o-Amino conjugation effect on the photochemistry of trans-aminostilbenes

The excited-state behavior of a series of trans-2-(N-arylamino)stilbenes (aryl = phenyl (o1H), 4-methylphenyl (o1Me), 4-methoxyphenyl (o1OM), and 4-cyanophenyl (o1CN)) and trans-2-(N,N-diphenylamino)stilbene (o2) in both nonpolar and polar solvents is reported and compared to that of the parent trans-2-aminostilbene and the corresponding meta- and para-isomers (m1R and p1R, where R = H, Me, OM, and CN, and m2 and p2). Two types of torsional motions, the D-A torsion that results in a nonfluorescent twisted intramolecular charge transfer (TICT) state and the C═C torsion that leads to the cis isomers, account for the radiationless decays of o1R and o2. The relative efficiencies of these torsions can be readily evaluated from their quantum yields for fluorescence (Φ(f)) and trans → cis isomerization (Φ(tc)). The propensities of the D-A torsion are similar for the ortho and meta isomers, which is 1OM > 1Me and negligible for 1H, 1CN, and 2. The activation parameters determined from temperature-dependent fluorescence lifetimes suggest that the C═C torsion occurs mainly via the triplet state for the ortho systems, a behavior again similar to that of the meta isomers. Whereas the intersystem crossing in o1R, m1R, and m2 is essentially a nonactivated process, it encounters a barrier of 2.7-3.8 kcal mol(-1) in o2. As a result of the barriers that decelerate the radiationless decays and the slow fluorescence rate for o2 in acetonitrile, the observed long fluorescence lifetime 24.5 ns at room temperature reaches a new record for unconstrained trans-stilbenes.     

Meta conjugation effect on the torsional motion of aminostilbenes in the photoinduced intramolecular charge-transfer state

The photochemical behavior of a series of trans-3-(N-arylamino)stilbenes (m1, aryl = 4-substituted phenyl with a substituent of cyano (CN), hydrogen (H), methyl (Me), or methoxy (OM)) in both nonpolar and polar solvents is reported and compared to that of the corresponding para isomers (p1CN, p1H, p1Me, and p1OM). The distinct propensity of torsional motion toward a low-lying twisted intramolecular charge-transfer (TICT) state from the planar ICT (PICT) precursor between the meta and para isomers of 1CN and 1Me reveals the intriguing meta conjugation effect and the importance of the reaction kinetics. Whereas the poor charge-redistribution (delocalization) ability through the meta-phenylene bridge accounts for the unfavorable TICT-forming process for m1CN, it is such a property that slows down the decay processes of fluorescence and photoisomerization for m1Me, facilitating the competition of the single-bond torsional reaction. In contrast, the quinoidal character for p1Me in the PICT state kinetically favors both fluorescence and photoisomerization but disfavors the single-bond torsion. The resulting concept of thermodynamically allowed but kinetically inhibited TICT formation could also apply to understanding the other D-A systems, including trans-4-cyano-4'-(N,N-dimethylamino)stilbene (DCS) and 3-(N,N-dimethylamino)benzonitrile (3DMABN).     

Substituent-dependent photoinduced intramolecular charge transfer in N-aryl-substituted trans-4-aminostilbenes

The photochemical behavior of trans-4-(N-arylamino)stilbene (1, aryl = 4-substituted phenyl) in solvents more polar than THF is strongly dependent on the substituent in the N-aryl group. This is attributed to the formation of a twisted intramolecular charge transfer (TICT) state for those with a methoxy (1OM), methoxycarbonyl (1CO), or cyano (1CN) substituent but not for those with a methyl (1Me), hydrogen (1H), chloro (1Cl), or trifluoromethyl (1CF) substituent. On the basis of the ring-bridged model compounds 3-6, the TICT states for 1CN and 1CO result from the twisting of the anilino-benzonitrilo C-N bond, but for 1OM it is from the twisting of the stilbenyl-anilino C-N bond, both of which are distinct from the TICT states previously proposed for N,N-dimethylaminostilbenes.     

