Zhiyun Zhang

Excited-State Conformational/Electronic Responses of Saddle-Shaped N,N′-Disubstituted-Dihydrodibenzo[a,c]phenazines: Wide-Tuning Emission from Red to Deep Blue and White Light Combination

A tailored strategy is utilized to modify 5,10-dimethylphenazine (DMP) to donor-acceptor type N,N-disubstituted-dihydrodibenzo[a,c]phenazines. The representative compounds DMAC (N,N-dimethyl), DPAC (N,N-diphenyl), and FlPAC (N-phenyl-N-fluorenyl) reveal significant nonplanar distortions (i.e., a saddle shape) and remarkably large Stokes-shifted emission independent of the solvent polarity. For DPAC and FlPAC with higher steric hindrance on the N,N-substituents, normal Stokes-shifted emission also appears, for which the peak wavelength reveals solvent-polarity dependence. These unique photophysical behaviors are rationalized by electronic configuration coupled conformation changes en route to the geometry planarization in the excited state. This proposed mechanism is different from the symmetry rule imposed to explain the anomalously long-wavelength emission for DMP and is firmly supported by polarity-, viscosity-, and temperature-dependent steady-state and nanosecond time-resolved spectroscopy. Together with femtosecond early dynamics and computational simulation of the reaction energy surfaces, the results lead us to establish a sequential, three-step kinetics. Upon electronic excitation of N,N-disubstituted-dihydrodibenzo[a,c]phenazines, intramolecular charge-transfer takes place, followed by the combination of polarization stabilization and skeletal motion toward the planarization, i.e., elongation of the π-delocalization over the benzo[a,c]phenazines moiety. Along the planarization, DPAC and FlPAC encounter steric hindrance raised by the N,N-disubstitutes, resulting in a local minimum state, i.e., the intermediate. The combination of initial charge-transfer state, intermediate, and the final planarization state renders the full spectrum of interest and significance in their anomalous photophysics. Depending on rigidity, the N,N-disubstituted-dihydrodibenzo[a,c]phenazines exhibit multiple emissions, which can be widely tuned from red to deep blue and even to white light generation upon optimization of the surrounding media.

A silole copolymer containing a ladder-type heptacylic arene and naphthobisoxadiazole moieties for highly efficient polymer solar cells

We report a combination of a silole containing ladder-type heptacylic arene and naphthobisoxadiazole moieties for highly efficient polymer solar cells. This new class of PSiNO polymer possesses a planar, rigid backbone and a low-ordering framework. This unique feature facilitates chain extension, leading to high hole mobility and hence a high PCE of 8.37% without further thermal annealing.

Locked ortho- and para-core chromophores of green fluorescent protein; dramatic emission enhancement via structural constraint.

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.

Snapshotting the Excited-State Planarization of Chemically Locked N,N′-Disubstituted Dihydrodibenzo[a,c]phenazines

For deeper understanding of the coupling of electronic processes with conformational motions, we exploit a tailored strategy to harness the excited-state planarization of N,N-disubstituted dihydrodibenzo[a,c]phenazines by halting the structural evolution via a macrocyclization process. In this new approach, 9,14-diphenyl-9,14-dihydrodibenzo[a,c]phenazine (DPAC) is used as a prototype, in which the para sites of 9,14-diphenyl are systematically enclosed by a dialkoxybenzene-alkyl-ester or -ether linkage with different chain lengths, imposing various degrees of constraint to impede the structural deformation. Accordingly, a series of DPAC-n (n = 1-8) derivatives were synthesized, in which n correlates with the alkyl length, such that the strength of the spatial constraint decreases as n increases. The structures of DPAC-1, DPAC-3, DPAC-4, and DPAC-8 were identified by the X-ray crystal analysis. As a result, despite nearly identical absorption spectra (onset ∼400 nm) for DPAC-1-8, drastic chain-length dependent emission is observed, spanning from blue (n = 1, 2, ∼400 nm) and blue-green (n = 3-5, 500-550 nm) to green-orange (n = 6) and red (n = 7, 8, ∼610 nm) in various regular solvents. Comprehensive spectroscopic and dynamic studies, together with a computational approach, rationalized the associated excited-state structure responding to emission origin. Severing the linkage for DPAC-5 via lipase treatment releases the structural freedom and hence results in drastic changes of emission from blue-green (490 nm) to red (625 nm), showing the brightening prospect of these chemically locked DPAC-n in both fundamental studies and applications.

