Pyrromethene–BF2 complexes as laser dyes: 2

Pyrromethene–BF2 complexes (P–BF2) 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–BF2 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–BF2 dyes depended on the number of methylene units in the n-alkyl substituent, -(CH2)nH, 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–BF2 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–BF2 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–BF2 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–BF2 complex (PMP–BF2) 7a and in 1,7-dimethoxy-2,3,5,6,8-pentamethylpyrromethene–BF2 complex 7n, RE 30. Diethyl 1,3,5,7-tetramethyl-8-cyanopyrromethene-2,6-dicarboxylate–BF2 complex, 7aa, and 1,3,5,7-tetramethyl-2,6-diethyl-8-cyanopyrromethene–BF2 complex, 7bb, offered examples of P–BF2 dyes with electron withdrawing substituents at the 8-position. The dye 7aa, λlas 617 nm, showed nearly twice the power efficiency that was obtained from rhodamine B, λlas 611 nm.