C₁₅H₁₀ and C₁₅H₁₂ Thermal Chemistry: Phenanthrylcarbene Isomers and Phenylindenes by Falling Solid Flash Vacuum Pyrolysis of Tetrazoles

2-Phenyl-5-(phenylethynyl)­tetrazole <b>44</b> provides a new entry to the C₁₅H₁₀ energy surface. Flash vacuum pyrolysis of <b>44</b> using the falling solid flash vacuum pyrolysis (FS-FVP) method afforded cyclopenta­[<i>def</i>]­phenanthrene <b>31</b> and cyclopenta­[<i>jk</i>]­fluorene <b>52</b> as the principal products. The products are explained in terms of the formation of <i>N-</i>phenyl-<i>C</i>-phenylethynylnitrile imine/(phenylazo)­(phenylethynyl)­carbene <b>45</b> and its cyclization to 3-(phenylethynyl)-3<i>H-</i>indazole <b>46b</b>. Pyrolytic loss of N₂ from <b>46b</b> generates C₁₅H₁₀ intermediate <b>48</b>. Cyclization of <b>48</b> to a dibenzocycloheptatetraene derivative and further rearrangements with analogies in the chemistry of phenylcarbene and the naphthylcarbenes leads to the final products. Similar pyrolysis of 2-phenyl-5-styryltetrazole <b>43</b> afforded 3-styrylindazole <b>58</b>, which on further pyrolysis eliminated N₂ to generate 3- and 2-phenylindenes <b>61</b> and <b>62</b> via C₁₅H₁₂ intermediates