Role of hydrogen bonds in molecular packing of photoreactive crystals: Templating photodimerization of protonated stilbazoles in crystalline state with a combination of water molecules and chloride ions

Article abstract of DOI:10.1039/c4pp00221k

A difference in photobehavior and molecular packing between hydrated and anhydrous crystals of protonated trans-stilbazoles has been identified. While stilbazoles are not photoreactive in the crystalline state, upon protonation with HCl in the solid state they dimerized to a single dimer (anti-head-tail) when exposed to UV light. In these photoreactive crystals the protonated stilbazole molecules are arranged in a ladder-like format with the rungs made up of water molecules and chloride ions. A combination of water and chloride ion holds the protonated trans-stilbazoles through either N-H...O or N-H...Cl^- interactions. Anhydrous protonated stilbazole crystals prepared by heating the 'wet' crystals under reduced pressure were inert upon exposure to UV light. As per X-ray crystal structure analyses these light stable crystals did not contain water molecules in their lattice. The current investigation has established that water molecules are essential for photodimerization of crystalline protonated trans-stilbazoles. To compare the reactivity of protonated trans-stilbazoles with that of protonated cis-stilbazoles, photoreactivity and packing arrangement of cis-4-iodo stilbazole?HCl salt was examined. This molecule in the crystalline state only isomerized to the trans isomer and did not dimerize. Thus, while the trans isomer dimerized and did not isomerize, the cis isomer only isomerized and did not dimerize in the crystalline state. To probe the role of cation...π interaction in the packing of protonated trans-stilbazoles, energies of various types of packing in the gas phase were estimated by MP-2 calculations and cation...π interaction was found to be unimportant in the packing of protonated trans-stilbazole crystals investigated here.

Full text of DOI:10.1039/c4pp00221k

Photochemical &
Photobiological Sciences
View Article Online
View Journal | View Issue
PAPER
Role of hydrogen bonds in molecular packing
of photoreactive crystals: templating
photodimerization of protonated stilbazoles
in crystalline state with a combination of
water molecules and chloride ions†
Cite this: Photochem. Photobiol. Sci.,
2014, 13, 1509
Barnali Mondal, Tingting Zhang, Rajeev Prabhakar, Burjor Captain* and
V. Ramamurthy*
A difference in photobehavior and molecular packing between hydrated and anhydrous crystals of proto-
nated trans-stilbazoles has been identified. While stilbazoles are not photoreactive in the crystalline state,
upon protonation with HCl in the solid state they dimerized to a single dimer (anti-head–tail) when
exposed to UV light. In these photoreactive crystals the protonated stilbazole molecules are arranged in a
ladder-like format with the rungs made up of water molecules and chloride ions. A combination of water
and chloride ion holds the protonated trans-stilbazoles through either N–H⋯O or N–H⋯Cl− interactions.
Anhydrous protonated stilbazole crystals prepared by heating the ‘wet’ crystals under reduced pressure
were inert upon exposure to UV light. As per X-ray crystal structure analyses these light stable crystals did
not contain water molecules in their lattice. The current investigation has established that water mole-
cules are essential for photodimerization of crystalline protonated trans-stilbazoles. To compare the reac-
tivity of protonated trans-stilbazoles with that of protonated cis-stilbazoles, photoreactivity and packing
arrangement of cis-4-iodo stilbazole·HCl salt was examined. This molecule in the crystalline state only
isomerized to the trans isomer and did not dimerize. Thus, while the trans isomer dimerized and did not
isomerize, the cis isomer only isomerized and did not dimerize in the crystalline state. To probe the
role of cation⋯π interaction in the packing of protonated trans-stilbazoles, energies of various types of
packing in the gas phase were estimated by MP-2 calculations and cation⋯π interaction was found to be
unimportant in the packing of protonated trans-stilbazole crystals investigated here.
Received 18th June 2014,
Accepted 21st July 2014
DOI: 10.1039/c4pp00221k
www.rsc.org/pps
systematic study combining X-ray crystallography and photo-
reactions in the 1960s reignited this interest.5 The term
Introduction
The first report of supramolecular photochemistry1 whose ‘topochemical postulate’ originally used by Kohlschutter in
history dates back to the late 1800s is probably the photo- 1911 and Hertel in 1931 reworded by Schmidt as ‘reaction in
dimerization of cinnamic acid derivatives in solid sate reported the solid state occurs with a minimum amount of atomic or mole-
by Libermann in 1871.2 The immense interest in solid state cular movement’ which is now known as ‘Schmidt’s topochem-
photochemistry evident from reports between 1900 and 1930 ical postulate’ forms the basis of organic solid state
waned due to the lack of experimental techniques to uncover photochemistry.6 According to the original topochemical pos-
mechanisms of such reactions.3,4 Schmidt and co-workers’ tulate, photodimerization in the solid state is likely to occur
when the separation between the reacting double bonds is
less than 4.2 Å, are parallel, and one above the other.3 Several
identified exceptions since the work of Schmidt have led to the
conclusion that less than ideal atomic and orbital orientations
Department of Chemistry, University of Miami, Coral Gables, FL 33146, USA.
E-mail: murthy1@miami.edu
†Electronic supplementary information (ESI) available: All crystallographic data can still lead to dimerization if the surrounding lattice can tole-
(CIF files) of published structures of seven reactive stilbazole·HCl salts (1–5,
rate motions that would steer the molecules to proper mutual
7 and 8) have been deposited previously with CCDC and their numbers are
orientation, i.e., some amount of tolerance in terms of
813287–818801. Structures of unreactive trans crystals and the cis crystals have
distance (4.2 Å) and parallel arrangement of CvC bonds.7 One
been solved and their CIF files are deposited. CCDC 1008122–1008125. For ESI
of the major problems encountered in the current efforts
devoted to establishing reliable strategies that would steer
and crystallographic data in CIF or other electronic format see DOI: 10.1039/
c4pp00221k
This journal is © The Royal Society of Chemistry and Owner Societies 2014
Photochem. Photobiol. Sci., 2014, 13, 1509–1520 | 1509