Phenazine-Based Ratiometric Hg2+ Probes with Well-Resolved Dual Emissions: A New Sensing Mechanism by Vibration-Induced Emission (VIE)

Phenazines exhibit intriguing vibration-induced emission (VIE) owing to the fast intrinsic vibration of benzo[a,c]phenazine moiety. For the first time, a phenazine-based ratiometric fluorescent probe DBPST is developed for recognizing Hg2+ via restriction of VIE. Upon binding with Hg2+ , DBPST demonstrates two well-resolved emission peaks (over 130 nm) with a wide tuning color and affords a large signal-to-background ratio.

The Quest of Excited-State Intramolecular Proton Transfer via Eight-Membered Ring π-Conjugated Hydrogen Bonding System

Searching for eight-membered ring π-conjugated hydrogen bonding (8-MR H-bonding) systems with excited-state intramolecular proton transfer (ESIPT) property is seminal and synthetically challenging. In this work, a series of π-conjugated molecules (8-HB-1, 8-HB-L1 and 8-HB-2) potentially possessing 8-MR H-bonding are strategically designed, synthesized and characterized. The configurations of these three potential molecules are checked by their X-ray structures, among which 8-HB-L1 (a structurally locked 8-HB-1 core chromophore) is proved to be an 8-MR H-bonding system, whereas 8-HB-1 and 8-HB-2 are too sterically hindered to form the 8-MR intramolecular H-bond. The ESIPT property of 8-HB-L1 is confirmed by the dual fluorescence consisting of normal and proton-transfer tautomer emissions. The insight into the ESIPT process of 8-HB-L1 is provided by femtosecond fluorescence upconversion measurements together with computational simulation. The results demonstrate for the first time a successful synthetic route to attain the 8-MR H-bonding molecule 8-HB-L1 with ESIPT property.

Tuning the Conformation and Color of Conjugated Polyheterocyclic Skeletons by Installing ortho-Methyl Groups.

ortho-Methyl effects are exploited to tune steric hindrance between side-chain N,N-diaryls and polycyclic dihydrodibenzo[a,c]phenazine, and in turn control the conformations of N,N-diphenyl-dihydrodibenzo[a,c]phenazine (DPAC) and its ortho-methyl derivatives Mx-My (x=0, 1 or 2, y=1 or 2, x and y correlate with the number of methyl groups in the ortho-positiond of N,N-diphenyl). The magnitude of steric hindrance increases as x and y increase, and the V-shaped dihydrodibenzo[a,c]phenazine skeleton is gradually tuned from a bent (DPAC) to planar (M2-M2) structure in the ground state. As a result, the relaxation of the excited-state structure of DPAC and its numerous analogues could be mimicked by model structures Mx-My, demonstrating for the first time the the conformation change from bent-to-planar and hence a large range of energy-gap tuning of polycyclic conjugated structures controlled by the steric hindrance.

Ratiometric Indicator Based on Vibration-Induced Emission for in Situ and Real-Time Monitoring of Gelation Processes

Monitoring specific processes such as gelation in a ratiometric and visual manner is of scientific value and has practical implications but remains challenging. Herein, an innovative fluorescent low-molecular-weight gelator (DPAC-CHOL) capable of revealing and self-revealing the gelation processes in situ and in real time via the ratiometric fluorescence change from orange-red to blue has been developed. By virtue of its vibration-induced emission attribute, the gelation point, critical gelation concentration, and the internal stiffness of the gel networks of DPAC-CHOL and other gelation systems could be facilely evaluated in a ratiometric and naked-eye-observable fashion. Noteworthily, the DPAC-CHOL-doped gelation system Ph-CHOL can quantitatively identify the environmental temperature in a daily-concerned range (i.e., 20-55 °C). This work not only provides a versatile advanced material but also opens up a new avenue for the investigation of gelation systems.