The invention claims a compound of 6, 7-dihydroxy coumarin phosphonium amphiphile and negatively charged agent. The complex is self-assembled into a nanoparticle or non-viral vector to facilitate the delivery of the negatively charged agent. The negatively charged agent may be a therapeutic or diagnostic agent. Thus, the present invention provides a method of delivering such negatively charged agent to a target cell, and in particular to promote the delivery of a therapeutic or diagnostic agent through the plasma membrane, as well as a method of treating or diagnosing by delivering the therapeutic or diagnostic agent. The 6,7-dihydroxy coumarin phosphonium amphiphile can be Mito-Esc, and the negatively charged agent can be a nucleic acid, such as siRNA. In addition, the 6, 7-dihydroxy coumarin phosphonium amphiphile is considered to be effective in the treatment of cancer.