Efficient emission of purely organic room-temperature phosphorescence (RTP) is of great significant for potential application in optoelectronics and photobiology. Herein, we report an uncommon phosphorescent effect of organic single molecule enhanced by resulting supramolecular assembly of host-guest complexation. The chromophore bromophenyl-methyl-pyridinium (PY) with different counterions as guests displayvarious phosphorescence quantum yields from 0.4 %t o2 4.1 %. Single crystal X-rayd iffraction results indicate that the chromophore with iodide counterion (PYI) exhibits the highest efficiency maybe due to the halogen-bond interactions.S ignificantly,t he nanosupramolecular assembly of PY chloride complexation with the cucurbit[6]uril gives agreatly enhanced phosphorescent quantum yield up to 81.2 % in ambient. Such great enhancement is because of the strict encapsulation of cucurbit[6]uril, which prevents the nonradiative relaxation and promotes intersystem crossing (ISC). This supramolecular assembly concept with counterions effect provides anovel approach for the improvement of RTP.Organic room-temperature phosphorescent (RTP) has got tremendous attention due to their extensive application in optoelectronics [1] and photobiology, [2] such as organic lightemitting diodes [3] and bioimaging. [4] So far, most of organic RTPm aterials are organometallic complexes.C onsidering their high cost and restriction in resources,t he development of purely organic (metal-free) phosphorescent materials are of great concern.[5] However,p hosphorescence of purely organic molecules are fairly weak owing to their inefficient spin-orbit coupling, [6] hence massive efforts have been devoted to achieving efficient RTPb yd eveloping new methods,s uch as special design of structure, [7] embedding into proper matrix [8] and careful crystallization. [9] Recently, aprinciple of using directed heavy atom effect to facilitate the efficiency of phosphorescence (phosphorescence quantum yield 55 %) has been reported by Kim and co-workers. [10] Moreover,external heavy atom effect is proven to be another effective method in fabricating efficient phosphorescent materials (phosphorescence quantum yield 36 %). [11] On the other hand, supramolecular approach have been used extensively to construct functional materials by complexing or assembly.N oncovalent interactions,s uch as host-guest interaction, [12] hydrogen bonding, [13] p-p stacking [14] can alter the properties of guests greatly.Recently,Huang and co-workers reported asupramolecular strategy to enhance the efficiency (phosphorescence quantum yield 24.3 %) and extend the lifetime of phosphorescence by constructing crystalline framework. [13] Moreover,i ti sw ell known that the complexation between macrocycles (especially for cyclodextrin [15] and cucurbituril [16] )a nd luminous guests have great impact on photoluminescent characters.M ore recently,T ian and coworkers reported adecent quantum yield (16.9 %) of organic RTPinduced by hydrogen bonding of chromophore modified cyclodextrin...

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