Thermally　activated　delayed　fluorescence　(TADF)　materials　are　promising　for　the　next　generation　of　organic　light-emitting　diodes.　To　realize　reverse　intersystem　crossing　(RISC)　effectively　for　TADF　molecules　requires　basically　small　energy　gap　(ΔEST)　between　the　lowest　singlet　(S1)　and　triplet　(T1)　excited　states.　In　this　work,　1,8-naphthalimide　was　selected　as　a　fixed　acceptor　(A)　because　of　its　large　rigid　conjugate　structure　with　a　high　radiative　decay　rate.　Twelve　naphthalimide-based　intramolecular　charge　transfer　(ICT)　compounds　were　constructed　by　the　conjunction　of　naphthalimide　moiety　with　each　of　donors　(D)　including　9,9-dimethyl-9,10-dihydroacridine,　phenoxazine,　etc.　Their　energy　gaps　were　calculated　with　the　Gaussian　09　package　at　TD-DFT　level　using　the　optimal　Hartree-Fock　exchange　method　reported　by　Huang　Shuping　recently.　The　results　show　that　D-A　structure　of　naphthalimide-based　ICT　compounds　has　smaller　ΔEST　and　longer　starting　fluorescence　emission　wavelengths　(λ)　than　those　for　corresponding　D-phenyl-A　type.　However,　strong　electron　donor　like　9-(9H-carbazol-3-yl)-9H-carbazole　(DCZ)　seems　to　have　no　significant　effect　on　ΔEST　and　λ　at　high　CT　amount.　To　achieve　small　ΔEST　of　these　naphthalimide-based　ICT　compounds,　increasing　the　twisting　angle　between　D　and　A,　together　with　enhancing　electron　donating　ability　of　D　is　found　to　be　a　practical　strategy　to　stabilize　the　energy　of　the　lowest　locally-excited　triplet　state　(3LE)　and　substantially　lower　the　energy　of　its　lowest　CT　triplet　excited　state　(3CT).　As　a　result,　4-(9,9-dimethyl-9,10-dihydroacridine)-N-phenyl-1,8-naphthalimide　(4b)　and　4-(phenoxazine)-N-phenyl-1,8-naphthalimide　(5b)　show　small　ΔEST　of　0.01　eV　and　0.02　eV,　respectively.　Their　λ　and　oscillator　strengths　of　vertical　absorption　from　ground　state　(S0)　to　S1　are　575　nm　and　0.0002　for　4b,　while　621　nm　and　0.0025　for　5b,　respectively.　In　addition,　the　λ　of　compound　5b　obtained　from　its　photoluminescence　curve　is　600　nm,　which　consists　with　the　computed　one　with　an　error　of　0.07　eV.　Therefore,　compounds　4b　and　5b　can　be　expected　to　be　potential　reddish-orange　and　red　TADF　emitters.　©　2015　Shanghai　Institute　of　Organic　Chemistry,　Chinese　Academy　of　Sciences.