Locked ortho- and para-Core Chromophores of Green Fluorescent Protein; Dramatic Emission Enhancement via Structural Constraint

Abstract

We report the design strategy and synthesis of a structurally locked GFP core chromophore p-LHBDI, its ortho-derivative, o-LHBDI, and H2BDI possessing both para- and ortho-hydroxyl groups such that the inherent rotational motion of the titled compounds has been partially restricted. o-LHBDI possesses a doubly locked configuration, i.e., the seven-membered ring hydrogen bond and five-membered ring C(4-5-10-13-14) cyclization, from which the excited-state intramolecular proton transfer takes place, rendering a record high tautomer emission yield (0.18 in toluene) and the generation of amplified spontaneous emission. Compared with their unlocked counterparts, a substantial increase in the emission yield is also observed for p-LHBDI and H2BDI in anionic forms in water, and accordingly the structure versus luminescence relationship is fully discussed based on their chemistry and spectroscopy aspect. In solid, o-LHBDI exhibits an H-aggregate-like molecular packing, offers narrow-bandwidth emission, and has been successfully applied to fabricate a yellow organic light emitting diodes (λmax = 568 nm, ηext = 1.9%) with an emission full width at half-maximum as narrow as 70 nm.

Excited‐State Intramolecular Proton Transfer in a Blue Fluorescence Chromophore Induces Dual Emission

“…Finally,o ur work addresses the mechanistic photochemistry of the BFP chromophores and sets the fundamental stage for furthere xperimental and theoretical studies on similarG FP and BFP chromophores and chemically synthesized variants in the future. [20,23,[46][47][48][49][50]…”

“…The BFP chromophoree xhibits am uch slower ESIPT rate of about 18 ps, which is significantly longer than in GFP analogues with ESIPT,f or example, 25 fs for o-HBDI. [46][47][48][49] Experimentally,t his is rationalized in terms of the HOMO and LUMO structures. For the 2-BFP chromophore, the HOMO and LUMO involved in the S 0 !S 1 electronic transition are both spread over the whole moleculars pace;t hus, the S 1 electronic state has much less charge-transfer character and favors fluorescence emission.…”

“…[20,23] The fluorescence quantum yield of the 2-BFP chromophore is almost two orders of magnitude highert han its GFP analogues (0.1761 for 2-BFP in dioxanev s. 0.0033 for o-HBDI in toluene). [46,48,49] Even thought he fluorescence quantum yield of the BFP chromophorei se nhanced comparedw ith that of GFP chromophores, its absolutev alue remainss mall( 0.1761 in dioxane). [46] Experimentally,Y ang et al inferred that there are still somec omplicated relaxation pathways, rather than ESIPT, that are responsible for the partial loss in fluorescence quantum yield.…”

“…As ac onsequence, the S 1 de-excitation of the BFP chromophorei sheavily suppressed and the fluorescenceq uantum yield is increased compared with that of GFP chromophores, in which the excited-state deactivation channel is barrierless and ultrafast (withinh undreds of femtoseconds for o-HBDI. [20,23,[47][48][49] The early fast picosecond decay of the S 1 tautomer ( % 3ps) is not unambiguously assigned in experiments. [46] Fang et al inferred that it possibly arises from, for example, intramolecular vibrational redistributiona nd torsional relaxation to give ap lanar and relaxed configuration.…”

“…More interestingly,c ompared with GFP analogues with ESIPT,t his BFP chromophore exhibits ag reatlye nhanced fluorescence quantum yield and am uch slowerE SIPT rate of about 18 ps, which is significantly longert han 25 fs for 4-(2-hydroxybenzylidene)-1,2-dimethyl-1H-imidazol-5(4H)-one (o-HBDI). [47][48][49] Even Compared with green fluorescencep rotein (GFP) chromophores,t he recently synthesized blue fluorescencep rotein (BFP) chromophore variant presentsi ntriguing photochemical properties, for example, dual fluorescencee mission, enhanced fluorescenceq uantumy ield, and ultra-slow excited-state intramolecular proton transfer (ESIPT; J. Phys. Chem.…”