View Article Online
Paper
Photochemical & Photobiological Sciences
organic molecules toward a predetermined form is the lack of respective CIF files deposited with CCDC, we did not elaborate
complete understanding of the intra- and intermolecular inter- on the forces that may be involved in the packing of proto-
actions that would lead to the observed crystal packing.8–12 nated stilbazoles and did not correlate reactivity with packing.
With this knowledge designing template groups, perhaps tem- Following our brief communication, we have carefully exam-
porarily attached to the functional molecules to guide photo- ined the packing arrangements and photochemistry of the
chemically reactive groups into appropriate juxtaposition in reactive, non-reactive and dry (water-free) protonated stilba-
crystals, would be feasible.
zoles and the results are presented in this article. In addition,
Three of the several distinct strategies that have been since the stilbazole is protonated and carries a positive charge
employed to bring the reactive molecules into proper orien- we were curious to examine the role of cation–π interaction on
tations have yielded promising results:4,13–17 (a) intramolecular crystal packing.44,45 In this context, we have examined the
substitution, (b) templation with host structures and (c) steer- location of the positive charge in the protonated stilbazole
ing crystallization with a ditopic molecule capable of hydrogen molecule by computation and estimated the energies of the
bonding. In this regard chloro and fluoro substituents as steer- various arrangements for a pair of protonated stilbazoles. The
ing groups and well-defined host systems such as inorganic results of these studies are presented and discussed below.
hosts (zeolites and clays) and organic hosts (bis-urea and
cucurbituril) to template photodimerization in the solid state
have been exploited. The more popular approach of direct rele-
vance to the current presentation is the ‘templation’ strategy.
Experimental
Pioneering reports on this topic came from the group of Toda
who used various 1,n-diols to orient olefins through weak
intermolecular hydrogen bonds.18–21 Scheffer, Ito and their co-
workers exploited the stronger Coulombic acid–base inter-
action between the template amine and the reactant acid.22,23
MacGillivray and co-workers’ report on the templating
strategy24,25 garnered the attention of a number of groups
and restored interest to the area of solid state
photodimerization.26–28 They identified 1,3-dihydroxybenzene
as a template to pre-orient 4,4′-bispyridylethylene (4,4′-BPE)
towards dimerization in the crystalline state. Along this line we
recently demonstrated the value of thiourea as a template to
pre-orient eleven stilbazoles and azastilbenes in the crystalline
state toward a single dimer product.29 Despite the established
utility of thiourea and 1,3-dihydroxybenzene as templates, no
single universal template that would steer ‘majority’ of mole-
cules toward a dimer has been identified. Search for templates
and understanding their function therefore continue unabated
in organic solid state photochemistry.
Synthesis and sample characterization
trans-Stilbazole derivatives were synthesized following litera-
ture procedures.46–48 The corresponding HCl salts were pre-
pared by adding 2 equivalents of conc. HCl to stilbazoles in a
mortar and grinding with a pestle to give a paste. The HCl salt
whose formation was inferred from the color change in stilba-
zoles upon addition of conc. HCl was air dried overnight to
give a powder of trans-stilbazole·HCl salt. These powders with
10–20% water as determined from thermogravimetric analysis
(TGA) using Netzsch Thermal Analyzer were considered to be
hydrated salts. The TGA data are provided in Table 1. The
anhydrous stilbazole·HCl salts obtained by drying the hydrated
salts at ∼110 °C under reduced pressure (10−2 torr), for 7–9 h
were stored in vials in a desiccator. The anhydrous nature was
confirmed by recording the weight loss by TGA measurements.
For TGA experiments 2–4 mg of powder sample was used and
the temperature range was set between 25–120 °C with a
heating ramp of 5° min−1 to confirm.
In this article we present our results on the solid state
photodimerization of stilbazoles. Like cinnamic acids, stilba-
zoles have played historically an important role in photodimer-
ization. When Schmidt was exploring the dimerization of
cinnamic acids at the Weizmann Institute,5 Williams of Kodak
Research Laboratories, concurrently irradiating several proto-
nated stilbazoles in the solid state noted differences in reactiv-
ity between solution and solid state, and between various
substituted derivatives in the solid state.30–32 It is interesting
to note that while the former explored from a ‘basic science’
perspective, Williams did it for lithographic applications.33
Photodimerization of protonated stilbazoles has since been
investigated in monolayer assemblies, micelles, membranes
and within various host structures such as cyclodextrins, calix-
arenes and cucurbiturils.34–42 We recently communicated in
this journal that HCl salts of numerous stilbazoles dimerized
readily in the crystalline state while neutral stilbazoles were
inert.43 Although the packing arrangements of seven reactive
and two non-reactive stilbazoles were presented, with the
Photoreaction and product characterization
Irradiations were performed using a 450 W medium pressure
mercury arc lamp kept in a water-cooled Pyrex immersion well
(transmission λ ≥ 290 nm). Hydrated and anhydrous crystals
were irradiated by uniformly spreading 8–10 mg of salt
Table 1 Water content in the crystal before irradiationa
% H2O loss from TGA
experiment
% H2O present in
crystal structure
Sample
1·HCl·3H2O
2·HCl·3H2O
3·HCl·2H2O
4·HCl·2H2O
5·HCl·2H2O
7·HCl·8/3H2O
8·HCl·3H2O
19.3
13.6
11
7.8
6.8
9.8
13.1
19
19
12
10
9
15
17b
a Microcrystalline powder for TGA measurements and single crystal for
crystal structure analysis single were used. b Data taken from ref. 43.
1510 | Photochem. Photobiol. Sci., 2014, 13, 1509–1520
This journal is © The Royal Society of Chemistry and Owner Societies 2014

