The photophysics of naphthalimide (NI)-phenothiazine (PTZ) dyads were investigated as electron donor-acceptor (D-A) thermally activated delayed fluorescence (TADF) emitters. Femtosecond transient absorption (fs-TA) spectra show that the photophysical processes in non-polar solvents are in singlet localized state (1LE, tau = 0.8 ps) -> Franck-Condon singlet charge separation state (1CS, tau = 7.8 ps) -> 1CS state (tau = 2.2 ns) -> triplet state (3LE, tau = 16 mu s). The 3LE state is formed via the spin-orbit charge transfer-intersystem crossing (SOCT-ISC) mechanism rather than the spin-orbit (SO)-ISC mechanism. In a polar solvent, the CS state has a much lower energy than the 3LE state; thus, the 3LE state is absent from the photophysical processes and no TADF was observed. Moreover, we found that the delayed fluorescence lifetime is related to the low-lying triplet state (3LE or 3CS states). When the 3CS state is the low-lying triplet state, the TADF lifetime is shorter than that of the 3LE state as the low-lying triplet state. In the time-resolved electron paramagnetic resonance (TREPR) spectra, both 3LE (zero field splitting parameter D = 2250 MHz, E = -150 MHz) and 3CS (D = 430 MHz, E = 0 MHz) states were observed. It is noteworthy that the electron spin polarization (ESP) phase pattern of the 3CS state was inverted at longer delay times as a consequence of the selective transition between the 3LE and 3CS states and a faster decay of one sublevel of the 3CS state. These results are strong and direct experimental evidence for the spin-vibronic coupling mechanism of TADF.

We studied a series of thermally activated delayed fluorescence (TADF) emitters based on phenothiazine-dibenzothiophene-S,S-dioxide (PTZ-DTO) electron donor-acceptor (D-A) dyads, using femtosecond/nanosecond transient absorption (fs/ns-TA) spectroscopy and pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectroscopy. In the analyzed compounds, a Se atom replaced the S atom in the phenothiazine (PTZ) unit to study the heavy atom effect on the reverse intersystem crossing (RISC). Moreover, oxidation of the PTZ to sulfoxide and sulfone was also used to tune the energy of the charge separated (CS) state, while keeping the other factors (3LE state energy, LE = locally excited, and electronic coupling between the donor and acceptor) intact to large extent. Fs-TA spectra show that charge separation occurs rapidly (ca. 7.6 ps) in non oxidized compunds, while for the compounds with the PTZ unit oxidized the CS is slightly slower (10.9 ps). Ns-TA spectroscopy demonstrated the coexistence of 3CS and 3LE states for the non oxidized dyads and the absence of heavy atom effect on RISC. The oxidation of the PTZ unit increased the CS state energy, so that only the 3LE state remains observed. All our results demonstrated that RISC rate constants are not enhaced in the presence of heavy atoms for the studied dyads. TREPR spectra show the presence of the 3LE state, and that the triplet state is formed most likely via spin-orbit charge transfer intersystem crossing (SOCT-ISC) because the electron spin polarization (ESP) phase pattern is (e, a, e, a, e, a). These studies are useful for an in-depth understanding of the photophysics of the electron D-A TADF emitters.

To study the effect of thionation of the carbonyl groups in a chromophore, i. e. replacing the O atom with S atom, on the photophysics, we studied two thionated coumarin derivatives (Cou-S and Cou-6-S) with various steady state and transient spectroscopic methods. Both compounds exhibit red-shifted absorption (up to 4900 cm-1) and strong fluorescence quenching as compared to the unthionated analogues. Femtosecond transient absorption spectra show fast ISC (ca. 10 ps) in the thionated coumarin derivatives, while negligible ISC was observed in the unthionated coumarin. Interestingly, triplet excited state lifetimes of the thionated coumarin (0.14 μs) is much shorter than the unthionated analogues (53.4 μs). Time-resolved electron paramagnetic resonance (TREPR) spectra indicate much larger zero field splitting (ZFS) D parameters (up to 0.287 cm-1) for the T1 state of the thionated coumarins than the unthionated analogues (D=0.1001 cm-1). This large D value is attributed to the strong spin orbital coupling effect. These results demonstrate the advantage and the drawback of thionation-enhanced ISC, i. e. the ISC is efficient, but triplet state lifetimes become substantially shorter. This information is useful for the future design of heavy atom-free triplet photosensitizers for photodynamic therapy, photon upconversion, photocatalytic organic synthesis and photopolymerization, etc.