The photochemical characteristics of aromatic enediyne compounds substituted with electron donating and electron withdrawing groups

trans- and cis-1-(4-Dimethylaminophenyl)-6-(4-nitrophenyl)hex-3-ene-1,5-diynes (trans- and cis-DANE) were synthesized and their photochemical properties were studied. The absorption spectra of trans-DANE red-shifted compared with the parent compound bisphenylethynylethene (BEE) due to intramolecular charge transfer. The fluorescence spectra, Stokes shift, fluorescence lifetime, fluorescence quantum yield, and quantum yield of trans-to-cis photoisomerization of trans-DANE showed strong dependence upon the solvent polarity in the less-polar region. No fluorescence emission from trans-DANE was observed in medium-polar and polar solvents. The quantum yield of cis-to-trans isomerization was almost solvent independent. The donor-acceptor substituents shifted the equilibrium between the trans perpendicular triplet state and the trans planar triplet state to the trans triplet state, and resulted in an increase in the triplet lifetime. Comparison of the photochemical properties of trans-DANE with trans-4-dimethylamino-4'-nitrostilbene (DANS) suggests that trans-DANE is a possible fluorescent probe in the non-polar region.     

Photophysics of trans-4-(dimethylamino)-4'-cyanostilbene and its use as a solvation probe

Electronic structure calculations, steady-state electronic spectroscopy, and femtosecond time-resolved emission spectroscopy are used to examine the photophysics of trans-4-(dimethylamino)-4'-cyanostilbene (DCS) and its solvent dependence. Semiempirical AM1/CI calculations suggest that an anilino TICT state is a potential candidate for the emissive state of DCS in polar solvents. But observation of large and solvent-independent absorption and emission transition moments in a number of solvents (M(abs) = 6.7 +/- 0.4 D and M(em) = 7.6 +/- 0.8 D) rule out the involvement of any such state, which would have a vanishingly small transition moment. The absorption and steady-state emission spectra of DCS evolve in a systematic manner with solvent polarity, approximately as would be expected for a single, highly polar excited state. Attempts to fit the solvatochromism of DCS using standard dielectric continuum models are only partially successful when values of the solute dipole moments suggested by independent measurements are assumed. The shapes of the absorption and emission spectra of DCS change systematically with solvent polarity in a manner that is semiquantitatively reproduced using a coupled-state model of the spectroscopy. Kerr-gate emission measurements show that the emission dynamics of DCS down to subpicosecond times reflect only solvent relaxation, rather than any more complicated electronic state kinetics. The spectral response functions measured with DCS are well correlated to those previously reported for the solvation probe coumarin 153, indicating DCS to be a useful alternative probe of solvation dynamics.     

Synthesis, dual fluorescence, and fluoroionophoric behavior of dipyridylaminomethylstilbenes

The synthesis, dual fluorescence, and fluoroionophoric behavior of two donor-sigma spacer-acceptor (D-s-A) compounds, trans-4-(N,N-bis(2-pyridyl)amino)methylstilbene (1H) and trans-4-(N,N-bis(2-pyridyl)amino)methyl-4'-cyanostilbene (1CN), are reported and compared to that of trans-4-(N,N-bis(2-pyridyl)amino)methyl-4'-(N,N-dimethylamino)stilbene (1DPA). To gain insights into the dual fluorescence properties for 1H and 1CN in polar but not in nonpolar solvents, model compounds resulting from a replacement of the stilbene group by alkyl (2R) or xylyl (2X) groups or from a replacement of the dipyridylamino (dpa) group by dianisoleamino (3AA), diethylamino (3EE), methylanilino (3MP), or diphenylamino (3PP) groups also have been investigated. In addition to 1H and 1CN, all four compounds of 3 display dual fluorescence. The locally excited (LE) fluorescence mainly results from the stilbene group and the ICT fluorescence from the through-bond interactions between the amino donor and the stilbene acceptors. In the presence of transition metal ions such as Zn(II), Ni(II), Cu(II), and Cd(II), the ICT processes are switched from dpa (D) --> stilbene (A) in 1H and 1CN to stilbene (D) --> dpa/metal ion (A) in their complexes. Whereas the ICT states for the complexes are generally nonfluorescent, an exception was found for the case of 1H/Zn(II). As a result, substituent-dependent fluoroionophoric behavior has been demonstrated by 1H, 1CN, and 1DPA in response to Zn(II).     

