Engineering solid-state molecular switches: N-salicylidene N-heterocycle derivatives

FULL PAPER

DOI: 10.1002/ejoc.200901175

Engineering Solid-State Molecular Switches: N-Salicylidene N-Heterocycle

Derivatives

François Robert,^[a]Anil D. Naik,^[a]Florence Hidara,^[a]Bernard Tinant,^[b]

Raphaël Robiette,^[c]Johan Wouters,^[d]and Yann Garcia*^[a]

Keywords: Molecular devices / Thermochromism / Photochromism / Crystal engineering

A supramolecular engineering approach has been developed

for a novel family of N-salicylidene aniline derivatives to con-

trol their thermo- and photochromic behaviours. Hsaltrz,

Habs, Hsalphen, Hsaltz and Ksaltz are versatile molecules

interactions. A theoretical study of the crystal packing, com-

bined with time-dependent diffuse reflectance studies of

Habs, have confirmed the appearance of photochromism at

room temperature completed with an unprecedented

built from N-heterocycles, which drive the molecular ar- “spring-type effect” observed during the photochemical re-

rangement to form a controlled crystal packing with prede-

signed optical properties. A complete structural, optical and

computational study of powders of these new molecular

nanoswitches is presented. An N-salicylidene aniline deriva-

tive possessing no thermo- or photoinduced chromic proper-

ties thanks to a specific molecular geometry was sought, and

Habs and Hsalphen were designed to enhance π–π stacking

laxation of the metastable trans-keto form. The absence of

thermochromism in an N-salicylidene aniline derivative, in-

duced by the absence of cis-keto form, is a unique behaviour,

firstly identified and explained for these types of compounds.

Finally, Hsaltz and Ksaltz are the result of a molecular deriva-

tion of Hsaltrz which allow enhancing the amplitude of

thermochromism of these functional materials.

connected through a large panel of supramolecular interac-

tions such as H-bonds,^[9]π–π stackings,^[10]hydrophilic,^[11]

ionic^[12]and halogen–halogen interactions.^[13]This diversity

of interactions explains the complexity of supramolecules

which can be either artificial (nanogrids,^[14–16]nanoracks,^[17]

nanocages^[18]) or natural^[19](DNA, recognition proteins).

Supramolecular interactions can be very stable (thermo-

dynamic) or very labile (kinetic),^[20]which allows the system

to (re)organize either itself^[21]or under a given constraint

(temperature,^[21]light irradiation,^[10,22]pH^[23]and solvent

change,^[21]concentration^[21]) to form complex self-assembl-

ies. The self-reorganization ability of supramolecular sys-

tems plays also an important role in molecular recognition

encountered in some host–guest complexes.^[20]The nature

of the supramolecular architecture can also be used to pre-

dict physical properties (cooperativity and spin-transition

temperature,^[24]magnetic behaviours,^[16]porosity,^[25]etc.)

thanks to crystal structure control. This approach, called

crystal engineering,^[26]based on supramolecular chemistry

concepts, allows simplifying the complex problem of struc-

ture prediction into a problem of network architecture^[26]

where molecules, metals, ions and so on are considered as

nodes and the intermolecular interactions or coordination

bonds represent node connections. In this report, we make

use of this strategy to obtain new switchable functional ma-

terials based on the N-salicylidene moiety, whose optical

properties could be finely tuned thanks to a proper elec-

tronic and structural control.

Introduction

Supramolecular chemistry constitutes a highly versatile

research field where new complex chemical systems are

made from simple components interacting through nonco-

valent intermolecular forces.^[1]Supramolecular chemistry

concepts have been largely used in materials science,^[2]poly-

mer science^[3]and biology^[4]and provide novel types of ap-

plications (solid-state receptors,^[5]chemoresponsive sen-

sors,^[6]zeolite analogs,^[7]etc). Supramolecular architectures

are constructed thanks to steric and electronic information

contained in basic molecular building blocks.^[8]These pri-

mary covalent units are made of an internal algorithm that

controls the final supramolecular assembly.^[1]In such archi-

tectures, molecular entities (made by covalent bonds) are

[a] Unité de Chimie des Matériaux Inorganiques et Organiques,

Département de Chimie, Université Catholique de Louvain,

Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium

Fax: +32-1047-2330

E-mail: yann.garcia@uclouvain.be

[b] Unité de Chimie Structurale et des Mécanismes Réactionnels,

Département de Chimie, Université Catholique de Louvain,

Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium

[c] Unité de Chimie Organique et Médicinale, Département de

Chimie, Université Catholique de Louvain,

Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium

[d] Laboratoire de Chimie biologique Structurale, Faculté des

Sciences, Facultés Notre-Dame de la Paix,

Rue de Bruxelles 61, 5000 Namur, Belgium

Supporting information for this article is available on the

WWW under http://dx.doi.org/10.1002/ejoc.200901175.

Eur. J. Org. Chem. 2010, 621–637

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Y. Garcia et al.

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N-Salicylidene aniline derivatives have been used in vari- fluorescent. It is assumed that the trans*–trans relaxation is

ous fields such as nonlinear optics,^[27]catalysis,^[28]bio- mostly nonradiative. Back reaction from trans-keto to cis-

logy^[29]and several applications have been proposed (chemi- keto and enol forms is either thermally or photochemically

cal sensor,^[30]stimuli responsive polymer,^[31]molecular ma- induced. The thermal lifetime of the metastable trans-keto

chines,^[32]information storage and display^[33,34]). This latter form strongly depends on the energy barrier between the

application is probably the most promising one thanks to trans- and cis-keto forms and leads to a fast or slow thermal

the thermo- and photochromism switching ability of a few relaxation.^[42]

N-salicylidene aniline derivatives^[35]that occurs in the solid

state and without fatigue.^[36]Thermochromism, which cor- sible candidates for light-induced information storage appli-

responds to temperature-induced reversible colour cations. Many others photochromic molecules have been

N-Salicylidene aniline derivatives are not the only pos-

a

change,^[37]is observed with a thermal tautomeric equilib- described by using a large range of chemical reaction such

rium between the uncoloured enol and the yellow cis-keto as redox reactions,^[43]electron-hole pair generation,^[44]spin

forms (Figure 1a). Photochromism, which is a similar op- crossover,^[45]coordination change,^[46]photocyclization,^[43]

tical phenomenon induced by light irradiation,^[37]arises photoisomerization,^[47]bond cleavage^[48]and so on. Never-

from photoisomerization of both the enol and cis-keto theless, solid-state photochromism appears to be rare and

forms (Figure 1a) to the red trans-keto form. For N-salicyli- only a few systems fulfill criteria for data storage applica-

dene aniline, photochromism is induced by light irradiation tions. A good thermal stability, low fatigue, rapid response

at 365 and 436 nm but derivatives can absorb at others and nondestructive readout capability of switchable forms

wavelengths, as demonstrated by the large range of colours are indeed necessary.^[49]N-Salicylidene aniline derivatives

shown by these molecules.^[38]The photochemical process are promising candidates but to fulfill these application

associated to the photochromism has been described.^[39–41]requirements, optical and structural properties of these

UV irradiation (Figure 1b) of the enol form leads to the molecules must be accurately controlled and hence fully

formation of the excited enol form (enol*). This species can understood.

fluoresce in the visible region in some rare cases^[42]or com-

N-Salicylidene aniline derivatives were firstly claimed to

plete a fast tautomerization to the excited cis-keto form be thermochromic or photochromic but not both.^[38,50,51]

(cis*). The cis* to cis relaxation is mainly radiative and is This exclusive classification was recently clarified^[35]and N-

always detected by fluorescence spectroscopy.^[38]The cis* salicylidene aniline derivatives were shown to be either pho-

form can also undergoes photoisomerization to produce the tochromic and thermochromic or only thermochromic (ex-

excited trans-keto (trans*), which is described to be non- cept for few rare examples that are neither thermo- nor

Figure 1. (a) Thermochromism and photochromism of N-salicylidene aniline. (b) Photochemical process leading to the formation of the

trans-keto form from the enol form for N-salicylidene aniline.

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Engineering Solid-State Molecular Switches

photochromic, which present a strong deep red color- ionic interactions. All compounds are discussed as family

ation^[38]probably originating from a very large cis-keto members selected to lead to specific optical properties.

form population). Being aware of the importance to predict From nonfunctional Hsaltrz, we could make either thermo-

these optical properties, a structure–properties classification chromic molecules (Hsaltz and Ksaltz) by adding strong

was proposed.^[51]Thermochromic compounds were ob- intermolecular H-bonds and ionic interactions or a photo-

served as very planar molecules that are closely packed in chromic molecule (Habs) by isolating the triazole moiety

the crystal lattice, with a low dihedral angle between aro- by two pyridine rings. Hsalphen is similar to Habs from a

matic rings (Φ Ͻ 25°). These structures are stabilized by crystallographic point of view but however presents

strong π–π or CH–π supramolecular interactions. Aromatic thermochromic properties. Optical properties of these com-

rings and imine groups are particularly involved in these pounds have been fully discussed in light of the supramolec-

interactions, assisted by charge-transfer phenomenon.^[52]

A

ular interactions revealed by the crystal structures. A com-

typical example of this type of packing is represented by plementary computational study carried out on isolated

the N-salicylidene 2-aminopyridine crystal structure.^[51,53]molecules and on molecules in the crystal packing strongly

Thermo- and photochromic compounds were described as supports the optical properties of these materials.

highly twisted molecules, with large dihedral angles (Φ Ͼ

25°), packed within an open structure. Molecules appear to

be quite isolated because of long intermolecular distances,

as observed for instance in the model compound N-salicyl-

idene 2-chloroaniline.^[54]Some molecules such as N-salicyl-

idene aniline present polymorphism and can crystallize in

one thermochromic phase (β₁) as well as two photochromic

phases (α₁and α₂).^[55,56]Interestingly, N-salicylidene aniline

derivatives are only thermochromic in solution because of

the low stability of the trans-keto form in this medium. So-

lid-state optical properties of N-salicylidene aniline deriva-

tives are also influenced by the molecular environment, as

shown by the incorporation of these molecules in poly-

methyl methacrylate,^[48]MCM41 mesoporous silica,^[57–59]li-

quid crystals,^[60]organogelators and clathrates,^[61–67]Na-Y

zeolite^[59,68]or micelles.^[59]More recently, we have shown

that the crystal structure was not the only factor to take

into account to predict thermo- and photochromism of N-

salicylidene anilines in the solid state.^[42]Indeed, both elec-

tronic and structural aspects must be considered to properly

predict optical properties. In this paper, we propose a new

Scheme 1. Molecular scheme of Hsaltrz (a), Hsaltz (b), Habs (c),

strategy to gain a better control of the crystal structure Hsalphen (d) and Ksaltz (e).

using crystal packing directing effect moieties. This possibil-

ity was thought after the analysis of the crystal structure of

N-salicylidene 2-aminopyridine,^[52,69,70]where an intramo-

lecular H-bond between imine and pyridine ring locks the

molecule in a planar geometry and gives a closed structure.

