Synthesis and optical properties of new tricyano-p-quinodimethane dyes: Molecular and polymeric systems

Article abstract of DOI:10.1016/S0040-4039(00)00610-9

The reaction of N,N-bis(2-hydroxyethyl)aniline 1 with tetracyano-p- quinodimethane (TCNQ) 2 yields the photochromic red chromophore 3 [λ(max) (acetone/dichloromethane) 497 nm] which upon UV irradiation is converted into the blue zwitterionic form 3' (λ(max) 680 nm) by a novel intramolecular charge transfer process involving a D-quinoid-aryl-A→D⁺-aryl-quinoid-A^- conversion. The analogous process occurs in the polyimide derived system 8. Different products are isolated from reactions of 1 with the stronger electron acceptors TCNQBr₂ and TCNQF₄. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0040-4039(00)00610-9

Tetrahedron Letters 41 (2000) 4645±4649
Synthesis and optical properties of new tricyano-p-
quinodimethane dyes: molecular and polymeric systems
Naveed A. Zaidi,a,b Martin R. Brycea,* and Graham H. Crossb,*
aDepartment of Chemistry, University of Durham, South Road, Durham DH1 3LE, UK
bDepartment of Physics, University of Durham, South Road, Durham DH1 3LE, UK
Received 16 February 2000; accepted 10 April 2000
Abstract
The reaction of N,N-bis(2-hydroxyethyl)aniline 1 with tetracyano-p-quinodimethane (TCNQ) 2 yields
the photochromic red chromophore 3 [lmax (acetone/dichloromethane) 497 nm] which upon UV irradiation
is converted into the blue zwitterionic form 30 (lmax 680 nm) by a novel intramolecular charge transfer
process involving a D-quinoid-aryl-A!D+-aryl-quinoid-A^ conversion. The analogous process occurs in
the polyimide derived system 8. Di€erent products are isolated from reactions of 1 with the stronger electron
acceptors TCNQBr2 and TCNQF4. # 2000 Elsevier Science Ltd. All rights reserved.
Keywords: tetracyano-p-quinodimethane; charge-transfer; photochromism; polyimides.
In the search for organic chromophores with electron accepting properties, derivatives of
tricyanoethene (TCE)1 and tricyano-p-quinodimethane (TCQ)2 have attracted considerable
attention, and some of these systems display high second order nonlinear optical e€ects.3 The
TCQ systems o€er the advantage of an extended p-system and gain in aromaticity upon charge
transfer. Furthermore, the TCQ acceptor properties can be ®nely tuned by the electronic nature
of substituents attached to the precursor tetracyano-p-quinodimethane (TCNQ) framework.4
Herein we report the synthesis and optical properties of new TCQ acceptor systems bearing an
N,N-disubstituted aniline group to provide donor-p-acceptor chromophores. We have also varied
the electron anity of the TCQ moiety and embeded the chromophores within a polymer matrix.
We initially studied monomeric species, and in accord with other aniline derivatives,5 the
reaction of N,N-bis(2-hydroxyethyl)aniline 1 with TCNQ 2 in chlorobenzene gave product 36 (51%
yield, Scheme 1) as a red solid [lmax (acetone/dichloromethane) 497 nm] formed by nucleophilic
displacement of cyanide ion from TCNQ. Compound 3 is photochromic: upon irradiation with
UV light (Xe lamp, using a CuSO4 thermal ®lter) in dichloromethane solution, the colour changed
to dark blue and a new absorption band emerged at lmax 680 nm assigned to the zwitterionic
structure 30.
* Corresponding author. E-mail: m.r.bryce@durham.ac.uk
0040-4039/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(00)00610-9

