization (i.e., the
olefins have to be within a bonding distance of <4.2 Å) occurring
via the metastable cyclobutane intermediate [5]. These cis-
stilbazole.HCl salts 1b–4b are however not the first such examples.
Schmidt himself noted that trans-dibenzoylethylene isomerized to
the cis although adjacent C
¼
C bonds were separated by 5 Å and
the cis-trans isomerization of a,b-unsaturated amides and
claimed it to be a single crystal-single crystal process [27,28,36]. In
the case of 2-ethoxy-cis-cinnamic acid whose photochemistry was
earlier examined by Schmidt and co-workers and said to isomerize
via the metastable dimer intermediate was reported by Bryan and
thioamides independently came to a similar conclusion regarding
the role of free space by estimating the packing co-efficient in the
crystal using OPEC program by Gavezzotti and pointed out that
looser packing in the crystal favored cis-trans isomerization [16]. It
is important to note that looser packing would not assure free
Hartley to have adjacent C C bonds separated by 5.18 Å from X-ray
¼
structural studies [29]. We reported about a decade ago several
examples of diarylethylenes isomerizing in the crystalline state
space where the rotation occurs. Cavity sizes near the C C bond for
¼
the two examples were calculated and they are represented in
Fig. 10. Interestingly, in both reactive and unreactive crystals there
were cavities. Careful analysis of the cavity size and location and
other weak interactions between the molecules in crystals is
needed to understand the reactivity of molecules in the crystalline
state. Obviously, the packing co-efficient (loose or dense packing)
is not a direct reflection of the location of the cavity in the crystal.
Based on examples of others and ours in the literature and the
current ones we conclude that the most important criterion albeit
where adjacent C C bonds were separated by >5.2 Å.[17] Based on
¼
the above examples, one can confidently state that crystal packing
does not have to be tight and metastable dimers need not be
involved for the cis-trans isomerization to occur in the crystalline
state. The adjacent C
a role in this process.
¼
C bond separation distance also may not play
Having ruled out a metastable dimer pathway we needed to
identify how exactly the isomerization proceeded in crystals. We
Please cite this article in press as: B. Mondal, et al., Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline
state: Probing the role of a metastable dimer, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.07.017

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JPC 9974 No. of Pages 9
8
B. Mondal et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
Fig. 10. Cavity plot estimated using PLATON program of
photochemistry refer to Ref. [16]. The silvery sphere represents the available free space. The red arrow indicates the C
a
,
b
unsaturated amides (a) photostable racemic amide and (b) photoreactive reactive R—amide; for details on the
C bond.
¼
not the only one for cis-trans isomerization is the presence of free
space at an appropriate site. While the presence of free space
assures initiation of the isomerization, continued rotation and
completion of the reaction requires the crystal packing be soft and
passive, the former ensuring the surroundings accommodate the
cis and trans isomers form a solid solution leading to phase
separation after which the trans isomer recrystallizes itself in its
own packing arrangement. The formation of anti-head-tail dimers
in our case during this process could be resulting from the trans
isomer absorbing moisture from the atmosphere to pack in a
manner as in Figs. 4 and 5. The details of phase rebuilding and
phase separation related to organic solid-state reactions as
investigated by Kaupp and co-workers using AFM has not been
performed by us at this stage [43]. We believe powder X-ray
photographs recorded at various stages of irradiation would throw
light on this process and plan to pursue them in the future.
change and allow a complete 180ꢀ rotation of the C
and the latter indicating the absence of strong intermolecular
interactions (e.g., hydrogen bond, cation- interaction, ionic
¼
C substituent,
p
interaction etc.) that could resist any rotational motion. In the
past, several groups recognizing the importance of free space in
various reactions in crystals have developed their own method to
estimate the energy involved in molecular motion within a crystal
lattice. Amongst these contributions of Gavezzotti [38], McBride
[39], Ohashi [40], Scheffer [41], Venkatesan and Ramamurthy [3]
and Zimmerman [42] should be noted.
The last point we address concerns the formation of anti-head-
tail dimers as the final product. We have previously shown
formation of these dimers from hydrated crystals of trans 5b–8b.
As shown in Figs. 4 and 5 trans 5b–8b are packed such that the
adjacent pairs are within 4.0 Å and centrosymmetrically related
favoring the formation of anti-head-tail dimers. However, in the
crystals of 6b–8b without water molecules the adjacent pairs are
6. Conclusion
We have explored the photochemistry of four cis-stilbazole salts
in the crystalline state. To understand their photochemical
behavior X-ray crystal structures were obtained. Based on the
packing arrangements we claim that cis-trans isomerization in the
crystalline state does not involve a metastable dimer. Ideally for
the cis-trans isomerization to occur the molecule in question must
be well separated from its neighbors and there be ‘free space’
around the substituent that undergoes 180ꢀ rotation. In addition
this isomerization is complicated by the formation of a secondary
product (cyclobutane dimer) from the primary product the trans
isomer. This process requires phase separation and recrystalliza-
tion of the product in the lattice of the reactant. We have not made
any attempt to understand this important aspect of solid-state
photochemistry. We plan to focus on this aspect in the near future.
separated by more than 4.5 Å, the C C bonds displaced with
¼
pseudo mirror symmetry. Thus, crystals that do not contain water
molecules are not expected to dimerize. Although the crystals of cis
isomers 1b–4b we irradiated did not contain water molecules the
final products obtained, and only after about 25% accumulation of
trans isomers in the cis crystals, were anti-head-tail dimers. The
absence of dimer at earlier stages suggested that the precursor for
anti-head-tail dimer was the excited trans isomer and not the
excited cis isomer. This begs the question of how the trans isomer
recrystallized itself in the anhydrous cis lattice with a packing
arrangement suitable for dimerization but was not explored in this
study. For the present we go with Schmidt's mechanism assuming
Acknowledgements
VR is grateful to the National Science Foundation for continued
financial support that enabled us to keep our interest in solid-state
photochemistry. VR dedicates this article to Professor Yoshihisa
Please cite this article in press as: B. Mondal, et al., Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline
state: Probing the role of a metastable dimer, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.07.017

G Model
JPC 9974 No. of Pages 9
B. Mondal et al. / Journal of Photochemistry and Photobiology A: Chemistry xxx (2015) xxx–xxx
9
Inoue, Osaka University,
dedicated researcher, and a gracious and generous host during
VR’s multiple visits to Japan.
a
dear friend, esteemed colleague,
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Please cite this article in press as: B. Mondal, et al., Volume demanding geometric isomerization of cis-4-stilbazole. HCl salts in the crystalline
state: Probing the role of a metastable dimer, J. Photochem. Photobiol. A: Chem. (2015), http://dx.doi.org/10.1016/j.jphotochem.2015.07.017