Delineating Conformation Control in the Photophysical Behaviour of a Molecular Donor‐Acceptor‐Donor Triad. Issue 22 (23rd September 2021)

• Record Type: Journal Article
• Authors: Garain, Bidhan Chandra; Das, Shubhajit; Pati, Swapan K.
• Is Part Of: Chemphyschem. Volume 22:Issue 22(2021)
• Page Start: 2297
• Page End: 2304
• Publication Date: 2021-09-23
• Subjects: mechanochromic luminescence -- thermally activated delayed fluorescence -- phosphorescence -- spin-orbit coupling; Chemistry, Physical and theoretical -- Periodicals; 541.05
• DOI: 10.1002/cphc.202100518
• Languages: English
• ISSNs: 1439-4235

Abstract

Abstract: Mechanochromic luminescent materials, exhibiting a change in luminescence behavior under external stimuli have emerged as one of the promising candidates for upcoming efficient OLEDs. Recently mechanochromic luminescence was reported in a donor‐acceptor‐donor (D‐A‐D) triad featuring two phenothiazine units separated by a dibenzo[ a, j ]phenazine motif. The triad follows different emissive routes ranging from phosphorescence to TADF based on the conformational switching of the D units. In this article, we investigate such conformation‐dependent photophysical behavior of this triad through theoretical calculations. By analyzing the nature of ground state, excited state and factors determining the reverse ISC crossing rates associated with the relative orientation of the D and A units, we delineate the effect of the conformational changes on their photophysical properties. Our findings reveal that axial orientation of both the donor groups enhances the overlap between HOMO and LUMO leading to a large singlet‐triplet gap ( Δ E S T ) which drives phosphorescence emission. On the contrary, the equatorial orientation of the donor groups minimizes Δ E S T to facilitate rISC making the conformers TADF active. The role of several geometric factors affecting the photophysical properties of the conformers is also highlighted. Finally, we show how to regulate the population difference among the conformers by functionalizing the triad to harvest the maximum TADF efficiency. Abstract : Recently synthesized D‐A‐D system shows mechanochromic luminescence . The resulting four conformers are quasi axial – quasi axial (AA) with syn: and anti‐variants, quasi axial – quasi equatorial (AE) and quasi equatorial – quasi equatorial (EE). AA shows phosphorescence and EE shows thermally activated delayed fluorescence (TADF), while AE exerts both. With lesser overlap between HOMO and LUMO in AE and EE, transitions are charge transfer in nature and decreases the singlet‐triplet gap (ΔEST ) favoring TADF, while higher ΔEST and lower reorganization energy of T1 in AA boosts phosphorescence.