Excitation Wavelength-Dependent Charge Stabilization in Highly Interacting Phenothiazine Sulfone-Derived Donor–Acceptor Constructs

Abstract

Prolonging the lifetime of charge-separated states (CSS) is of paramount importance in artificial photosynthetic donor−acceptor (DA) constructs to build the next generation of light-energy-harvesting devices. This becomes especially important when the DA constructs are closely spaced and highly interacting. In the present study, we demonstrate extending the lifetime of the CSS in highly interacting DA constructs by making use of the triplet excited state of the electron donor and with the help of excitation wavelength selectivity. To demonstrate this, π-conjugated phenothiazine sulfone-based push−pull systems, PTS2−PTS6 have been newly designed and synthesized via the Pdcatalyzed Sonogashira cross-coupling followed by [2 + 2] cycloaddition−retroelectrocyclization reactions. Modulation of the spectral and photophysical properties of phenothiazine sulfones (PTZSO 2 ) and terminal phenothiazines (PTZ) was possible by incorporating powerful electron acceptors, 1,1,4,4-tetracyanobutadiene (TCBD) and cyclohexa-2,5-diene-1,4-diylidene-expanded TCBD (exTCBD). The quadrupolar PTS2 displayed solvatochromism, aggregation-induced emission, and mechanochromic behaviors. From the energy calculations, excitation wavelength-dependent charge stabilization was envisioned in PTS2−PTS6, and the subsequent pump−probe spectroscopic studies revealed charge stabilization when the systems were excited at the locally excited peak positions, while such effect was minimal when the samples were excited at wavelengths corresponding to the CT transitions. This work reveals the impact of wavelength selectivity to induce charge separation from the triplet excited state in ultimately prolonging the lifetime of CCS in highly interacting push−pull systems.

Broad-Band-Capturing Tetracyanobutadiene-Incorporated Phenothiazine-azaBODIPY Push–Pull Systems: Excited State Charge Separation and Relaxation Dynamics

“…For example, TCBD exhibits reversible first and second reductions at – 0.21 and – 0.72 V, respectively, impacting the reductions of azaBODIPY being anodically shifted to – 0.02 and – 0.68 V, corresponding to its first and second reductions of compound 2 , suggesting electronic impact induced by the TCBD unit (Figures B and S19B). Similarly, the first and second reversible oxidations that appeared at 0.91 and 1.08 V can be attributed to the PTZ unit far and close to the TCBD entity, respectively . Like PTZ, quasireversible azaBODIPY oxidation was also found to be slightly anodically shifted, appearing at 1.39 V, and such a positive shift of PTZ oxidation and azaBODIPY redox potentials upon TCBD incorporation was consistent with previous studies .…”

“…The reversible second reductions of azaBODIPY and TCBD were located at – 0.50 and – 0.68 V, respectively, and were found to be slightly more anodically shifted than that observed in 2 . The symmetrical alignmentof TCBD units in 3 resulted in a single reversible oxidation peak of PTZ at 1.09 V since both PTZs are now close to TCBD . The azaBODIPY oxidation on the other hand appeared at 1.46 V along with the second oxidation of the chemically equivalent PTZ entities at 1.74 V, both of which were anodically shifted when compared to their unsymmetrical analog 2 .…”

“…A comparison of time constants associated with each component achieved after CT and azaBODIPY-centered excitation in both solvents was summarized in Table . It can be mentioned that, due to the isolated (noninteracting) spectral origin, CT and azaBODIPY-centered excitation likely lead to the formation of different excited state species initially, which in turn causes a difference in lifetime and SAS of the photoinduced species. , …”

Shedding Light on Highly Emissive 1,4‐Dihydropyrrolo[3,2‐b]pyrrole Derivatives: Synthesis and Aggregate‐Dependent Emission

“…On the other hand, the CT emission is more commonly observed in molecules that have a good spatial separation between these orbitals. [6] Regarding the OLEDs emissive layer, typically, two types of molecules are used as emitters: small molecules and polymers. [7] The advantages of using non-polymeric emitters include defined structure, easier synthetic routes and more efficient purification, higher photoluminescence quantum yields, and the ability to be deposited by spin coating or evaporation, the latter important in the production of more efficient devices.…”

“…To favour LE emission, the decay must involve the Highest occupied molecular orbital (HOMO) and Lowest unoccupied molecular orbital (LUMO), with both of them being located in the same part of the molecule. On the other hand, the CT emission is more commonly observed in molecules that have a good spatial separation between these orbitals [6] …”