Design and syntheses of efficient and thermally stable diamino chromophores for NLO application

A novel series of diamino imidazole-type chromophores has been synthesized, which can be chemically incorporated into polymer matrices. Furthermore, their molecular optical nonlinearity, transparency and thermal stability were determined and studied comparatively. Theoretical calculation and experimental results indicate that good nonlinearity-transparency-thermal stability is well balanced in chromophore 4 , which possesses a µ g β 0 of 554.8 × 10 -48 esu, a rather blue-shifted absorption peak at 384 nm in THF, and a high decomposition temperature T d of 377 ° C. It is strongly suggested from these results that multiple substitution strategy should be useful for designing novel NLO chromophores.


Introduction
Organic molecules with large first hyperpolarizabilities (β) have drawn considerable attention over the last 20 years for their potential applications in optical data storage, telecommunications and optical signal processing 1,2 .It is already well known that molecules that contain electrondonor and electron-acceptor groups separated by a large conjugated framework possess large values of β. 3 Many of these chromophores fall into the category of substituted benzenes, biphenyls, Schiff bases, stilbenes, azo stilbenes and tolans 4,5 .Molecular nonlinearities can be enhanced significantly by increasing the donor and acceptor strength or the length of the conjugation pathway. 6lthough significant progress has accrued in the design and synthesis of such onedimensional dipole chromophores 7,8 , other properties, such as transparency, thermal stability and capability to be incorporated into bulk materials must also be optimized.Most NLO chromophores synthesized for electro-optic and frequency doubling application typically exhibit trade-off between nonlinearities and each of the other properties. 9,10For those traditional D-π-A chromophores, the increase in NLO response is usually accompanied by bathochromic shifts of the maximum absorption peak and decrease in thermal stability.
In order for the nonlinear response to be stable during processing and operation, the chromophores need to be chemically stable at all temperatures that the system encounters.However, it is extremely difficult to achieve thermal stabilities of 300 °C or higher without concomitant compromise in the nonlinearity of the chromophore.Furthermore, in order to make a useful device, NLO chromophores must be incorporated into a fabric version, such as poled polymers, LB films, self-assembled or crystal environments, etc.Among these, chemical incorporation of chromophores into a polymer backbone is probably the most important and widely useful.For incorporation into polymers, the NLO chromophores must bear suitable reactive groups in the molecular structure.Up to now few such chromophores have been exploited, due to the difficulties in their synthesis.There exists a need to explore and develop such reactive chromophores with efficient nonlinearities, desired transparency and high thermal stabilities.
Herein we report initial studies on the synthesis and properties of a novel series of imidazolebased diamino chromophores, which can be used to form high T g NLO poled polymers such as polyureas and polyimides for optimized thermal and physical properties.