The photophysical properties of two new Bodipy dimers are investigated using a variety of techniques, including steady-state UV-vis absorption and fluorescence spectroscopy, femtosecond and nanosecond transient absorption spectroscopy, and pulse laser-excited time-resolved electron paramagnetic resonance (TREPR) spectroscopic methods. The dimers are formed by the Bodipy units rigidly linked by the orthogonal phenylene bridge. One of the dimers is composed of iodinated units, and the other is not. The dimerization of the non-iodinated Bodipy leads to a strong quenching of the fluorescence compared to the monomer, indicating the presence of a new electronic state relaxation pathway, which we assign to charge transfer processes. This conclusion is supported by the femtosecond transient absorption (fs-TA) spectroscopy and TREPR studies. However, the triplet yield of this dimer formed by charge recombination is not high (4.4%). The iodination of the Bodipy units leads to a significant difference in the photophysical properties of the studied chromophores as a result of the enhanced intersystem crossing (ISC, the singlet oxygen quantum yield: 69.8%) induced by the spin-orbit coupling of the iodine atom due to the heavy atom effect. As a result, the metastable T1 triplet state is formed after photoexcitation (the rate constant for S1 → T1 is 0.2 ps-1), and the processes of charge separation and recombination have a low quantum yield (3.2%). TREPR spectra reveal that IDPB exhibits a weak charge transfer (CT) signal in polar solvents. The absence of heavy atoms in DPB decreases the triplet state formation efficiency and enhances the formation of the CT state. This observation is consistent with fs-TA spectral data. Overall, these results suggest that the triplet state may form indirectly through the CT state. Low-temperature TREPR spectra observed for the triplet and CT states in a polar solvent exhibit similar decay rates. This indicates that these two states may have similar energies and could achieve dynamic equilibrium under certain conditions. The spin density of the metastable T1 state of the studied dimers is localized on one Bodipy unit, as confirmed by the analysis of the zero-field splitting (ZFS) parameters derived from the TREPR spectra. This information is useful for an in-depth understanding of the triplet state and intersystem crossing in chromophore dimers or oligomers.

A compact electron donor-acceptor dyad, NI-ICz, with 6,12-diphenyl-indolo[3,2-b]carbazole (ICz) as the electron donor and naphthalimide (NI) as the acceptor, was prepared to study its electron transfer (ET) and thermally activated delayed fluorescence (TADF) properties. The rigid and bulky electron donor reduced the reorganization energy (lambda = 0.96 eV) for ET, which facilitated the formation of a long-lived charge separated (CS) state in NI-ICz by exploiting the Marcus inverted region effect on charge recombination (CR). Transient absorption (TA) spectroscopy revealed the formation of CS triplet (3CS) states (tau = 8.6 mu s in n-hexane, 1.4 mu s in toluene, and 0.13 mu s in acetonitrile). TADF of this dyad was observed in n-hexane but was absent in polar solvents, indicating that 3CS -> 1CS reversed ISC (RISC) was inefficient, which is solid experimental evidence for the spin-vibronic coupling mechanism of TADF. The small zero-field splitting (ZFS) parameter (|D| = 900 MHz) for the triplet state of NI-ICz confirmed the formation of the 3CS state. These results demonstrate the feasibility of achieving a long-lived CS state in compact electron donor-acceptor dyads. The findings also highlight the crucial role of the closely lying 3LE, 1CS and 3CS states in enabling TADF, where the 3LE state serves as an essential intermediate state to facilitate RISC in TADF systems.

