Firefly luminescent understand: the appearance and mechanism of fluorescein solid luminescent materials

Nature is always selflessly dedicated to human beings living in it In addition to all kinds of materials necessary for life, various phenomena in nature also provide continuous inspiration for researchers Through their research, people can not only better understand the nature behind the phenomenon, but also develop various tools to improve human living standards Among them, luminescent materials have attracted people's attention since ancient times, such as the night pearl, which is regarded as a treasure, is a kind of luminescent material With the development of science and technology, people's understanding of the principle of luminescent has been different Scientists have synthesized countless molecules with luminescent properties by chemical means and applied them to all aspects of human life However, for some natural bioluminescent materials, such as firefly fluorescein, the principle of bioluminescent is not well understood Figure 1 A) firefly fluorescein and its derivatives; b) synthesis of firefly fluorescein 3 (picture source: angelw Chem Int ed.), pan TCH e Naumov, New York University, Abu Dhabi, USA Associate professor and collaborators studied the phenomenon of fluorescence enhancement of firefly fluorescein derivatives under blue light irradiation, and explored the mechanism behind this phenomenon The team found that the crystal of firefly fluorescein derivative 3 (Fig 1) would burst under blue light irradiation, and its fluorescence intensity would increase significantly with the increase of irradiation time By means of crystallography, computer simulation and other means, the team found that 3 decarboxylation occurred under light, resulting in another firefly fluorescein derivative 4 (Fig 1), and the accumulation of 4 would produce strong fluorescence This achievement was published in German Applied Chemistry (DOI: 10.1002 / anie 201803424) under the title of "turning on solid state fluorescence with light" Fig 2 Changes of surface structure and luminous characteristics of crystal 3 after illumination: a) SEM of 3 before illumination; b) SEM of 3 after illumination for 10s; C-E) optical micrograph of crystal 3 under white illumination; f-I) fluorescence micrograph of crystal after illumination for 0, 5, 10 and 30 minutes; J-M) local enlarged Atlas of figure H (source: angel Chem Int ed.) The synthesis of 3 is relatively simple, using 6-n, N '- dimethylamino-2-cyanobenzothiazole reacts with D-cysteine in basic condition, then adjust the pH value of the reaction solution to acid by hydrochloric acid, and finally extract with ethyl acetate to obtain crude product, which can be recrystallized in dichloromethane / methanol 4:1 solution to obtain relatively pure crystal After obtaining 3, the author found that blue light (LED light, 25 MW) or 405 nm light (laser, 50 MW) would cause 3 crystal to break and bounce If the crystal is fixed, parallel cracks can be observed on its surface (Figure 2a-e) When the crystal is placed in a liquid environment for the same test, gas generation can be seen (Fig 2e) In addition, with the increase of irradiation time, the fluorescence intensity of the crystal will gradually increase, and the fluorescence will appear from the outside to the inside (Fig 2f-i) Fig 3 Molecular stacking of compounds 3 and 4: A, b) SEM and molecular arrangement diagram of compounds 3 before and after irradiation; c) molecular stacking diagram of compounds 4 (photo source: angelw Chem Int ed.) preliminary test shows that there are three phenomena of crystal 3 by light note: appearance of parallel crack, generation of gas and change of fluorescence intensity After that, the author explains these three phenomena in turn The chemical reaction of 3 under the light is likely to destroy the crystal structure of 3, so it is not surprising that the cracks appear It is worth studying why the cracks are parallel? In order to explain this problem, the author analyzed the single crystal of 3 in detail, and found that in the crystal structure, the molecules present a mutually perpendicular spatial structure due to the interaction between carboxyl groups; in this structure, all carboxyl groups are in the same column Therefore, when these carboxyl groups react, they show parallel cracks on the macroscopic view After that, the author analyzed the reason why bubbles appeared in the light by infrared spectrum With the increase of illumination time, the vibration peak of CO at 1738cm-1 decreased significantly, while that of CO2 at 2342cm-1 increased gradually At the same time, the dynamics study shows that the enhancement speed of CO vibration peak is opposite to that of CO 2 Therefore, the author speculates that the bubble observed before is due to the decarboxylation of 3 under the light and the formation of CO2 This conjecture is not groundless The author found that the molecular weight of the product is the same as that of 4 by high-resolution mass spectrometry In addition, the author synthesized 4 by a similar reaction and obtained its single crystal structure (Fig 3C) Different from the structure of 3 which is perpendicular to each other, 4 shows a layer by layer structure, which is more conducive to the charge transfer between molecules So it can also explain why the fluorescence of 3 increases gradually under the light Figure 4 Theoretical calculation results of compounds 3 and 4 (picture source: angel Chem Int ed.) finally, the author proposed a reaction mechanism combining experimental results and theoretical calculation (Figure 4) The energy (Erel, S1 = 308.7 kJ / mol) of excited molecule 3 * is much higher than that of transition state formation (Erel, TS = 287.7 kJ / mol) in decarboxylation reaction Therefore, the decarboxylation of molecule 3 * will take place directly and generate 4 (E = - 56.6 kJ / mol) with lower energy The excited state molecules generated after illumination do not return to the ground state and emit photons, so the fluorescence of 3 can not be observed in macroscopic Pan ć e Naumov's team studied biologically inspired firefly fluorescein derivatives Through crystallography, computer simulation and kinetic spectrum analysis, the reason for the fluorescence phenomenon was found This research has a broad application prospect in sensing Stefan Schramm, Durga Prasad karothu, gijo Raj, Sergey P laptenok, Kyril M solntsev, and pan ć e Naumov