Solid State Room Temperature Dual Phosphorescence from 3-(2-Fluoropyridin-4-yl)triimidazo[1,2-a:1′,2′-c:1″,2″-e][1,3,5]triazine

Abstract

Organic room temperature persistent luminescence is a fascinating but still largely unexplored phenomenon. Cyclic-triimidazole and its halogenated (Br, I) derivatives have recently revealed as intriguing phosphors characterized by multifaceted emissive behavior including room temperature ultralong phosphorescence (RTUP) associated with the presence of H-aggregates in their crystal structure. Here, we move towards a multicomponent system by incorporating a fluoropyridinic fragment on the cyclic-triimidazole scaffold. Such chromophore enhances the molecular properties resulting in a high photoluminescence quantum yield (PL QY) in solution but preserves the solid-state RTUP. By means of X-ray diffraction (XRD) analysis, theoretical calculations, steady-state and time-resolved spectroscopy on solutions, polymethylmethacrylate (PMMA) blends and crystals, the nature of the different radiative deactivation channels of the compound has been disclosed. In particular, the molecular fluorescence and phosphorescence, this latter observed in frozen solution and in PMMA blends, are associated to deactivation from S1 and T1 respectively, while the low energy RTUP, observed only for crystals, is interpreted as originated from H aggregates.

Unravelling the intricate photophysical behavior of 3-(pyridin-2-yl)triimidazotriazine AIE and RTP polymorphs

“…Particularly relevant in considering the molecular origin of these emissions is the comparison with the analogue TT-4pyF compound. 21 For the latter, a photophysical behavior comprising fluorescence (deactivation from S 1 ) together with molecular phosphorescence of T 1 –S 0 origin at about 420 nm and a long lived T H –S 0 H-aggregated supramolecular component at about 560 nm, has been observed. In agreement, the HEF band of TT-Py can be explained by deactivation from S 1 and the compound's MEP could be as well interpreted as originated from T 1 as also supported by theoretical calculations (see discussion below).…”

“…The three polymorphs share the leitmotif of TT moiety π···π stacking, which is already observed in both the TT prototype and its previously investigated derivatives. 9 , 10 , 19 – 21 In TT-Py-A , adjacent TT units along the stacks are largely shifted, with respect to one another, as denoted by the distance, 5.358 Å, between respective triazinic centroids. However, several C···C close contacts (3.234, 3.301 Å to cite the shorter ones) are indicative of strong π···π stacking interactions.…”

“…These multiple emissions are collectively activated in the solid compound with an overall quantum efficiency F of 25%, showing that the 2-uoropyridine fragment provides increased molecular performances with respect to TT itself, comprising uorescence and heavy atom free phosphorescence, but it preserves the solid state RTUP which is red-shied (by about 70 nm) with respect to the TT parent compound. 21 Here we report on the multifaceted emissive properties of 3-(pyridin-2-yl)triimidazotriazine (TT-Py), the pyridine derivative of TT where the nitrogen atom is located in the ortho position with respect to TT. It will be demonstrated how this particular geometrical disposition is responsible for different metastable states which result, in the crystalline phase, in the formation of three polymorphs and in aggregated phase in a peculiar photophysical behaviour.…”

“…Encouraging results in this direction have just been obtained with TT-4pyF , 3-(2-fluoropyridin-4-yl)triimidazo[1,2- a :1′,2′- c :1′′,2′′- e ][1,3,5]triazine, obtained by inserting the 2-fluoropyridine moiety in TT . 21 This compound possesses molecular fluorescence and green phosphorescence together with aggregate RTUP at almost 600 nm. These multiple emissions are collectively activated in the solid compound with an overall quantum efficiency Φ of 25%, showing that the 2-fluoropyridine fragment provides increased molecular performances with respect to TT itself, comprising fluorescence and heavy atom free phosphorescence, but it preserves the solid state RTUP which is red-shifted (by about 70 nm) with respect to the TT parent compound.…”

“…These multiple emissions are collectively activated in the solid compound with an overall quantum efficiency Φ of 25%, showing that the 2-fluoropyridine fragment provides increased molecular performances with respect to TT itself, comprising fluorescence and heavy atom free phosphorescence, but it preserves the solid state RTUP which is red-shifted (by about 70 nm) with respect to the TT parent compound. 21 …”