4-Amino-2,2,6,6-tetramethyl-1-piperidinyloxyl (TEMPO) radical was linked to perylene-3,4:9,10-bis(dicarboximide) (PBI) at varying distances and orientations. PBI-TEMPO dyads with the radical linked at the bay-position show a charge transfer absorption band in the UV−vis absorption spectra. With increasing solvent polarity, a fluorescence quenching is observed for these dyads, whereas for a derivative with TEMPO attached at the imide-position, such polarity dependency for fluorescence spectra was not observed. Steady state and femtosecond/nanosecond time-resolved optical spectroscopy confirmed the occurrence of radical-enhanced intersystem crossing (REISC. k ISC = (23 ps) −1 − (0.5 ns) −1 ). The lifetime of the 3* PBI state ( τ T = 1.0–7.6 μs) depends on the distance and orientations between TEMPO and PBI units. The results indicate that stronger electron spin–spin dipolar interaction ( v dd ) between the radical and the chromophore improve REISC efficiency. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy demonstrates different electron exchange interactions ( J TR ) in the dyads, varying from ferromagnetic interaction corresponding to strong exchange regime to weak antiferromagnetic exchange interaction with increasing the distance between PBI and TEMPO units. Transient-nutation experiments further clarify the TREPR signals. DFT calculations indicate that changes in the dyad structure alter the exchange coupling from ferromagnetic ( J TR = 0.47 cm −1 ) to antiferromagnetic ( J TR = −0.03 cm −1 and −0.01 cm −1 ).

The photophysics of thermally activated delayed fluorescence emitters based on the multiple resonance effect was studied.

We prepared thermally activated delayed fluorescence (TADF) emitter dyads, NI-PTZ, NI-PTZ-2Br and NI-PSeZ, with naphthalimide (NI) as electron acceptor and 10H-phenothiazine (PTZ) or 10H-phenoselenazine (PSeZ) as electron donor to study the heavy-atom effect on the intersystem crossing (ISC) and reverse ISC (rISC) in the TADF emitters. The delayed fluorescence lifetimes of the dyads containing heavy atoms ( τ D F ${{\tau }_{{\rm D}{\rm F}}}$ =5.9 μs for NI-PSeZ and τ D F ${{\tau }_{{\rm D}{\rm F}}}$ =16.5 μs for NI-PTZ-2Br, respectively) are longer than the heavy atom-free counterpart NI-PTZ ( τ D F ${{\tau }_{{\rm D}{\rm F}}}$ =2.0 μs). Nanosecond transient absorption (ns-TA) spectral study and the time-resolved electron paramagnetic resonance (TREPR) spectra show the presence of both 3LE and 3CS states. These findings represent solid experimental evidences for the spin-vibronic coupling mechanism of TADF. Moreover, the ns-TA spectra show that the heavy atoms don't have a significant effect since the lifetime of the triplet transient species (1.3 μs for NI-PTZ) is not shortened in their presence (4.5 μs for NI-PSeZ and 5.3 μs for NI-PTZ-2Br). These results show that the previously claimed heavy-atom effect on rISC and TADF is not a universal principle. The femtosecond transient absorption (fs-TA) spectra of the compounds indicate the occurrence of fast charge separation within 1-2 ps, and the charge recombination is slow (>4 ns).

Herein we studied the excited state dynamics of two bis-amino substituted anthraquinone (AQ) derivatives. Femtosecond transient absorption spectra show that intersystem crossing (ISC) takes place in 190-320 ps, and nanosecond transient absorption spectra demonstrated unusually short triplet state lifetime (2.1-5.4 us) for the two AQ derivatives at room temperature. Pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra shows an inversion of the electron spin polarization (ESP) phase pattern of the triplet state at longer delay time. Spectral simulations show that the faster decay of the Ty sublevel (x = 15.0 us, y = 1.50 us, z = 15.0 us) rationalizes the short T1 state lifetime and the ESP inversion. Computations taking into account the electron-vibrational coupling, i.e., the Herzberg-Teller effect, successfully rationalize the TREPR experimental observations.