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Scheme 1.
The conversion of 3 into 30 is a novel photochemically-induced intramolecular charge transfer
process in a donor-p-acceptor system, comprising the interchange of aromatic and quinoid
systems (i.e. D-quinoid-aryl-A!D+-aryl-quinoid-A^). This process is fundamentally di€erent
from diquinoid!diaryl transformations (i.e. D-quinoid-quinoid-A!D+-aryl-aryl-A^) which are
well documented, e.g. in diphenoquinoid derivatives.7
We have explored the analogous reactions of 1 with 2,5-dibromo- and 2,3,5,6-tetra¯uoro-TCNQ:
the electronegative halogen substituents increase the electron anity relative to unsubstituted
TCNQ (E1 values: TCNQ +0.17, TCNQBr2 +0.41, TCNQF4 +0.53 V versus SCE in MeCN).4a
We reasoned that this e€ect should also apply to the TCQ moiety, thereby stabilising the zwitterionic
state if analogues of 30 could be obtained. The reaction of compound 1 with TCNQBr2 and
TCNQF4 followed di€erent courses from the reaction with TCNQ. With TCNQBr2, instead of
the red covalent analogue of 3, a dark charge-transfer salt 4 was isolated (45% yield), comprising
the anilinium cation radical and the TCNQBr2 anion radical. EPR studies on salt 4 revealed a
strong signal with a g value of 2.0028, which is typical of a TCNQ anion radical.8 UV irradiation
of salt 4 in DMF a€orded a blue covalent species (lmax 680 nm) assigned the zwitterionic structure
50 which was epr silent.
Reaction of 1 with TCNQF4 under identical conditions used for TCNQ and TCNQBr2 gave
a stable yellow solid (45% yield, lmax 460 nm) to which we assign the covalent zwitterionic
diradical structure 6. The compound gave a strong epr signal with a g value of 2.0026, and a spin
concentration of two unpaired electrons per molecule [referenced to 1,1-diphenyl-2-picryhydrazyl
(DPPH)]. Compound 6 was converted into the blue zwitterionic compound 60 (lmax 685 nm; epr
silent) on UV irradiation.
It is very interesting that the initial products isolated from the reactions of 1 with TCNQ,
TCNQBr2 and TCNQF4 are di€erent in each case. It is reasonable to propose that each reaction
initially a€ords an intermolecular charge-transfer complex between 1 and the TCNQ acceptor, as

4647
suggested earlier for the interaction of N,N-dimethylaniline with TCNQ,9 but the only complex of
this type isolated in the present series of reactions is the TCNQBr2 salt 4. This is reminiscent of
the unusually high stability of CT salts of TCNQBr2, compared to the TCNQ and TCNQF4
counterparts, observed in previous work with other electron donor molecules.10 The TCNQ and
TCNQF4 complexes of 1 react readily, with loss of HCN, to form the covalent products. This
comparable reaction for 4 requires photochemical activation. The direct formation of structure 6
is a consequence of the very strong electron acceptor properties (i.e. high stability of the anion
radical) of the TCQF4 moiety.
We sought to attach covalently these new chromophores to a polymeric system with the aim of
improving their processability and stability to photo- and oxidative degradation. Polyimides have
been widely used as a backbone for pendant chromophores due to their ease of synthesis, solubility
in organic solvents, good mechanical properties and high glass transition temperatures (Tg).1b,11
Polymer 7 reacted with TCNQ in DMF as above, to yield a red polymer 8 (Scheme 2) [90% yield,
MW>100,000, lmax (DMF) 497 nm]. The Tg for polymer 7 was 156ꢀC; for polymer 8 this value
was raised to 175ꢀC, and the solubility in organic solvents was signi®cantly reduced. These
observations can be attributed to functionalisation of the polymer.
Scheme 2.
By direct analogy with compound 3, the irradiation of a solution of polymer 8 in DMF resulted
in a blue solution (lmax 680 nm) signifying conversion to the zwitterionic species 80 (Fig. 1).
Compounds 30 and 50 are stable for several weeks in solution and in the solid state; polymer 80
reverts to 8 after a few days at 0ꢀC; 80 can be regenerated upon photolysis, and this cycle can be
performed several times, without observable decomposition of 8 or 80.
Reaction of polymer 7 with TCNQBr2 yielded an insoluble polymeric CT salt,12 which was
unchanged upon UV irradiation. Reaction of polymer 7 with TCNQF4 gave a yellow polymer 9
(90% yield) analogous to monomer 6, which (in contrast to 6) was unchanged upon UV irradia-
tion. Clearly the polyimide host structure is stabilising the TCNQBr2 CT salt and TCQF4 anion
radical structures, preventing the photochemical transformations which occur in the monomeric
counterparts 4 and 6.

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Figure 1. UV±vis spectra of 8, in DMF solution, before and after photolysis
In summary, new photochromic TCQ chromophores have been synthesised, and incorporated
into polymers. These results should encourage further studies on functionalised TCQ systems
with the aim of obtaining derivatives suitable for exploitation in optoelectronic devices.
Acknowledgements
We thank EPSRC for funding this work.
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4649
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