Results and Discussion

Here, a rational crystal engineering approach is applied for

the first time to the N-salicylidene aniline derivatives sys-

Syntheses and Crystallization

tem. New N-heterocycle derivatives that are predesigned to

Purification of Hsaltz and Hsalphen was very difficult

lead to a controlled crystal structure with selected supra- because the crude products systematically contained a large

molecular interactions have been designed and synthesized. amount of unreacted salicylaldehyde in contrast to other

In the crystal network, N-salicylidene aniline derivatives are N-salicylidene aniline derivatives. These impurities were

considered as nodes and supramolecular interactions (H- successfully extracted by sonication in hexane and single

bonds, π–π stacking, CH–π and ionic interactions) as link- crystals were obtained by slow evaporation of concentrated

ers. N-Salicylidene 4-amino-1,2,4-triazole (Hsaltrz) and N- ethyl ether solutions. Slow sublimation of Habs at 110 °C

salicylidene 5-aminotetrazole (Hsaltz) (Scheme 1a,b) pres- under vacuum led to the growing of single crystals on the

ent dense supramolecular networks based on intermo- cold finger condenser of our sublimator setup. Single crys-

lecular H-bonding interactions, although N-salicylidene 4- tals of Hsaltrz were obtained by an unconventional method.

amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole (Habs) and N- For this experiment, we used the nucleophilicity of 4-

salicylidene

(1,10)-phenanthrolin-5-amine

(Hsalphen) amino-1,2,4-triazole, which can trap salicylaldehyde mole-

(Scheme 1c,d) reveal mostly π–π stacking interactions. The cules, slowly released by the hydrolysis of a sodium salt of

potassium salt of N-salicylidene 5-aminotetrazolate N-salicylidene p-aminobenzenesulfonate in methanol. A

(Ksaltz) (Scheme 1e) was studied to probe the impact of Cu^IIsalt, inserted in the reaction medium, is assumed to

Eur. J. Org. Chem. 2010, 621–637

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Y. Garcia et al.

FULL PAPER

ease the production of a pure white product of Hsaltrz, of the usual intramolecular H-bond, the system has coun-

which slowly crystallizes (Scheme S1, Supporting Infor- terbalanced it by the formation of a strong intermolecular

mation).

H-bond between a nitrogen atom of the triazole ring and

the alcohol function [O¹⁴–H¹⁴···N¹⁰²1.74(2) Å, O¹¹⁴

–

H¹¹⁴···N¹1.76(2) Å] (Table 2 and Figure 2). The triazole

ring has a strong structure-directing effect that drives mole-

cules to leave their normal molecular geometry to allow the

formation of strong intermolecular H-bonds. The packing

is also highly modified because of the formation of supra-

molecular zigzag double chains (leading to a supramolec-

ule; Figure 2). In each chain, a sequence of ···–Hsaltrz-2–

Hsaltrz-1–Hsaltrz-2–Hsaltrz-1–··· is identified (Table 1 and

Structural Aspects

Crystal Structure of N-Salicylidene 4-Amino-1,2,4-triazole

(Hsaltrz)

Hsaltrz crystallizes in the monoclinic space group P2₁/n

(Table S1, Supporting Information). Two molecules in the

enol form were distinguished in the asymmetric part of the

unit cell (Figure 2). The bond lengths C⁹–O¹⁴and C¹⁰⁹

–

O¹¹⁴of 1.351(2) and 1.350(2) Å respectively, confirm the

presence of the enol form (Table 1, Scheme 2). In addition,

the lengths of C⁸–C⁷[1.458(2) Å] and C¹⁰⁸–C¹⁰⁷

[1.456(6) Å] are in the range for single bonds, whereas the

lengths for C⁷–N⁶[1.279(2) Å] and C¹⁰⁷–N¹⁰⁶[1.279(2) Å]

are in the range for double bonds^[71](Table 1). No bond

length difference is noted between the two molecules of the

asymmetric part of the unit cell. In contrast, a change in

the dihedral angle between the aromatic rings is clearly dis-

tinguished [Φ₁= 16(1)°, Φ_1Ј= 6(1)°, where Φ₁and Φ_1Јare

measured on molecules 1 and 2, respectively] and is ac-

Table 1. Selected bond lengths [Å] and angles [°] for Hsaltrz, Habs,

Hsalphen, Hsaltz and Ksaltz.

Compound

Hsaltrz

5.10

Habs

3.80

Hsalphen Hsaltz

Ksaltz

4.06

Intermolecular

distance [Å]^[a]

Φ₁[°]^[b]

4.72

4.13

16 (1)

6 (1)

–

41 (1)

48 (1)

35 (1)

6 (1)

Φ₂[°]^[c]

Φ₃[°]^[d]

Γ [°]^[e]

10 (1)

9 (1)

62 (1)

–

1 (1)

–

15 (1)

6 (1)

45 (1)

33 (1)

companied by different torsion angles C⁽¹⁰⁾⁵–N⁽¹⁰⁾⁴–N⁽¹⁰⁾⁶

–

C⁴–O⁵[Å]^[f]

C²–N¹[Å]^[f]

C²–C³[Å]^[f]

1.351 (2) 1.352 (2) 1.355 (3) 1.354 (3) 1.347 (5)

1.350 (2)

1.279 (2) 1.279 (3) 1.283 (3) 1.293 (3) 1.283 (5)

1.279 (2)

1.458 (2) 1.452 (3) 1.450 (3) 1.431 (4) 1.437 (6)

1.456 (6)

2.15 (2)

C⁽¹⁰⁾⁷: Γ₁= 15(1)° and Γ_1Ј= 6(1)° (Table 1). The distances

between H², H¹⁰²and the nearest H from the triazole ring

are 2.15(2) Å and 2.19(2) Å (Table 1). According to some

authors,^[61]these differences can be interpreted as a slight

change in the electronic conjugation of the molecule. Inter-

estingly, Hsaltrz develops a new type of molecular geometry

for the two molecules included in the asymmetric part of

the unit cell where the intramolecular H-bond between the

alcohol and imine functions is broken by rotation of the

phenolic ring (Figure 2) and replaced by softer intramolecu-

lar H-bonds between the CH_imineand O_alcoholatoms [C⁷–

H⁷···O¹⁴2.38(2) Å and C¹⁰⁷–H¹⁰⁷···O¹¹⁴2.40(2) Å]

(Table 2). Considering the loss of energy led by the absence

H²···nearest H

[Å]^[f]

–

2.39 (2)

–

2.19 (2)

N⁰···H²[Å]^[f]

K··N²[Å]^[g]

K···N³[Å]^[g]

K···N^3#2[Å]^[g]

K···N⁴[Å]^[g]

K···N^4#2[Å]^[g]

K···N⁵[Å]^[g]

K···N^5#2[Å]^[g]

–

2.80 (3)

–

2.64(3)

2.49 (5)

2.940 (3)

2.806 (3)

3.170 (4)

2.853 (4)

3.100 (4)

2.886 (3)

2.869 (4)

–

[a] Distances between nearest molecular centroids. [b] Dihedral an-

gle between phenolic plane and triazole (for Hsaltrz and Habs) or

tetrazole (for Hsaltz and Ksaltz) or benzoic rings (for Hsalphen).

[c] Dihedral angle between C¹⁵–N¹⁶–C¹⁷–C¹⁸–C¹⁹–C²⁰pyridine ring

and the triazole ring. [d] Dihedral angle between C²¹–N²²–C²³–

C²⁴–C²⁵–C²⁶pyridine ring and triazole ring. [e] Torsion angle

centred on the imine function. [f] Distances with respect to

Scheme 2. [g] Distances between K and the nearest nitrogen atoms.

Figure 2. ORTEP view of the asymmetric part of the unit cell of

Hsaltrz, showing 50% probability displacement ellipsoids. The in-

set shows the crystal packing involving intermolecular H-bonds

(dotted line) and the formation of supramolecular double chains.

The organization by pairs with (molecule 1/molecule 1) and (mole-

cule 2/molecule 2) is clearly identified.

Scheme 2. Topology of the N-salicylidene moiety in Hsaltrz (a),

Hsalphen (b) and in Habs and Hsaltz (c).

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Engineering Solid-State Molecular Switches

Table 2. Intramolecular and intermolecular H-bonds in Hsaltrz, Habs, Hsalphen, Hsaltz and Ksaltz.

D–H···A

D–H [Å]

H···A [Å]

D···A [Å]

D–H···A [°]

Hsaltrz

O¹⁴–H¹⁴···N^102[a]

O¹¹⁴–H¹¹⁴···N^1[b]

C⁷–H⁷···O¹⁴

0.96 (2)

0.93 (2)

0.97 (2)

0.96 (2)

1.74 (2)

1.76 (2)

2.38 (2)

2.40 (2)

2.45 (2)

2.667 (2)

2.685 (2)

2.736 (3)

2.743 (2)

3.244 (2)

163 (2)

174 (2)

101 (2)

100 (1)

140 (2)

C¹⁰⁷–H¹⁰⁷···O¹¹⁴

C¹¹¹–H¹¹¹···O^114[c]

Habs

O¹⁴–H¹⁴···N⁶

O¹⁴–H¹⁴···N¹⁶

C⁷–H⁷···N^22[d]

C²⁶–H²⁶···N^2[e]

0.89 (2)

0.97 (3)

1.00 (3)

1.95 (3)

2.36 (3)

2.45 (3)

2.60 (3)

2.682 (2)

3.098 (3)

3.400 (3)

3.333 (3)

138 (2)

140 (2)

167 (2)

131 (2)

Hsalphen

O²³–H²³···N¹⁵

C⁶–H⁶···N^1[f]

C¹⁶–H¹⁶···N^1[g]

0.97 (3)

1.00 (3)

1.02 (3)

1.80 (3)

2.51 (3)

2.46 (2)

2.650 (3)

3.352 (3)

3.401 (3)

146 (2)

141 (2)

152 (2)

Hsaltz

Ksaltz

O¹⁴–H¹⁴···N⁶

0.94 (3)

0.92 (3)

1.75 (3)

1.90 (3)

2.602 (3)

2.805 (3)

151 (3)

167 (3)

N⁴–H⁴···N^1[h]

O¹–H¹···N¹

C⁸–H⁸···N²

0.96 (5)

0.93

1.76 (5)

2.49

2.615 (4)

2.837 (6)

147 (5)

102

[a] Symmetry operations: x – 1, y, z. [b] x – 3/2, –y + 1/2, z + 1/2. [c] x – 1/2, 1/2 – y, 1/2 + z. [d] x + 1, y, z. [e] 1 – x, 2 – y, –z. [f] 1/2 –

x, 1/2 + y, z. [g] –x, 2 – y, 1 – z. [h] x, 1 + y, z.