View Article Online
Photochemical & Photobiological Sciences
Paper
between two Pyrex plates sealed with parafilm and by turning ithm. Corrections for Lorentz and polarization effects were
the plates around every 1 h for uniform exposure. After also applied with SAINT+. An empirical absorption correction
irradiation the powder was dissolved in water and neutralized based on the multiple measurement of equivalent reflections
with 1 N NaOH, extracted four times with chloroform and the was applied using the program SADABS. All structures were
combined organic layer dried over anhydrous Na2SO4. Product solved by a combination of direct methods and difference
was obtained upon solvent evaporation and was characterized Fourier syntheses, and refined by full-matrix least-squares on
by 1H NMR in CDCl3. 1H NMR spectra of the dimers are F2, by using the SHELXTL software package.50,51 All non-hydro-
available in the ESI† of our earlier communication.43 The gen atoms were refined with anisotropic displacement
isomer was confirmed by solving the structure by X-ray parameters. Hydrogen atoms were placed in geometrically
crystallography.
idealized positions and included as standard riding atoms
during the least-squares refinements. Crystal data, data collec-
tion parameters, and the results of the analyses are listed in
Table 2.
Crystallographic analyses
Details regarding the hydrated crystals were presented in our
previous communication and will not be repeated here. This
article reports crystal structures of three anhydrous trans
isomers and a cis isomer of protonated substituted stilbazoles.
Yellow single crystals of anhydrous 3·HCl and 4·HCl, suitable
for X-ray diffraction analyses were obtained by evaporation of
solvent from an anhydrous methanol solution at 25 °C in a
glove box. Yellow single crystals of anhydrous 5·HCl suitable
for X-ray diffraction analyses were obtained by evaporation of
solvent from an anhydrous ethanol solution at 25 °C in a glove
box. Light yellow single crystals of 11·HCl suitable for X-ray
diffraction analyses were obtained by evaporation of solvent of
a mixture of ethyl acetate–chloroform solution at 25 °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
Compounds 3·HCl and 4·HCl crystallized in the monoclinic
crystal system. The systematic absences in the intensity data
were consistent with either of the space groups P21 or P21/m.
The latter was chosen and confirmed by the successful refine-
ment and solution of the structure. With Z = 2, both molecules
have crystallographic mirror symmetry. The alkene group of
the cationic stilbazole molecule in both complexes is dis-
ordered over two orientations, which were refined in a 50 : 50
ratio. One molecule of methanol cocrystallized with the
complex and was included in the analysis. Compound 5·HCl
ˉ
crystallized in the triclinic crystal system. The space group P1
was assumed and confirmed by the successful refinement and
solution of the structure. The asymmetric crystal unit contains
two molecules of 5·HCl. Compound 11·HCl crystallized in the
monoclinic crystal system. The systematic absences in the
intensity data were consistent with the unique space groups
P21/c.
CCD-based diffractometer using Mo Kα radiation (λ
=
0.71073 Å).49 The raw data frames were integrated with the
SAINT+ program by using a narrow-frame integration algor-
Table 2 Crystallographic data for protonated stilbazoles 3, 4, 5 and 11
3·HCl
4·HCl
5·HCl
11·HCl
Empirical formula
Formula weight
Crystal system
Lattice parameters
a (Å)
b (Å)
c (Å)
C13H11NCl·Cl CH3OH
284.17
Monoclinic
C13H11NBr·Cl CH3OH
328.63
Monoclinic
C13H11NI·Cl
343.58
Triclinic
C13H11NI·Cl
343.58
Monoclinic
4.7953(5)
8.5668(8)
17.3529(18)
90
90.321(2)
90
713.15(12)
P21/m (# 11)
2
1.323
0.443
296
50.00
4.8200(5)
8.7182(9)
17.3278(17)
90
90.278(2)
90
728.14(13)
P21/m (# 11)
2
1.499
2.994
296
53.98
8.6445(5)
14.0901(6)
8.0051(3)
12.3594(5)
90
107.537(1)
90
1329.26(9)
P21/c (# 14)
4
1.717
2.583
296
57.98
12.0822(6)
13.2869(7)
103.255(1)
107.303(1)
97.032(1)
α (°)
β (°)
γ (°)
V (Å3)
1262.22(12)
ˉ
Space group
Z value
P1 (# 2)
4
ρcalc (g cm−3
)
1.808
2.720
296
56.00
μ (Mo Kα) (mm−1
Temperature (K)
2Θmax (°)
)
No. obs. (I > 2σ(I))
No. parameters
Goodness of fit
1090
109
1.054
1270
109
1.025
5290
289
1.035
3064
145
1.015
Max. shift in cycle
Residualsa: R1; wR2
Absorption correction, Max/min
0.000
0.002
0.002
0.003
0.0379; 0.0988
Multi-scan
0.9825/0.8291
0.290
0.0370; 0.0841
Multi-scan
0.7457/0.5597
0.469
0.0272; 0.0654
Multi-scan
0.8538/0.4764
0.606
0.0262; 0.0662
Multi-scan
0.7461/0.6207
1.019
Largest peak in final diff. map (e− Å−3
)
] .
a R = ∑hkl(kFobs| − |Fcalck)/∑hkl|Fobs|; Rw = [∑hklw(|Fobs| − |Fcalc|)2/∑hklwFobs2 1/2, w = 1/σ2(Fobs); GOF = [∑hklw(|Fobs| − |Fcalc|)2/(ndata − nvari)]1/2
This journal is © The Royal Society of Chemistry and Owner Societies 2014
Photochem. Photobiol. Sci., 2014, 13, 1509–1520 | 1511

View Article Online
Paper
Photochemical & Photobiological Sciences
Computational details
All calculations were performed using the Gaussian 09
program.52 The geometry of stilbazole·H+ monomer was opti-
mized at the B3LYP/6-311+G(d,p) level.53–55 To study the inter-
actions between two stilbazole·H+ molecules, four different
arrangements (a, b c, and d in Fig. 12) were optimized at the
second-order Møller–Plesset (MP2) level of theory using the
6-31G(d) basis set.56–58 All aforementioned structures were
optimized in the gas phase without any symmetry constraints.
Results and discussion
In this article we focus our discussion on ten (1–10; Scheme 1)
of the sixteen protonated trans-stilbazoles whose reactivity and
X-ray structures we had previously reported in a communi-
cation.43 Eight (1–8) of the ten (1–10) stilbazole·HCl salts upon
irradiation in the crystalline state dimerized to a single dimer
product (anti-head–tail; Scheme 2). The 1H NMR spectral
pattern was consistent with that of anti-head–tail dimer whose
structure was confirmed by single crystal X-ray diffraction ana-
lysis for the dimer from stilbazole (Fig. 1). Since 1H NMR
spectra of the dimers are available in the ESI† of our earlier
Fig. 1 1H NMR (CDCl3) signals of cyclobutane protons in the dimers
obtained from 1–8·HCl xH2O. Dimers from irradiation of 1–8·HCl xH2O
are shown in order (a)–(h) respectively.
Scheme 1 Structures of stilbazoles investigated. All stilbazoles were
protonated with HCl before irradiation.
Fig. 2 (a) 1·HCl·3H2O (b) 8·HCl·3H2O. Oxygen of H2O represented as
red sphere and Cl− as green sphere. Data for (b) taken from ref. 44.
communication43 they are not reproduced here. The packing
arrangements of eight reactive (1–8) (structure for 8 was taken
from the literature44) and two non-reactive (9 and 10) stilba-
zole·HCl salts are presented in Fig. 2–5. It is clear from these
Scheme 2 Compound 12 (shown in green) is obtained while the other
possible products 13–16 (shown in red) are not obtained from irradiation
of hydrated crystals of 1–10. In the product structures H = head; T = tail.
1512 | Photochem. Photobiol. Sci., 2014, 13, 1509–1520
This journal is © The Royal Society of Chemistry and Owner Societies 2014

View Article Online
Photochemical & Photobiological Sciences
Paper
Fig. 4 (a) 2·HCl·3H2O (b) 3·HCl·2H2O (c) 7·HCl·8H2O. Oxygen of H2O
and –OCH3 represented as red sphere and Cl− as green sphere.
Fig. 3 (a) 4·HCl·3H2O (b) 5·HCl·2H2O (c) 6·HCl·3H2O. Oxygen of H2O
represented as red sphere and Cl− as green sphere.
figures that the reactive ones contain water in their lattice
(Fig. 2–4) while the two unreactive stilbazole salts do not
contain water molecules (Fig. 5) suggesting the possible role of
water for their reactivity.
To examine the role of water in the above reactive crystals,
seven reactive stilbazole·HCl salts (1–5, 7 and 8) were crystal-
lized under dry conditions and examined for reactivity by
irradiation (for the same time periods by which time the
hydrated crystals dimerized). They were found to be inert. The
progress of the reaction of the dry and wet crystals of 4-iodo-
stilbazole, an example, is presented in Fig. 6. Examination of
Fig. 6a–c indicates that while the wet sample has dimerized to
about 49%, (with 5 h of irradiation) the dry sample has not
reacted. Similar results were observed with other samples as
well. Quantitative conversion to dimer was achieved in about
15 h in all cases. For comparative (dry and wet samples)
studies the solids were irradiated for a shorter time to prevent
the dry samples from absorbing moisture from the humid
Fig. 5 Non-reactive derivatives (a) 9·HCl (b) 10·HCl. Cl− is represented
as green sphere.
This journal is © The Royal Society of Chemistry and Owner Societies 2014
Photochem. Photobiol. Sci., 2014, 13, 1509–1520 | 1513