Intramolecular charge transfer in 4-aminobenzonitriles does not necessarily need the twist

In electron donor/acceptor species such as 4-(dimethylamino)benzonitrile (DMABN), the excitation to the S(2) state is followed by internal conversion to the locally excited (LE) state. Dual fluorescence then becomes possible from both the LE and the twisted intramolecular charge-transfer (TICT) states. A detailed mechanism for the ICT of DMABN and 4-aminobenzonitrile (ABN) is presented in this work. The two emitting S(1) species are adiabatically linked along the amino torsion reaction coordinate. However, the S(2)/S(1) CT-LE radiationless decay occurs via an extended conical intersection "seam" that runs almost parallel to this torsional coordinate. At the lowest energy point on this conical intersection seam, the amino group is untwisted; however, the seam is accessible for a large range of torsional angles. Thus, the S(1) LE-TICT equilibration and dual fluorescence will be controlled by (a) the S(1) torsional reaction path and (b) the position along the amino group twist coordinate where the S(2)/S(1) CT-LE radiationless decay occurs. For DMABN, population of LE and TICT can occur because the two species have similar stabilities. However, in ABN, the equilibrium lies in favor of LE, as a TICT state was found at much higher energy with a low reaction barrier toward LE. This explains why dual fluorescence cannot be observed in ABN. The S(1)-->S(0) deactivation channel accessible from the LE state was also studied.     

Mechanism of the Photodissociation of 4-Diphenyl(trimethylsilyl)methyl-N,N-dimethylaniline

On irradiation in hexane (248- and 308-nm laser light) 4-diphenyl(trimethylsilyl)methyl-N,N-dimethylaniline, 2, undergoes photodissociation of the C-Si bond giving 4-N,N-dimethylamino-triphenylmethyl radical, 3(*) (lambda(max) at 343 and 403 nm), in very high quantum yield (Phi = 0.92). The intervention of the triplet state of 2 (lambda(max) at 515 nm) is clearly demonstrated through quenching experiments with 2,3-dimethylbuta-1,3-diene, styrene, and methyl methacrylate using nanosecond laser flash photolysis (LFP). The formation of 3(*) is further demonstrated using EPR spectroscopy. The detection of the S(1) state of 2 was achieved using 266-nm picosecond LFP, and its lifetime was found to be 1400 ps, in agreement with the fluorescence lifetime (tau(f) = 1500 ps, Phi(f) = 0.085). The S(1) state is converted almost exclusively to the T(1) state (Phi(T) = 0.92). In polar solvents such as MeCN, 2 undergoes (1) photoionization to its radical cation 2(*)(+), and (2) photodissociation of the C-Si bond, giving radical 3(*) as before in hexane. The formation of 2(*)(+) occurs through a two-photon process. Radical cation 2(*)(+) does not fragment further, as would be expected, to 3(*) via a nucleophile(MeCN)-assisted C-Si bond cleavage but regenerates the parent compound 2. Obviously, the bulkiness of the triphenylmethyl group prevents interaction of 2(*)(+) with the solvent (MeCN) and transfer to it of the electrofugal group Me(3)Si(+). The above results of the laser flash photolysis are supported by pulse radiolysis, fluorescence measurements, and product analysis.     

The N‐Arylamino Conjugation Effect in the Photochemistry of Fluorescent Protein Chromophores and Aminostilbenes

