Intersystem crossing involving strongly spin exchange-coupled radical ion pairs in donor-bridge-acceptor molecules

Intersystem crossing involving photogenerated strongly spin exchange-coupled radical ion pairs in a series of donor-bridge-acceptor molecules was examined. These molecules have a 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) donor either connected directly or connected by a phenyl bridge (Ph), to pyromellitimide (PI), 1 and 2, respectively, or naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptors, 3 and 4, respectively. Femtosecond transient optical absorption spectroscopy shows that photodriven charge separation produces DMJ(+?)-PI(-?) or DMJ(+?)-NI(-?) quantitatively in 1-4 (τ(CS) ≤ 10 ps), and that charge recombination occurs with τ(CR) = 268 and 158 ps for 1 and 3, respectively, and with τ(CR) = 2.6 and 10 ns for 2 and 4, respectively. Magnetic field effects (MFEs) on the neutral triplet state yield produced by charge recombination were used to measure the exchange coupling (2J) between DMJ(+?) and PI(-?) or NI(-?), giving 2J > 600 mT for 1-3 and 2J = 170 mT for 4. Time-resolved electron paramagnetic resonance (TREPR) spectroscopy revealed that the formation of (3)*An upon charge recombination occurs by spin-orbit charge transfer intersystem crossing (SOCT-ISC) and/or radical-pair intersystem crossing (RP-ISC) mechanisms with the magnitude of 2J determining which triplet formation mechanism dominates. SOCT-ISC is the exclusive triplet formation mechanism in 1-3, whereas both RP-ISC and SOCT-ISC are active for 4. The triplet sublevels populated by SOCT-ISC in 1-4 depend on the donor-acceptor geometry in the charge separated state. This is consistent with the fact that the SOCT-ISC mechanism requires the relevant donor and acceptor orbitals to be nearly perpendicular, so that electron transfer results in a large orbital angular momentum change that must be compensated by a fast spin flip to conserve overall system angular momentum.     

Exponential distance dependence of photoinitiated stepwise electron transfer in donor-bridge-acceptor molecules: implications for wirelike behavior

Donor-bridge-acceptor (D-B-A) systems in which a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) chromophore and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor are linked by oligomeric 2,7-fluorenone (FN(n)) bridges (n = 1-3) have been synthesized. Selective photoexcitation of DMJ-An quantitatively produces DMJ(+?)-An(-?), and An(-?) acts as a high-potential electron donor. Femtosecond transient absorption spectroscopy in the visible and mid-IR regions showed that electron transfer occurs quantitatively in the sequence: DMJ(+?)-An(-?)-FN(n)-NI → DMJ(+?)-An-FN(n)(-?)-NI → DMJ(+?)-An-FN(n)-NI(-?). The charge-shift reaction from An(-?) to NI(-?) exhibits an exponential distance dependence in the nonpolar solvent toluene with an attenuation factor (β) of 0.34 ?(-1), which would normally be attributed to electron tunneling by the superexchange mechanism. However, the FN(n)(-?) radical anion was directly observed spectroscopically as an intermediate in the charge-separation mechanism, thereby demonstrating conclusively that the overall charge separation involves the incoherent hopping (stepwise) mechanism. Kinetic modeling of the data showed that the observed exponential distance dependence is largely due to electron injection onto the first FN unit followed by charge hopping between the FN units of the bridge biased by the distance-dependent electrostatic attraction of the two charges in D(+?)-B(-?)-A. This work shows that wirelike behavior does not necessarily result from building a stepwise, energetically downhill redox gradient into a D-B-A molecule.     