View Article Online
Paper
Photochemical & Photobiological Sciences
Fig. 7 Anhydrous structures (a) 3·HCl·MeOH (b) 4·HCl·MeOH. Oxygen
of methanol represented as red sphere and Cl− as green sphere.
Fig. 6 (a) 1H NMR (300 MHz) in CDCl3 of 5 (b) neutral extract of anhy-
drous 5·HCl, irradiated for 5 h in solid state, (c) neutral extract of
5·HCl·2H2O, irradiated for 5 h in solid state. (d) neutral extract of 11·HCl,
irradiated for 5 h in solid state. The anhydrous 5·HCl shows no dimer
formation, while the hydrous salts shows 49% conversion to dimer and
the 11·HCl shows 11% conversion to trans. Aromatic proton ortho to N in
pyridyl ring is denoted by ‘ ’ for trans, ‘ ’ for cis and ‘ ’ for anti H–T
dimer. Solvent residual peak of chloroform is denoted by ‘ ’.
laboratory. Furthermore, inclusion of water in the wet crystals
and the lack of it in the dry crystals in the solid state were con-
firmed by estimating the water content by TGA (Table 1). The
water content as measured by thermogravimetry and estimated
from crystal unit cell are close and in the range of 10–20%.
The difference in water content in some cases between TGA
measurements and that estimated from X-ray crystal structures
is possibly due to the nature of samples used for these two
studies; powder for TGA and single crystals for X-ray data col-
lection. To shed more light on the lack of reactivity of anhy-
drous crystals attempts were made to obtain single crystals of
dry 1–7 HCl salts. However, only three anhydrous samples
(3–5) gave single crystals suitable for X-ray analysis and the
packing arrangements for them as obtained from X-ray struc-
ture determination are presented in Fig. 7 and 8. As evident
from the figures two of these crystals contained methanol
from the crystallization solvent instead of water in the lattice.
First we discuss the packing arrangements of the photoreac-
tive stilbazole·HCl salts (1–8) with water molecules in their
lattice. Examination of Fig. 2–4 leads to the following con-
clusions: (a) adjacent pairs of stilbazole molecules are within
4.2 Å and parallel to one another. (b) The two olefins are
arranged in a head to tail centrosymmetric fashion with
respect to one another (i.e., α-packing as per Schmidt’s cin-
namic acid nomenclature59). (c) The pairs of molecules as
arranged above upon excitation could smoothly transform to
anti-head–tail dimer (consistent with the photochemical
results mentioned in the previous paragraph). (d) Although the
Fig. 8 Anhydrous structures (a) 5·HCl (b) 6·HCl·Cl− is represented as
green sphere. Data for (b) taken from ref. 45.
elegant ladder-like stacking of stilbazole molecules in the
lattice seem similar between the diagrams, the side rails
holding the rungs of the ladder differ significantly between
the eight reactive molecules. This observation is quite different
from the thiourea template we reported earlier29 where the
rungs and side rails looked closely similar for the ten different
stilbazoles. (e) As illustrated in Fig. 9 there are six different
arrangements for the side rails made up of water molecules
and chloride ions for the eight protonated stilbazoles. (f) In all
cases the ladder-like packing of the reactive protonated stilba-
zoles are held by the rungs made up of water molecules or
chloride ions. A combination of water and chloride ion holds
the stilbazole–H+ through either N–H⋯O or N–H⋯Cl− inter-
actions. Of the eight packing arrangements five are held by
N–H⋯O bonding (Fig. 2 and 3)60,61 and the remaining by
N–H⋯Cl− (Fig. 4). (g) The hydrogen bonding between the
three, pyridinium N–H, water and chloride ions are inter-
dependent for the stability and arrangement of the lattice.
1514 | Photochem. Photobiol. Sci., 2014, 13, 1509–1520
This journal is © The Royal Society of Chemistry and Owner Societies 2014

View Article Online
Photochemical & Photobiological Sciences
Paper
crystals (3–6) suitable for X-ray diffraction studies. Their crystal
packing shown in Fig. 7 and 8 account for their inertness. In
all cases the adjacent olefins are >4.7 Å apart and the olefinic
π-bonds are slipped with respect to each other. From the above
experiments it is clear that water molecules play a critical role
in templating the protonated stilbazoles toward photodimeri-
zation in crystals. We were unable to test whether Cl− is essen-
tial or any other anion would function as a template along
with water molecules. When HBr, HI and HClO4 were used to
protonate the stilbazole a wet paste was obtained that could
not be dried to a powder suitable for irradiation. Therefore at
this stage we assume that Cl− in combination with water is a
useful template for protonated stilbazole. We plan to pursue
experiments to probe whether Cl− is unique or whether any
other anion in combination with H2O would be able to
template the olefins.
Examination of the packing arrangements shown in
Fig. 2–4 reveals that the hydrogen bonding between N–H⋯O
and N–H⋯Cl− control the packing. In addition hydrogen
bonding between Cl− and H2O is essential for building a
running template along both the sides of the reactive olefins.
As evident from Fig. 5, 7 and 8, without water, Cl− alone
cannot build a rail to hold the rungs of the ladder. The above
studies have established that templation of the protonated stil-
bazoles could be brought about with a composite of water and
chloride ion. Both are essential for the photodimerization of
protonated stilbazoles in the solid state.
Two common known photoreactions of olefins in solution
are unimolecular geometric isomerization and bimolecular
dimerization.62 In solution the geometric isomerization com-
petes effectively with the diffusion controlled dimerization
process. On the other hand, in crystals the need for diffusion
of the two molecules towards one another if eliminated
through pre-organization, the dimerization is the main reac-
tion. In addition, generally the geometric isomerization requir-
ing considerable free space around the excited molecule does
not occur in close packed crystals. Because of the horizontal
planar structure, trans-olefins tend to pack tightly as compared
to slightly twisted cis-olefins. This reduces the probability of
trans to cis isomerization in crystals63,64 while several examples
of cis to trans isomerization of olefins in crystals are
known.65–68 However, the mechanism of geometric isomeriza-
tion in crystals is less well established than in solution.
Following the success with the photodimerization, we
decided to examine the geometric isomerization of trans and
cis-stilbazole·HCl salts as crystals. None of the hydrated or
anhydrous trans-stilbazole·HCl salts upon excitation isomer-
ized to the cis-isomer; they either dimerized or were inert. This
is consistent with the known behavior of trans-cinnamic acids.
On the other hand, exposure of several cis-stilbazole·HCl salts
to light resulted in conversion to the trans isomer. In this
article, we present results on one system namely, cis-4-iodo stil-
bazole·HCl salt (11; Scheme 1). Crystallization of cis 4-iodo
stilbazole·HCl salt in a manner similar to the trans isomer
under normal laboratory conditions, surprisingly gave crystals
without inclusion of water molecules. Note that under such
Fig. 9 H2O and Cl− motifs formed. Oxygen of H2O represented as red
sphere and Cl− as green sphere.
Fig. 2–4 reveal that in each case the olefin in the middle
could react with the one above or below (the three olefins are
indicated with distances between them) to give the same
product anti-head–tail dimer. The distances between the two
possible reactive pairs are provided in these figures. In all
cases except Fig. 3(a) and (b) the distance between the two
pairs are close and less than 4.2 Å. In principle any one of the
two pairs could initiate the dimerization. In the case of
4·HCl·3H2O and 5·HCl·2H2O (Fig. 3(a) and (b)) one pair is
within 4.2 Å and the other pair is separated by more than
4.2 Å. It is likely that the pair with a distance of less than 4.2 Å
reacts to yield the anti-head–tail dimer. Regardless of the speci-
ficity of the reacting pair quantitative conversion would be
expected in all cases.
The above eight protonated stilbazoles, all containing water
in their crystal lattice photodimerized to a single anti-head–tail
dimer product within 5 h upon irradiation with 450 W
medium pressure lamp through a Pyrex glass plate. For total
conversion a much longer time was required. As illustrated in
Fig. 5, the protonated stilbazoles 9 and 10 which are unreactive
in the crystalline form do not contain water in the lattice and
the adjacent molecules are separated by >4.9 Å. One should
note that in reactive crystals the reactive molecules were separ-
ated by less than 4 Å. This difference between the two sets
highlights the importance of water in packing molecules in a
topochemically favorable arrangement. Our attempts at crystal-
lizing stilbazole·HCl 9 and 10 with water (in their lattices), to
test if the presence of water would make them reactive, were
not successful. We therefore decided to crystallize the reactive
protonated stilbazoles 1–8 without water and examine their
reactivity and packing. As mentioned above, seven (1–5, 7 and
8) of the eight protonated stilbazoles were obtained as
powders (without water) and upon excitation remained unreac-
tive. Of the seven inert protonated stilbazoles four gave single
This journal is © The Royal Society of Chemistry and Owner Societies 2014
Photochem. Photobiol. Sci., 2014, 13, 1509–1520 | 1515

