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Trivalent samarium (Sm) complexes show four emission bands due to the 4f–4f transition in the visible 550–750 nm region, where the 645 nm transition has the dominant intensity. In this paper we have synthesized the three Sm complexes with varying pybox and β-diketonato ligands, to investigate the relative intensity among the emission bands. We have shown that the β-diketonato ligands with asymmetric molecular structures enhance the relative intensity of the 645 nm transition by approximately 20% compared to that with symmetric ligands. By virtue of such a property, Sm complexes can be used as possible candidates for deep red emitting materials. As a strategy on the molecular design, it has also been confirmed that the introduction of heavy atoms such as fluorine to the β-diketonato moiety should be of essential to obtain a high emission quantum yield.