Room‐Temperature Phosphorescence in Metal‐Free Organic Materials

Abstract

Purely organic materials with room-temperature phosphorescence (RTP) have attracted a growing interest for their potential applications in biological imaging, digital encryption, optoelectronic devices, and so on. To date, many strategies have succeeded in designing efficient organic RTP materials by overcoming the spin-forbidden transition between singlet and triplet states. However, the underlying mechanisms of RTP still remain ambiguous. Such spin prohibition in phosphorescence are clarified, herein, from the perspective of perturbation theory, helping to understand the intrinsic relationship among various phosphorescence parameters, like phosphorescence efficiency, lifetime, intersystem crossing rate, as well as radiative and nonradiative rates. Taking into consideration the recent progress in organic RTP materials, these factors are further illustrated by a selection of the most relevant molecules. In addition, some novel RTP phenomena are also reviewed, thus providing an excellent guideline to constructing efficient RTP materials.

Two Are Better Than One: A Design Principle for Ultralong‐Persistent Luminescence of Pure Organics

“…Though access to and from the triplet state is a forbidden process and once thought to be too inefficient for effective use at room temperature, [ 17 ] recent advances have vastly increased intersystem crossing efficiency by enhancing spin–orbit coupling (SOC) with the use of heteroatoms, [ 18,19 ] the carbonyl functional group, [ 20–22 ] heavy atom effects, [ 23–27 ] and multimer‐enhanced intersystem crossing. [ 28–32 ] Equally important is protecting the long‐lived triplet after its generation, due to the fact that they are particularly sensitive to molecular vibrational quenching and atmospheric oxygen. In this regard, recent works have accomplished this through the use of crystals, [ 33,34 ] metal–organic frameworks, [ 35 ] H‐aggregation, [ 36 ] and others.…”

Organic Room Temperature Phosphorescence Materials for Biomedical Applications

“…One way is transferring from S 1 to S 0 directly, namely, fluorescence. The other way is from the lowest single state (S 1 ) to triple state (T n ) through intersystem crossing (ISC) firstly, then is internal conversion (IC) from T n to the lowest triplet state (T 1 ), generating phosphorescence, which this review focuses on (Figure ) . However, the transitions between the excited singlet states to triplet states, like from S 1 to T n , are normally spin‐forbidden, leading to low efficiency of phosphorescence.…”

Room‐Temperature Phosphorescence from a Series of 3‐Pyridylcarbazole Derivatives

“…Shorter t T values were obtained for CzPyI (t P = 0.05 s) and CzPyBr (t P = 0.15 s), respectively, whereas longer t T values were observed for CzPyCl (t P = 0.21 s) and CzPyF (t P = 1.1 s), respectively.T he t T is reportedly inversely proportional to the square of the SOC matrix elements of z (S 0 ,T 1 ). [1,5,21] In fact, larger SOC matrix elements z (S 0 ,T 1 )a pparently induce shorter t P and larger k P ( Table 1). Introduction of aheavy atom into the 3-pyridylcarbazole skeleton is ineffective for obtaining longer RTP.A pparently,t he introduction of as mallerh alogen atom with larger electronegativity,s uch as Cl and F, is effective for obtaining longerRTP.Asaresult, CzPyF,which has the smallest pendanth alogen atom, exhibited the longest t T of 1.1 s. It should be noted that the fluorines ubstitution is effectivem ost likely due to the formation of strong CÀH···F hydrogen bonds ( Figure S11, Supporting Information).…”

“…In fact, much smaller SOC elements, z (S 1 , T 3 )o f1 .23 cm À1 and z (S 1 ,T 6 )o f2 .00 cm À1 ,w ere obtained for CzPhBr as compared to those of the pyridine counterpart. The DE ST was calculatedt ob er elatively large at approximately 0.48 eV.A ss uch, accordingt ot he Felmi's goldenr ule, [1,5,21] the ISC rate (k ISC )o ft he pyridine counterpart can be two orderso f magnitude higher than that of the phenyl counterpart, which To gain deeper insights into the emission characteristics of these molecules, we investigated their intermolecular interactions in the crystal state at room temperature (296 K; Figures S10-S15 and Ta ble S2 in the Supporting Information). Singlecrystal X-ray crystallographic analysis of CzPyBr revealed that it has ad enselyp acked structure with p stacking of carbazole units (3.715 )a nd pyridyl units (3.574 ), respectively (Figure S12, Supporting Information).…”