View Article Online
Paper
Photochemical & Photobiological Sciences
Fig. 10 Partial packing arrangement of 11·HCl. (a) Distance between the two planes defined by the two pyridinium rings is 3.397 Å. Cl− is rep-
resented as green sphere. (b) Distance between the CvC bonds is 8.005 Å. Cl− are omitted for clarity.
conditions the trans isomer crystals included water molecules the mechanism of geometric isomerization of olefins in
(Fig. 3b). trans 4-Iodo stilbazole·HCl salt could also be crystal- crystals.
lized without water as shown in Fig. 8a. Thus, these three crys-
In the literature cation–π interaction is suggested to be
tals presented an opportunity to compare the packing and important in packing of protonated trans-stilbazole molecules
photoreactivity of trans 4-iodo stilbazole·HCl salt under in a photodimerizable arrangement.44,45,69,70 The fact that the
hydrated and anhydrous conditions and the cis 4-iodo stilbazole· crystals of 3–6 HCl not containing water molecules are not
HCl salt under anhydrous conditions. The packing arrange- reactive suggested that cation–π interaction is less important
ment of cis 4-iodo stilbazole·HCl salt is presented in Fig. 10. than hydrogen bonding of N–H with water molecules and
The packing is facilitated by N–H⋯Cl− and π⋯π interactions chloride ions. More importantly, we reasoned that in proto-
(see Fig. 10a); interestingly the protonated pyridyl rings are nated stilbazoles the positive charge would be localized on the
arranged parallel in a head–tail fashion within 3.97 Å. Prob- N–H σ-bond rather than on the π-cloud of the pyridyl ring. If
ably π⋯π interaction favors stacking of the two rings, and elec- this is true we believed that the two reactive olefins would be
tron deficiency of the pyridyl ring due to protonation prompts perpendicular to each other and would not be in the correct
head–tail arrangement. Also noteworthy is that since the adja- geometry for dimerization. To examine the location of the
cent CvC bonds are far apart (>8 Å; Fig. 10b), dimerization is positive charge in the protonated stilbazole, we optimized its
not expected. Such loose packing of these crystals as compared structure at the B3LYP/6-311+G(d,p) level and calculated the
to the trans 4-iodo stilbazole·HCl (anhydrous and hydrated; charges on all atoms. The charges on the optimized structure
Fig. 3b and 8a) is likely to favor geometric isomerization upon are shown in Fig. 11. It is important to note that the carbon
excitation. As expected, irradiation of cis 4-iodo stilbazole·HCl atoms carry the negative charge and all the hydrogens atoms
crystals gave the corresponding trans isomer as the only are positively polarized. As intuitively predicted the hydrogen
1
product; no dimers were obtained. The H NMR spectra of the atom bound to the nitrogen atom (N–H) carries the largest
three irradiated samples (hydrated and anhydrous trans 4-iodo positive charge (+0.43) and the negative charge (−0.47) resides
stilbazole·HCl salts and anhydrous cis 4-iodo stilbazole·HCl on the nitrogen atom. This clearly suggests that the protonated
salt) presented in Fig. 6 clearly highlights the reactivity differ- stilbazole is positively polarized along the N–H σ bond.
ences between the three crystals. The hydrated trans crystal
dimerized to a single anti-head–tail dimer (12 in Scheme 2;
Fig. 6c); the anhydrous trans crystals were photostable (Fig. 6b)
and the anhydrous cis crystals isomerized to the trans (Fig. 6d).
The photochemical behavior is consistent with the expec-
tations based on crystal packing. It is important to note the
crystal packings of both trans and cis 4-iodo stilbazole·HCl
salts do not favor dimerization (Fig. 8a and 10). Thus a geo-
metric isomerization pathway is open to both molecules in the
excited state. However, only the cis isomer isomerizes to trans,
but the trans isomer does not react. This suggests that careful
analysis of the packing is required to understand the differ-
Fig. 11 Charge distribution in the case of protonated trans-stilbazole as
ence. We are currently carrying out experiments to probe
calculated by at the B3LYP/6-311+G(d,p) level.
1516 | Photochem. Photobiol. Sci., 2014, 13, 1509–1520
This journal is © The Royal Society of Chemistry and Owner Societies 2014