Controlling electron transfer in donor-bridge-acceptor molecules using cross-conjugated bridges

Photoinitiated charge separation (CS) and recombination (CR) in a series of donor-bridge-acceptor (D-B-A) molecules with cross-conjugated, linearly conjugated, and saturated bridges have been compared and contrasted using time-resolved spectroscopy. The photoexcited charge transfer state of 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) is the donor, and naphthalene-1,8:4,5-bis(dicarboximide) (NI) is the acceptor in all cases, along with 1,1-diphenylethene, trans-stilbene, diphenylmethane, and xanthone bridges. Photoinitiated CS through the cross-conjugated 1,1-diphenylethene bridge is about 30 times slower than through its linearly conjugated trans-stilbene counterpart and is comparable to that observed through the diphenylmethane bridge. This result implies that cross-conjugation strongly decreases the π orbital contribution to the donor-acceptor electronic coupling so that electron transfer most likely uses the bridge σ system as its primary CS pathway. In contrast, the CS rate through the cross-conjugated xanthone bridge is comparable to that observed through the linearly conjugated trans-stilbene bridge. Molecular conductance calculations on these bridges show that cross-conjugation results in quantum interference effects that greatly alter the through-bridge donor-acceptor electronic coupling as a function of charge injection energy. These calculations display trends that agree well with the observed trends in the electron transfer rates.     

Ultrafast energy transfer between the 3MLCT state of [Ru(II)(dmb)2(bpy-an)]2+ and the covalently appended anthracene

The time scale for triplet-triplet energy transfer (EnT) between a Ru(II) chromophore and a ligand bound anthracene acceptor in [Ru(II)(dmb)2(bpy-an)]2+ (dmb = 4,4'-dimethyl-2,2-bipyridine; bpy-an = 4-(9-anthrylethylene), 4-methyl-2,2-bipyridine) has been measured using femtosecond transient absorption spectroscopy. The appearance of the anthracene excited state is monitored following photoexcitation to a metal-to-ligand charge transfer (MLCT) state via the pi pi* absorption of the triplet excited state of anthracene. Our time-resolved experiments show the presence of fast, sub-100 ps energy transfer to the anthracene occurring on two characteristic time scales of 23 and 72 ps.     

Engineering the Charge-Transfer State to Facilitate Spin–Orbit Charge Transfer Intersystem Crossing in Spirobis[anthracene]diones

Spiro conjugation has been proposed to dictate the efficiency of charge transfer, which could directly affect the spin–orbit charge transfer intersystem crossing (SOCT-ISC) process. However, this process has yet to be exemplified. Herein, we prepared three spirobis[anthracene]diones, in which two benzophenone moieties are locked in close proximity and differentially functionalized to fine-tune the charge transfer state. Its feasibility for SOCT-ISC was theoretically predicted, then experimentally evaluated. Through fine-tuning the spiro conjugation coupling and varying the solvent dielectric constants, ISC rate constants were engineered to vary in a dynamic range of three orders of magnitude, from 7.8×10⁸ s⁻¹ to 1.0×10¹¹ s⁻¹, which is the highest ISC rate reported for SOCT-ISC system to our knowledge. Our findings substantiate the key factors for effective SOCT-ISC and offer a new avenue for the rational design of heavy atom free triplet sensitizers.     

Temperature dependence of spin-selective charge transfer pathways in donor-bridge-acceptor molecules with oligomeric fluorenone and p-phenylethynylene bridges

The temperature dependence of spin-selective intramolecular charge recombination (CR) in a series of 2,7-fluorenone (FN(1-2)) and p-phenylethynylene (PE(1-2)P) linked donor-bridge-acceptor molecules with a 3,5-dimethyl-4-(9-anthracenyl) julolidine (DMJ-An) electron donor and a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor was studied using nanosecond transient absorption spectroscopy in the presence of a static magnetic field. Photoexcitation of DMJ-An into its charge transfer band and subsequent electron transfer to NI results in a nearly quantitative yield of (1)(DMJ(+?)-An-FN(n)-NI(-?)) and (1)(DMJ(+?)-An-PE(n)P-NI(-?)), which undergo rapid radical pair intersystem crossing (RP-ISC) to produce the triplet RPs, (3)(DMJ(+?)-An-FN(n)-NI(-?)) and (3)(DMJ(+?)-An-PE(n)P-NI(-?)), respectively. The CR rate constants, k(CR), in toluene were measured over a temperature range from 270 to 350 K, and a kinetic analysis of k(CR) in the presence of an applied static magnetic field was used to extract the singlet and triplet charge recombination rate constants, k(CRS) and k(CRT), respectively, as well as the intersystem crossing rate constant, k(ST). Plots of ln (kT(1/2)) versus 1/T for PE(1)P show a distinct crossover at 300 K from a temperature-independent singlet CR pathway to a triplet CR pathway that is positively activated with a barrier of 1047 ± 170 cm(-1). The singlet CR pathway via the FN(1) bridge displays a negative activation energy that results from donor-bridge and bridge-acceptor torsional motions about the single bonds joining them. In contrast, the triplet CR pathway via the FN(1-2) and PE(1-2)P bridges exhibits positive activation energies. The activation barriers to these torsional motions range from 1100 to 4500 cm(-1) and can be modeled by semiclassical electron transfer theory.     

