Interplay of Steric Effects and Aromaticity Reversals to Expand the Structural/Electronic Responses of Dihydrophenazines

Abstract

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.

A Series of D–A–D Structured Disilane-Bridged Triads: Structure and Stimuli-Responsive Luminescence Studies

“…The activation energy between ⟨bf-eq⟩ and ⟨bf-ax⟩ was calculated as +5.6 kcal mol –1 , which was slightly higher than that of N -phenylphenothiazine (+4.2 kcal mol –1 ) (Figure S13­(b)). , These results suggest the existence of ⟨bf-eq⟩, ⟨ax-bf⟩, and ⟨pl-eq⟩ conformer in an equilibrium state. The frontier orbitals and energy levels of these conformers were shown in Figure S13­(c).…”

Photoconductance from the Bent-to-Planar Photocycle between Ground and Excited States in Single-Molecule Junctions

“…As investigated by both photophysical measurements and theoretical calculations in previous studies, , DPAC derivatives possess photoinduced bent-to-planar motions along the elastic N–N axis (as depicted in Figure a). In this case, we elaborately selected the DPAC derivative DPAC-SMe (Figure b) featuring the phenanthrene moiety as the molecular bridge with SMe anchors to construct the single-molecule junctions, as the excited-state planarization of DPAC-SMe could lead to improvements in the charge transport ability along the phenanthrene chain without altering the geometry of the charge transport path.…”

“…As an ideal candidate to investigate conformational evolution in the excited state, photoinduced bent-to-planar transformation has gained increasing attention, showing great potential in photoelectronic materials. − Among these, 9,14-diphenyl-9,14-dihydrodibenzo­[ a , c ]­phenazine ( DPAC ) derivatives may serve as proof-of-concept molecules to explore the role of the bent-to-planar photocycle of conjugated molecules in photoconductance (see Figure a), because of their significant π-delocalization changes under continuous light irradiation within the unimolecular framework. , To achieve this goal, long-term endeavors have focused on investigating the interactions between charge transport and the light within the molecular junctions by characterizing photoconductance. − Investigating the photoconductance of single-molecule junctions could provide a unique tool for studying the inherent structural/electronic changes in the photoexcited state (see Figure a).…”

Tailor-Made Dynamic Fluorophores: Precise Structures Controlling the Photophysical Properties

“…Recently, a series of sterically hindered dihydrophenazine derivatives (DPP-10− DPP-22) with different numbers of methyl groups on the 1,4,6,9-sites were synthesized via base-promoted nucleophilic aromatic substitution reactions of diamines and electrondeficient o-difluorobenzenes (Figure 5e). 6 Single-crystal X-ray diffraction analysis revealed the progressive planar-to-bent structure changes from DPP-00 to DPP-22 (Figure 5f). Interestingly, optical measurements indicated that the absorption spectra of the molecules dramatically blue-shifted from 416 to 324 nm, while the emission bands exhibited a reverse shift to longer wavelengths from 459 to 584 nm as the number of methyl substituents increased (Figure 5g,h), which is rarely reported in most organic fluorophores.…”

“…Organic fluorophores with dynamic conformations in the excited state are fascinating molecules with respect to their unique photophysical properties and versatile functions. − Among them, the bent-to-planar excited-state dynamics have been reported in conjugated 8π-electron systems, such as sterically hindered dihydrophenazines, , cyclooctatetraenes, , and oxepins. , The changes of molecular conformations in the excited state have created unique photophysical properties, such as a large Stokes shift, dual emission, and environment-dependent fluorescence, for use in various applications such as ratiometric fluorescent probes, stimulus-responsive materials, and white organic light-emitting diodes. , …”