Table 3. Intermolecular π–π stacking and C–H···π interactions in Hsaltrz, Habs, Hsalphen, Hsaltz and Ksaltz.

Moiety 1

Moiety 2

Distance between centroids

Angle between moieties

[Å]

[°]

Hsaltrz

Habs

C⁸–C⁷–N⁶–N⁴

N¹–N²–C³–N⁴–C⁵

3.782 (2)

3.552 (2)

3.934 (2)

3.481 (2)

2.87 (2)

12 (1)

6 (1)

4 (1)

12 (1)

3 (1)

C¹⁰⁸–C¹⁰⁷–N¹⁰⁶–N¹⁰⁴

C⁸–C⁷–N⁶–N⁴

N¹⁰¹–N¹⁰²–C¹⁰³–N¹⁰⁴–C¹⁰⁵

C⁸–C⁹–C¹⁰–C¹¹–C¹²–C¹³

C¹⁰⁸–C¹⁰⁹–C¹¹⁰–C¹¹¹–C¹¹²–C¹¹³

C¹⁰⁵–H¹⁰⁵

C¹⁰⁸–C¹⁰⁷–N¹⁰⁶–N¹⁰⁴

C¹⁰⁸–C¹⁰⁹–C¹¹⁰–C¹¹¹–C¹¹²–C¹¹³

C¹⁵–N¹⁶–C¹⁷–C¹⁸–C¹⁹–C²⁰

C²¹–N²²–C²³–C²⁴–C²⁵–C²⁶

N¹–N²–C³–N⁴–C⁵

C¹⁵–N¹⁶–C¹⁷–C¹⁸–C¹⁹–C²⁰

N¹–N²–C³–N⁴–C⁵

3.805 (2)

0 (1)

C⁸–C⁹–C¹⁰–C¹¹–C¹²–C¹³

C⁷–C⁸–N⁴–N⁶

C⁸–C⁹–C¹⁰–C¹¹–C¹²–C¹³

C⁷–C⁸–N⁴–N⁶

Hsalphen

C⁵–C⁶–C¹¹–C¹²–C¹³–C¹⁴

N¹–C²–C³–C⁴–C¹²–C¹³

C⁵–N¹⁵–C¹⁶–C¹⁷

C⁵–C⁶–C¹¹–C¹²–C¹³–C¹⁴

N¹⁰–C⁹–C⁸–C⁷–C¹⁴–C¹¹

C¹⁷–C¹⁸–C¹⁹–C²⁰–C²¹–C²²

N¹–C²–C³–C⁴–C¹²–C¹³

C²–H²

3.603 (2)

3.985 (2)

4.228 (2)

4.239 (2)

3.713 (2)

2.86 (3)

0 (1)

2 (1)

1 (1)

6 (1)

8 (1)

3 (1)

N¹⁰–C⁹–C⁸–C⁷–C¹⁴–C¹¹

Hsaltz

Ksaltz

N¹–N²–N³–N⁴–C⁵

3.468 (2)

0 (1)

C⁹–N¹–C⁸–C⁷

C⁷–C³–C²–C⁴–C⁵–C⁶

N⁵–N⁴–N³–C⁹–N²

3.694 (2)

3.504 (2)

5 (1)

2 (1)

Figure 2). The double chain architecture is not only stabi- Crystal Structure of N-Salicylidene 4-Amino-3,5-

lized by H-bonds but also by π–π stacking interactions bis(pyridine-2-yl)-1,2,4-triazole (Habs)

(Figure 2) between the imine function of a molecule of a

¯

chain and salicyl or a triazole ring of a molecule of the

Habs crystallizes in the triclinic space group P1

other chain (Table 3). In contrast, double chains are almost (Table S1, Supporting Information). The asymmetric part

isolated from each other. Only a weak intermolecular H- of the unit cell contains one molecule in the enol form (Fig-

bond between the alcohol function of a molecule of one ure 3). It is possible to clearly identify it: the C⁹–O¹⁴

chain and a CH from the salicyl ring of a molecule of an- [1.352(2) Å] and C⁸–C⁷[1.452(3) Å] bond lengths are char-

other chain [C¹¹¹–H¹¹¹···O¹¹⁴2.45(2) Å] is found (Table 2). acteristic of single bonds and the N⁶–C⁷[1.279(3) Å] bond

A weak CH–π interaction between a salicyl ring of a mole- length reveals a double bond^[71](Table 1). Small dihedral

cule in a chain and a CH of a triazole ring of the nearest angles are observed between the triazole and pyridine rings

chain [C¹⁰⁸–C¹⁰⁹–C¹¹⁰–C¹¹¹–C¹¹²–C¹¹³···H¹⁰⁵–C¹⁰⁵2.87(2) [Φ₂= 10(1)° between C¹⁵–N¹⁶–C¹⁷–C¹⁸–C¹⁹–C²⁰pyridine

Å] is also noted (Table 3).

and the triazole ring and Φ₃= 9(1)° between C²¹–N²²–C²³–

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C²⁴–C²⁵–C²⁶pyridine and triazole ring] (Table 1). In con- (Table 1). A strong intramolecular H-bond [O²³–H²³···N¹⁵

trast, a large dihedral angle is identified between the salicyl 1.80(3) Å] is observed between the alcohol and imine func-

and the triazole rings [Φ₁= 41(1)°] (Table 1). In addition, tions as in Habs (Figure 4 and Table 2). Although the dihe-

the imine function is observed to be away from the “pyr- dral angle with the salicyl ring is similar in Habs and

idine-triazole-pyridine” plane [N²²···H⁷2.80(3) Å]. The tor- Hsalphen, the distance between H²³and N¹⁵is significantly

sion angle C³–N⁴–N⁶–C⁷(Γ) is equal to 62(1)° (Table 1). A shorter [1.80(3) Å] in Hsalphen in comparison to the dis-

strong intramolecular H-bond exists between the alcohol tance between H¹⁴and N⁶in Habs [1.95(3) Å] (Table 2).

and imine functions, as commonly identified in N-salicylid- This observation constitutes another proof of the absence

ene aniline derivatives [O¹⁴–H¹⁴···N⁶1.95(3) Å] (Figure 3 of a link between Φ and the intramolecular H-bond

and Table 2).^[51]Another soft intramolecular H-bond is also strength.^[42]Crystal packing of Hsalphen is directed by the

observed between the alcohol function and the nearest ni- formation of strong intermolecular π–π stacking interac-

trogen of one pyridine ring [O¹⁴–H¹⁴···N¹⁶2.36(3) Å]. Fi- tions between phenanthroline moieties, imine functions

nally, two weak intermolecular H-bonds are observed be- and salicyl rings (phen···phen 3.90 Å between centroids;

tween the imine group and a pyridine ring of the nearest d_{between molecular centroids}= 4.72 Å) (Table 1 and Figure 4). In

molecule [C⁷–H⁷···N²²2.45(3) Å] and between triazole and addition, two weak intermolecular H-bonds [C⁶–H⁶···N¹

the pyridine ring of another molecule [C²⁶–H²⁶···N²2.51(3) Å and C¹⁶–H¹⁶···N¹2.46(2) Å] (Table 3) and a

2.60(3) Å] (Figure 3 and Table 2). These interactions ex- CH···π interaction between phenanthroline moieties (C²–

plain the position of the nitrogen of the pyridine rings in H²···N¹⁰–C¹¹–C¹⁴–C⁷–C⁸–C⁹) complete such a dense supra-

the molecule. Crystal packing is very compact because of molecular network (Table 3).

short intermolecular distances (d_{centroid–centroid}= 3.80 Å)

(Table 1). Strong π–π stacking interactions are noted be-

tween all aromatic rings of one molecule and the same rings

in nearest symmetric molecules (Figure 3 and Table 3). In

contrast to Hsaltrz, no CH–π interaction is observed.

Figure 4. ORTEP view of the asymmetric part of the unit cell of

Hsalphen, showing 50% probability displacement ellipsoids. Intra-

molecular H-bond is indicated as a dotted line. View along the b

axis of the crystal packing showing intermolecular π–π stacking

interactions (indicated as dotted lines).

Crystal Structure of N-Salicylidene 5-Aminotetrazole

(Hsaltz)

Hsaltz crystallizes in the monoclinic space group C2/c

(Table S1, Supporting Information). In the asymmetric part

of the unit cell, one highly twisted molecule in the enol form

is observed (Figure 5). The C⁹–O¹⁴[1.354(3) Å] and C⁷–C⁸

[1.431(4) Å] bond lengths indicate single bonds and C⁷–N⁶

[1.293(3) Å] reveals a double bond^[71](Table 1). The mole-

cule is characterized by a large dihedral angle [Φ₁= 35(1)°],

a large torsion angle N¹–C⁵–N⁶–C⁷Γ₁= 33(1)° and a

Figure 3. ORTEP view of the asymmetric part of the unit cell of

Habs, showing 50% probability displacement ellipsoids. Intramo-

lecular bifurcated H-bonds are indicated as dotted lines. View

along the b axis of the crystal packing showing extended intermo-

lecular π–π stacking interactions (indicated as dotted lines).

Crystal Structure of N-Salicylidene 5-Amino-1,10-

phenanthroline (Hsalphen)

Hsalphen crystallizes in the orthorhombic space group strong intramolecular H-bond [O¹⁴–H¹⁴···N⁶1.75(3) Å] be-

Pbca (Table S1, Supporting Information). As in Habs, one tween the imine and alcohol functions (Tables 2 and 3, Fig-

molecule in the enol form is observed in the asymmetric ure 5), which is clearly stronger than in Hsalphen and Habs.

part of the unit cell (Figure 4). The C²²–O²³[1.355(3) Å] Crystal packing is directed, as in Hsaltrz, by the formation

and C¹⁶–C¹⁷[1.450(3) Å] bond lengths indicate single bonds of an intermolecular H-bond between the proton of the

and C¹⁶–N¹⁵[1.283(3) Å] reveals

a

double bond^[71]tetrazole moiety and the nearest tetrazole ring [N⁴–H⁴···N¹

(Table 1). Two planes are present in the molecule: the first 1.90(3) Å] (Figure 5 and Table 2). The distance between

one is formed by the salicyl ring and the imine function, molecular centroids is evaluated as 4.13 Å and the distance

which are coplanar, and the second one is created by the between N¹and H⁷was found as 2.64(3) Å (Table 1). Inter-

phenanthroline moiety, which is perfectly planar. A large molecular H-bonds lead to a supramolecular network

dihedral angle Φ₁= 48(1)° and a large torsion angle (C⁶– where supramolecular double chains (supramolecules) are

N⁵–N¹⁵–C¹⁶) Γ₁= 45(1)° are noted between them (Table 1). clearly distinguished (Figure 5). Cohesion between these

The distance between H¹⁶and H⁶is found as 2.39(2) Å two chains originates from intermolecular π–π stacking in-

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teractions between the tetrazole moieties (tz···tz 3.47 Å be- A strong H-bond is present between the alcohol and the

tween centroids) (Figure 5 and Table 3). No CH–π interac- imine functions [O¹H¹···N¹1.76(5) Å], which is highly sim-

tion is observed.

ilar to the intramolecular H-bond observed in Hsaltz. A

soft intramolecular H-bond is also observed between a ni-

trogen of the tetrazolate ring and the hydrogen of the imine

function (C⁸–H⁸···N²2.49 Å), which was not present in

Hsaltz because of the large dihedral angle. Crystal packing

is clearly directed by ionic interactions between potassium

and tetrazolate ions. The structure is packed in planes: a

stack of ionic planes and organic planes with salicyl and

imine moieties as observed in Figure 6. Potassium ions are

present within disorganized atomic spheres and no clear

geometric environment can be derived (Table 1). The closed

packing is not only stabilized by ionic interactions but also

by strong π–π stacking interactions between the imine

group and the salicyl ring [d_{inter planes}= 3.694(2) Å] and be-

tween the imine group and the tetrazolate ring [d_{inter planes}

= 3.504(2) Å].

