A novel fluorophore with dual fluorescence: local excited state and photoinduced electron-transfer-promoted charge-transfer state.

Absorption and emission spectra of 9-N,N-dimethylaniline decahydroacridinedione (DMAADD) have been studied in different solvents. The fluorescence spectra of DMAADD are found to exhibit dual emission in aprotic solvents and single emission in protic solvents. The effect of solvent polarity and viscosity on the absorption and emission spectra has also been studied. The fluorescence excitation spectra of DMAADD monitored at both the emission bands are different. The presence of two different conformation of the same molecule in the ground state has lead to two close lying excited states, local excited (LE) and charge transfer (CT), and thereby results in the dual fluorescence of the dye. A CTstate involving the N,N-dimethylaniline group and the decahy droacridinedione chromophore as donor and acceptor, respectively, has been identified as the source of the long wavelength anomalous fluorescence. The experimental studies were supported by ab initio time dependent-density functional theory (TDDFT) calculations performed at the B3LYP/6-31G* level. The molecule possesses photoinduced electron transfer (PET) quenching in the LE state, which is confirmed by the fluorescence lifetime and fluorescent intensity enhancement in the presence of transition metal ions.     

Charge Transfer in 1,8-Naphthalimide: A Combined Theoretical and Experimental Approach

Photophysics of 1,8-naphthalimide (NAPMD) in different solvents has been delineated in this paper. Theoretically calculated bond distance of N–H and C=O groups rule out any intramolecular proton transfer in the excited state. Concomitant increase in negative charge on O atom compared to N atom and dipole moment hints at possible intramolecular charge transfer. Progressive redshift with polarity of solvents in emission and absorption spectra also confirms the theoretical prediction. Weakening of N–H bond helps hydrogen abstraction and anion formation in water with decay time of 2.54 ns through intermolecular proton transfer. This was corroborated from the ground state photoexcitation of laboratory synthesized anion of NAPMD. Amide hydrolysis in higher pH and excess proton availability at low pH are responsible for anion emission quenching. A possible electron transfer diminishes phosphorescence at 77 K with changing pH.     

Spectral and photophysical properties of intramolecular charge transfer fluorescence probe: 4'-dimethylamino-2,5-dihydroxychalcone

The spectral and photophysical properties of a new intramolecular charge transfer (ICT) probe, namely 4′-dimethylamino-2,5-dihydroxychalcone (DMADHC) were studied in different solvents by using steady-state absorption and emission spectroscopy. Whereas the absorption spectrum undergoes minor change with increasing polarity of the solvents, the fluorescence spectrum experiences a distinct bathochromic shift in the band position and the fluorescence quantum yield increases reaching a maximum before decrease with increasing the solvent polarity. The magnitude of change in the dipole moment was calculated based on the Lippert–Mataga equation. These results give the evidence about the intramolecular charge transfer character in the emitting singlet state of this compound.     

2-吡唑啉类化合物光物理行为的研究──分子内两种电荷转移机制的竞争

近年来,1,3,5-三芳基-2-吡唑啉类化合物的光物理及光化学问题研究引起了广泛兴趣^[1,2],这是由于2-吡唑啉化合物不仅是一种良好的荧光增白剂,在纺织工业中得到普遍应用,而且还是一种可在静电复印中应用的良好光导材料^[3,4].     

Solvent and Media Effects on the Photophysics of Naphthoxazole Derivatives

The photophysical properties of 2‐phenyl‐naphtho[1,2‐d][1,3]oxazole, 2(4‐N,N‐dimethylaminophenyl)naphtho[1,2‐d][1,3]oxazole and 2(4‐N,N‐diphenylaminophenyl) naphtho[1,2‐d][1,3]oxazole were studied in a series of solvents. UV–Vis absorption spectra are insensitive to solvent polarity whereas the fluorescence spectra in the same solvent set show an important solvatochromic effect leading to large Stokes shifts. Linear solvation energy relationships were employed to correlate the position of fluorescence spectra maxima with microscopic empirical solvent parameters. This study indicates that important intramolecular charge transfer takes place during the excitation process. In addition, an analysis of the solvatochromic behavior of the UV–Vis absorption and fluorescence spectra in terms of the Lippert–Mataga equation shows a large increase in the excited‐state dipole moment, which is also compatible with the formation of an intramolecular charge‐transfer excited state. We propose both naphthoxazole derivatives as suitable fluorescent probes to determine physicochemical microproperties in several systems and as dyes in dye lasers; consequence of their high fluorescence quantum yields in most solvents, their large molar absorption coefficients, with fluorescence lifetimes in the range 1–3 ns as well as their high photostability.     