To understand the nonradiative decay mechanism of fluorescent protein chromophores in solutions, a systematic comparison of a series of (Z)‐4‐(N‐arylamino)benzylidene‐2,3‐imidazolinones (ABDIs: 2P , 2PP , 2OM , and 2OMB ) and the corresponding trans‐4‐(N‐arylamino)‐4′‐cyanostilbenes (ACSs: 1P , 1PP , 1OM , and 1OMB ) was performed. We have previously shown that the parameter Φ_f+2 Φ_tc, in which Φ_f and Φ_tc are the quantum yields of fluorescence and trans→cis photoisomerization, respectively, is an effective probe for evaluating the contribution of twisted intramolecular charge transfer (TICT) states in the excited decays of trans‐aminostilbenes, including the push–pull ACSs. One of the criteria for postulating the presence of a TICT state is Φ_f+2 Φ_tc?1.0, because its formation is decoupled with the C?C bond (τ) torsion pathway and its decay is generally nonradiative. Our results show that the same concept also applies to ABDIs 2 with the parameter Φ_f+2 Φ_ZE in which Φ_ZE is the quantum yield of Z→E photoisomerization. We conclude that the τ torsion rather than the C? C bond (φ) torsion is responsible for the nonradiative decays of ABDIs 2 in aprotic solvents (hexane, THF, acetonitrile). The phenyl‐arylamino C? N bond (ω) torsion that leads to a nonradiative TICT state is important only for 2OM in THF and acetonitrile. If the solvent is protic (methanol and 10–20 % H₂O in THF), a new nonradiative decay channel is present for ABDIs 2 , but not for ACSs 1 . It is attributed to internal conversion (IC) induced by solvent (donor)–solute (acceptor) hydrogen‐bonding (HB) interactions. The possible HB modes and the concept of τ torsion‐coupled proton transfer are also discussed.     

Photophysical properties of coumarin-7 dye: role of twisted intramolecular charge transfer state in high polarity protic solvents

Photophysical studies on coumarin-7 (C7) dye in different protic solvents reveal interesting changes in the properties of the dye on increasing the solvent polarity (Deltaf; Lippert-Mataga solvent polarity parameter) beyond a critical value. Up to Deltaf approximately 0.31, the photophysical properties of the dye follow good linear correlations with Deltaf. For Deltaf > approximately 0.31, however, the photophysical properties, especially the fluorescence quantum yields (Phi(f)), fluorescence lifetimes (tau(f)) and nonradiative rate constants (k(nr)), undergo large deviations from the above linearity, suggesting an unusual enhancement in the nonradiative decay rate for the excited dye in these high polarity protic solvents. The effect of temperature on the tau(f) values of the dye has also been investigated to reveal the mechanistic details of the deexcitation mechanism for the excited dye. Studies have also been carried out in deuterated solvents to understand the role of solute-solvent hydrogen bonding interactions on the photophysical properties of the dye. Observed results suggest that the fluorescence of the dye originates from the planar intramolecular charge transfer (ICT) state in all the solvents studied and the deviations in the properties in high polarity solvents (Deltaf > approximately 0.31) arise due to the participation of a new deexcitation channel associated with the formation of a nonfluorescent twisted intramolecular charge transfer (TICT) state of the dye. Comparing present results with those of a homologous dye coumarin 30 (C30; Photochem. Photobiol., 2004, 80, 104), it is indicated that unlike in C30, the TICT state of the C7 dye does not experience any extra stability in protic solvents compared to that in aprotic solvents. This has been attributed to the presence of intramolecular hydrogen bonding between the NH group (in the 3-benzimidazole substituent) of the C7 dye and its carbonyl group, which renders an extra stability to the planar ICT state, making the TICT state formation relatively difficult. Qualitative potential energy diagrams have been proposed to rationalize the differences observed in the results with C7 and C30 dyes in high polarity protic solvents.     

Twisted intramolecular charge-transfer state of trans-3-(N,N-Dimethylamino)-4'-cyanostilbene: The C−C bond twisting

Whether a twisted intramolecular charge-transfer (TICT) state is formed is an important issue for understanding the fluorescence properties of a push-pull organic dye. Here we report a position effect of the donor substituent on the TICT state formation of aminostilbenes: namely, trans-3-(N,N-dimethylamino)-4′-cyanostilbene ( mDCS ) behaves differently from its TICT-free para isomer DCS and forms a TICT state in acetonitrile (MeCN). The TICT state of mDCS also differs from that of many previously reported aminostilbenes by twisting a C−C bond instead of a C−N bond. In addition, among the ring-bridged model compounds mDCS-N , mDCS-C1 , and mDCS-C2 , we observed an unusual electronic effect of the ring-bridging in mDCS-C2 that mitigates the impact of the TICT state on the fluorescence properties. Both the C−C bond twisting in mDCS and the ring-bridging electronic effect in mDCS-C2 provide new insights into the TICT chemistry of aminostilbenes.     

