Preparation Of Color-developing Rhodamine Containing Urea Structure And Its Application In Environmentally Friendly Inks

Due to the extensive use of inks and paper,there are many environmental problems in the daily printing process,such as the emission of large amounts of volatile organic compounds（VOCs）in inks,the increase in total carbon emissions in paper production,the increase in energy consumption in printing,and the depletion of forest resources.Photochromic molecules offer an opportunity to address these issues.The unique structure of photochromic molecules,which changes the molecular structure before and after light irradiation,causes significant changes in absorption and fluorescence emission spectra,making them easy to use in the design and production of photochromic inks and rewritable papers.Compared with other color-changing molecules,rhodamine dyes have good photophysical and photochemical properties,high molar extinction coefficients,large fluorescence quantum yields,resistance to photobleaching,and can undergo UV-promoted heterocleavage bond cleavage to form amphiphilic structures with large conjugation.However,the application of rhodamine dyes as photochromic molecules is severely limited by their very short open-loop lifetime.Therefore,there is an urgent need to solve the challenge of short color-changing lifetime of rhodamine dyes in environmentally friendly inks and reproduction papers.This paper focuses on the control method of rhodamine spiro ring switching to provide material support for application in eco-friendly inks.By extending and changing the structure of the spiro ring,different regulating groups are introduced into the six-membered spiro ring rhodamine to develop a new series of reversible photochromic systems with good photoresponsivity and stability.First,hexameric spirocyclic rhodamine containing urea structure was constructed,and rhodamine isocyanate was reacted with 2-aminomethylpyridine and 2-thiophenemethanamine,respectively,to prepare two new hexameric spirocyclic rhodamine SRh-PD and SRh-TP.They were subjected to p H titration test,metal ion response test,anion interference test and light response test.p Ka（Abs）for SRh-PD=4.82（±0.02）,p Ka（Int）=4.99（±0.01）;p Ka（Abs）=3.64（±0.11）,p Ka（Int）=2.66（±0.06）for SRh-TP.SRh-PD and SRh-TP were stable in an alkaline medium.The compounds in the pure acetonitrile system were more sensitive to metal ions,while in the CH₃CN-HEPES buffer solution compounds SRh-PD and SRh-TP were largely insensitive to metal ions and both compounds were not interfered by common anions.Moreover,in the presence of metal ions,Al³⁺,Cr³⁺,Fe³⁺,Cu²⁺,Pb²⁺and other metal ions could complex with hexameric spirocyclic rhodamine to form complexes,and the aqueous solutions of SRh-PD and SRh-TP changed from colorless to pink after irradiation with 313 nm UV light,accompanied by the appearance of red fluorescence,both at The maximum absorbance of SRh-PD reached 0.021 and the fluorescence intensity reached 133.27;the absorbance intensity of SRh-TP reached 0.079 and the fluorescence intensity could reach 86.54.Among them,SRh-TP reached its maximum emission after 30 minutes of UV irradiation.The maximum emission intensity of SRh-TP could be reached after 30 min of UV irradiation,and the initial state could be restored after 15 min of illumination withdrawal,which has the reversibility of photochromism.It provides a new idea for the design of subsequent photochromic rhodamine systems.In order to solve the problem that the photochromic property depends on the assistance of metal ions,two new six-membered spirocyclic rhodamine SRh-n PR and SRh-AP were prepared.p H titration tests,metal ion and anion interference tests,light response tests,etc.were also performed.p Ka（Abs）for SRh-n PR=3.25（±0.24）,p Ka（Int）=4.34（±0.27）;p Ka（Abs）=3.98（±0.02）and p Ka（Int）=3.88（±0.03）for SRh-AP.According to the performance of their absorption and fluorescence spectra,both compounds underwent ring-opening reactions in the six-membered spiro ring structure before and after UV irradiation,and the compound SRh-AP showed better color change and faster response than SRh-n PR with reversibility of color change.Although metal ions also promoted the photochromic property of SRh-AP,the photochromic effect of the solution was sufficiently obvious in the presence of SRh-AP alone to achieve the excellent photochromic behavior without the help of metal ions.After 30 min of UV irradiation,SRh-n PR showed an absorption peak at 562 nm with a maximum absorbance of 0.01 and an emission peak at 581 nm with a maximum fluorescence intensity of 481.91,while SRh-AP showed an absorption peak at 562 nm with a maximum absorbance of 0.039 and an emission peak at 581 nm with a maximum fluorescence intensity of 82.08.In addition,SRh-AP showed an absorption peak at 562nm with a maximum absorbance of 0.039 and an emission peak at 581 nm with a maximum fluorescence intensity of 82.08 after 10 minutes of UV illumination.The maximum emission intensity of SRh-AP could be reached after 10 min of UV light,and the initial state could be restored after 10 min of light withdrawal,which has the reversibility of photochromism.Subsequently,we focused on the application of SRh-AP in anti-counterfeit ink printing and information record rewritability.Based on the above study,a series of hexameric spirocyclic rhodamine containing urea structures were designed and synthesized by introducing different modulating groups,and their photochromic mechanism was investigated to prove their photochromic properties,the existence of regular photochromic behaviors and fading phenomena,and the free dynamic modulation of photochromic properties without the help of metal ions,which provides a new strategy for the study of new smart materials in optical storage media and data encryption applications.