N-Pyrrolobenzonitrile (PBN), its ester derviative PBAEE, and a more twisted model compound DPBN have been compared regarding absorption and fluorescence properties. A long wavelength absorption shoulder in DPBN has been assigned to the charge transfer (CT) state. In polar solvents, single-fluorescence bands with a strong solvatochromic effect establish a CT nature of the emission for all compounds. The long radiative lifetime (ca. 130 ns for PBN, 700 ns for DPBN in CH 2 Cl 2 ) points to a forbidden emission, and the 5-fold longer value for DPBN indicates a difference in CT nature tentatively assigned to conformations differing in the twist angle. Even in low-temperature polar solvent glasses, the single-fluorescence band of PBN is of CT nature and develops into a dual fluorescence by thermal activation. Also in alkanes, the increased fluorescence rate constants and the temperature effect on spectral structuring indicate emission from an equilibrium involving a CT state with an unstructured spectrum and a less polar locally excited state with structured emission.

The polarity dependent fluorescence quantum yields and lifetimes at room temperature have been measured for two squaraine dyes, SQ-DMA and its crown ether derivative SQ-CR. The unstructured fluorescence bands and the polarity independent k f values indicate fluorescence from one single species. The nonradiative decay rates increase strongly with solvent polarity, indicating a polarity-induced intramolecular fluorescence quenching process. The latter could be due to the formation of a nonemissive twisted intramolecular charge transfer (TICT) state. Low-temperature measurements and the comparison of SQ-DMA and SQ-CR support this view.

The fluorescence of dimesityl-pyrroloboranes with different ground state twist angles is investigated by absorption, steady state, and time-resolved fluorescence spectroscopies. Only one strongly Stokes shifted charge transfer fluorescence band is found, characterized by monoexponential decays. This fluorescence is connected with extremely low fluorescence rate constants (k f ) (1-2) × 10 6 s -1 ) which are nearly solvent and temperature independent and indicate the forbidden character of the emission. The pretwisted compound shows reduced k f values and only a lengthening of the lifetimes with decreasing temperature whereas the planar compound shows a lifetime maximum (increase followed by decrease) upon cooling. This is interpreted as evidence that the planar compound relaxes to a more twisted structure in the excited state. Large-scale ab initio CI calculations on a smaller model system confirm the nearly full charge-transfer character present for all twist angles and the energy minimum in the excited state for twisted geometries.

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