Results and Discussion
Our approach to the design of superior nonlinear optical chromophores is based on the use of electron-rich five-membered heteroaromatics such as imidazole in the conjugation pathway.It is recognized that imidazole derivatives exhibit excellent thermal stability and solubility in conventional organic solvents. 11Meanwhile, it is possible to incorporate the chromophore into a polymer backbone, if two aminophenyl groups are introduced into the 4-and 5-positions of the imidazole ring.
By connecting all four chromophores to an imidazole ring, we were able to produce extensively conjugated compounds that possess a stilbene moiety, an azo stilbene moiety, a thio stilbene moiety and another imidazole ring in the conjugation pathway.Amino groups were employed as the electron donor and nitro groups as the electron-acceptor.Chromophore 4, which bears two imidazole groups, is an X-type dual charge transfer (CT) system, which is favorable to obtain balanced nonlinearity-transparency-thermal stability properties.
To form the imidazole ring, condensation of benzil with aldehyde in the presence of ammonium acetate in glacial acetic acid solution is employed as a general method.The method to introduce amino groups into the structure becomes the key step for the syntheses.Marks and co-workers reported the use of 4,4-diphthalimidobenzil as a starting dicarbonyl compound. 12The condensation product bearing protected amino groups was deprotected by hydrazinolysis to afford the target chromophore.Since the use of bulky phthaloyl group for protecting amino group could result in poor solubility of the substrates and harsh reaction conditions for hydrazinolysis, the chromophores were produced in poor overall yield.The main modification is the use of the acetyl group, which can be removed under mild conditions, to protect amino functions.Compound 6 was prepared simply through N-acylation of p-aminobenzaldehyde 5 with acetic anhydride.The syntheses of chromophores 1-4 are summarized in the above schemes.Phosphonate 10 was prepared by the Michaelis-Arbuzov reaction using benzyl chloride 9 and trimethyl phosphite.The nitro group in 4-position of the phenyl ring was introduced by nitration of 10 to afford 11.The Wadsworth-Emmons reaction of p-nitrophosphonate 11 with one equivalent of aldehyde 12 yielded aldehyde 13, which bears a trans-carbon-carbon double band and can be used to form an imidazole structure in the following step.Reduction of p-nitrobenzaldehyde 14 with zinc powder gave p-nitrosobenzaldehyde 15.Subsequently, Mills condensation of 15 with p-nitroaniline followed by treatment with acetic acid afforded aldehyde 16.
Wadsworth-Emmons reaction of aldehyde 17 13 with p-nitrophosphonate 11 gave product 18.Vilsmeier-Haack formylation reaction of 18 formed aldehyde 19 with a small amount of other isomer.
Theoretical investigation can play an important role in understanding structure-property relationship, which is able to assist in designing novel NLO chormophores.The electrostatic first hyperpolarizability β and dipole moment µ of the four chromophores have been calculated by employing the semi-empirical AM1/FF (finite field) calculations.Characterization results for chromophores 1-4 are presented in Table 1.Characterization results for 2-(4-nitrophenyl)-4,5di(aminophenyl)imidazole (I) also are listed in Table 1 for comparison 12 .It is found that all the four chromophores show larger µ g β 0 value than that of I, which is attributed to the positive contribution of their elongated conjugation lengths.The azo chromophore 2 exhibits larger nonlinearity than the olefin chromophore 1, which indicates that the effect of -N=N-moiety on the value of µ g β 0 is superior to that of -CH=CH-, but the λ max of the former is red-shifted.
Comparison of chromophore 3 with chromophores 1and 2 reveals that the replacement of the benzene ring by thiophene ring in conjugation pathway results in a greatly enhanced charge transfer properties and nonlinear responses.Chromophore 3 has the value of µ g β 0 : 405.8×10 -48 esu, which is larger than that of chromophore 1 (342×10 -48 esu) and 2 (283.7×10 -4 esu), but its λ max is not the largest of the three.This can be explained as follow: since the delocalization energy of thiophene is lower than that of benzene 15 , the introduction of the thiophene ring as spacer between donor and acceptor would result in the loss of aromaticity in the ground state.However, this loss can be compensated by aromacity in the charge-separated form 16 .Most stilbene-type chromophores in which a benzene ring was replaced by a delocalized fivemembered heteroaromatic ring show significant increase in optical nonlinearities.These results clearly demonstrate the importance of thiophene-like rings in the design of push-pull chromophores.For one-dimensional push-pull chromophores, either increasing the conjugation length or changing the π-conjugation system inevitably results in enhancement of their chargetransfer properties.
For the two-level model of chromophore, it is clear that the hyperpolarizability is a strong function of the absorption maximum.Since even a small absorption at the operating wavelength of electron-optic devices can be detrimental, it is important to make NLO chromophores as transparent as possible without compromising the molecule's nonlinearity.From the results listed in Table 1, it can be seen that chromophore 4 performs rather well in the nonlinearitytransparency trade-off.Its µ g β value is ca.twice that of chromophore 1, but its λ max value is blueshifted by ca. 17 nm due to the presence of two charge-transfer states in similar energy levels, each of which can contribute to the hyperpolarizability.The results indicate that β of chromophore 4 arises from multiple excited configurations as reported by Ye and co-workers 17 .Obviously, due to its poor co-planarity and the multiple substitution strategy, chromophore 4 does not have a strengthened CT structure.Thermal stability of the four chromophores was estimated by thermogravimetric analysis (TGA).All samples show very high decomposition temperature (T d ) above 250 °C measured at a heating rate of 20 °C/min, which makes them ideal candidates for incorporation into high temperature polymers for device application.For chromophore 3, the lowest T d (279 °C) may be due to the introduction of thiophene ring.Comparison of T d of chromophore 1 with that of chromophore 2 reveals that azo moiety is thermally more stable than the olefin moiety in this type of compounds.Among the four chromophores, chromophore 4 in particular possesses very high T d (377 °C), which is attributed to the absence of azo, olefin, thiophene groups in its structure.
In summary, a novel series of diamino imidazole-type chromophores has been synthesized.The molecular optical nonlinearity, transparency and thermal stability were determined and studied comparatively.The theoretical calculation and experimental results indicate that nonlinearity-transparency-thermal stability of chromophore 4 is well balanced, which possesses a µβ 0 of 555×10 -48 esu, rather blue-shifted absorption peak at 384 nm in THF and high decomposition temperature T d of 377 °C.All the four novel chromophores can be used to form high Tg polymers for nonlinear optical applications.