Figure 5. ORTEP view of the asymmetric part of the unit cell of

Hsaltz, showing 50% probability displacement ellipsoids. Intramo-

lecular H-bond is indicated as a dotted line. View of the crystal

packing showing the formation of supramolecular double chains

built with intermolecular π–π stacking interactions (indicated as

dotted lines) and with intermolecular H-bonds (indicated as dotted

lines).

Optical Properties

Thermochromic and Photochromic Phenomena

Crystal Structure of the Potassium Salt of N-Salicylidene

5-Aminotetrazolate (Ksaltz)

As shown in Figure 7, Hsaltrz and Habs remain white

over the whole investigated temperature range (298–77 K).

In contrast, Hsalphen, Hsaltz and Ksaltz exhibit moderate

to strong thermochromism upon cooling to 77 K. Hsalphen

has a yellow/orange coloration that evolves from dark to

light orange when the temperature is decreased. Hsaltz is

only slightly thermosensitive. Its colour varies from very

pale yellow at room temperature to white at liquid-nitrogen

temperature. In contrast, Ksaltz clearly changes its colour:

the sample is yellow at room temperature and turns to white

at low temperature. Colour difference between Hsaltz and

Ksaltz is clearly distinguished by naked eyes at room tem-

perature.

Ksaltz also crystallizes in the monoclinic space group

C2/c (Table S1, Supporting Information). However, it is not

isostructural to Hsaltz. An almost planar molecule is ob-

served in the asymmetric part of the cell. A small dihedral

angle between the aromatic rings (Φ₁) of 6(1)° was mea-

sured (Figure 6 and Table 1), which is clearly lower than

that in the Hsaltz structure [35(1)°]. The molecule is in the

fundamental enol form (Figure 6). The C²–O¹and C⁷–C⁸

bond lengths [1.347(5) and 1.437(6) Å] are typical of single

bonds and C⁸–N¹[1.283(5) Å] of a double bond^[71]

(Table 1). Two intramolecular H-bonds are noted in Ksaltz.

Figure 7. Photograph of powders of Hsaltrz (a) and Habs (b) over

the temperature range 298–77 K and of Hsalphen (c), Hsaltz (d)

and Ksaltz (e) at 298 and 77 K. Thermochromism is identified for

c, d (very weak) and e (strong).

All compounds were analyzed by diffuse reflectance

spectroscopy as pure solid powder samples^[72]to avoid ma-

trix and environment effects that are known to intensively

modify the optical properties of N-salicylidene aniline de-

rivatives.^[48,58–68]Figure 8a shows the Kubelka Munk spec-

tra of Hsaltrz, Hsaltz and Ksaltz. A bathochromic shift is

observed from Hsaltrz to Hsaltz and Ksaltz. Hsaltrz only

Figure 6. ORTEP view of the asymmetric part of the unit cell of

Ksaltz, showing 50% probability displacement ellipsoids. Intramo-

lecular H-bond is indicated as a dotted line. View of the crystal

packing, along the b axis, showing a planar arrangement of ionic

planes containing potassium and tetrazolate ions and organic

planes containing others functions.

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absorbs in the UV range (below 400 nm). This absorption

Figure 8b shows the Kubelka Munk spectra of Habs and

is typical of the enol form, which is largely predominant as Hsalphen. Habs presents an intense band in the UV range,

described in the crystallographic section. In Hsaltz, the enol a feature that fits with the white colour of the compound.

band is shifted by approximately 18 nm to lower energies Irradiation of the sample at 254 nm during 1 h leads to a

(an intense band is noted at approximately 430 nm), which clear spectral change in the visible range. A new maximum

leads to the slightly yellow coloration of the product. No is noted at 465 nm and is attributed to the formation of a

band from the keto forms is observed. The intense band in photoinduced metastable form: the trans-keto isomer.^[42]

the UV range of the spectrum (at approximately 430 nm) is The absence of the cis-keto form is assumed for this com-

attributed to the enol form. The colour difference between pound. For Hsalphen, a nonresolved intense band in the

Hsaltz and Ksaltz is ascribed to the presence of a band UV range of the spectrum is noted with a bathochromic

with a maximum in the visible range of the spectrum at shift of approximately 50 nm. A second broad band is ob-

approximately 460 nm for Ksaltz. This band is typical of served in the visible (maximum unresolved at approximately

the cis-keto form of the molecule. Hsaltrz, Hsaltz and 450 nm) and is attributed to the cis-keto form. Irradiation

Ksaltz are observed to be nonphotochromic at room tem- at 254 nm for 1 h leads to a very slight change in the Kub-

perature. Indeed, irradiation at 254 nm for 1 h does not lead elka Munk function intensity around 500 nm, which is at-

to modification of the Kubelka Munk spectra.

tributed to the formation of the trans-keto isomer. The in-

crease in the trans-keto band is less intense in Hsalphen

than in Habs but is still detected with the naked eye.

Thanks to the clear spectral change under UV irradiation

of Habs, a study of the thermal relaxation of the system in

the trans-keto form was conducted by using time dependant

diffuse reflectance spectroscopy. Figure 9 shows the evol-

ution of the Kubelka Munk function at 465 nm, which is

near the band maximum of the trans-keto form (Figure 8b).

Irradiation at 254 nm for 20 min induces a large increase in

F. After stopping the irradiation, leading to rapid thermal

relaxation, complete recovery of the initial value in approxi-

mately 4 h is observed. This cycle is fully reproducible, as

shown in Figure 9. The relaxation appears to be elaborated

and contains more than one chemical relaxation pathway.

Attempts to fit this curve with an exponential function

failed. The photoisomerization process is also photochemi-

cally reversible, as shown in Figure 10. After photoexci-

tation at 254 nm and thermal relaxation over 10 min, illu-

mination at 450 nm induces a fast return to the initial F

value (Figure 10). Afterwards, an unexpected “spring-type”

effect is observed without further stimulus. Over a few min-

utes, the signal increased to approximately 20% of the

maximum value reached. The system reacts spontaneously

to the energy flow induced by the second irradiation by

Figure 8. Kubelka Munk spectra at 298 K of Hsaltrz, Hsaltz and

Ksaltz (a) and of Habs and Hsalphen before and after UV irradia-

tion (b). Photochromism is clearly identified for Habs. The inset in

Figure 8b reveals weak photochromism for Hsalphen at 298 K.

Figure 9. Time dependence of the Kubelka Munk function (F) of

Habs at 465 nm, showing the reproducible thermal relaxation of

the metastable trans-keto form.

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trying to return to the metastable state. After this abnormal A first intense emission band, centred at 523 nm and

increase, a subsequent thermal relaxation of the trans-keto 569 nm for Hsaltrz and Hsalphen, respectively, is attributed

form is observed.

to the radiative relaxation of the excited cis-keto* form

(Table S2, Supporting Information). Thus, a shift of this

band is clearly noted between Hsaltrz and Hsalphen. A sec-

ond band is observed in both spectra at very narrow wave-

lengths, 441 and 438 nm for Hsaltrz and Hsalphen, respec-

tively (Table S2, Supporting Information). This emission is

attributed to the radiative relaxation of the excited enol*

form. Finally, it is important to notice that a third emission

contribution is only observed in Hsalphen, centred at

420 nm, which is believed to be a radiative relaxation of an

enol form in a more relaxed conformation (Table S2, Sup-

porting Information). Figure 11c represents the emission

spectrum of Habs at 400 nm. The main emission band

noted at 544 nm is attributed to the excited cis-keto* radia-

tive relaxation. A second major contribution is observed at

476 nm (Table S2, Supporting Information). This less-in-

tense band could originate from three excited molecules: the

enol form, a more relaxed cis-keto form or the molecule

in an intermediate state as described in the next section.

Unfortunately, we are not able to distinguish between these

three molecules because of the proximity or the superposi-

tion of some absorption bands. It is important to notice

that in all spectra (Hsaltrz, Habs, Hsalphen, Hsaltz and

Ksaltz), a low emission is observed at approximately

650 nm that cannot be assigned with confidence. The origin

of this emission can be the radiative de-excitation of either

an excited cis-keto form higher in energy than the previous

one or the excited trans-keto* form.

Figure 10. Time dependence of the Kubelka Munk function (F) of

Habs at 465 nm, showing the reproducible photochemical relax-

ation of the metastable trans-keto form leading to an unexpected

“spring-type” effect.

Emission Spectroscopy

Fluorimetry was used to study energy levels involved

during the thermo- and photoinduced processes. Analyses

were conducted on pure solid powder samples. Figure 11a

shows the emission spectra at 400 nm of Hsaltz and Ksaltz.

Photoisomerization Model in Single Crystals

In order to gain a better understanding of factors that

One intense emission is observed, centred at 536 nm for control the intriguing optical properties of Hsalphen and

Hsaltz and at 540 nm for Ksaltz, which is assigned to the Habs, we have investigated their photoisomerization by

emission of the excited cis-keto* form (Table S2, Support- computational means (see Experimental Section). Firstly,

ing Information). Thus, the two compounds seem to have we have examined the enol, cis-keto and trans-keto equilib-

very similar emission properties. Figure 11b presents the ria for isolated Hsalphen and Habs molecules in the ground

emission spectra of Hsaltrz and Hsalphen at 350 nm, which state (GS) and in the first singlet (S¹) and the first triplet

are clearly different. More than one contribution is noted. (T¹) excited states. Table S3 (Supporting Information) re-

Figure 11. Solid-state emission spectra at 298 K of (a) Hsaltz and Ksaltz at 400 nm (b) Hsaltrz and Hsalphen at 350 nm and of (c) Habs

at 400 nm.