Computational study of thioflavin T torsional relaxation in the excited state

Quantum-chemical calculations of the Thioflavin T (ThT) molecule in the ground S0 and first excited singlet S1 states were carried out. It has been established that ThT in the ground state has a noticeable nonplanar conformation: the torsion angle phi between the benzthiazole and the dimethylaminobenzene rings has been found to be approximately 37 degrees. The energy barriers of the intramolecular rotation appearing at phi = 0 and 90 degrees are quite low: semiempirical AM1 and PM3 methods predict values approximately 700 cm-1 and ab initio methods approximately 1000-2000 cm(-1). The INDO/S calculations of vertical transitions to the S1(abs) excited state have revealed that energy ES1(abs) is minimal for the twisted conformation with phi = 90 degrees and that the intramolecular charge-transfer takes place upon the ThT fragments' rotation from phi = 0 to 90 degrees. Ab initio CIS/RHF calculations were performed to find optimal geometries in the excited S1 state for a series of conformers having fixed phi values. The CIS calculations have predicted a minimum of the S1 state energy at phi approximately 21 degrees; however, the energy values are 1.5 times overestimated in comparison to experimental data. Excited state energy dependence on the torsion angle phi, obtained by the INDO/S method, reveals that ES1(fluor) is minimal at phi = approximately 80-100 degrees, and a plateau is clearly observed for torsion angles ranging from 20 to 50 degrees. On the basis of the calculation results, the following scheme of photophysical processes in the excited S1 state of the ThT is suggested. According to the model, a twisted internal charge-transfer (TICT) process takes place for the ThT molecule in the excited singlet state, resulting in a transition from the fluorescent locally excited (LE) state to the nonfluorescent TICT state, accompanied by torsion angle phi growth from 37 to 90 degrees. The TICT process effectively competes with radiative transition from the LE state and is responsible for significant quenching of the ThT fluorescence in low-viscosity solvents. For viscous solvents or when the ThT molecule is located in a rather rigid microenvironment, for example, when it is bound to amyloid fibrils, internal rotation in the dye molecule is blocked due to steric hindrance, which results in suppression of the LE --> TICT quenching process and in a high quantum yield of fluorescence.     

Excited-state behavior of N-phenyl-substituted trans-3-aminostilbenes: where the "m-amino effect" meets the "amino-conjugation effect"

The electronic spectroscopy and photochemistry of the trans isomers of 3-(N-phenylamino)stilbene (m1c), 3-(N-methyl-N-phenylamino)stilbene (m1d), 3-(N,N-diphenylamino)stilbene (m1e), and 3-(N-(2,6-dimethylphenyl)amino)stilbene (m1f) and their double-bond constrained analogues m2a-m2c and m2e are reported. When compared with trans-3-aminostilbene (m1a), m1c-m1e display a red shift of the S0 --> S1 absorption and fluorescence spectra, lower oscillator strength and fluorescence rate constants, and more efficient S1 --> T1 intersystem crossing. Consequently, the N-phenyl derivatives m1c-m1e have lower fluorescence quantum yields and higher photoisomerization quantum yields. The corresponding N-phenyl substituent effect in m2a-m2e is similar in cyclohexane but smaller in acetonitrile. This is attributed to the weaker intramolecular charge transfer character for the S1 state of m2 so that the rates for intersystem crossing are less sensitive to solvent polarity. It is also concluded that N-phenyl substitutions do not change the triplet mechanism of photoisomerization for m1 in both nonpolar and polar solvents. Therefore, the "m-amino conjugation effect" reinforces the "m-amino effect" on fluorescence by further reducing its rate constants and highlights the N-phenyl-enhanced intersystem crossing from the "amino-conjugation effect" by making S1 --> T1 the predominant nonradiative decay pathway.     

氧鎓盐分子在受限制体系中的电荷转移和发光行为

2,4,6-三芳基氧盐是一类类似于三苯甲烷染料的化合物^[1].其结构中存在两种跃迁矩,分别对应于分子的X-轴方向(2,6-位取代基)和Y-轴方向(4-位取代基).因此,2,4,6-三芳基氧盐的吸收光谱中存在着两个较具特征的吸收峰^[2].     