View Article Online
Photochemical & Photobiological Sciences
Paper
Fig. 12 Structures and energies of pairs of protonated trans-stilbazole molecules in the gas phase optimized at the second-order Møller–Plesset
(MP2) level of theory using the 6-31G(d) basis set.
To probe how this positively polarized N–H bond would inter- conclude that the packing in protonated stilbazoles are stabil-
act with another protonated stilbazole we optimized four struc- ized by hydrogen bonds and charge transfer interaction43
tures (Fig. 12a–d) at the second-order Møller–Plesset (MP2) rather than by cation–π interaction. The approach we have
level of theory using the 6-31G(d) basis set.56–58 Structures and adopted also illustrates that the quantum chemical calcu-
energies of the four structures are shown in Fig. 12. Two struc- lations could be a valuable tool in solid-state photochemistry
tures (a and d) correspond to the N–H bond being oriented to infer molecular packing in crystals. Combining crystallo-
towards the π cloud of the two rings (cation–π interaction), graphy, photochemistry, quantum chemistry and solid state
pyridyl and phenyl and the other two structures correspond to techniques could be valuable in solving problems in solid state
the two olefin molecules being one on top of each other in a photochemistry that seem to be complex and challenging.
parallel fashion in head to tail (b) and head to head (c)
fashion. Because the pyridyl part is electron deficient and the
phenyl part is relatively electron rich the structure (b) could be
more stable due to charge transfer interaction and the arrange-
Conclusions
ment (d) would be least stable due to charge repulsion. Of the
four structures shown in Fig. 12 only the head–tail arrange-
ment shown in Fig. 12(b) would give the observed dimer.
According to MP2 calculations, of the four arrangements the
most stable one in the gas phase is the structure shown in
Fig. 12(a) in which the two olefins are nearly perpendicular to
each other. The next most stable one is the head–tail arrange-
ment shown in Fig. 12(b). In this case the stability (although
lower than cation–π interaction) results from charge-transfer
interaction between the electron deficient pyridyl and electron
rich phenyl rings. The fact that this arrangement is preferred
in crystals, according to crystal structure analysis (Fig. 2–4),
suggests that hydrogen bonds between stilbazole–H+ and water
molecules or chloride ions provide enough energy to overcome
the cation–π interaction. Thus based on MP2 calculations we
Photodimerization continues to play a key role in the develop-
ment of the field of organic solid state photochemistry. The
unfortunate unpredictability of solid-state photoreactions,
with its history dating back to the late 1800, is intimately
linked to crystal engineering which despite the considerable
interest and development, is yet to become a predictable
science. One of the current methods of organic solid-state
photodimerization termed templated photodimerization
involves the use of a template whose bonding properties with
the reactant molecules are better understood than the inter-
molecular interactions between the reactants themselves. In
this article we have identified a mixture of water and chloride
ions that serve as a template to orient protonated stilbazoles
toward photodimerization in the solid state. It is important to
note that unlike thiourea, 1,3-dihydroxybenzene and other
This journal is © The Royal Society of Chemistry and Owner Societies 2014
Photochem. Photobiol. Sci., 2014, 13, 1509–1520 | 1517

View Article Online
Paper
Photochemical & Photobiological Sciences
ditopic templates the system identified here is unlikely to
serve as a universal template. The weak interactions identified
here is specific to the olefins investigated here. However, two
common molecules, water and chloride ion, together are able
to template a number of protonated stilbazoles is interesting
8 G. R. Desiraju, Crystal Engineering: The Design of Organic
Solids, Elsevier, Amsterdam, 1989.
9 G. R. Desiraju and T. Steiner, The Weak Hydrogen Bond in
Structural Chemistry and Biology, Oxford University Press
Inc., New York, United States, 1999.
and calls for in further understanding of principles of crystal 10 G. R. Desiraju, J. J. Vittal and A. Ramanan, Crystal Engineer-
engineering.
ing A Textbook, World Scientific Publishing Co. Pte. Ltd.,
By combining X-ray crystallography and photochemistry we
Singapore, 2011.
have established that water molecule and chloride ion has the 11 J. D. Dunitz, Are Crystal Structures Predictable, Chem.
potential to be a general template (13 of the 16 protonated stil- Commun., 2003, 545–548.
bazoles reacted; success rate 81%). The templation is facili- 12 J. D. Dunitz and A. Gavezzoti, How molecules stick together
tated by weak interaction between PyN+–H and water or PyN+–
in organic crystals: weak intermolecular interactions,
Chem. Soc. Rev., 2009, 38, 2622–2633.
H and Cl−. In spite of the reported success, it is not obvious
which of the two interactions is going to be dominant in a 13 J. Svoboda and B. Konig, Templated Photochemistry:
given case and it is also not clear whether this approach would
apply to other olefins as well. During the last decade several
templates have been identified, each one with its own advan-
Toward Catalyst Enhancing the Efficiency and Selectivity of
Photoreactions in Homogeneous Solutions, Chem. Rev.,
2006, 106, 5413–5430.
tages and disadvantages, but they all suggest that further work 14 L. S. Shimizu, S. R. Salpage and A. A. Korous, Functional
is needed. The field of organic solid state photochemistry com-
bined with crystal engineering has plenty of opportunities for
Materials from Self-Assembled Bis-urea Macrocycles, Acc.
Chem. Res., 2014, 47, 2116–2127.
those interested in combining crystallography and photochem- 15 B. Bibal, C. Mongin and D. M. Bassani, Template effects
istry. More importantly, the combination of quantum chemical
calculations with crystallography and photochemistry, should
and supramolecular control of photoreactions in solution,
Chem. Soc. Rev., 2014, 43, 4179.
help in the understanding of weak interactions in solids, and 16 N. Vallavoju and J. Sivaguru, Supramolecular photocataly-
predicting reactivity of molecules in the crystalline state.
sis: combining confinement and non-covalent interactions
to control light initiated reactions, Chem. Soc. Rev., 2014,
43, 4084–4101.
17 V. Ramamurthy and A. Parthasarathy, Chemistry in
Restricted Spaces: Select Photodimerizations in Cages,
Cavities, and Capsules, Isr. J. Chem., 2011, 51, 817–829.
18 K. Tanaka, E. Mochizuki, N. Yasui, Y. Kai, I. Miyahara,
K. Hirotsu and F. Toda, Single-Crystal-to-Single-Crystal
Enantioselective [2 + 2] Photodimerization of Coumarin,
Thiocoumarin and Cyclohex-2-enone in the Inclusion
Complexes with Chiral Host Compounds, Tetrahedron,
2000, 56, 6853–6865.
19 K. Tanaka and F. Toda, Selective Photodimerization of
Coumarin in Crystalline Inclusion Compounds, J. Chem.
Soc., Perkin Trans. 1, 1992, 943–944.
20 F. Toda, Solid State Organic Chemistry: Efficient Reactions,
Remarkable Yields, and Stereoselectivity, Acc. Chem. Res.,
1995, 28, 480–486.
Acknowledgements
VR is grateful to the National Science Foundation, USA
(CHE-0848017 and CHE-1411458) for continued generous
financial support.
References
1 N. J. Turro, V. Ramamurthy and J. C. Scaiano, Modern
Molecular Photochemistry of Organic Molecules, University
Science Books, Sausalito, 2010, ch. 13.
2 C. T. Libermann, Ann. Chem. Pharm., 1871, 158, 300.
3 V. Ramamurthy and K. Venkatesan, Photochemical reac-
tions of organic chemistry, Chem. Rev., 1987, 87, 433–481.
4 A. Natarajan and V. Ramamurthy, Solvent-free photosyn-
thesis of cyclobutanes: Photodimerization of crystalline
olefins, in The Chemistry of Cyclobutanes, ed. Z. Rappoport
21 F. Toda, K. Tanaka and M. Yagi, Highly Selective Photoreac-
tions of alpha-Oxoamides and alpha-Tropolone alkyl ethers
in crystalline inclusion complexes, Tetrahedron, 1987, 43,
1495–1502.
and J. F. Liebman, John Wiley & Sons, Ltd, Chichester, 22 Y. Ito, B. Borecka, G. Olovsson, J. Trotter and J. R. Scheffer,
2005, pp. 807–872.
5 D. Ginsburg and G. M. J. Schmidt, et al., Solid State Photo-
chemistry, Verlag Chemie, GmbH, Weinheim, 1976.
Control of the solid-state photodimerization of some
derivatives and analogs of trans-cinnamic acid by ethylene-
diamine, Tetrahedron Lett., 1995, 36, 6087–6090.
6 M. D. Cohen and G. M. J. Schmidt, Topochemistry. Part 1. 23 Y. Ito, B. Borecka, M. Trotter and J. R. Scheffer, Control of
A Survey, J. Chem. Soc., 1964, 1996–2000.
7 G. S. Murthy, P. Arjunan, K. Venkatesan and
V. Ramamurthy, Consequences of lattice relaxability in
solid-state photodimerization of trans-cinnamic acid by
double salt formation with diamines, Tetrahedron Lett.,
1995, 36, 6083–6086.
solid state photodimerizations, Tetrahedron, 1987, 43, 24 L. R. MacGillivray, G. S. Papaefstathiou, T. Friscic,
1225–1240.
T. D. Hamilton, D.-K. Bucar, Q. L. Chu, D. B. Varshney and
1518 | Photochem. Photobiol. Sci., 2014, 13, 1509–1520
This journal is © The Royal Society of Chemistry and Owner Societies 2014

