In the course of a spectroscopic investigation carbonium ions with aromatic compounds (11, we have of complexes of stable isolated a I:1 complex of phenothiazine and dibenzotropylium tetrafluoroborate. We believe this is the first example of a crystalline molecular complex (Tf-complex) of a carbonium ion salt. Dibensotropylium fluoroborate cyclohepten-5-01 (Aldrich Chemical in acetic anhydride. The deep red was prepared from 5H-dibenzo[a,d]-Co.) and 45% aquecus fluoroboric acid salt, m. p. ill', dec., dissolves in acetonitrile, acetic anhydride or methylene chloride to give red solutions, but it is destroyed by atmospheric moisture, acetone, dimethylformamide, dimethyl sulfoxide and more basic solvents. The visible spectrum of the salt in acetonitrile (X, 502 y, logt 3.95; 530 m p 1ogE 3.93) resembles that of the corresponding alcohol in concentrated H2S04 (2). :hen aromatic compounds are added to solutions of the salt in methylene chloride, a color change is noted immediately; for example, solutions of pyrene and dibenzotropylium fluoroborate are dark green. !de believe that these colors denote the formation of donor-acceptor complexes in solution, and the results of our spectroscopic investigation of these complexes, as well as those of other stable carbonium ions will be presented soon. >Jhsn a .?ethylene chloride solution of an equimolar mixture of phenc-thisLine and dibenzctropylium fluoroborate is concentrated and petroleum