Tunable Linear and Nonlinear Optical Properties from Room Temperature Phosphorescent Cyclic Triimidazole‐Pyrene Bio‐Probe

Abstract

Organic materials with multiple emissions tunable by external stimuli represent a great challenge. TTPyr, crystallizing in different polymorphs, shows a very rich photophyisics comprising excitation-dependent fluorescence and phosphorescence at ambient conditions, and mechanochromic and thermochromic behavior. Transformation among the different species has been followed by thermal and X-ray diffraction analyses and the emissive features interpreted through structural results and DFT/TDDFT calculations. Partic-ularly intriguing is the polymorph TTPyr(HT), serendipitously obtained at high temperature but stable also at room temperature, whose non-centrosymmetric structure guarantees an SHG efficiency 10 times higher than that of standard urea. Its crystal packing, where only the TT units are strongly rigidified by π-π stacking interactions while the Pyr moieties possess partial conformational freedom, is responsible for the observed dual fluorescence. The potentialities of TTPyr for bioimaging have been successfully established.

Mono-, Di-, Tri-Pyrene Substituted Cyclic Triimidazole: A Family of Highly Emissive and RTP Chromophores

“…Pyrene was crystallized from hot toluene solution before photoluminescence measurements. Triimidazo[1,2-a:1 ,2 -c:1",2"-e][1,3,5]triazine (TT) [28], its dibromoand tribromo-derivatives (namely 3,7-dibromotriimidazo[1,2-a:1 ,2 -c:1",2"-e][1,3,5]triazine, TTBr 2 , and 3,7,11-tribromotriimidazo[1,2-a:1 ,2 -c:1",2"-e][1,3,5]triazine, TTBr 3 ) [19] and TTPyr 1 [24] were prepared according to literature procedures. 1 H and 13 C NMR spectra were recorded on a Bruker AVANCE-400 instrument (400 MHz).…”

“…Geometry optimization of TTPyr 2 has been performed at the ωB97X/6-311++G(d,p) level of theory [30], in agreement with the computational protocol previously developed to study TT [7] and its derivatives, in particular TTPyr 1 [24]. The adopted functional was in fact demonstrated to be able to accurately describe ground and excited states properties of TT and its derivatives, besides dispersive intermolecular interactions which are essential to interpret the multi-faceted properties of this family of compounds.…”

“…In order to get insight on the structural features of the investigated compounds, DFT calculations were performed on the smaller derivative, TTPyr 2 (see optimized structures in Figure 1). Owing to the lack of the crystal structure of this compound, suitable models were built up starting from structural information previously obtained on TTPyr 1 by both X-ray diffraction analysis and theoretical calculations [24]. For this latter compound, crystallizing in different phases and polymorphs, two possible conformations were detected, which correspond to different orientation of pyrene with respect to the TT moiety as denoted by the (H)C1-C2-C10-C11(H) torsion angle τ (C2-C10 being the bond connecting TT with pyrene).…”

“…Introduction of one or multiple halogen atoms on the TT scaffold results in a complex excitation dependent photoluminescent behavior including dual fluorescence, molecular and supramolecular phosphorescences, together with ultralong components [8,[19][20][21]. The introduction of one chromophoric fragment, namely 2-fluoropyridine, 2-pyridine and pyrene (TTPyr 1 ) has further enriched the photophysical behavior, preserving the solid state ultralong component [22][23][24]. In particular, the pyrene functionality was chosen due to its favorable emissive properties (i.e., high fluorescence quantum efficiency and good hole-transporting ability) which can be exploited in a large number of applications [25][26][27].…”

“…Moreover, differently from the room temperature phases, the non-centrosymmetric one displays not only second order nonlinear optical features (SHG ten times that of standard urea), but also an additional fluorescence associated with the presence of two possible almost isoenergetic conformers in its crystals. Additionally, the material in aggregate-state was tested for cellular and bacterial imaging in view of its applications in bioimaging [24]. Here, we report the synthesis, characterization and photophysical studies of the analogue derivatives with two and three pyrene moieties on the TT scaffold, TTPyr 2 and TTPyr 3 , respectively.…”