Pyrromethene–BF₂ complexes as laser dyes: 2

Abstract

Abstract Pyrromethene–BF 2 complexes (P–BF 2 ) 7 were obtained from α‐unsubstituted pyrroles 5 by acylation and condensation to give intermediate pyrromethene hydrohalides 6 followed by treatment with boron trifluoride etherate. Conversion of ethyl α‐pyrrolecarboxylates 4 to α‐unsubstituted pyrroles 5 was brought about by thermolysis in phosphoric acid at 160°C, or by saponification followed by decarboxylation in ethanolamine at 180°C, or as unisolated intermediates in the conversion of esters 4 to pyrromethene hydrobromides 6 by heating in a mixture of formic and hydrobromic acids. Addition of hydrogen cyanide followed by dehydrogenation by treatment with bromine converted 3,5,3′,5′‐tetramethyl‐4,4′‐diethylpyrromethene hydrobromide 9 to 3,5,‐3′,5′‐tetramethyl‐4,4′‐diethyl‐6‐cyanopyrromethene hydrobromide 6bb , confirmed by the further conversion to 1,3,5,7‐tetramethyl‐2,6‐diethyl‐8‐cyanopyrromethene–BF 2 complex 7bb on treatment with boron trifluoride etherate. An alternation effect in the relative efficiency (RE) of laser activity in 1,3,5,7,8‐pentamethyl‐2,6‐di‐n‐alkylpyrromethene–BF 2 dyes depended on the number of methylene units in the n‐alkyl substituent, ‐(CH 2 ) n H, to give RE ≥ 100 when n = 0,2,4 and RE 65, 85 when n = 1,3. (The RE 100 was arbitrarily assigned to the dye rhodamine 6G). The absence of fluorescence and laser activity in 1,3,5,7‐tetramethyl‐2,6‐diethyl‐8‐isopropylpyrromethene–BF 2 complex 7p and a markedly diminished fluorescence quantum yield (Φ 0.23) and lack of laser activity in 1,3,5,7‐tetramethyl‐2,6‐diethyl‐8‐cyclohexylpyrromethene–BF 2 complex 7q were attributed to molecular nonplanarity brought about by the steric interference between each of the two bulky 8‐substituents with the 1,7‐dimethyl substituents. An atypically low RE 20 for a peralkylated dye without steric interference was observed for 1,2,6,7‐bistrimethylene‐3,5,8‐trimethylpyrromethene–BF 2 complex 7j . Comparisons with peralkylated dyes revealed a major reduction in RE 0–40 for the six dyes 7u–z lacking substitution at the 8‐position. Low laser activity RE was brought about by functional group (polar) substitution in the 2,6‐diphenyl derivative 7I , RE 20, and the 2,6‐diacetamido derivative 7m , RE 5, of 1,3,5,7,8‐pentamethylpyrromethene–BF 2 complex (PMP–BF 2 ) 7a and in 1,7‐dimethoxy‐2,3,5,6,8‐pentamethylpyrromethene–BF 2 complex 7n , RE 30. Diethyl 1,3,5,7‐tetramethyl‐8‐cyanopyrromethene‐2,6‐dicarboxylate–BF 2 complex, 7aa , and 1,3,5,7‐tetramethyl‐2,6‐diethyl‐8‐cyanopyrromethene–BF 2 complex, 7bb , offered examples of P–BF 2 dyes with electron withdrawing substituents at the 8‐position. The dye 7aa , λ las 617 nm, showed nearly twice the power efficiency that was obtained from rhodamine B, λ las 611 nm.