Direct observation of the preference of hole transfer over electron transfer for radical ion pair recombination in donor-bridge-acceptor molecules

Understanding how the electronic structures of electron donor-bridge-acceptor (D-B-A) molecules influence the lifetimes of radical ion pairs (RPs) photogenerated within them (D+*-B-A-*) is critical to designing and developing molecular systems for solar energy conversion. A general question that often arises is whether the HOMOs or LUMOs of D, B, and A within D+*-B-A-* are primarily involved in charge recombination. We have developed a new series of D-B-A molecules consisting of a 3,5-dimethyl-4-(9-anthracenyl)julolidine (DMJ-An) electron donor linked to a naphthalene-1,8:4,5-bis(dicarboximide) (NI) acceptor via a series of Phn oligomers, where n = 1-4, to give DMJ-An-Phn-NI. The photoexcited charge transfer state of DMJ-An acts as a high-potential photoreductant to rapidly and nearly quantitatively transfer an electron across the Phn bridge to produce a spin-coherent singlet RP 1(DMJ+*-An-Phn-NI-*). Subsequent radical pair intersystem crossing yields 3(DMJ+*-An-Phn-NI-*). Charge recombination within the triplet RP then gives the neutral triplet state. Time-resolved EPR spectroscopy shows directly that charge recombination of the RP initially produces a spin-polarized triplet state, DMJ-An-Phn-3*NI, that can only be produced by hole transfer involving the HOMOs of D, B, and A within the D-B-A system. After the initial formation of DMJ-An-Phn-3*NI, triplet-triplet energy transfer occurs to produce DMJ-3*An-Phn-NI with rate constants that show a distance dependence consistent with those determined for charge separation and recombination.     

Intersystem Crossing and Electron Spin Selectivity in Anthracene-Naphthalimide Compact Electron Donor-Acceptor Dyads Showing Different Geometry and Electronic Coupling Magnitudes

Anthracene-naphthalimide (An-NI) compact electron donor-acceptor dyads were prepared, in which the orientation and distance between the two subunits were varied by direct connection or with intervening phenyl linker. Efficient intersystem crossing (ISC) and long triplet state lifetime (Φ_Δ=92 %, τ_T=438 μs) were observed for the directly connected dyads showing a perpendicular geometry (81°). This efficient spin-orbit charge transfer ISC (SOCT-ISC) takes 376 fs, inhibits the direct charge recombination (CR) to ground state (¹CT→S₀, takes 3.04 ns). Interestingly, efficient SOCT-ISC for dyads with intervening phenyl linker (Φ_Δ=40 % in DCM) was also observed, although the electron donor and acceptor adopt almost coplanar geometry (dihedral angle: 15°). Time-resolved electron paramagnetic resonance (TREPR) spectroscopy shows that the electron spin polarization of the triplet state, i. e. the electron spin selectivity of ISC, is highly dependent on the dihedral angle and the linker. For the dyads showing weaker coupling between the donor and acceptors, the charge separation and the intramolecular triplet energy transfer are inhibited at 80 K (frozen solution), because both the ³An and ³NI states were observed and the ESP are same as compared to the native anthracene and naphthalimide, which unravel their origin. The dyads were used as triplet photosensitizers for triplet−triplet annihilation upconversion (TTA UC). High UC quantum yield (Φ_UC=12.9 %) as well as a large anti-Stokes shift (0.72 eV) was attained by excitation into the CT absorption band.     