We prepared a series fully rigid spiro electron donor-acceptor orthogonal dyads, with closed form of rhodamine (Rho) as electron donor and naphthalene (Np)/anthraquinone (AQ) as electron acceptor, to access the long-lived triplet charge separation (3CS) state, via the electron spin control method. We found strong dependency of the photophysical property of the dyads on the amino substitution positions of the Np chromophores in the dyads 1,8-DaNp-Rho and 2,3-DaNp-Rho. Nanosecond transient absorption (ns-TA) spectra show the population of the 3LE state (lifetime: 47 μs) for 2,3-DaNp-Rho, however, long-lived 3CS state was observed (τCS=0.62 μs) for AQ-Rho, with a CS quantum yield of ΦCS=58 %. Based on femtosecond transient absorption (fs-TA) spectra, spin orbit charge transfer ISC (SOCT-ISC) is proposed to be responsible for the formation of the triplet states. Time-resolved electron paramagnetic resonance (TREPR) spectra of AQ-Rho indicate the presence of two states, a 3LE state with zero field splitting (ZFS) D parameter of 1400 MHz and E parameter of -410 MHz, formed via radical pair ISC (RP-ISC) and SOCT-ISC mechanism; and a 3CS state with the electron spin-spin interaction in the regime of spin-correlated radical pair (SCRP).

We prepared a series of phenothiazine (PTZ)-anthraquinone (AQ) electron donor-acceptor dyads to study the relationship between molecular structures and the possibility of charge transfer (CT) and intersystem crossing (ISC). As compared to the previously reported PTZ-AQ dyad with a direct connection of two units via a C-N single bond, the PTZ and AQ units are connected via a p-phenylene or p-biphenylene linker. Conformation restriction is imposed by attaching ortho-methyl groups on the phenylene linker. UV-vis absorption spectra indicate electronic coupling between the PTZ and AQ units in the dyads without conformation restriction. Different from the previously reported PTZ-AQ, thermally activated delayed fluorescence (TADF) is observed for the dyads containing one phenylene linker (PTZ-Ph-AQ and PTZ-PhMe-AQ). The prompt fluorescence lifetime in cyclohexane is exceptionally long (τPF = 62.0 ns, population ratio: 99.2%) and 245.0 ns (93.5%) for PTZ-Ph-AQ and PTZ-PhMe-AQ, respectively (normally τPF <20 ns); the delayed fluorescence lifetimes for these two dyads were determined as τDF = 2.4 μs (6.5%) and 7.6 μs (0.8%), respectively. For the dyad containing a biphenylene linker (PTZ-Ph2Me-AQ), no TADF was observed. Charge-separated (CS) states were observed for PTZ-Ph-AQ and PTZ-PhMe-AQ, and the lifetimes were determined as 7.0 and 1.3 μs, respectively, indicating the triplet spin multiplicity of the CS state. The 3CS state lifetimes are shortened to 100 ns and 440 ns for the two dyads, respectively, in the polar solvent acetonitrile. For dyads with a longer linker, i.e., PTZ-Ph2Me-AQ, the CS state lifetime is not sensitive to solvent polarity (τCS = 1.8 and 1.3 μs in cyclohexane and acetonitrile, respectively). In reference dyads, where the PTZ unit is oxidized to sulfoxide, no CT absorption band and TADF were observed, which is attributed to the increased CS state energy (>3 eV) becoming higher than that of the AQ triplet (3AQ*) state (ca. 2.7 eV). These experimental evidence show that the presence of 1CS, 3CS, and 3LE (LE: locally excited) states sharing similar energy is essential for the occurrence of TADF. Population of the long-lived 3CS state (with a lifetime of a few μs) does not produce by itself TADF, because the ISC process of 1CS→3CS is nonsufficient. Femtosecond transient absorption spectra show that charge separation (CS) occurs readily (<5 ps) for most dyads, even in nonpolar solvents. Nanosecond pulsed laser-excited time-resolved electron paramagnetic resonance (TREPR) spectra show that either a spin correlated radical pair (SCRP) is formed, with the electron exchange energy 2J = +2.14 mT, or radical pairs with stronger interaction, |2J| > 6.57 mT. These studies are useful for in-depth understanding of the CS and ISC in compact electron donor-acceptor dyads and for design of efficient TADF emitters.