Photophysical properties of a rhodium tetraphenylporphyrin-tin corrole dyad. The first example of a through metal-metal bond energy transfer

The luminescence spectroscopy study and the determination of the photophysical parameters for the M-M'-bonded rhodium meso-tetraphenylporphyrin-tin(2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole) complex, (TPP)Rh-Sn(Me6Et2Cor) 1, was investigated. The emission bands as well as the lifetimes (tau(e)) and the quantum yields (Phi(e); at 77 K using 2MeTHF as solvent) were compared with those of (TPP)RhI 2 (TPP = tetraphenylporphyrin) and (Me6Et2Cor)SnCl 3 (Me6Et2Cor = 2,3,7,13,17,18-hexamethyl-8,12-diethylcorrole) which are the two chemical precursors of 1. The energy diagram has been established from the absorption, fluorescence and phosphorescence spectra. The Rh(TPP) and Sn(Me6Et2Cor) chromophores are the energy donor (D) and acceptor (A), respectively. The total absence of fluorescence in 1 (while fluorescence is observed in the tin derivative 3) indicates efficient excited state deactivation, presumably due to heavy atom effect and intramolecular energy transfer (ET). The large decreases in tau(P) and Phi(P) of the Rh(TPP) chromophore going from 2 to 1 indicate a significant intramolecular ET in the triplet states of 1 with an estimated rate ranging between 10(6) and 10(8) s(-1). Based on the comparison of transfer rates with other related dyads that exhibit similar D-A separations and no M-M' bond, and for which slower through space ET processes (10(2)-10(3) s(-1)) operate, a through M-M' bond ET has been unambiguously assigned to 1.     

Skeletal relaxation effect on the charge transfer state formation of 4-dimethylamino,4'-cyanostilbene

Ab initio complete active space self-consistent field (CASSCF) calculations combined with polarized continuum model (PCM) have been performed to examine the charge transfer (CT) state formation of trans-4-dimethylamino,4'-cyanostilbene (DCS) in a solvent. In a polar solvent, the globally stable geometry in S1 takes a twisted conformation where the electron-donating dimethylanilino group is highly twisted against the other part of the electron-withdrawing 4-cyanostyryl group. In addition, skeletal relaxation where the aromatic benzene rings turn to be a nonaromatic quinoid structure is essential to stabilize the CT state. In a nonpolar solvent, the stable geometry in S1 takes a nontwisted conformation, though the skeletal relaxation is also an essential factor. By means of the free energy decomposition analysis, it is found that the stable CT geometry which depends on solvent polarity mainly comes from two factors: the linkage bond between the dimethylanilino and the 4-cyanostyryl group and the electrostatic interaction. In a polar solvent, the linkage bond has a single bond character to slightly prevent the torsional motion. This twist geometrically assists the charge separation so as to reinforce the electrostatic interaction. In consequence, the twisted internal CT (TICT) conformation is stable. In a nonpolar solvent, on the other hand, a nontwisted CT state is stable because the linkage bonds greatly increase a double bond character so as to prevent the torsional motion, while the electrostatic interaction is not so enhanced even by the geometrical twist.     

Orientation of a TICT probe trapped in the peripheral confined water created by ionic surfactant envelope around silver nanoparticles

Ionic surfactants are known to aggregate around the surface of a nanoparticle as a single layer in premicellar and a double layer in micellar concentrations. This motif of arrangement indicates the development of a layer of confined water of lower polarity than bulk water around the surface of the nanoparticle. We have demonstrated the behavior of a twisted intramolecular charge transfer (TICT) probe, trans-2-[4-(dimethylamino)styryl]benzothiazole (DMASBT), in the confined aqueous layer developed at the surface of spherical silver nanoparticles (Ag NPs) at and above the critical micellar concentrations (CMC) of a cationic and an anionic surfactant, namely, cetyltrimethylammonium bromide (CTAB) and sodium dodecyl sulfate (SDS). It is observed that the presence of charged surfactant head groups affects DMASBT differentially in the ground and the excited states. In presence of CTAB, DMASBT turns over in the excited state and interacts with the Ag NP surface, whereas in SDS the probe remains in its original orientation during the interaction. Steady-state and time-resolved fluorescence spectral studies provide enough evidence for orientation of the TICT probe in the peripheral water of Ag NP created by the surfactants. The results were confirmed by steady-state anisotropy measurements. The data show the difference between the properties of the confined peripheral water and the bulk aqueous environment. The TICT probe, DMASBT, is proved to be an excellent marker for the phenomenon.     