Long-Lived Triplet Excited State Accessed with Spin–Orbit Charge Transfer Intersystem Crossing in Red Light-Absorbing Phenoxazine-Styryl BODIPY Electron Donor/Acceptor Dyads

Orthogonal phenoxazine-styryl BODIPY compact electron donor/acceptor dyads were prepared as heavy atom-free triplet photosensitizers (PSs) with strong red light absorption (ϵ=1.33×10⁵ M⁻¹ cm⁻¹ at 630 nm), whereas the previously reported triplet photosensitizers based on the spin-orbit charge transfer intersystem crossing (SOCT-ISC) mechanism show absorption in a shorter wavelength range (<500 nm). More importantly, a long-lived triplet state (τ_T=333 μs) was observed for the new dyads. In comparison, the triplet state lifetime of the same chromophore accessed with the conventional heavy atom effect (HAE) is much shorter (τ_T=1.8 μs). Long triplet state lifetime is beneficial to enhance electron or energy transfer, the primary photophysical processes in the application of triplet PSs. Our approach is based on SOCT-ISC, without invoking of the HAE, which may shorten the triplet state lifetime. We used bisstyrylBodipy both as the electron acceptor and the visible light-harvesting chromophore, which shows red-light absorption. Femtosecond transient absorption spectra indicated the charge separation (109 ps) and SOCT-ISC (charge recombination, CR; 2.3 ns) for BDP-1 . ISC efficiency of BDP-1 was determined as Φ_T=25 % (in toluene). The dyad BDP-3 was used as triplet PS for triplet-triplet annihilation upconversion (upconversion quantum yield Φ_UC=1.5 %; anti-Stokes shift is 5900 cm⁻¹).     

Role of bridge energetics on the preference for hole or electron transfer leading to charge recombination in donor-bridge-acceptor molecules

The impact of donor-acceptor electronic coupling and bridge energetics on the preference for hole or electron transfer leading to charge recombination in a series of donor-bridge-acceptor (D-B-A) molecules was examined. In these systems, the donor is 3,5-dimethyl-4-(9-anthracenyl)-julolidine (DMJ-An) and acceptor is naphthalene-1,8:4,5-bis(dicarboximide) (NI), while the bridges are either oligo(p-phenyleneethynylene) (PE(n)P, where n = 1-3) 1-3 or oligo(2,7-fluorenone) (FN(n), where n = 1-3) 4-6. Photoexcitation of 1-3 and 4-6 produces DMJ(+?)-An-PE(n)P-NI(-?) and DMJ(+?)-An-FN(n)-NI(-?), respectively, which undergo radical pair intersystem crossing followed by charge recombination to yield both (3*)An and (3*)NI, which are observed by time-resolved electron paramagnetic resonance (TREPR) spectroscopy. (3*)NI is produced by hole transfer from DMJ(+?) to NI(-?), while (3*)An is produced by electron transfer from NI(-?) to DMJ(+?), using the agency of the bridge HOMOs and LUMOs, respectively. By monitoring the initial population of (3*)NI and (3*)An in 1-6, the data show that charge recombination occurs preferentially by selective hole transfer when the bridge is PE(n)P, while it occurs by preferential electron transfer when the bridge is FN(n). Over time, the initial population of (3*)NI decreases, while that of (3*)An increases, indicating that triplet-triplet energy transfer (TEnT) occurs. The observed distance dependence of TEnT from (3*)NI to An is weakly exponential with a decay parameter β = 0.08 ?(-1) for the PE(n)P series and β = 0.03 ?(-1) for the FN(n) series. In the PE(n)P series, this weak distance dependence is attributed to a transition from the superexchange regime to hopping transport as the energy gap for triplet energy injection onto the bridge becomes significantly smaller as n increases, while in the FN(n) series the corresponding energy gap is small for all n resulting in triplet energy transport by the hopping mechanism.     