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ports the computed relative energy of the different species toisomerization must take place through the S¹state as pre-

and the activation barrier for cis-keto/trans-keto isomeriza- viously reported (Table S3, Supporting Information; Fig-

tion in the respective states. The energy profiles for this lat- ure 12).^[42]For Hsalphen, the activation barrier in the S¹

ter step are depicted in Figure 12.

state (21.0 kcal/mol) nicely corresponds to those of N-sal-

icylidene aminopyridine derivatives (20–21 kcal/mol),^[42]

whereas in the case of Habs it is much lower (16.9 kcal/

mol). This result supports the observation of photochro-

mism for these molecules.

It is interesting to note that the energy profile for photo-

isomerization of Habs in the ground state unexpectedly re-

veals the presence of an intermediate in the pathway to the

trans-keto form (Figures 12 and 13). Formation of a cy-

clized intermediate can be accounted for by the developing

zwitterionic character of Habs (heterolytic breaking of the

C⁷–C⁸double bond) with rotation (see Table S5, Support-

ing Information, for charges). Indeed, the presence of a nu-

cleophilic nitrogen atom (N²²) in this case allows addition

of this latter onto the developing positive charge on C⁷,

which is rapidly followed by addition of the phenolic oxy-

gen (O¹⁴) onto the pyridine ring (C²³) to form a six-mem-

bered ring (Figure 13b). This cyclized intermediate can then

lead to the trans-keto form through a very low activation

Figure 12. Calculated energy evolution for the ground state (GS)

and for the first singlet (S¹) and first triplet excited states (T¹) dur-

ing the photoisomerization of (a) Hsalphen and (b) Habs. The tor-

sion angle C⁴–C³–C²–N¹is given relative to Scheme 2.

These results reveal that, in both cases, the proton trans-

fer step (enol to cis-keto) is energetically disfavoured in the

ground state and favoured in the first singlet and first triplet

excited states.^[73]Interestingly, the proton transfer in the GS

for Habs is significantly more endothermic (11.3 kcal/mol)

than for Hsalphen (7.6 kcal/mol) or for previously studied

aminopyridines derivatives (7–9 kcal/mol).^[42]This could ac-

count for the absence of thermochromism in Habs, whereas

all the other considered molecules, except Hsaltrz, show

thermochromism.

Figure 13. (a) Optimized structure of the cyclized form of Habs.

(b) Suggested mechanism for the formation of the cyclized form

The computed activation barrier for cis-keto to trans-

keto isomerization indicates that, for both molecules, pho- involving a zwitterionic form of Habs.

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Engineering Solid-State Molecular Switches

barrier (0.1 kcal/mol), which suggests that the equilibrium actions. Interestingly, the close crystal packing made with

between these two forms must be thermally accessible. The highly planar molecules is not sufficient to properly predict

influence of this new cyclized form on the experimental re- the optical properties of Habs, because, despite the fact it

sults will be discussed in the next section. Note that, even is expected to be thermochromic following the classification

if this mechanism allows a low activation barrier in the GS of Ogawa,^[35,51]it is exclusively photochromic. This intri-

(lower than that for Hsalphen or aminopyridine deriva- guing situation was recently met in the case of N-salicylid-

tives^[42]), no thermochromism upon heating is observed for ene aminopyridines.^[42]Our calculations suggest that this

this compound (≈100 °C).

unexpected absence of thermochromism associated with an

Because of the expected importance of crystal packing absence of coloration of the compound in the whole studied

on the photochromic properties of Habs and Hsalphen, we temperature range (77–400 K), which is observed for the

also investigated, by computational means, the cis-keto/ first time for this class of molecules, could be due to a

trans-keto isomerization by taking in account a model of higher reaction energy (more endothermic) for proton

the crystal packing obtained by single-crystal X-ray diffrac- transfer in the ground state. On the other hand, in the first

tion analyses (Figures 3 and 4). The influence of crystal singlet excited state, the proton transfer is found to be exo-

packing on the photoisomerization was probed by estimat- thermic, as for other studied derivatives, and the barrier

ing the relative stabilization induced by placing the different to cis-keto/trans-keto isomerization is consistent with the

species in their crystal (see Experimental Section).^[74]Ob- observed photochromism at room temperature.

tained stabilization energies (from 0 to 7.2 kcal/mol) indi-

Kinetic study of the thermal relaxation of the photoin-

cate that although the packing is closed, due to the strong duced metastable trans-keto form indicates that the relax-

π–π stacking network, it does not prevent isomerization. ation pathway is very complex. Our computational calcula-

Analysis of the transition-state structure shows that, as pre- tions have revealed the presence of a cyclized intermediate

viously observed for N-salicylidene aminopyridine deriva- on the energy profile of Habs (Figure 12). This intermediate

tives, the preferred mechanism for isomerization resembles is predicted to be in equilibrium with the trans-keto form

the pedal motion of a bicycle.^[42]In both cases (although through a very low activation barrier. This new molecular

to a larger extent for Hsalphen), the TS structure is better scheme allows a rationalization of the complex thermal re-

stabilized by crystal packing than for the cis-keto form. laxation kinetics and in particular the unprecedented

This means that crystal packing should lead to an acti- “spring-type” effect observed during the photochemical re-

vation of the isomerization, which is in good agreement versibility study to be proposed (Figure 10). Figure 14 sug-

with the observed photochromism at room temperature.

gests a schematic evolution of the population of the enol,

the cyclized and the trans-keto forms during each step ob-

served in Figure 9. We assume that the irradiation at

254 nm (step 1) leads to a large increase in the trans-keto

form population and the formation of a small amount of

Discussion

Novel, nitrogen-rich N-salicylidene aniline derivatives the cyclized form. During the first minutes of the relaxation

have been synthesized. Each molecule has been selected to (step 2), the cyclized form rapidly relaxes to the trans-keto

promote a given supramolecular interaction (intra- or inter- form. The input of the trans-keto form into the system

molecular H-bonds, π–π stacking, ionic interaction) that al- slightly compensates for the thermal relaxation of this form

lows their optical properties to be tuned.

to the enol form and then modifies the shape of the thermal

Hsaltrz is distinguished from other N-salicylidene aniline relaxation from a normal exponential to a curve as ob-

derivatives thanks to a novel molecular geometry. Complete served in Figure 9. Finally, during the last step (step 3), nor-

rotation of the salicyl ring leads to breakage of the intramo- mal kinetic relaxation from the trans-keto to the enol form

lecular H-bond between the OH and N_iminemoieties and is observed. The unexpected “spring-type” effect can also

allows the formation of supramolecular double chains with be thought by a similar reasoning (Figure 14b). Although

π–π stacking interactions. Thus, a crystal-directing effect of steps 1 and 2 are the same as those described in Figure 14a,

the electron-rich triazole ring is effective. This feature asso- irradiation at 450 nm (step 3) leads to a zeroing of the trans-

ciated with phenolic ring rotation leads to a large change keto form population, but we assume that a significant pop-

in the optical properties because the prototropic phenome- ulation of the cyclized form remains present. A small

non is no longer thermally accessible. Photochromism at amount of trans-keto form could be then reformed by relax-

room temperature is also quenched because the molecular ation of the cyclized form after a few minutes without any

geometry and packing do not allow the photoisomerization external stimulus (step 4), as observed in the experimental

to proceed.

data (Figure 10, “spring-type” effect). Finally, the trans-

Habs could be seen as a derivative of Hsaltrz, where the keto form population then relaxes to the enol form to re-

triazole ring is isolated thanks to two pyridine rings. The turn to the initial situation (step 5).

intermolecular H-bond between the triazole rings in Hsaltrz

Hsaltz is closely related to Hsaltrz. The tetrazolate ring

is no longer possible and thus no rotation of the salicyl ring is indeed more electron rich than a triazole ring, but it con-

is observed. However, the presence of triazole and pyridine tains an N–H moiety that has an important crystal packing

rings linked to each other leads to a close molecular pack- directing effect. Indeed, crystal packing is directed by the

ing directed by the formation of strong π–π stacking inter- formation of an intermolecular H-bond between the tetra-

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served in absorption optical properties. Ksaltz is shown to

be strongly thermochromic, whereas Hsaltz is only slightly

thermochromic. Surprisingly, this difference, associated to

stabilization of the cis-keto form with respect to the enol

form, is not correlated to a difference in intramolecular H-

bonding strength. No change in emission properties is ob-

served between both tetrazole compounds. Finally,

Hsalphen appears to be similar to Habs due to its crystal

packing directed by strong π–π stacking interactions. The

highly close structure obtained is typical of thermochromic

compounds^[35]except that the dihedral angle is large [Φ₁=

48(1)°]. The π–π stacking interactions seem to decrease the

photochromism at room temperature but do not quench it.

This observation is supported by the height of the energy

barrier predicted by our calculations.

Fluorimetry on solid samples is a very interesting

method to analyze N-salicylidene aniline derivatives be-

cause of their strong fluorescence, which allows probing en-

ergy levels involved during the photochemical processes. All

molecules have a strong emission around 540 nm (from 523

to 569 nm) that can be attributed to the radiative relaxation

of the excited cis-keto form. Habs and Hsalphen have more

bathochromic emissions, whereas Hsaltrz has a more ener-

getic emission (523 nm). This phenomenon can be attrib-

uted to the enlarged electronic conjugation generated by the

presence of pyridine moieties in Habs and of aromatic rings

placed side by side in Hsalphen. Interestingly, the emission

of the enol form appears only in Hsaltrz and Hsalphen at

very narrow wavelengths (438 and 441 nm, respectively). We

thus suspect that the optical properties of the enol forms

are less affected by described molecular changes than the

ones of keto forms. We assume that the presence or the

absence of enol emission is controlled by the energy gap

between excited enol and cis-keto form levels.^[42]Indeed, if

Figure 14. Suggested evolution of the population of the enol, cy-

clized and trans-keto forms of Habs during each step of the (a)

thermal relaxation study (see Figure 9) and of the (b) photochemi-

cal relaxation study (see Figure 10). Populations have been fixed

arbitrarily.

zole rings, which replaces the intermolecular H-bond be- this gap is large, as predicted by our calculations for Habs,

tween the OH function and the triazole ring noted in excited enol molecules are very rapidly relaxed to the ex-

Hsaltrz. We observe the formation of supramolecular cited cis-keto form by tautomerism, which quenches the

double chains made by π–π stacking interactions with enol emission (Figure 1b).

highly twisted molecules. Because of the absence of stacking

of salicyl rings, Hsaltz can be classified as an open struc-

Conclusions

ture^[51]that is typical of photochromic compounds but no

photochromism was observed at room temperature with the

powder sample of this compound.

