Anti-Kasha behavior has been the subject of intense debate in the last few years, as demonstrated by the high number of papers appearing in the literature on this topic, dealing with both mechanistic and applicative aspects of this phenomenon. Examples of anomalous emitters reported in the last 10 years are collected in the present review, which is focused on strictly anti-Kasha organic molecules displaying radiative deactivation from S_n and/or T_n, with n greater than 1.

The development of purely organic materials showing multicolor fluorescent and phosphorescent behaviour represents a formidable challenge in view of practical applications. Herein the rich photophysical behaviour of 3-(pyridin-2-yl)triimidazotriazine (TT-Py) organic molecule, comprising excitation-dependent fluorescence and phosphorescence under ambient conditions in both blended films and the crystalline phase, is investigated by means of steady state, time resolved and ultrafast spectroscopies and interpreted on the basis of X-ray diffraction studies and DFT/TDDFT calculations. In particular, by proper excitation wavelength, dual fluorescence and dual phosphorescence of molecular origin can be observed together with low energy phosphorescences resulting from aggregate species. It is demonstrated that the multiple emission properties originate from the copresence, in the investigated system, of an extended polycyclic nitrogen-rich moiety (TT), strongly rigidified by π–π stacking interactions and short C–H⋯N hydrogen bonds, and a fragment (Py) having partial conformational freedom.

Recently developed luminescent neutral 0D mono-, di-, tri- and tetranuclear Cu( i ) complexes with N-donor ligands are gathered and analyzed with a focus on their synthesis, luminescent behavior and responsiveness to external stimuli.

The remarkable emissive properties of cyclic triimidazole (TT), showing crystallization-induced emissive behavior and, in particular, room temperature phosphorescence (RTP), are here combined with its versatility in assembling coordination compounds. A series of Ag(I) coordination polymers (CPs), comprising a 1D chain ([Ag(TT)I]_n, 1-Ag) and 3D networks ([Ag(TT)Cl]_n, 2-Ag, and [Ag₃(TT)₄]_n(NO₃)_3n·6nH₂O, 3-Ag), have been synthesized and their photophysical behavior thoroughly investigated. They show both fluorescence and multiple RTPs, all simultaneously activated but varied in intensity by changing the excitation energy. Based on DFT/TDDFT calculations and analysis of the X-ray crystal structures, the origin of the different phosphorescences has been ascribed to H-aggregation of the ligand (in 1-Ag and 2-Ag), intermolecular electronic coupling by an extrinsic heavy-atom effect (in 1-Ag) and ligand-centered emissive states (in all three compounds). Comparison with isostructural 1-Cu and 2-Cu CPs reveals that, different from the Ag(I) analogues, non-thermally equilibrated XMLCT and ligand-centered emissive states are active. The isostructural Ag(I) and Cu(I) compounds show comparable emission efficiency, while the phosphorescence lifetimes are longer for the former (ms regime) than the latter (μs regime). A Quantum Theory of Atoms In Molecules (QTAIM) topological analysis of electron density allows the interpretation of the different nature of the emissive states of Ag(I) and Cu(I) compounds on the basis of larger shell-shared character of the Cu–N bond with respect to the Ag–N one.

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. Particularly 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.

Purely organic luminophores that exhibit room temperature phosphorescence (RTP) are receiving an ever‐growing attention. The design of efficient RTP materials and the understanding of their emissive behaviour, however, remain a challenging task. Herein, to gain further insights into metal‐free RTP, we report the synthesis, characterization, and photophysical study of chloro‐derivatives of [1,2‐a:1′,2′‐c:1′′,2′′‐e][1,3,5]triazine or cyclic triimidazole (TT). Mono‐ (1Cl), di‐ (2Cl) and tri‐chloro‐TT (3Cl) derivatives are prepared by mild chlorination with N‐chlorosuccinimide (NCS). The three compounds are characterized by aggregation‐induced emission (AIE) comprising dual fluorescence and multiple phosphorescences, whose origin is disclosed by crystal structure analysis together with accurate DFT/TDDFT calculations. Comparison with previously reported bromine and iodine TT derivatives reveals the new chlorinated compounds as the best performing halogenated AIE members of the family.