The metalla-Pinner reaction between Pt(IV)-bound nitriles and alkylated oxamic and oximic forms of hydroxamic acids

The nitrile ligands in the platinum(IV) complexes trans-[PtCl4(RCN)2] (R=Me, Et, CH2Ph) and cis/trans-[PtCl4(MeCN)(Me2SO)] are involved in a metalla-Pinner reaction with N-methylbenzohydroxamic acid (N-alkylated form of hydroxamic acid, hydroxamic form; F1), PhC(=O)N(Me)OH, to achieve the imino species [PtCl4[NH=C(R)ON(Me)C(=O)Ph]2 (1-3) and [PtCl4[NH=C(Me)ON(Me)C(=O)Ph](Me2SO)] (7), respectively. Treatment of trans-[PtCl4(RCN)2] (R=Me, Et) and cis/trans-[PtCl4(MeCN)(Me2SO)] with the O-alkylated form of a hydroxamic acid (hydroximic form), i.e. methyl 2,4,6-trimethylbenzohydroximate, 2,4,6-(Me3C6H2)C(OMe)=NOH (F2A), allows the isolation of [PtCl4[NH=C(R)ON=C(OMe)(2,4,6-Me3C6H2)]2] (5, 6) and [PtCl4[NH=C(Me)ON=C(OMe)(2,4,6-Me3C6H2)](Me2SO)] (8), correspondingly. In accord with the latter reaction, the coupling of nitriles in trans-[PtCl4(EtCN)2] with methyl benzohydroximate, PhC(OMe)=NOH (F2B), gives [PtCl4[NH=C(Et)ON=C(OMe)Ph]2] (4). The addition proceeds faster with the hydroximic F2, rather than with the hydroxamic form F1. The complexes 1-8 were characterized by C, H, N elemental analyses, FAB+ mass-spectrometry, IR, 1H and 13C[1H] NMR spectroscopies. The X-ray structure determinations have been performed for both hydroxamic and hydroximic complexes, i.e. 2 and 6, indicating that the imino ligands are mutually trans and they are in the E-configuration.     

How the pi conjugation length affects the fluorescence emission efficiency

How the pi conjugation length affects the fluorescence emission efficiency is elucidated by examination of the theoretical and experimental relationship between absolute quantum yield (Phi(f)) and magnitude (Api) of the pi conjugation length in the excited singlet state, which provides a novel concept for molecular design for highly fluorescent organic compounds. As a tool to predict Phi(f) from a structural model, (nu(a) - nu(f))1/2 x a3/2 (nu(a): wavenumber of absorption maximum, nu(f): wavenumber of emission maximum, a: molecular radius) could be used instead of Api. The concept should be valuable for potential applications to (1) examination of an excited singlet state structure (for example, coplanarity of excited-state molecules) and (2) molecular design of novel materials, in which the excited singlet state plays an important role, such as highly efficient fluorophores, electroluminescent materials, photoconducting materials, and nonlinear optical materials. A remarkably intense green fluorophore (Phi(f) 0.88, log epsilon 4.72, lambda(em) 527 nm) is created based on this concept, which is of great interest in relation to a green fluorescent protein (Topaz, T203Y type, Phi(f) 0.60, log epsilon 4.98, lambda(em) 527 nm).     

分子内扭转电荷转移的胶束效应

研究了水溶液中对二甲氨基苯甲(DMABA)分子内扭转电荷转移(TICT)的胶束效应.胶束能促进DMABA的TICT过程,并导致DMABA的总荧光量子产率提高和TICT荧光峰的显著蓝移,DMABA的TICT荧光强度与正常荧光带的强度之比与DMABA的浓度之间有线性关系,胶束存在时该直线的斜率提高,认为DMABA的TICT激发态涉及两个DMABA分子,并且影响DMABA的TICT激发态相对布居的主要因素是其与相应的三重态和基态的能隙.讨论了DMABA的TICT激发态的可能结构及胶束效应的本质.