Carbon　dots　(CDs)　as　the　advancing　fluorescent　carbon　nanomaterial　have　superior　potential　and　prospective.　However,　the　ambiguous　photoluminescence　(PL)　mechanism　and　intricate　structure-function　relationship　become　the　greatest　hindrances　in　the　development　and　applications　of　CDs.　Herein,　red　emissive　CDs　were　synthesized　in　high　yield　from　o-phenylenediamine　(oPD)　and　catechol　(CAT).　The　PL　mechanism　of　the　CDs　is　considered　as　the　molecular　state　fluorophores　because　5,14-dihydroquinoxalino[2,3-b]　phenazine　(DHQP)　is　separated　and　exhibits　the　same　PL　properties　and　behavior　as　the　CDs.　These　include　the　peak　position　and　shape　of　the　PL　emission　and　PL　excitation　and　the　emission　dependence　on　pH　and　solvent　polarity.　Both　of　them　display　close　PL　lifetime　decays.　Based　on　these,　we　deduce　that　DHQP　is　the　fluorophore　of　the　red　emissive　CDs　and　the　PL　mechanism　of　CDs　is　similar　to　DHQP.　During　the　PL　emission　of　CDs,　the　electron　of　the　molecule　state　can　transfer　to　CDs.　The　formation　process　of　DHQP　is　further　confirmed　by　the　reaction　intermediates　(phthalazine,　dimers)　and　oPD.　These　findings　provide　insights　into　the　PL　mechanism　of　this　type　of　CDs　and　may　guide　the　further　development　of　tunable　CDs　for　tailored　properties.