Pyrromethene–BF 2 complexes as laser dyes:1

Condensations between 3‐X‐2,4‐dimethylpyrroles (X = H, CH 3 , C 2 H 5 , and CO 2 C 2 H 5 ) and acyl chlorides gave derivatives of 3,5,3′,5′‐tetramethylpyrromethene (isolated as their hydrochloride salts): 6‐methyl, 6‐ethyl, 4,4′,6‐trimethyl, 4,4′‐diethyl‐6‐methyl, and 4,4′‐dicarboethoxy‐6‐ethyl derivatives for conversion on treatment with boron trifluoride to 1,3,5,7‐tetramethylpyrromethene–BF 2 complex (TMP–BF 2 ) and its 8‐methyl (PMP–BF 2 ), 8‐ethyl, 2,6,8‐trimethyl (HMP–BF 2 ),2,6,‐diethyl‐8‐methyl (PMDEP–BF 2 ), and 2,6‐dicarboethoxy‐8‐ethyl derivatives. Chlorosulfonation converted, 1,3,5,7,8‐pentamethylpyrromethene–BF 2 complex to its 2,6‐disulfonic acid isolated as the lithium, sodium (PMPDS–BF 2 ), potassium, rubidium, cesium, ammonium, and tetramethylammonium disulfonate salts and the methyl disulfonate ester. Sodium 1,3,5,7‐tetramethyl‐8‐ethylpyrromethene‐2,6‐disulfonate–BF 2 complex was obtained from the 8‐ethyl derivative of TMP–BF 2 . Nitration and bromination converted PMP–BF 2 to its 2,6‐dinitro‐(PMDNP–BF 2 ) and 2,6‐dibromo‐ derivatives. The time required for loss of fluorescence by irradiation from a sunlamp showed the following order for P–BF 2 compounds (10 −3 to 10 −4 M) in ethanol: PMPDS–BF 2 , 7 weeks; PMP–BF 2 , 5 days; PMDNP–BF 2 , 72 h; HMP–BF 2 , 70 h; and PMDEP–BF 2 , 65 h. Under similar irradiation PMPDS–BF 2 in water lost fluorescence after 55 h. The dibromo derivative was inactive, but each of the other pyrromethene–BF 2 complexes under flashlamp excitation showed broadband laser activity in the region λ 530–580 nm. In methanol PMPDS–BF 2 was six times more resistant to degradation by flashlamp pulses than was observed for Rhodamine‐6G (R‐6G). An improvement (up to 66%) in the laser power efficiency of PMPDS–BF 2 (10 −4 M in methanol) in the presence of caffeine (a filter for light