We have presented new compounds that represent rare

examples of thermo- and photochromic azole-based mole-

Ksaltz is an interesting molecule because of its similarity cules. A supramolecular approach was used to select these

to Hsaltz from an electronic point of view. The main differ- new N-salicylidene aniline derivatives and to gain better

ence between Hsaltz and Ksaltz originates from the differ- control of the intermolecular interactions present in their

ence in the involved supramolecular interactions. In Hsaltz, molecular packings. Crystal engineering was developed

N–H intermolecular H-bonds appear to be very important with H-bonds (Hsaltrz and Hsaltz), π–π stacking (Habs and

in contrast to the ionic interactions present in Ksaltz. In Hsalphen) and ionic interactions (Ksaltz) to build dense

addition, the appearance of ions leads to a drastic change supramolecular networks. Electronic properties have been

in the medium polarity, which can have a large influence on properly analyzed by computational means on isolated

the photochromic behaviour.^[68]Whereas the intramolecular molecules and molecules incorporated into the real single-

H-bonding between the alcohol and imine functions is sim- crystal packing. Optical properties in absorption and emis-

ilar in both structure, the Ksaltz structure contains planar sion were then fully discussed and interesting links between

molecules [Φ₁= 6(1)°] although Hsaltz is constituted of the supramolecular interactions, electronic properties and

highly twisted ones [Φ₁= 35(1)°]. In addition to structural the optical properties were fully discussed.

differences and, more precisely, to supramolecular network

Starting from a nonfunctional molecule (Hsaltrz) that is

linkers, the major change between both compounds is ob- neither thermochromic nor photochromic, chemical “sub-

632

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Eur. J. Org. Chem. 2010, 621–637

Engineering Solid-State Molecular Switches

134.14 (CH-CH-C-OH), 133.37 (CH-C-C-OH), 127.54 (CH-C-

SO₃), 121.44 (CH-C-N), 120.07 (C-C-OH), 119.94 (CH-CH-CH-

C-OH), 117.36 (CH-C-OH) ppm. C₁₃H₁₀NNaO₄S (299.28): calcd.

C 52.17, H 3.61, N 4.68, S 10.71; found C 52.31, H 3.30, N 4.40,

S 10.47. MS: m/z (%) = 276 [M^–], 212 [M^–– SO₂], 196 [M^–– SO₃],

118 [C₇H₄NO^–]. X-ray powder diffraction: 3.24 (s), 6.42 (m), 9.64

(s), 12.8 (w), 16.04 (w), 18.38 (w), 19.67 (s), 20.81 (w), 21.68 (w),

23.30 (w), 23.58 (w), 25.40 (w), 25.80 (w), 27.08 (w), 25.80 (w),

26.00 (w), 27.08 (w), 30.34 (w), 31.12 (w), 32.40 (w), 32.62° (w).

stitutions” were made to lead to novel functional materials

that are either thermochromic or photochromic. Hsaltz and

Ksaltz are new thermochromic molecules where the ther-

mosensibility is tuned by changing the type of supramolec-

ular interactions (H-bonds–ionic interaction) and the me-

dium polarity. Habs is a novel molecule that is exclusively

photochromic. Our calculations show that this behaviour,

hitherto not observed, can be explained by the high energy

difference between the enol and cis-keto forms. Hsalphen

FTIR (KBr disk): ν = 418 (w), 442 (w), 459 (w), 494 (w), 528 (w),

˜

appears to be thermochromic and photochromic although 552 (m), 565 (m), 577 (s), 634 (m), 650 (s), 716 (s), 731 (s), 779 (w),

812 (m), 835 (s), 850 (s), 912 (m), 935 (w), 982 (m), 1011 (m), 1032

(m), 1051 (s), 1107 (m), 1032 (s), 1150 (m), 1174 (s), 1186 (s), 1236

(s), 1261 (w), 1281 (s), 1319 (w), 1362 (m), 1404 (m), 1456 (m),

1489 (s), 1528 (w), 1576 (s), 1591 (m), 1626 (s), 1664 (w), 3250 (m,

br.) cm^–1.

the packing is closed by the strong π–π stacking network.

This strongly supports the idea that it is difficult to predict

the optical properties of N-salicylidene aniline derivatives

considering exclusively single-crystal packing analyses.^[42]

Energetic factors, properly analyzed by computational me-

ans, were then used to complete the optical properties pre-

diction process.

Finally, Habs was presented as a highly interesting trist-

able molecule. In addition to the observed unstable cis-keto

form, which is only photochemically accessible (observed

by fluorimetry), the fundamental enol, the metastable trans-

keto and cyclized forms were predicted by computational

means. This new molecular scheme allows us to propose an

explanation for the complex photochromic decay as well as

the unexpected “spring-type” effect. The suggested complex

kinetic scheme involved during thermal and photochemical

relaxation was then presented and fully discussed in light

of our computational study.

N-Salicylidene 4-Amino-1,2,4-triazole (Hsaltrz): A solution of sali-

cylaldehyde (1.25 mL, 1.18 mmol, 1 equiv.) dissolved in ethanol

(5 mL) was added to a solution of 4-amino-1,2,4-triazole (1 g,

1.18 mmol, 1 equiv.) in ethanol (5 mL). The resulting slightly yel-

low solution was stirred for 15 min and, afterwards, heated at reflux

for 45 min. The resulting pale-yellow solution was allowed to cool

to room temperature to give a crystalline white powder. The prod-

uct was filtered, washed with ethanol (1 mL) and ethyl ether (1 mL)

and dried under vacuum (2 g, 90%). ¹H NMR (300 MHz, [D₆]

DMSO, 298 K): δ = 10.50 (s, 1 H, H¹⁴), 9.20 (m, 3 H, H³, H⁵and

H⁷), 7.63 (m, 1 H, H¹³), 7.46 (m, 1 H, H¹¹), 7.01 (m, 2 H, H¹⁰and

H¹²) ppm. ¹³C NMR (300 MHz, [D₆]DMSO, 298 K): δ = 158.93

(C⁹), 155.71 (C⁷), 139.76 (C³and C⁵), 134.70 (C¹¹), 128.50 (C¹³),

120.45 (C¹⁰), 118.99 (C⁸), 117.52 (C¹²) ppm. C₉H₈N₄O (188.19):

calcd. C 57.44, H 4.28, N 29.77; found C 57.44, H 4.30, N 29.80.

M.p. onset temperature by DSC: 206(1) °C. Degradation tempera-

ture (onset) by DSC: 220(1) °C. MS: m/z = 189 [M + H⁺], 120

[C₇H₆NO^·+], 70 [C₂N₃H₅^·+]. X-ray powder diffraction: 7.86 (s), 9.48

(m), 11.72 (m), 14.26 (m), 14.86 (s), 15.78 (m), 16.2 (w), 17.0 (m),

16.88 (w), 18.06 (w), 18.78 (w), 19.08 (s), 19.84 (m), 20.48 (w), 21.2

(m), 22.06 (m), 22.76 (m), 23.74 (w), 26.1 (w), 28.38 (w), 28.8 (w),

Experimental Section

Starting Materials: Solvents (absolute ethanol, chloroform and di-

ethyl ether, analytical reagent and methanol HPLC grade from Pro-

labo; [D₆]DMSO 99.9 atom-%D, n-hexane HPLC grade from Ald-

rich and ethylene glycol 99% from Acros Organics) and reagents

[4-amino-4H-1,2,4-triazole 99%, salicylaldehyde 99%, copper(II)

chloride dehydrate 98%, 2-cyanopyridine 99%, hydrazine dihydro-

chloride 99% from Acros Organics; Sulfanilic acid 99%, (1,10)-

phenanthrolin-5-amine 97%, hydrazine monohydrate 98%, 5-ami-

notetrazole 97% from Aldrich; potassium hydroxide analytical rea-

gent from Riedel-de Haën and sodium hydroxide, analytical rea-

gent from Fisher Scientific] were obtained commercially and used

as received.

28.7 (w), 30.96 (w), 34.94° (w). FTIR (KBr disk): ν = 461 (m), 515

˜

(w), 598 (s), 623 (s), 729 (w), 762 (s), 833 (w), 854 (m), 949 (w),

974 (w), 1057 (s), 1105 (w), 1155 (s), 1194 (m), 1259 (s), 1300 (s),

1373 (m), 1412 (w), 1456 (s), 1516 (s), 1605 (s), 2710 (s, br.), 3117

(s), 3435 (s, br.) cm^–1. Single crystals were obtained following an

unconventional procedure: Nasalsulfo (0.30 g, 1 mmol, 2 equiv.)

was added to a solution of CuCl₂·2H₂O (0.12 g, 0.5 mmol, 1 equiv.)

in methanol (10 mL). 4-Amino-1,2,4-triazole (0.13 g, 1.5 mmol,

3 equiv.), dissolved in methanol (10 mL), was allowed to slowly dif-

fuse into the copper solution through a diffusion H-tube filled with

methanol (100 mL). After 1 year, diffusion was assumed to be com-

plete and the whole solution was allowed to slowly evaporate at

room temperature. Long uncolored needles of diffractable quality

were obtained within the resulting concentrated green solution af-

ter 6 months. These crystals were then analyzed by X-ray diffrac-

tion.

Sodium Salt of N-Salicylidene para-Aminobenzenesulfonate (Nasal-

sulfo): Prepared as a precursor for single-crystals growing of

Hsaltrz. Sulfanilic acid (5 g, 28.8 mmol, 1 equiv.) was suspended in

methanol (50 mL) and deprotonated by adding solid sodium hy-

droxide (1.16 g, 28.8 mmol, 1 equiv.) under reflux for 1 h. After-

wards, salicylaldehyde (3 mL, 28.8 mmol, 1 equiv.) was added to

the white suspension to give a yellow solution. The mixture was

stirred for 2 h at room temperature and then concentrated. The

resulting solution was filtered to give 11.94 g of a crystalline yellow

product that was washed with methanol (10 mL) and dried under

vacuum. The pure product (7.44 g, 86%) was obtained after

recrystallization in hot methanol (100 mL). ¹H NMR (300 MHz,

[D₆]DMSO, 298 K): δ = 13.06 (s, 1 H, OH), 9.00 (s, 1 H, CH=N),

7.70 (m, 3 H, CH-C-CH=N and CH-CSO₃), 7.42 (m, 3 H, CH-

CH-OH and CH-CH-SO₃), 7.00 (m, 2 H, CH-OH and CH-CH-

CH-OH) ppm. ¹³C NMR (300 MHz, [D₆]DMSO, 298 K): δ =

4-Amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole (abpt): Obtained by

following a reported procedure.^[75]Yield: 2.76 g (55%). ¹H NMR

(300 MHz, [D₆]DMSO, 298 K): δ = 8.78 (m, 2 H, H¹⁷and H²³),

8.24 (m, 2 H, H²⁰and H²⁶), 8.08 (dt, J = 7.8, 1.77 Hz, 2 H, H¹⁹

and H²⁵), 7.83 (s, 2 H, NH₂), 7.59 (m, 2 H, H¹⁸and H²⁴) ppm. ¹³

C

NMR (300 MHz, [D₆]DMSO, 298 K): δ = 149.67 (C²³and C¹⁷),

149.52 (C³and C⁵), 147.78 (C¹⁵and C²¹), 138.52 (C¹⁹and C²⁵),

125.30 (C¹⁸and C²⁴), 123.55 (C²⁰and C²⁶) ppm.