We prepared a rhodamine-TEMPO chromophore-radical dyad (RB-TEMPO) to study the radical enhanced intersystem crossing (REISC). The visible light-harvesting chromophore rhodamine is connected with the TEMPO (a nitroxide radical) via a C–N bond. The UV-vis absorption spectrum indicates negligible electron interaction between the two units at the ground state. Interestingly, the fluorescence of the rhodamine moiety is strongly quenched in RB-TEMPO, and the fluorescence lifetime of the rhodamine moiety is shortened to 0.29 ns, from the lifetime of 3.17 ns. We attribute this quenching effect to the intramolecular electron spin–spin interaction between the nitroxide radical and the photoexcited rhodamine chromophore. Nanosecond transient absorption spectra confirm the REISC in RB-TEMPO, indicated by the detection of the rhodamine chromophore triplet excited state; the lifetime was determined as 128 ns, which is shorter than the native rhodamine triplet state lifetime (0.58 μs). The zero-field splitting (ZFS) parameters of the triplet state of the chromophore were determined with the pulsed laser excited time-resolved electron paramagnetic resonance (TREPR) spectra. RB-TEMPO was used as a photoinitiator for the photopolymerization of pentaerythritol triacrylate (PETA). These studies are useful for the design of heavy atom-free triplet photosensitizers, the study of the ISC, and the electron spin dynamics of the radical-chromophore systems upon photoexcitation.

Herein, a spiro rhodamine (Rho)-thionated naphthalimide (NIS) electron donor-acceptor orthogonal dyad (Rho-NIS) was prepared to study the formation of a long-lived charge separation (CS) state via the electron spin control approach. The transient absorption (TA) spectra of Rho-NIS indicated that the intersystem crossing (ISC) occurs within 7-42 ps to produce the 3NIS state via the spin orbit coupling ISC (SOC-ISC). The energy order of 3CS (2.01 eV in n-hexane, HEX) and 3LE states (1.68 eV in HEX) depended on the solvent polarity. The 3NIS state having n-π* character and a lifetime of 0.38 μs was observed for Rho-NIS in toluene (TOL). Alternatively, in acetonitrile (ACN), the long-lived 3CS state (0.21 μs) with a high CS state quantum yield (ΦCS, 97%) was produced with the 3NIS state as the precursor and the CS took 134 ps. On the contrary, in the case of the reference Rho-naphthalimide (NI) Rho-NI dyad without thionation of its carbonyl group, a long-lived CS state (0.94 μs) with a high energy level (ECS = 2.12 eV) was generated even in HEX with a lower ΦCS (49%). In the presence of an acid, the Rho unit in the Rho-NIS adopted an open form (Rho-o) and the 3NIS state was produced within 24-47 ps with the 1Rho-o state as the precursor. Subsequently, slow intramolecular triplet-triplet energy transfer (TTET, 0.11-0.60 μs) produced the 3Rho-o state (9.4-13.6 μs). According to the time-resolved electron paramagnetic resonance (TREPR) spectra of NIS-NH2, the zero-field splitting (ZFS) parameter |D| and E of the triplet state were determined to be 6165 MHz and -1233 MHz, respectively, indicating that its triplet state has significant nπ* character, which was supported by its short triplet state lifetime (6.1 μs).