Synthesis and spectroscopy of anthracene-containing linear and ‘T’-shaped π-conjugated ligands

A range of new π-conjugated ethynyl- and diethynyl-benzene ligands has been synthesised and their spectroscopic characterisation carried out, most notably via IR and ¹H NMR. X-ray crystal structures were obtained for three of these ligands and one unusual ruthenium complex. Both the 4-ethynyl- and 2,5-diethynyl-benzene cores of these compounds have been functionalised through organic transformations by addition of an 9-anthracenyl. This has been attached via a range of linker moieties that vary in both their length and degree of π-conjugation. This has given rise to two groups of compounds with either a linear, e.g., 9-(2-(4-ethynylphenyl)ethynediyl)anthracene and 9-(2-(4-ethynylphenyl)ethyl)anthracene, or ‘T’-shaped morphologies, e.g., 9-(2-(2,5-diethylnylphenyl)ethyl)anthracene.     

Highly twisted triarylamines for photoinduced intramolecular charge transfer

9-(N,N-Dianisylamino)anthracene (9DAAA), 9-(N,N-dianisylamino)dinaphth([1,2-a:2'-1'-j]-anthracene (9DAAH), and 9,10-bis(N,N-dianisylamino)anthracene (910BAA) were synthesized as highly twisted triarylamines with potential for photoexcited internal charge transfer. Crystallography of 9DAAA shows its dianisylamino group to be twisted nearly perpendicular to its anthracene unit, similar to a report for 910BAA. The solution fluorescence spectra show strong bathochromic shifts for each of the three molecular systems with strongly decreased quantum efficiency in higher polarity solvents. Solution-phase (ensemble) time-resolved photoluminescence measurements show up to 4-fold decreases in fluorescence lifetime in acetonitrile compared to hexane. The combined results are consistent with photoinduced, transient intramolecular charge-transfer from the bis-anisylamine unit to the polycyclic aromatic unit. Computational modeling is in accord with intramolecular transfer of electron density from the bis-anisylamino unit to the anthracene, based on in comparisons of HOMO and LUMO.     

Spiroconjugated Intramolecular Charge‐Transfer Emission in Non‐Typical Spiroconjugated Molecules: The Effect of Molecular Structure upon the Excited‐State Configuration

A set of terfluorenes and terfluorene‐like molecules with different pendant substitutions or side groups were designed and synthesized, their photophysical properties and the excited‐state geometries were studied. Dual fluorescence emissions were observed in compounds with rigid pendant groups bearing electron‐donating N atoms. According to our earlier studies, in this set of terfluorenes, the blue emission is from the local π–π* transition, while the long‐wavelength emission is attributed to a spiroconjugation‐like through‐space charge‐transfer process. Herein, we probe further into how the molecular structures (referring to the side groups, the type of linkage between central fluorene and the 2,2′‐azanediyldiethanol units, and—most importantly—the amount of pendant groups), as well as the excited‐state geometries, affect the charge‐transfer process of these terfluorenes or terfluorene‐like compounds. 9‐(9,9,9′′,9′′‐tetrahexyl‐9H,9′H,9′′H‐[2,2′:7′,2′′‐terfluoren]‐9′‐yl)‐1,2,3,5,6,7‐hexahydropyrido[3,2,1‐ij]quinolone (TFPJH), with only one julolidine pendant group, was particularly synthesized, which exhibits complete “perpendicular” conformation between julolidine and the central fluorene unit in the excited state, thus typical spiroconjugation could be achieved. Notably, its photophysical behaviors resemble those of TFPJ with two pendant julolidines. This study proves that spiroconjugation does happen in these terfluorene derivatives, although their structures are not in line with the typical orthogonal π fragments. The spiroconjugation charge‐transfer emission closely relates to the electron‐donating N atoms on the pendant groups, and to the rigid connection between the central fluorene and the N atoms, whereas the amount of pendant groups and the nature of the side chromophores have little effect. These findings may shed light on the understanding of the through‐space charge‐transfer properties and the emission color tuning of fluorene derivatives.     

