Photochemical synthesis and photophysical features of ethynylphenanthrenes studied by emission and transient absorption measurements

Abstract

Phenanthrenes substituted with trimethylsilylethynyl and phenylethynyl groups were photochemically prepared, and their photophysical properties were systematically investigated based on measurements of fluorescence quantum yields, lifetimes, and transient absorption. Introducing ethynyl groups into the phenanthrene skeleton caused an increase in the fluorescence quantum yields compared to that of phenanthrene. The quantum yields and rates of fluorescence were dependent on the substituting position(s) and the terminating group for the C-C triple bond. The observation of the triplet-triplet absorption of the substituted phenanthrenes was evident for the nonradiative process being intersystem crossing competitive with the fluorescence process. The mechanism of increasing the fluorescence abilities by substituting with the ethynyl group(s) was discussed with the aid of TD-DFT calculations.

Remarkable effect of alkynyl substituents on the fluorescence properties of a BN-phenanthrene

“…A similar increase (Φ f = 0.45) is observed when the alkynyl group is attached to C1 ( 5 ), thus indicating that the positive influence of the triple bond on the fluorescence quantum yield of BN-phenanthrene seems to be a general effect, irrespective of its position. It should be noted that the introduction of ethynyl groups into the all-carbon phenanthrene skeleton results in a slight increase in the fluorescence quantum yields compared to that of phenanthrene [27]. However, the effect observed here for BN-phenanthrenes is much more pronounced [31].…”

“…In order to gain a deeper understanding of the photophysical properties of the BN-phenanthrene core, we decided to evaluate the influence of substituents located in other positions. We were particularly interested in the effect of alkynyl substituents, as their presence in PAHs is known to alter the fluorescence properties thereof markedly [27–29]. In this regard, we have recently described a methodology for the synthesis of a chloro-substituted BN-benzo[ c ]phenanthrene and its subsequent derivatization via palladium-catalyzed cross-coupling reactions [30], and we envisioned that this reaction could be used to prepare C7 substituted BN-phenanthrenes (Fig.…”

Photophysical Properties of Silyl‐Substituted Stilbene Derivatives

“…Melting points were determined on a Gallenkamp MFB-595 melting point apparatus. 1 H, 13 C, and 29 Si NMR spectra were recorded using a JEOL JMN LA-400 ( 400MHz ( 1 H) and 100 MHz ( 13 C), respectively) or a JEOL ECA-500 ( 500MHz ( 1 H), 125 MHz ( 13 C), and 100 MHz ( 29Si), respectively) spectrometer with Me4Si as an internal standard. IR spectra were determined using a Shimadzu FTIR-8300 spectrometer.…”

“…The organic layer was separated, dried over Na2SO4, filtered, and concentrated in vacuo, giving a residue that was recrystallized from EtOH to give (E)-1,2-bis [2-(trimethylsilyl)phenyl]ethene (3, 0.175 g, 36% yield). White solid; mp 94-95 °C; 1 H NMR (400 MHz, C6D6)  0.33 (s, 18H), 7.13 (t, J = 6.6 Hz, 2H), 7.25 (t, J = 6.4 Hz, 2H), 7.53 (d, J = 7.3 Hz, 2H), 7.59 (s, 2H), 7.81 (d, J = 7.8 Hz, 2H) ppm; 13 C{ 1 H} NMR (100 MHz, CDCl3)  0.4, 125.1, 126.8, 129.7, 130.9, 134.7, 139.1, 143.6 ppm; 29 Si NMR (100 MHz, CDCl3)  -4.3 ppm; IR (KBr) 459, 621, 691, 725, 764, 837, 964, 1072, 1123, 1250, 1435, 1470, 2959, 3047 cm -1 ; MS (EI+) m/z (relative intensity, %) = 235 (41), 250 (20), 324 (100, M + ); HRMS (EI+) calcd for C20H28Si2: 324.1730, found: 324.1728.…”

“…This finding is consistent with the results of earlier studies showing that the presence of silyl substituents in aromatic compounds leads to increase fluorescence efficiencies. [11] We have also found that trimethylsilyl-and trimethylsilylethynyl-substituted pyrenes, [12] phenanthrenes [13] and naphthalenes [12e, 14] have increased fluorescence quantum yields. However, no effort has been carried out thus far to assess the detailed absorption and fluorescence properties of compounds having silyl groups introduced at other positions of stilbene.…”