N-Salicylidene 4-Amino-3,5-bis(pyridine-2-yl)-1,2,4-triazole (Habs):

164.47 (C-OH), 161.05 (CH=N), 148.73 (C-N), 147.61 (C-SO₃), abpt (2.5 g, 10.5 mmol, 1 equiv.) was dissolved in hot methanol

Eur. J. Org. Chem. 2010, 621–637

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633

Y. Garcia et al.

FULL PAPER

(25 mL) to give a clear solution, to which was added salicylalde-

11.38 (m), 11.7 (s), 12.7 (w), 13.42 (w), 14.24 (w), 14.94 (w), 15.4

hyde (1.1 mL, 10.5 mmol, 1 equiv.) to afford a clear pale-yellow (m), 15.98 (m), 16.74 (m), 17.08 (m), 17.54 (m), 18.36 (m), 18.96

solution. The mixture was heated at reflux for 48 h and then al-

lowed to cool down to room temperature. The resulting crystalline

yellow precipitate was filtered and recrystallized from hot methanol

(60 mL) to give the pure product (2.58 g, 7.5 mmol, 72%). ¹H

NMR (300 MHz, [D₆]DMSO, 298 K): δ = 10.66 (s, 1 H, H¹⁴), 8.98

(s, 1 H, H⁷), 8.66 (m, 2 H, H²⁰and H²⁶), 8.18 (d, J = 7.89 Hz, 2

H, H¹⁷and H²³), 8.06 (dt, J = 7.76, 1.69 Hz, 2 H, H¹⁹and H²⁵),

7.59 (m, 3 H, H¹², H¹⁸and H²⁴), 7.47 (dt, J = 7.80, 1.68 Hz, 1 H,

H¹¹), 6.97 (m, 2 H, H¹⁰and H¹³) ppm. ¹³C NMR (300 MHz, [D₆]-

DMSO, 298 K): δ = 168.30 (C⁷), 159.54 (C⁹), 150.65 (C¹⁵and C²¹),

150.34 (C²⁰and C²⁶), 147.29 (C³and C⁵), 138.48 (C¹⁹and C²⁵),

135.40 (C¹¹), 130.48 (C¹⁸and C²⁴), 125.76 (C¹⁷and C²³), 125.33

(C¹²), 120.61 (C¹³), 118.52 (C¹⁰), 117.74 (C⁸) ppm. C₁₉H₁₄N₆O

(342.36): calcd. C 66.66, H 4.12, N 24.55; found C 66.34, H 4.06,

N 24.68. MS: m/z = 343 [M + H⁺], 224 [C₁₂H₁₀N₅]. FTIR (KBr

(m), 19.8 (s), 20.28 (w), 20.32 (w), 21.6 (m), 22.1 (m), 22.44 (m),

23.44 (s), 24.58 (m), 25 (m), 26.4 (s), 27.36 (s), 28.26 (w), 28.68 (w),

29.34 (m), 30.06 (w), 31.92 (w), 33.02 (w), 33.56 (w), 34.36 (w),

37.96 (w), 38.86° (w). White single crystals of X-ray quality were

obtained by slow evaporation of a saturated solution in ethyl ether.

N-Salicylidene 5-Aminotetrazole (Hsaltz): Synthesis was performed

under a dried argon atmosphere. 5-Aminotetrazole (5.0023 g,

58.8 mmol, 1 equiv.) was degassed under vacuum. Ethanol

(50 mL), dried and kept over molecular sieves, was bubbled with

argon for 30 min. 5-Aminotetrazole was dissolved in hot ethanol

by using Schlenk techniques. Salicylaldehyde was slowly added,

leading to a pale-green solution. The mixture was heated at reflux

for 5 h under an atmosphere of argon and then filtered, washed

with a few milliliters of degassed and dried ethanol and then dried

under vacuum. The resulting yellow precipitate was suspended in

ethyl ether (75 mL) and sonicated in an ultrasonic bath for 1 h. The

pure product (6 g, 31.7 mmol, 54%) was obtained and dried under

disk): ν = 472 (w), 554 (w), 604 (m), 623 (m), 642 (m), 690 (s), 731

˜

(w), 739 (s), 760 (s), 787 (s), 914 (w), 895 (w), 910 (m), 964 (w),

993 (m), 1036 (w), 1091 (w), 1115 (w), 1150 (m), 1196 (m), 1254

(w), 1271 (s), 1367 (m), 1406 (m), 1429 (s), 1448 (s), 1460 (s), 1487

(s), 1519 (w), 1568 (s), 1587 (s), 1622 (s), 3040 (m), 3153 (m, br.)

cm^–1. M.p. onset temperature by DSC: 161(1) °C. Degradation

temperature by TGA (onset point): 261(1) °C. X-ray powder dif-

fraction: 4.92 (w), 7.86 (s), 9.9 (m), 10.62 (m), 14.48 (w), 14.88 (m),

15.34 (w), 16.1 (s), 18.2 (m), 18.96 (m), 19.8 (w), 21.3 (w), 23.18

(m), 24.16 (w), 24.88 (w), 26.1 (w), 32.54 (w), 34.3 (w), 35.1 (w),

38.34 (w), 39.84 (w), 42.68 (w), 45.14 (m), 45.32 (w), 51.28° (w).

Single crystals were obtained by sublimation under reduced pres-

sure (110 °C, over a period of 3 months) on the cold finger con-

denser of a sublimator setup.

1

vacuum. H NMR (300 MHz, [D₆]DMSO, 298 K): δ = 11.52 (s, 1

H, H¹⁴), 9.53 (s, 1 H, H⁷), 7.92 (d, J = 7.32 Hz, 1 H, H¹³), 7.52 (t,

J = 7.23 Hz, 1 H, H¹¹), 7.03 (m, 2 H, H¹⁰and H¹²), 6.48 (s, 1 H,

H⁴) ppm. ¹³C NMR (300 MHz, [D₆]DMSO, 298 K): δ = 167.79

(C⁷), 162.64 (C⁹), 161.35 (C⁵), 157.60 (C⁸), 136.39 (C¹¹), 131.45

(C¹³), 120.70 (C¹²), 117.90 (C¹⁰) ppm. MS: m/z = 190 [M], 162 [M –

N₂]. X-ray powder diffraction: 7.40 (w), 8.98 (s), 10.38 (w), 10.48

(w), 12.34 (w), 12.48 (w), 13.92 (w), 14.64 (w), 15.88 (m), 16.86 (w),

17.82 (w), 19.00 (m), 20.16 (m), 20.28 (w), 21.44 (w), 22.98 (w),

23.56 (m), 23.74 (w), 24.34 (w), 25.28 (m), 25.16 (w), 26.60 (m),

27.12 (m), 27.84 (w), 28.32 (m), 28.38 (m), 29.1 (w), 31.56 (w),

35.48° (w). White single crystals of good quality were obtained by

slow evaporation of a saturated solution of ethyl ether.

N-Salicylidene (1,10)-Phenanthrolin-5-amine (Hsalphen): (1,10)-

Phenanthrolin-5-amine (0.5026 g, 2.6 mmol, 1 equiv.) was dissolved

in hot methanol (15 mL). Salicylaldehyde (3 mL, 28.5 mmol,

11 equiv.) was added to give a pale-yellow solution. The mixture

was heated at reflux for 16 h and then allowed to cool to room

temperature. The unreacted (1,10)-phenanthrolin-5-amine was fil-

tered, and the solvent was removed under vacuum. The residual

yellow oil was triturated with hexane (20 mL) and sonicated for

30 min. The supernatant containing an excess amount of salicylal-

dehyde was pipetted out without disturbing the oil. After repeating

Potassium Salt of N-Salicylidene 5-Aminotetrazolate (Ksaltz): In a

first step, 5-aminotetrazole (2.51 g, 29.5 mmol, 1 equiv.) was dis-

solved in distilled water (50 mL). Potassium hydroxide pellets

(1.66 g, 29.5 mmol, 1 equiv.) were slowly added to the solution. The

mixture was heated at reflux overnight, concentrated under vacuum

and then the resulting white precipitate was dried (3.9332 g, 78%).

The product was dissolved in methanol (50 mL) and salicylalde-

hyde (3.36 mL, 32 mmol, 1 equiv.) were added. The mixture was

heated at reflux overnight to give a clear yellow solution, which was

6ϫ, a pure yellow precipitate (0.6638 g, 2.21 mmol, 86%) was iso- concentrated under vacuum. The crude product contained 10% of

lated from the oil, filtered and dried under vacuum. ¹H NMR

(300 MHz, [D₆]DMSO, 298 K): δ = 12.37 (s, 1 H, H²³), 9.21 (d, J

= 1.68 Hz, 1 H, H⁹), 9.20 (s, 1 H, H¹⁶), 9.09 (dd, J = 4.32, 1.71 Hz,

1 H, H²), 8.70 (dd, J = 8.28, 1.71 Hz, 1 H, H⁷), 8.52 (dd, J = 8.13,

1.68 Hz, 1 H, H⁴), 7.89 (m, 3 H, H⁶, H⁸and H¹⁸), 7.81 (m, 1 H,

unreacted salicylaldehyde. This mixture was sonicated (3ϫ) with

hexane (50 mL) to give the pure product (5.51 g, 76%). Data for the

potassium salt of 5-aminotetrazolate: ¹H NMR (300 MHz, [D₆]-

DMSO, 298 K): δ = 4.73 (s, 2 H, NH₂) ppm. ¹³C NMR (300 MHz,

[D₆]DMSO, 298 K): δ = 163.68 (C-NH₂) ppm. Data for Ksaltz: ¹H

H³), 7.53 (m, 1 H, H²¹), 7.08 (m, 2 H, H¹⁹and H²⁰) ppm. ¹³C NMR (300 MHz, [D₆]DMSO, 298 K): δ = 13.25 (br. s, 1 H, H¹),

NMR (300 MHz, [D₆]DMSO, 298 K): δ = 165.10 (C¹⁶), 160.87 9.35 (s, 1 H, H⁸), 7.72 (dd, J = 7.98, 1.50 Hz, 1 H, H⁶), 7.43 (dt, J

(C²²), 151.20 (C⁹), 150.12 (C²), 146.48 (C⁵), 145.64 (C¹¹), 145.34 = 7.30, 1.62 Hz, 1 H, H⁴), 6.98 (m, 2 H, H³and H⁵) ppm. ¹³C

(C¹²), 136.96 (C⁴), 134.81 (C²¹), 132.77 (C¹⁸), 132.44 (C⁷), 129.40

(C¹⁴), 125.83 (C¹³), 124.46 (C⁶), 124.31 (C³), 120.76 (C¹⁷), 120.33

(C²⁰), 117.54 (C¹⁹), 113.72 (C⁸) ppm. MS: m/z = 300 [M + H⁺],

599 [M₂], 322 [M + Na], 621 [M₂+ Na], 107 [C₇H₇O]. FTIR (KBr

NMR (300 MHz, [D₆]DMSO, 298 K): δ = 166.06 (C²), 163.04 (C⁸),

160.97 (C⁹), 133.64 (C⁴), 132.78 (C⁶), 119.98 (C⁷), 119.71 (C⁵),

117.13 (C³) ppm. MS: m/z = 188 [M]. C₈H₆KN₅O (227.27): calcd.

