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.

Treatment with 2,3-dichloro-5,6-dicyano-l,4-benzoquinone (DDQj oxidized 2,6-diethyl-l,3,5,7,8-pentamethylpywomethene-BF, complex 1, 13,14-trimethyl-2, 3, 4, 5,9,10,11 ,I 2-octahydroindomethene-BF2 complex 5, and 1,3,5,7,8-pentamethyl-1,2,3, 5,6,7-hexahydropyn-omethene-BF, complex 8 to the weakly fluorescent 3-formy1, 5 0 x 0 , and 8-formy1 derivatives 4, 6 , and 9, respectively. The dye 1 was oxidized by lead tetraacetate to 1,7, complex 12 [Af (ethanol) INTROD UCTI0.YIn 1993, 2,6-diethyl-l,3,5,7,8-pentamethylpyrromethene-BF2 complex 1, A,,, (ethanol) 567 nm, became the undisputed champion in laser dye activity insofar as it afforded twice the power efficiency obtained from Rhodamine-6G (R-6G), unsurpassed in laser dye activity for over 25 years [ l -41. Other pyrromethene-BF, complexes (P-BF,) also surpassed R-6G in laser activity [l] Apart from their fluorescence, properties of P-BF2 complexes have received minimal attention. This investigation was undertaken to determine the effect on fluorescence brought about by functionalization of P-BF2 alkyl substituents. An exploration of the oxidation of alkyl P-BF2 derivatives treated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and in one instance with lead tetraacetate, was carried out.Oxidation of an arylalkane by DDQ generally proceeded by an initial abstraction of hydride anion from a benzylic or similar position, leading to a carbenium ion intermediate. An alcohol was produced when the carbenium ion was trapped by water and was, of course, subject to further oxidation to a carbonyl compound. The availability of multiple alkyl substituents generally led to product mixtures with partial regioselectivity determined by initial formation of the most stable carbenium ion [7,8]. A general conversion of methylaryl compounds to aryl acetates by treatment with lead tetraacetate has been known for over 70 years [9]. RESULTS AND DISCUSSIONAlkyl derivatives of P-BF2 were readily attacked by DDQ. The reaction with 1,3,5,7,8-~entamethyl-and 1,2,3,5,6,7-hexamethylpyrromethene-BF, complexes 2 and 3, examples of P-BF2 derivatives with unsubstituted positions, led to intractable mixtures. On the other hand, certain peralkylated P-BF, compounds were efficiently oxidized to single products.Treatment with DDQ brought about an oxidation of strongly fluorescent 2,6-diethyl-l,3,5,7,8-pentamethylpyrromethene-BF2 complex 1 to the weakly fluorescent 3-formyl derivative 4 (80%) and

Boron trifluoride reacted with dipyrid-2-ylamine 6, its N-methyl and 6,6'-dimethyl derivatives 8 and 10, and 3,3',5,5'-tetraphenyE6-azapynmethene 13 to give fluorescent P-azavinamidine (1,3,5-triazapenta-l,3-diene)

No abstract

No abstract