Experimental and Theoretical Studies of the Photophysical Properties of 2-and 2,7-Functionalized Pyrene Derivatives

Pyrene derivatives substituted at the 2- and 2,7-positions are shown to display a set of photophysical properties different from those of derivatives substituted at the 1-position. It was found that, in the 2- and 2,7-derivatives, there was little influence on the S-2 <- S-0 excitation, which is described as "pyrene-like", and a strong influence on the S-1 <- S-0 excitation, which is described as "substituent-influenced". In contrast, the 1-substituted derivatives display a strong influence on both the S-1 <- S-0 and the S-2 <- S-0 excitations. These observations are rationalized by considering the nature of the orbitals involved in the transitions. The existence of a nodal plane passing through the 2- and 7-positions, perpendicular to the molecular plane in the HOMO and LUMO of pyrene, largely accounts for the different behavior of derivatives substituted at the 2- and 2,7-positions. Herein, we report the photophysical properties of a series of 2-R-pyrenes {R = C3H6CO2H (1), Bpin (2; pin = OCMe2CMe2O), OC3H6CO2H (3), O(CH2)(12)Br (4), C CPh (5), C6H4-4-CO2Me (6), C6H4-4-B(Mes)(2) (7), B(Mes)(2) (8)} and 2,7-R-2-pyrenes {R = Bpin (9), OH (10), C C(TMS) (11), C CPh (12), C C-C6H4-4-B(Mes)(2) (13), C C-C6H4-4-NMe2 (14), C6H4-4-CO2C8H17 (15), N(Ph)-C6H4-4-OMe (16)} whose syntheses are reported elsewhere. Furthermore, we compare their properties to those of several related 1-R-pyrene derivatives {R = C3H6CO2H (17), Bpin (18), C CPh (19), C6H4-4-B(Mes)(2) (20), B(Mes)(2) (21)}. For all derivatives, modest (0.19) to high (0.93) fluorescence quantum yields were observed. For the 2- and 2,7-derivatives, fluorescence lifetimes exceeding 16 ns were measured, with most being ca. 50-80 ns. The 4-(pyren-2-yl)butyric acid derivative (1) has a long fluorescence lifetime of 622 ns, significantly longer than that of the commercially available 4-(pyren-1-yl)butyric acid (17). In addition to measurements of absorption and emission spectra and fluorescence quantum yields and lifetimes, time-dependent density functional theory calculations using the B3LYP and CAM-B3LYP functionals were also performed. A comparison of experimental and theoretically calculated wavelengths shows that both functionals were able to reproduce the trend in wavelengths observed experimentally