Optical switching and antenna effect of dendrimers with an anthracene core

Dendrimers 6G(i) (i=1-4) consisting of an anthracene core and Fréchet dendrons which are attached via a CH(2)OCH(2) chain in the 9-position undergo quantitative and completely reversible intramolecular [4pi+4pi] cycloaddition. The process can be monitored by absorption and fluorescence measurements. The Fréchet dendrons act as an energy funnel that collects and focuses the photon energy but does not change the photostationary states, which for both directions are completely on the product side when the separate chromophores are selectively irradiated. The quantum yields of anthracene fluorescence and of singlet energy transfer from the dendrons to the core were studied as a function of dendrimer generation.     

Spin–Orbit Charge-Transfer Intersystem Crossing (ISC) in Compact Electron Donor–Acceptor Dyads: ISC Mechanism and Application as Novel and Potent Photodynamic Therapy Reagents

Spin–orbit charge-transfer intersystem crossing (SOCT-ISC) is useful for the preparation of heavy atom-free triplet photosensitisers (PSs). Herein, a series of perylene-Bodipy compact electron donor/acceptor dyads showing efficient SOCT-ISC is prepared. The photophysical properties of the dyads were studied with steady-state and time-resolved spectroscopies. Efficient triplet state formation (quantum yield Φ_T=60 %) was observed, with a triplet state lifetime (τ_T=436 μs) much longer than that accessed with the conventional heavy atom effect (τ_T=62 μs). The SOCT-ISC mechanism was unambiguously confirmed by direct excitation of the charge transfer (CT) absorption band by using nanosecond transient absorption spectroscopy and time-resolved electron paramagnetic resonance (TREPR) spectroscopy. The factors affecting the SOCT-ISC efficiency include the geometry, the potential energy surface of the torsion, the spin density for the atoms of the linker, solvent polarity, and the energy matching of the ¹CT/³LE states. Remarkably, these heavy atom-free triplet PSs were demonstrated as a new type of efficient photodynamic therapy (PDT) reagents (phototoxicity, EC₅₀=75 nm ), with a negligible dark toxicity (EC₅₀=78.1 μm ) compared with the conventional heavy atom PSs (dark toxicity, EC₅₀=6.0 μm, light toxicity, EC₅₀=4.0 nm ). This study provides in-depth understanding of the SOCT-ISC, unveils the design principles of triplet PSs based on SOCT-ISC, and underlines their application as a new generation of potent PDT reagents.     

Research on the Synthesis and Optical Properties of Isomeric Bichromophoric Compounds --3-or 4- Phenyl 7-Coumarin ω-9-Anthracene-Poly-methene Carboxylate

Six bichromophoric compounds-substituted coumarin ω -9-anthra-cene-poly-methene carboxylate and five model compounds were synthesized. Among them, eight compounds are new ones. Intramolecular singlet energy transfer has been demonstrated in the bichromophoric compounds 1? and studied in some detail. The absorption spectra of the compounds 1-6 bear evidence that the π-electron systems of coumarin and anthracene ring do not overlap appreciably. The coumarin moiety of the bichromophoric compounds molecule was excited at 314nm and the resulting fluorescence was characteristic of the anthracene group. The efficiency of transfer of singlet excitation from coumarin moiety to the anthracene grou is about 100% in both calculation and observation and the rate of the singlet energy transfer is about 1014 sec in the compounds 4-6. A possible mechanism of intramolecular energy transfer was suggested. The difference between 4-phenyl compounds 1-3 and 3-phenyl compounds 4-6 in spectra was com -pared.     

