Structure and spectroscopic properties of donor-acceptor pair with guanidinium and carboxylate group

U ovom radu modeliran je donorsko akceptorski sustav u kojem su donor i akceptor elektrona povezani karboksilatno-gvanidinskim mostom koji omogućava samoudruživanje prepoznavanjem karboksilne i gvanidinske skupine. Kao akceptor odabrana je 3,5-dinitrofenilna skupina, a kao donor fenilna, metoksifenilna, dimetilaminofenilna i naftalenska skupina. TD-DFT metodom su za 40 singletnih pobuđenih stanja izračunate vertikalne pobudne energije, pripadne oscilatorske jakosti, određena elektronska konfiguracija i simuliran apsorpcijski spektar. Utjecaj otapala na pobudne energije modeliran je korištenjem PCM modela. Zaključeno je da obje elektron donorske skupine na donoru stabiliziraju stanja s prijenosom elektrona (CT stanja) neovisno o orijentaciji solnog mosta. Također utjecaj dimetilamino skupine zbog veće -elektronske donorske sposobnosti na pobudnu energiju CT stanja je značajniji od utjecaja metoksilne skupine. U slučaju kompleksa s naftilgvanidinom dolazi do djelomične interne neutralizacije naboja na obje strane kompleksa što stabilizira CT stanje. Optimizacijom geometrije u pobuđenom CT stanju u dva odabrana kompleksa pokazano je da dolazi do velike geometrijeske reorganizacije približavanjem donorske i akceptorske podjedinice zbog jakih Coulombovih interakcija kroz prostor.Electron donor-acceptor pairs connected by the guanidinium-carboxylate salt bridge, which allows recognition and self-assembly, were studied by using TD-DFT calculations and PCM solvation model. The 3,5-dinitrophenyl group was selected as acceptor, and phenyl, metoxyphenyl, dimethylaminophenyl and naphthyl functional groups were used as donors enabling pull and push-pull effects. Vertical excitation energies, oscillator strengths and electronic configurations were determined for the 40 lowest laying singlet states. Also, the UV/Vis absorption spectra were simulated. It was concluded that both metoxy and dimethylamino groups stabilize charge-transfer (CT) states regardless of the salt bridge orientation. Dimethylamino group was more efficient in this stabilisation than metoxy group due to higher -electron donor ability. In complexes with naphthalene as donor, partial internal neutralisation of charges in CT states were detected. Geometry optimisation in the excited CT state in two selected examples showed that huge relaxation energy was a consequence of the geometry reorganisation by increasing edge to edge distance