Singlet excited state properties. Fluorescence emission spectra of AL(DIPY)3 complexes were recorded, and the fluorescence quantum yield was calculated by using rhodamine 6G or coumarine 153 as reference compounds. The emission profile of the fluorescence spectra of the AL(DIPY)3 complexes is rather similar displaying one broad band which is relatively weak. The emission band of the AL(DIPY)3 occurs between 530-590 nm, and spans across a range of more than 100 nm (Error! Reference source not found., SI S3.2), being a mirror image of the absorption band at ~500 nm. Conjugate 20 displayed one broad peak ~570 nm indicating its origin from the emission of precursor 13c, rather than that of the BODIPY 19. Excitation either at 450 or 500 nm had no influence on the emission profile of 20, which means the typical strong BODIPY emission at λmax ~520 nm is attenuated, since no emissive BODIPY feature is observed. Fluorescence quantum yields for the majority of the compounds were calculated in the range of 0.01-0.07 (Φf) following an ascending order: 16, 17 < 13d, 13e, 13k, 20 < 13a, 13c, 13f < 13h < 13i < 13j < 13b (Table ). The highest yields were displayed by 13b, 13i, and 13j probably due to the bulky 5substitution and steric hindrance. It has been previously reported that mesityl substitution on the meso position and restriction of the internal rotation can drastically increase the fluorescence quantum yields of dipyrrinato metal complexes. Together with the trends observed in In(III) and Ga(III) homoleptic complexes this asserts that Group 13 complexes are less luminescent than the zinc or boron complexes. For 17, attempts to use DMSO, THF, or 2-propanol were unsuccessful, whilst in ethanol or DMF a weak band appeared with negligible yield (< 0.001). Presumably, in polar media there is a prominent possibility of either solvent fluorescence quenching or formation of non-emissive ICT states which lead to a more efficient triplet formation via charge recombination and nonradiative decay as the dominant pathway. On the other hand, in non-polar solvents the ICT state will not stabilize and therefore, the excited state S1 will undergo ordinary ISC (in competition with the IC). Indeed, all the aluminum complexes display moderate triplet state lifetimes in air equilibrated solutions (see below). The emission peaks are equating to Stokes shifts between a minimum 1308 cm -1 for 13b and a maximum 3114 cm -1 for 13k (Table ). Complexes 13b, 13i, and 13j have smaller Stokes shifts pointing to a reduced number of molecular rearrangements reflecting less conformational freedom in the ground and/or excited states due to the steric hindrance by the substituents. The considerable Stokes shifts and the moderately low fluorescence quantum yields support that non-radiative decay pathways are the most dominant processes (ISC and IC). TCSPC was performed and fluorescence lifetimes were determined to assess the singlet photophysical properties of the complexes (Table ). The singlet state lifetimes of the AL(DIPY)3 complexes in toluene (16 in THF) ranged from 1.3 to 4.7 ns with the ascending order: 13e, 16 < 13k, 20 < 13c < 13f < 13a < 13d < 13h < 13i < 13j < 13b. Error! Reference source not found. displays all the fluorescence decays. Comparing 13b with the respective mesityl-dipyrrinato Ga(III) and In(III) complexes, aluminum complex appears to have slightly increased Stokes shifts and longer singlet excited lifetime (+1 ns) whilst maintaining a moderately high fluorescence quantum yield equal to the In(III) counterpart: Ga(III) complex: Φf = 0.02, τs = 3.75 ns, Δν = 1220 cm -1 and In(III) complex: Φf = 0.07, τs = 1.93 ns, Δν = 1113 cm -1 (in hexane). As expected, the singlet state lifetime of 17 was not detected neither in DMSO nor ethanol or THF (< 1 ns).