辅酶NADH模型物还原活化烯烃反应机理研究

对本课题组近年来研究的辅酶NADH模型物还原活化烯烃的反应机理进行了综述。对于辅酶模型物还原2-溴-1-苯基亚乙基丙二腈类化合物的反应,依赖辅酶模型物和底物的结构,反应可以按一步的负氢转移机理或按电子转移机理进行。用手性辅酶模型物进行这一反应,可得到具有中等光学活性的环丙烷衍生物。实验结果表明辅酶模型物BNAH与1,1-二苯基-2,2-二硝基乙烯的反应的过渡态具有部分双自由基和部分共价键形成的特征,为Pross-Shaik“曲线交叉模型”所预测的“中间机理”提供了直接的证据。BNAH与9-亚芴基丙二腈的反应经历电子转移和负电荷在9-位碳上的碳负离子中间体,动力学同位素效应为2.6。     

十字交叉型π共轭分子3,6-二苯-1,2,4,5-(2′,2″-二苯基)-苯并二噁唑的电子结构和电荷传输性质的理论研究

采用密度泛函理论的B3LYP方法, 在6-31G(d)基组水平下研究了以三联苯和二苯基苯并噁唑构成的十字交叉型共轭分子3,6-二苯基-1,2,4,5-(2′,2″-二苯基)-苯并二噁唑的电子结构和电荷传输性质. 通过对分子的重组能和晶体中分子间电荷传输积分的计算得到该分子的空穴迁移率为0.31 cm²·V^-1·s^-1, 电子迁移率为0.11 cm²/(V·s). 计算结果表明, 空穴的传输主要是通过三联苯方向上两端苯环的“边对面”的相互作用以及分子中心π体系的错位重叠相互作用来实现的. 而电子的传输路径主要是通过苯并噁唑方向的π-π重叠相互作用来实现. 通过分析分子正负离子态的Mulliken电荷发现, 正电荷较多分布在三联苯方向上, 而负电荷较多分布在苯并噁唑方向上. 计算结果表明, 电子和空穴的传输分别在分子相互交叉的不同方向上, 有利于电子和空穴的平衡传输.     

Structural effects on the ground and excited-state properties of photoswitchable hydrogen-bonding receptors

The ground- and excited-state properties of a series of photochromic barbiturate receptors (N,N'-bis{6-[omega-(anthracen-9-yloxy)alkanoylamino]pyridin-2-yl}-5-t-butyl-isophthalamide, Tn), in which anthracene chromophores are tethered via (CH2)n (n = 1, 3-6) polymethylene linkers to the H-bond receptor moiety, are described. In these systems, the thermally reversible [4pi + 4pi] photodimerization of the anthracenes yields macrocyclic receptors (TnC) that possess significantly reduced affinity toward barbital as compared to their acyclic counterparts. The length of the tether not only determines the overall binding ability of the cyclized receptor but also has a profound influence on the photochemical and photophysical properties of the anthracene chromophores. The reduced mobility experienced by the covalently bound anthracenes controls the reactivity of a fluorescent excimer that is proposed to be an intermediate in the photocyclization process.     

不同位置取代的三氰基乙烯基蒽的光谱行为的研究

本文设计合成了两个典型的共轭的电子给体与电子受体(D-A)化合物:2-三氰基乙烯基蒽(2-TCVA)与9-三氰基乙烯基蒽(9-TCVA),通过极性效应,温度效应对它们基态与激发态的光谱行为进行了表征。研究表明:这两个化合物均表现出显著的电荷转移(CT)吸收峰,分子受光激发后,9-TCVA只能在非极性溶剂中产生分子内电荷转移(ICT)态荧光,而2-TCVA在极性与非极性溶剂中都能从ICT态发光。另外,温度效应显示冻结态下,2-TCVA只发射ICT态荧光,而9-TCVA既发射类蒽(anthracene-like)荧光又发射ICT态荧光,造成这一现象的主要原因可能是2-TCVA与9-TCVA分子平面性上的差异而引起分子内电荷转移相互作用不同所致。文中还利用了Bilot-Kawski公式估算了化合物2-TCVA在激发态与基态时偶极矩的差值为18.8D。