View Article Online
Photochemical & Photobiological Sciences
Paper
I. G. Georgiev, Supramolecular control of reactivity in the
solid state: From templates to ladderanes to metal–organic
frameworks, Acc. Chem. Res., 2008, 41, 280–291.
zoles with water-soluble calixarenes, Photochem. Photobiol.
Sci., 2006, 5, 925–930.
40 R. Kaliappan, M. V. S. N. Maddipatla, L. S. Kaanumalle and
V. Ramamurthy, Crystal engineering principles applied to
solution photochemistry: controlling the photodimeriza-
tion of stilbazolium salts within gamma-cyclodextrin and
cucurbit[8]uril in water, Photochem. Photobiol. Sci., 2007, 6,
737–740.
25 L. R. MacGillivray, J. L. Reid and J. A. Ripmeester, Supra-
molecular control of reactivity in the solid state using
linear molecular templates, J. Am. Chem. Soc., 2000, 122,
7817–7818.
26 M. Nagarathinam, A. M. P. Peedikakkal and J. J. Vittal,
Stacking of double bonds for photochemical [2 + 2] cyclo- 41 M. V. S. N. Maddipatla, L. S. Kaanumalle, A. Natarajan,
addition reactions in the solid state, Chem. Commun., 2008,
5277–5288.
M. Pattabiraman and V. Ramamurthy, Preorientation of
Olefins toward a Single Photodimer: Cucurbituril-Mediated
Photodimerization of Protonated Azastilbenes in Water,
Langmuir, 2007, 23, 7545–7554.
27 N. Shan and W. Jones, Identification of supramolecular
templates: design of solid-state photoreactivity using struc-
tural similarity, Tetrahedron Lett., 2003, 44, 3687–3689.
28 T. Caronna, R. Liantonio, T. A. Logothetis, P. Metrangolo,
T. Pilati and G. Resnati, Halogen Bonding and pi⋯pi Stack-
ing Control Reactivity in the Solid State, J. Am. Chem. Soc.,
2004, 126, 4500–4501.
42 K. Takagi, B. R. Suddaby, S. L. Vadas, C. A. Backer and
D. G. Whitten, Topological Control of Reactivity by Inter-
facial Orientation: Excimer Fluorescence of 4-Stilbazolium
Cations in Aerosol OT Reversed Micelles, J. Am. Chem. Soc.,
1986, 108, 7865–7867.
29 B. R. Bhogala, B. Captain, A. Parthasararthy and 43 B. Mondal, B. Captain and V. Ramamurthy, Photodimeriza-
V. Ramamurthy, Thiourea as a Template for Photodimeriza-
tion of Azastilbenes, J. Am. Chem. Soc., 2010, 132, 13434–
13442.
tion of HCl salts of azastilbenes in the solid state, Photo-
chem. Photobiol. Sci., 2011, 10, 891–894.
44 S. Yamada and Y. Nojiri, Water-assisted assembly of
(E)-arylvinylpyridine hydrochlorides: effective substrates for
solid-state [2 + 2] photodimerization, Chem. Commun.,
2011, 47, 9143–9145.
30 J. L. R. Williams, Photodimerization of 2-Styrylpyridine,
J. Org. Chem., 1960, 25, 1839–1840.
31 J. L. R. Williams, J. M. Carlson, G. A. Reynolds and
R. E. Adel, Photodimers of 4′-Substituted 2-Styrylpyridines, 45 S. Yamada, N. Sako, M. Okuda and A. Hozumi, A dual-
J. Org. Chem., 1963, 28, 1317–1320.
synthon in pyridinium chloride: formation of ladder-like
and columnar motifs through hydrogen bonds and cation-
pi interactions, CrystEngComm, 2013, 15, 199–205.
32 J. L. R. Williams, S. K. Webster and J. A. V. Allian, cis and
trans Isomers of 2-Styrylpyridine, J. Org. Chem., 1961, 26,
4893–4895.
33 G. W. Leubner, J. L. R. Williams and C. C. Unruh, Light-
Sensitive Polymeric Stilbazoles and Quaternary Salts Thereof.
2,811,510, 1957.
46 S. M. N. Efange, R. H. Michelson, R. P. Remmel,
R. J. Boudreau, A. K. Dutta and A. Freshler, Flexible
N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine analog: syn-
thesis and monoamine oxidase catalyzed bioactivation,
J. Med. Chem., 1990, 33, 3133–3138.
34 F. H. Quina and D. G. Whitten, Medium Effects on
Photochemical Reactions. Photochemistry of Surfactant 47 S. Brasselet, F. Cherioux, P. Audebert and J. Zyss, New Octu-
Alkyl-4-stilbazole Salts in solution, in the Solid State, and
in Monolayer Assemblies, J. Am. Chem. Soc., 1975, 97,
1602–1603.
polar Star-Shaped Structures for Quadratic Nonlinear
Optics, Chem. Mater., 1999, 11, 1915–1920.
48 J. G. Rodriguez, R. Martin-Villamil and A. Lafuente, pi-
extended conjugate phenylacetylenes. Synthesis of 4-[(E)
and (Z)-2-(4-ethenylphenyl)ethenyl]pyridine. Dimerization,
quaternation and formation of charge-transfer complexes,
Tetrahedron, 2003, 59, 1021–1032.
35 F. H. Quina and D. G. Whitten, Photochemical Reactions
in Organized Monolayer Assemblies. 4. Photodimerization,
Photoisomerization, and Excimer Formation with Surfac-
tant Olefins and Dienes in Monolayer Assemblies, Crystals,
and Micelles, J. Am. Chem. Soc., 1977, 99, 877–883.
36 J. Bolt, F. H. Quina and D. G. Whitten, Solid State Photo-
49 Apex2 Version 2.2–0 and SAINT+ Version, Bruker Analytical
X-Ray System, Inc., Madison, Wisconsin, USA, 2007.
dimerization of Surfactant Esters of Cinnamic Acid, Tetra- 50 G. M. Sheldrick, SHELXTL Version 6.1, Bruker Analytical
hedron Lett., 1976, 30, 2595–2598. X-Ray Systems, Inc., Madison, Wisconsin, USA, 2000.
37 X.-H. Li, L.-Z. Wu, L.-P. Zang and C.-H. Tung, Controlled 51 G. M. Sheldrick, A short history of SHELX, Acta Crystallogr.,
Photocyclization, Photodimerization, and Photoisomeriza- Sect. A: Fundam. Crystallogr., 2008, 64, 112–122.
tion of Stilbazole Salts within Nafion Membranes, Org. 52 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
Lett., 2002, 4, 1175–1177.
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato,
X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng,
J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda,
J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao,
H. Nakai, T. Vreven, J. J. A. Montgomery, J. E. Peralta,
F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
38 M. Pattabiraman, A. Natarajan, R. Kaliappan, J. T. Mague
and V. Ramamurthy, Template directed photodimerization
of trans-1,2-bis(n-pyridyl)ethylenes and stilbazoles in
water, Chem. Commun., 2005, 4542–4544.
39 R. Kaliappan, L. S. Kaanumalle, A. Natarajan and
V. Ramamuthy, Templating photodimerization of stilba-
This journal is © The Royal Society of Chemistry and Owner Societies 2014
Photochem. Photobiol. Sci., 2014, 13, 1509–1520 | 1519

