Influence of molecular dipoles on the photoluminescence and electroluminescence of dipolar spirobifluorenes

This Full Paper investigates a series of strongly fluorescent donor-acceptor-substituted spirobifluorene compounds, red 2-diphenylamino-7-(2,2-dicyanovinyl)-9,9'-spirobifluorene (DCV), green 2-diphenylamino-9,9'-spirobifluorene7-carxoxaldehyde (CHO), and blue 2-diphenylamino-7-(2,2-diphenylvinyl)-9,9'-spirobifluorene (DPV), together with their spiro-linked dimeric analogs, 2DCV, 2,2'-bis(diphenylamino)-9,9'-spirobifluorene-7,7'-dicarboxaldehyde (2CHO), and 2,2'-bis(diphenylamino)-7,7'bis(2,2-diphenylvinyl)-9,9'-spirobifluorene (2DPV), respectively. The emission optical density and, hence, the intensity of photoluminescence (PL) or electroluminescence (EL) of the dimeric analogs is presumed to increase, which is beneficial for organic light-emitting diode (OLED) applications. The physical properties, including the dipole moments obtained from quantum chemistry calculations, emission solvatochromism, fluorescence quantum yield (Of) as well as the EL of these six spirobifluorene compounds have been examined in detail. We found that Of as well as OLED performance (EL efficiency and intensity) of the strongly dipolar DCV decrease significantly in the dimeric analog 2DCV, but less so in the moderately dipolar CHO and 2CHO, and only slightly in the weakly dipolar DPV and 2DPV. This is parallel to the intramolecular dipole moment, which is large for 2DCV, medium for 2CHO, and very small for 2DPV. Here, we show for the first time systematically that the luminescence intensity is closely correlated with the local electric field induced by the molecular dipole. A strong electric field may facilitate radiationless decay channels with a charge-transfer nature, leading to a high quenching rate. Consistent with this conclusion, which is derived from the red DCV/2DCV and green CHO/2CHO, our new blue fluorophore DPV with an essentially zero dipole moment has successfully achieved one of the best electrofluorescent blue OLEDs. At the same time, by doping the highly dipolar DCV into an isolated environment with the low-polarity Alq(3) as the host matrix, we obtained a very high performance of saturated yellow OLEDs as well, This is possibly due to the reduction of emission-quenching dipoles from the neighboring molecules. Our results have provided an important insight in designing luminescent materials, as follows: molecular dipole moments should be kept at a low magnitude to avoid quenching induced by a strong local electric field in the chromophore.