C 42.28, H 2.66, N 30.82; found C 42.92, H 2.73, N 31.14. X-ray

disk): ν = 517 (w), 565 (w), 608 (w), 621 (w), 656 (w), 702 (w), 742 powder diffraction: 13.34 (w), 13.86 (w), 14.80 (m), 15.44 (s), 16.46

˜

(s), 752 (m), 781 (w), 806 (w), 833 (w), 881 (w), 893 (w), 922 (w), (w), 18.52 (s), 18.84 (w), 19.82 (w), 20.68 (w), 22.58 (w), 23.22 (m),

991 (w), 1024 (w), 1036 (w), 1059 (m), 1084 (w), 1115 (w), 1140

(w), 1151 (w), 1197 (w), 1275 (s), 1300 (w), 1366 (w), 1394 (m),

1420 (m), 1460 (w), 1481 (w), 1560 (m), 1574 (m), 1601 (s), 1616

(s), 3022 (w, br.), 3448 (w, br.) cm^–1. M.p. onset temperature by

DTA: 185(1) °C. Degradation temperature by DTA (onset point):

254(1) °C. C₁₉H₁₃N₃O (299.10): calcd. C 76.23, H 4.38, N 14.04;

24.26 (m), 25.40 (m), 26.86 (s), 27.80 (s), 28.82 (s), 29.00 (w), 30.48

(w), 31.84 (w), 33.16 (w), 34.7 (w), 36.66 (m), 42.2 (w), 43.68 (m),

45.12 (w), 46.50 (w), 48.42 (w), 49.64 (w), 50.62° (w). Yellow single

crystals of good quality were obtained by slow evaporation of a

saturated solution in methanol. X-ray diffraction measurements

were done at room temperature because of the instability of crystals

found C 75.35, H 4.19, N 13.40. X-ray powder diffraction: 7.72 (s), at low temperature (120 K).

634

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Eur. J. Org. Chem. 2010, 621–637

Engineering Solid-State Molecular Switches

Instrumentation: Sonication was performed with a Branson 3510

ultrasonic bath. ¹H and ¹³C NMR spectra were recorded with a

Bruker AC 300 MHz instrument with the DMSO proton peak as

internal standard. Peaks assignments for Hsaltrz, Habs, Hsalphen,

Hsaltz and Ksaltz NMR spectra were carried out by using atomic

numbering from CIF files. Peaks assignments in abpt NMR spectra

were made by using the same atomic numbering as in the Habs CIF

file. Mass spectroscopic data were obtained with a Thermofinnigan

LCQ ion trap spectrometer by using an ESI ionization mode for

Hsaltz and Hsalphen and APCI ionization mode for Habs and

Hsaltrz. Infrared spectra were recorded with a Shimadzu FTIR-

8400S spectrometer with KBr disks. Elementary analyses were per-

formed at University College London. The TGA/DTA instrument

used was a TA instrument SDT2960 Simultaneous DSC-TGA with

alumina crucibles filled with approximately 10 mg of sample. Ap-

proximately 10 mg of dried aluminium oxide were used as reference

for DTA measurements. DSC measurements were carried out at a

scanning rate of 10 K/min, over the temperature range (298–873 K)

with a Mettler Toledo DSC20, calibrated with pure aluminium

(s/l) and indium (s/l), and with alumina sample holders. X-ray pow-

der diffractograms were performed with a Siemens D5000 X-ray

diffractometer, with a Cu anticathode (λ_K-α= 1.5418 Å) in Bragg–

Brentano geometry (θ/θ mode). Samples were deposited on silicon

after calibration with a quartz standard. Diffuse reflectance spectra

were obtained with a Varian Cary 5E spectrometer by using PTFE

as a reference. Spectra were measured on pure solids to avoid ma-

trix effects.^[48,57–68]Eventual distortions in the Kubelka Munk spec-

tra^[76]that could result from the study of pure compounds have not

been considered because no comparison with absorption spectra

was necessary. Solid-state emission spectra were obtained with a

Fluorolog-3 (Jobin–Yvon-Spex Company) spectrometer. Kubelka

Munk and emissions spectra were normalized to allow meaningful

comparisons. Light irradiations were carried out with a standard

lamp used for visualizing TLC plates (ROC; intensity at 15 cm from

filter: 440 µWcm^–2at 254 nm). The reversibility of the cis–trans

isomerization was checked with a LOT-ORIEL 200 W high-pres-

sure mercury arc lamp combined with a water cooling setup to

avoid infrared light exposure. In situ irradiations were carried out

for 10 min with appropriate filters. Data that were accumulated

each 30 s were normalized.

final R indices are presented in Table S1 (Supporting Information).

CCDC-748023 (for Hsalphen), -748024 (for Habs), -748025 (for

Hsaltz), -748026 (for Ksaltz) and -748027 (for Hsaltrz) contain the

supplementary crystallographic data for this paper. These data can

be obtained free of charge from the Cambridge Crystallographic

Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Computational Methods: For isolated molecules, the geometry opti-

mization of the ground and excited states was performed at the

UB3LYP/6-31G(d,p) and CIS/6-31G(d,p) levels of theory, respec-

tively, using Jaguar 6.5 pseudospectral program package.^[78]Enol,

cis-keto and trans-keto forms and cyclized intermediate (in the case

of Habs) were fully geometry optimized. Energy profile for cis-

keto/trans-keto isomerization was obtained by performing a set of

constrained geometry optimization at successively lower values of

the C⁴–C³–C²–N¹torsion angle (see Scheme 2). All energies are

given after single-point calculations at the CIS/6-31G(d) level using

the Gaussian 03 program.^[79]

For calculations taking crystal packing into account, systems were

taken so as to be constituted of one molecule surrounded by others

in every direction (4 and 5 for Habs and Hsalphen, respectively).

Crystal environment created by neighbouring molecules was taken

from X-ray crystal structures. Optimization was performed at the

CIS/6-31G(d,p) level of theory (S¹). The position of every atom of

such molecules was frozen during optimization calculations with

only the central molecule being fully geometry optimized, excepted

for the intermediates during cis-keto/trans-keto isomerization,

which were obtained by freezing the C⁴–C³–C²–N¹torsion angle

(see Scheme 2) at successive values of this torsion angle. Energies

were obtained by single-point calculations with the central mole-

cule and crystal packing (surrounding molecules) treated at the

B3LYP/6-31G(d) and B3LYP/6-31G level of theory, respectively.

Stabilization energy induced by the packing was estimated by

Equation (1).

E_stab= E_system– E_{isolated molecule}– E_{surrounding molecules}

(1)

where E_systemis the energy of the full system (i.e., central molecule

and surrounding molecules), E_isolatedmolecule is the energy

[B3LYP/6-31G(d)] of the central molecule of interest without its

surrounding, E_{surrounding molecules}is the energy (B3LYP/6-31G) of

the surrounding molecules (without the molecule of interest). The

relative stabilization energy respective to the enol form was esti-

mated according to Equation (2).

X-ray Structure Data Collection and Refinement: For Hsaltrz, Habs,

Hsalphen and Hsaltz, X-ray intensity data were collected at 120 K

with a MAR345 image plate using Mo-K_α(λ = 0.71069 Å) radia-

tion. The crystal was chosen, mounted in inert oil and transferred

quickly to the cold gas stream for flash cooling. For Ksaltz, the

data were collected at room temperature with a Xcalibur CCD dif-

fractometer (Gemini ultra Mo) using Mo-K_α(λ = 0.71069 Å) radia-

tion. Crystal data and data collection parameters are summarized

in Table S1 (Supporting Information). The unit cell parameters

were refined using all collected spots after the integration process.

Except for Ksaltz, the data were not corrected for absorption, but

the data collection mode partially takes the absorption phenomena

into account. (See the total number of collected reflections vs. the

independent number of reflections in Table S1, Supporting Infor-

mation). The five structures were solved by direct methods with

SHELX97.^[77]All the structures were refined by full-matrix least-

squares on F²using SHELX97.^[77]All the non-hydrogen atoms

were refined with anisotropic temperature factors. For Hsaltrz,

Habs, Hsalphen and Hsaltz, all hydrogen atoms were localized by

Fourier-difference synthesis. For Ksaltz, only H1 of the hydroxy

group was localized, the other H atoms were calculated with AFIX.

The H atoms were included in the refinement with a common iso-

tropic temperature factor. The details of the refinement and the

(2)

Calculations of interaction energies are susceptible to basis set

superposition error (BSSE). In order to eliminate this error, E_isolated

molecule and E_{surrounding molecules}were estimated by single-point cal-

culations using a mixed basis set containing “ghost orbitals” corre-

sponding to the surrounding molecules and the central molecule,

respectively. The values of E_stabare available in Table S3 (Support-

ing Information).

Supporting Information (see footnote on the first page of this arti-

cle): Crystal data and structure refinement data of the compounds

studied; emission maximum wavelengths of the compounds; calcu-

lated energies for Habs and Hsalphen as isolated molecules and in

the crystal packing; NPA charges for Habs; calculated structures

and energies for Habs and Hsalphen; synthetic pathway leading to

the growing of single crystals of Hsaltrz.

Eur. J. Org. Chem. 2010, 621–637

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635

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FULL PAPER

Acknowledgments

This work was partly funded by the Fonds Special de Recherche

de l’Université Catholique de Louvain, the Fonds de la Recherche

Scientifique-FNRS (FRFC project No.2.4508.08, No.2.4502.05

and No.2.4.511.07.F), the Interuniversity Attraction Pole IAP-VI

(P6/17) INANOMAT program and the Concerted Research Action

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émie Universitaire Louvain. We also thank the Fonds pour la For-

mation à la Recherche dans l’Industrie et dans l’Agriculture for a

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Received: October 16, 2009

Published Online: December 8, 2009

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www.eurjoc.org

637

Article Doi

Engineering solid-state molecular switches: N-salicylidene N-heterocycle derivatives

DOI: 10.1002/ejoc.200901175

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Article abstract of DOI:10.1002/ejoc.200901175

Full text of DOI:10.1002/ejoc.200901175

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