To study the charge separation (CS) and long-lived CS state, we prepared a series of dyads based on naphthalimide (NI, electron acceptor) and phenothiazine (PTZ, electron donor), with an intervening phenyl linker attached on the N-position of both moieties. The purpose is to exploit the electron spin control effect to prolong the CS-state lifetime by formation of the 3CS state, instead of the ordinary 1CS state, the spin-correlated radical pair (SCRP), or the free ion pairs. The electronic coupling magnitude is tuned by conformational restriction exerted by the methyl groups on the phenyl linker. Differently from the previously reported NI-PTZ analogues containing long and flexible linkers, we observed a significant CS emission band centered at ca. 600 nm and thermally activated delayed fluorescence (TADF) with a lifetime of 13.8 ns (population ratio: 42%)/321.6 μs (56%). Nanosecond transient absorption spectroscopy indicates that in cyclohexane (CHX), only the 3NI* state was observed (lifetime τ = 274.7 μs), in acetonitrile (ACN), only the CS state was observed (τ = 1.4 μs), whereas in a solvent with intermediate polarity, such as toluene (TOL), both the 3NI* (shorter-lived) and the CS states were observed. Observation of the long-lived CS state in ACN, yet lack of TADF, confirms the spin-vibronic coupling theoretical model of TADF. Femtosecond transient absorption spectroscopy indicates that charge separation occurs in both nonpolar and polar solvents, with time constants ranging from less than 1 ps in ACN to ca. 60 ps in CHX. Time-resolved electron paramagnetic resonance (TREPR) spectra indicate the existence of the 3NI* and CS states for the dyads upon photoexcitation. The electron spin-spin dipole interaction magnitude of the radical anion and cation of the CS state is intermediate between that of a typical SCRP and a 3CS state, suggesting that the long CS-state lifetime is partially due to the electron spin control effect.

In order to obtain long-lived charge separated (CS) states in electron donor-acceptor dyads, herein we prepared a series of anthraquinone (AQ)-phenothiazine (PTZ) dyads, with adamantane as the linker. UV-vis absorption spectra show negligible electronic interaction between the AQ and PTZ units at ground state, yet charge transfer (CT) emission bands were observed. Nanosecond transient absorption shows that the (3)AQ state is populated upon photoexcitation for AQ-PTZ in cyclohexane (CHX), but in acetonitrile (ACN) a (CS)-C-3 state is formed. Similar results were observed for AQ-PTZ-M. The (CS)-C-3 state lifetimes were determined as 0.52 & mu;s and 0.49 & mu;s, respectively. Upon oxidation of the PTZ unit, the (3)AQ state was observed in both polar and non-polar solvents. For AQ-PTZ, femtosecond transient absorption spectra show fast formation of the (3)AQ state in all solvents, with no charge separation in CHX, while formation of the (CS)-C-3 state takes 106 ps in ACN. For AQ-PTZ-M, a (CS)-C-3 state is formed in CHX within 241 ps. Time-resolved electron paramagnetic resonance (TREPR) spectra show that a radical ion pair with electron exchange energy of |2 J|& GE;5.68 mT was observed for AQ-PTZ and AQ-PTZ-M, whereas in the dyads with the PTZ unit oxidized, only the (3)AQ state was observed.

A twisted π-conjugated framework does not necessarily promote efficacious ISC, energy matching of S 1 /T n excited states offers a more promising route to triplet enhancement.

Three anthracene (An)-naphthalenediimide (NDI) compact electron donor-acceptor dyads were prepared. Femtosecond transient absorption (fs-TA) spectra show fast charge separation (ca. 0.9-1.7 ps) and relatively slow charge recombination (ca. 8-565 ps) upon photoexcitation; moreover, the 3An state was observed for 9-An-NDI, whereas the final state is 3NDI for both 9-An-Ph-NDI and 2-An-Ph-NDI, which have an intervening phenyl linker between the An and NDI units. Nanosecond transient absorption (ns-TA) spectra indicate that the lowest triplet state of all the dyads is 3An, with triplet lifetimes of 139-354 μs. An unusually slow intramolecular triplet-triplet energy transfer (TTET) was observed for 9-An-Ph-NDI and 2-An-Ph-NDI (32-85 ns). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy confirms that the intersystem crossing (ISC) mechanism is spin orbit charge transfer ISC (SOCT-ISC) for all the dyads; for 9-An-NDI, only the 3An state was observed, while for the other two dyads, both 3NDI and 3An states were observed, with their relative population changing with increasing delay time, which supports TTET.

