To
gain insights into the coupling of conformational and electronic
variables, we exploited steric hindrance to modulate a polycyclic
skeleton with a bent conformation in the S0 state and a
twisted conformation in the S1 state under the guidance
of photoexcited aromaticity reversals. Polycyclic 5,10-dihydrophenazine
(DHP) adopted a bent structure in S0 but involved
a bent-to-planar transformation in S1 due to the excited-state
aromaticity of the 8π-electron central ring. The N,N′-locations and 1,4,6,9-sites of the DHP skeleton provided a versatile chemical handle for fine-tuning
intramolecular steric hindrance. Specifically, N,N′-diphenyl-5,10-dihydrophenazine (DPP-00) and its derivatives DPP-10–DPP-22 were synthesized with different numbers of methyl groups on the
1,4,6,9-sites. X-ray crystal analyses suggested that the DHP skeletons of DPP-00–DPP-22 had
more bending configurations along the N···N axis with
an increase in the number of methyl groups. Following the bending-promoted
interruption of π-conjugation, the absorption spectra of DPP-00–DPP-22 significantly blue-shifted
from 416 to 324 nm. By contrast, the emission bands exhibited a reverse
shift to longer wavelengths from 459 to 584 nm as the number of methyl
substituents increased. Theoretical calculations revealed that introducing
methyl groups caused the planar DHP skeleton in S1 to further twist along the N···N axis, resulting
in a twisted high-strain conformation. The greater Stokes shift of
the more steric-hindered structure can be attributed to the release
of larger strain and aromatic stabilization energy. This research
highlighted the potential promise associated with the interplay of
steric effects and aromaticity reversals in a single fluorophore.
Simple donor−acceptor (D−A) molecules with large Stokes-shifted and bright near-infrared (NIR) emissions are extremely attractive for the high-resolution three-photon fluorescence (3PF) imaging of deep tissues. Herein, we present a new strategy for significantly extending the Stokes shifts of D−A fluorophores, relying on increasing the excited-state structural/electronic relaxation of the donor moiety. The prototype of the D−A structure DPCN, containing the planar donor N,N′diphenyl-dihydrophenazine (DHP) and the acceptor malononitrile, exhibits normal Stokes-shifted (∼60 nm) NIR emission. By introducing two methyl groups at the ortho sites of the fused DHP ring of DPCN, DMPCN (methyl-decorated DPCN) is developed with an obviously bent donor unit. Compared to DPCN, DMPCN exhibits significantly blueshifted absorption and keeps NIR luminescence with a large Stokes shift of ∼200 nm, mainly due to the remarkable bent-to-planar transformation of the donor upon photoexcitation. Additionally, the DMPCN NPs offer enhanced luminescence quantum yields (11%) and three-photon excitation cross sections. More importantly, DMPCN NPs exhibit excellent performances for in vivo 3PF imaging of the cerebrovascular tissue and lipid droplets (LDs) in a fatty liver with a depth of 1200 and 50 μm, respectively. This research provides a distinct strategy for extending the Stokes shifts of D−A fluorophores, inspiring the utilization of dynamic NIR fluorophores for in vivo 3PF imaging.
Type Ⅰ PDT has been demonstrated to be superior over type Ⅱ PDT due to its better hypoxia tolerance. Nowadays, photosensitizers (PSs) featuring type Ⅰ active free radicals (O2-·, ·OH)...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.