View Article Online
Paper
Photochemical & Photobiological Sciences
K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, 61 A. Mukherjee and G. R. Desiraju, Combinatorial
K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar,
J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene,
Exploration of the Structural Landscape of Acid-Pyridine
Cocrystals, Cryst. Growth Des., 2014, 14, 1375–1385.
J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, 62 N. J. Turro, V. Ramamurthy and J. C. Scaiano, Modern Mole-
R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin,
R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
cular Photochemistry of Organic Molecules, University
Science Books, Sausalito, CA, 2010.
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, 63 G. W. Griffin, E. J. O. Connell and J. M. Kelliher, On the
J. J. Dannenberg, S. Dapprich, A. D. Daniels, Ö. Farkas,
J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox,
Gaussian 09 (Revision A.1), 2009.
mechanism of the Solid state cis-trans Photoisomerisation
of the 1,2 Dibenzoylethylene, Proc. Chem. Soc., London,
1964, 337–338.
53 A. D. Becke, Density-functional exchange-energy approxi- 64 J. C. J. Bart and G. M. J. Schmidt, Topochemistry. Part XL.
mation with correct asymptotic behavior, Phys. Rev. A,
1988, 38, 3098–3100.
54 A. D. Becke, A new mixing of Hartree–Fock and local
The crystal structures of trans-1,2 dibenzoyl- and 1,2-di-
p-toluylethylene, Recl. Trav. Chim. Pays-Bas, 1978, 97, 231–
238.
density-functional theories, J. Chem. Phys., 1993, 98, 1372– 65 J. Bregman, K. Osaki, G. M. J. Schmidt and F. I. Sonntag,
1377.
Topochemistry. Part IV. The Crystal Chemistry of some cis-
55 R. Krishnan, J. S. Binkley, R. Seeger and J. A. Pople, Self
Cinnamic Acids, J. Chem. Soc., 1964, 2021–2030.
consistent molecular orbital methods. XX. A basis set for 66 J. N. Moorthy, P. Venkatakrishnan, G. Savitha and
correlated wave functions, J. Chem. Phys., 1980, 72, 650–
654.
56 R. Ditchfield, W. J. Hehre and J. A. Pople, Self Consistent
Molecular Orbital Methods. IX. An Extended Gaussian
R. G. Weiss, Cis-trans and trans-cis isomerizations of styryl-
coumarins in the solid state. Importance of the location of
free volume in the crystal lattices, Photochem. Photobiol.
Sci., 2006, 5, 903–913.
Type Basis for Molecular Orbital Studies of Organic Mole- 67 J. Saltiel, T. S. R. Krishna, S. Laohhasurayotin, K. Fort and
cules, J. Chem. Phys., 1971, 54, 724–728.
R. J. Clark, Photoisomerization of cis,cis- to trans,trans-
1,4-Diaryl-1,3-butadienes in the Solid State: The
Bicycle-Pedal Mechanism, J. Phys. Chem. A, 2007, 112, 199–
209.
57 W. J. Hehre, R. Ditchfield and J. A. Pople, Self-Consistent
Molecular Orbital Methods. XII. Further Extensions of
Gaussian-Type Basis Sets for Use in Molecular Orbital
Studies of Organic Molecules, J. Chem. Phys., 1972, 56, 68 A. Natarajan, J. T. Mague, K. Venkatesan, T. Arai and
2257–2261.
V. Ramamurthy, Volume-Demanding Cis-Trans Isomeriza-
tion of 1,2-Diaryl Olefins in the Solid State, J. Org. Chem.,
2006, 71, 1055–1059.
58 W. J. R. L. Hehre, P. v. R. Schleyer and J. A. Pople, Ab Initio
Molecular Orbital Theory, John Wiley & Sons, New York,
1986.
59 G. M. J. Schmidt, Topochemistry. Part III. The Crystal
Chemistry of some trans-cinnamic acids, J. Chem. Soc.,
1964, 2014–2021.
69 G. K. Kole, G. K. Tan and J. J. Vittal, Crystal engineering
studies on the salts of trans-4,4′-stilbenedicarboxylic acid
in the context of solid state [2 + 2] cycloaddition reaction,
CrystEngComm, 2011, 13, 3138–3145.
60 J. Bernstein, R. E. Davis, L. Shimoni and N.-L. Chang, 70 G. K. Kole, G. K. Tan and J. J. Vittal, Solid state photodimeri-
Patterns in Hydrogen Bonding: Functionality and Graph
Set Analysis in Crystals, Angew. Chem., Int. Ed. Engl., 1995,
34, 1555–1573.
zation of trans-2–4-pyridyl-4-vinylbenzoic acid via salt for-
mation and isomerisation of cyclobutane compounds in
solution, CrystEngComm, 2012, 14, 7438–7443.
1520 | Photochem. Photobiol. Sci., 2014, 13, 1509–1520
This journal is © The Royal Society of Chemistry and Owner Societies 2014