A long-lived triplet charge separated state (3CS state lifetime: 0.56 μs) was observed in a compact electron donor-acceptor dyad with electron donor phenothiazine (PTZ) and acceptor anthraquinone (AQ) directly connected by a single C-N bond (AQ-PTZ). The 1CS state energy (2.0 eV in cyclohexane) is lower than those of the 3AQ (2.7 eV) or the 3PTZ state (2.6 eV). By oxidation of the PTZ unit, thus increasing of the 1CS state energy (2.7 eV in cyclohexane), thermally activated delayed fluorescence (TADF) was observed [τ = 17.7 ns (99.9%)/1.5 μs (0.1%)]. Time-resolved electron paramagnetic resonance (TREPR) spectra confirm the electron spin multiplicity of the 3CS state, and the zero-field-splitting (ZFS) parameters |D| and |E| are 48.2 mT and 11.2 mT, respectively. These results are useful for design of compact electron donor-acceptor dyads to access the long-lived 3CS state and study the TADF mechanism.

Two Bodipy dimers were prepared (BDP-BDP andBDP-BDP-I2), with a phenyl unit as the linker between two Bodipymoieties. It was found that the triplet excited state of thefirst dimerdepopulates fast to the ground state, while the metastable tripletstate is well-detected in the second dimer. With two iodine atomsattached, a high singlet oxygen quantum yield (Phi Delta= 51.5%) and long intrinsic triplet state lifetime (Tau T= 210 mu s) of BDP-BDP-I2were observed in toluene, which are similar to those of diiodoBodipy monomer (Phi Delta= 54.9%,Tau T= 173 mu s). As determined byphosphorescence experiments, the T1-state of BDP-BDP-I2local-izes predominantly on the diiodo Bodipy subunit (BDP-I2, 92%)and a little on the diphenyl Bodipy subunit (BDP-Ph2, 8%). At lowtemperature (ca. 80-140 K), three types of triplet signals wereobserved for BDP-BDP-I2in time-resolved electron paramagnetic resonance spectroscopy (TREPR), which are assigned to two subunits (BDP-I2and BDP-Ph2) and the3CT state. Efficient energy transfer from the BDP-I2subunit to the BDP-Ph2subunit was also observed in BDP-BDP-I2. The local equilibrium between three metastable states observed within ca. 1 mu s after the laser flash is sensitive to the temperature and the polarity of the solvent. The observed properties imply the presence of the energy and electron transfer pathways between two moieties of the studied dimers

Naphthalimide (NI) homo- and hetero-dimers adopting orthogonal geometry were prepared to study photo-induced symmetry-breaking charge transfer (SBCT) and charge recombination (CR)-induced intersystem crossing (ISC). The two moieties in the dimer are connected either at the 3-C or 4-C position of the NI unit. The photophysical properties of the dimers were studied with steady-state and transient absorption spectroscopic methods. Significant CT only occurs for the hetero-dimer, in which one NI unit has a 4-amino substituent and the other NI unit is without it. The CR-induced ISC is most efficient for this dimer (singlet oxygen quantum yield ΦΔ = 50.3%). For the homo-dimer, in which both NI units did not present amino substitution, SBCT was not observed. Based on the electrochemical studies, we propose that the absence of SBCT for the homo-dimer is attributed to its high oxidation potential and low reduction potential. Femtosecond transient absorption (fs TA) spectra show that there is no charge separation (CS) for the homo-dimer. Nanosecond transient absorption spectroscopy indicate the formation of a triplet state with electron delocalization for the homo dimer, with a lifetime of 72.0 μs, while for the hetero dimer a triplet state with an intrinsic lifetime of 206.4 μs is observed. CS (11.6 ps) and slow CR-induced ISC (>1.5 ns) were observed for the hetero-dimer. Time-resolved electron paramagnetic resonance spectra give the zero-field splitting parameters (|D| = 1894 MHz and |E| = 111 MHz) and electron spin polarization patterns (e, e, e, a, a, a) for the triplet state of the hetero-dimer, inferring that the triplet state of the hetero-dimer is confined on the amino-substituted NI moiety.