Synthesis And Photophysical Properties Of Novel S-triazine Derivatives

Two–photon absorption is a third–order nonlinear optical effect wherein the molecules absorb two photons and pass through a virtual intermediate state to the excited state.Two–photon absorption materials have been applied in many fields such as fluorescence imaging,three–dimensional microfabrication,three–dimensional optical data storage,optical limiting,two–photon pumped up–converted lasing,and photodynamic therapy.In this paper,we designed and synthesized a series of novel s-triazine branched derivatives,their structures were characterized by FT–IR?¹H NMR,¹³C NMR,and HRMS.Their photophysical properties,including linear absorption,one–photon excited fluorescence,two–photon absorption,and two–photon excited fluorescence,were systematically investigated in various solvents.Finally,the relationships between the molecular structures and the photophysical properties were systematically studied.Ethanol,acetonitrile,and hydrogen chloride were used as the starting materials to realize the synthesis of ethyl acetimidate,and then followed by the trimerization reaction to synthesize trimethyltriazine.Trimethyltriazine was then reacted with4-diphenylbenzaldehyde,9-ethyl-9H-carbazole-3-carboxaldehyde,10-ethyl-10H-phenothiazine-3-carboxaldehyde,2-thiophenecarboxaldehyde,and 3,4-dimethoxy benzaldehyde by the condensation reaction to give three new single-branched compounds?BA-1,KZ-1,and FQ-1?and two new symmetric two-branched compounds?SF-2 and JY-2?.Three kinds of aldehydes?4-diphenylbenzaldehyde,9-ethyl-9H-carbazole-3-carboxaldehyde,and 10-ethyl-10H-phenothiazine-3-carbox aldehyde?were synthesized by the Vilsmeier reaction using N,N-diphenylbenzenamine,9-ethyl-9H-carbazole,and 10-ethyl-10H-phenothiazin as the starting materials.The two intermediates 2,5-dimethoxy-4-[?1E?-2-?4-pyridinyl?ethenyl]benzaldehyde?BDQ?and 4-[?1E?-2-?1-methylbenzimidazol-2-yl?ethenyl]-2,5-dimethoxybenzal dehyde?JMQ?were synthesized using p-dimethoxybenzene,paraformaldehyde,hydrobromic acid,4-methylpyridine,and 1,2-dimethyl-1H-benzimidazole as materials by the following three-step reactions:?1?Blanc reaction;?2?Sommelet reaction;?3?Condensation reaction.BDQ and JMQ were then reacted with the single–branched compounds BA-1 and KZ-1 by the unilateral or bilateral condensation reaction to give four new asymmetric two–branched compounds?BA-B2,BA-M2,KZ-B2,KZ-M2?and four new asymmetric three–branched compounds?BA-B3,BA-M3,KZ-B3,KZ-M3?.The linear absorption and one–photon excited fluorescence properties of the above-mentioned s-triazine derivatives were studied in different solvents.Their linear absorption wavelengths range from 355 nm to 444 nm;the molar absorption coefficients range from 1.340×10³–2.045×10⁴ mol^-1 L cm^-1;the one–photon fluorescence emission wavelengths range from 402 nm to 594 nm;the stokes shifts range from 2599 cm^-11 to 7439 cm^-1;BA-1 exhibits the largest fluorescence quantum yield,which is 0.56.The results indicate that the maximum absorption peaks??a_mb_as_x?of all the compounds change slightly with the increase of solvent polarity from toluene to N,N-dimethylformamide.When increase the solvent polarity,the one–photon excited fluorescence peak positions(?_max^OPEF)of the compounds show obvious red shifts,the Stokes shifts exhibit the same solvent polarity dependencies as the?_ma^OP _x^EF.With the increase of branch number,the?_m^{ab s}_axand?_max^OPEF show obvious redshifts,and both the maximum molar absorption coefficients(?_max)and the one–photon excited fluorescence intensity increase.Except for KZ-1,JY-2,and SF-2,the fluorescence quantum yields of the other compounds decrease significantly upon increasing the solvent polarity,whereas the fluorescence quantum yields of JY-2 and SF-2 change slightly and the fluorescence quantum yields of KZ-1 increase with increasing the solvent polarity.Under the excitation of Ti:sapphire femtosecond lasers,the two–photon induced fluorescence method was used to study the nonlinear optical properties of the s-triazine derivatives in two solvents.These compounds exhibit strong two–photon excited fluorescence and relatively large two–photon cross–sections,in which the two–photon cross–section of KZ-B3 reaches 803 GM.The relationships between the structures and nonlinear optical properties of the compounds were systematically studied.The results indicate that the peak wavelengths of the two–photon excited fluorescence are evidently red–shifted when compared with those of one–photon excited fluorescence because of the reabsorption effect.With the increase of branch number,the two–photon emission wavelengths were redshifted.Increasing the branch number,improving the strength of electronic donor/acceptor,and increasing the coplanar of molecular structure,the intramolecular charge transfer can be significantly improved,and finally enlarges the?value obviously.