Ultrafast photoinduced intramolecular charge transfer in push-pull distyryl furan and benzofuran: solvent and molecular structure effect

The excited state deactivation pathways of push-pull distyryl furan and benzofuran derivatives in several organic solvents were investigated in detail by using time-resolved transient absorption and fluorescence spectroscopies, with nano- and femto-second time resolution. Solvent polarity was found to play a key role in determining the efficiencies of fluorescence, intersystem crossing and internal conversion. The triplet yield gradually decreased, while the internal conversion increased upon increasing the solvent dielectric constant. However the fluorescence showed a different solvent polarity effect in the low and high solvent polarity region, with a reversal of the trend of fluorescence properties (quantum yield and lifetime). This fact points to an emitting state of a different nature (smaller and larger dipole moments) in the two cases, as also suggested by the huge fluorosolvatochromism. In fact the ultrafast spectroscopic investigation evidenced the presence of two transients characterized by peculiar spectral shapes assigned to a locally excited (LE) and a charge transfer (CT) state. In the more polar solvents the CT state was the longer lived, fluorescent one and an intramolecular charge transfer process was found to be operative and to become faster (up to ～200-250 fs) in the higher polarity media. On the contrary, distyrylfuran, which exhibits the same molecular skeleton without the push-pull character showed a similar excited state dynamics in solvents of different polarities.     

1,3,5-三芳基-2-吡唑啉化合物光物理行为的研究

合成了一组带不同取代基的三芳基吡唑啉化合物,对它们在不同极性溶剂中的光物理行为(如荧光量子产率,荧光寿命等)进行了测定指出:这类化合物在光的激发下除存在有分子内共轭条件下的电荷转移行为外,还存在着分子内非共轭条件下的电子转移,本工作还以三芳基吡唑啉化合物为猝灭剂对氧鎓盐的荧光猝灭能力进行了研究,并对所得结果作了讨论。     

THE FLUORESCENCE PROPERTIES OF ORTHO AMINOBENZOATE ANESTHETICS IN DEFINED SOLVENTS AND PHOSPHOLIPID VESICLES

Abstract— The fluorescence properties of three ortho aminobenzoate local anesthetics have been determined in a variety of solvents. Results from these studies have been used to deduce how these drugs interact with phosphatidylcholine bilayers. The emission energy, fluorescence quantum yield and lifetime exhibited a biphasic dependence on solvent polarity. In aprotic solvents, alcohols and in ethanol-water mixtures containing less than 40% water, quantum yields and lifetimes were high (approximately 0.55 and 8.5 ns respectively). In ethanol-water mixtures containing >40% water, the strong fluorescence quenching was primarily due to an increase in the rate of non-radiative deactivation of the excited state. Both the radiative ( k_r ) and non-radiative ( k_nr ) rate constants show a biphasic dependence on solvent polarity. These studies suggest the presence of two singlet excited states for these molecules, a polar singlet excited state, S_1-p and a charge transfer excited state, S_1-ct with the latter predominating in ethanol-water mixture containing >40% water. In egg phosphatidylcholine bilayers, the fluorescence, lifetime and quantum yield are consistent with the view that these drugs are localized within the lipid head group region where the charge-transfer excited state can be stabilized by intermolecular hydrogen bonding.     

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。     

Dual fluorescence of omeprazole: effect of solvents and pH

Absorption, steady state fluorescence and time-resolved fluorescence spectra of omeprazole (OMP) have been studied in solvents of different polarity and pH. With an increase in the polarity of the solvents, blue shift is observed in the longer wavelength whereas red shift is noticed in the shorter wavelength band. The dual emission observed in non-polar solvents suggests that the energy of the twisted intramolecular charge transfer (TICT) state is lower than that of the locally excited (LE) state. The normal Stokes-shifted band originates from the LE state, and the large Stokes-shifted band is due to the emission from a TICT state. The Stokes shift of OMP is correlated with various solvent polarity scales like E_T(30) and f?(D,n).     

Synthesis and fluorescence properties of 2-aryl-3-hydroxyquinolones, a new class of dyes displaying dual fluorescence

A series of 2-aryl-3-hydroxyquinolones (3HQs) with different electron donating aryl substituents at the position 2 were synthesized. Their absorption and fluorescence properties were studied in solvents of medium and high polarity. Almost all the synthesized 3HQs display dual fluorescence in the tested solvents, in line with an excited state intramolecular proton transfer reaction. For N-methyl substituted compounds, the intensity ratio of the two emission bands was found to be exquisitely sensitive to solvent polarity, with a two orders of magnitude change from toluene to dimethylsulfoxide. Consequently, these compounds appear as prospective polarity fluorescent labels for proteins and nucleic acids.     

Spectral properties of intramolecular charge transfer fluorescence probe 1-keto-2-(p-dimethylaminobenzal)-tetrahydronaphthalene

The photophysical properties of a new compound 1-keto-2-(p-dimethylaminobenzal)-tetrahydronaphthalene in various solvents at room temperature were characterized by the absorption and steady-state fluorescence technique. The bathochromic shift on the emission spectra, the broad halfwidth of the fluorescence band and the increase in the excited state dipole moment occurred. These results gave the evidence about the intramolecular charge transfer (ICT) character in the emitting singlet state of the compound.     

取代苄叉亚胺类化合物溶液电子光谱的研究

研究了二甲氨基苄叉亚胺类化合物溶液的电子光谱。结果表明一端为电子给体另一端为电子受体的共轭型取代苄叉对硝基苯胺3,在基态下发生了分子内电荷转移,其跃迁吸收波长比化合物1,2(315 nm)红移90 nm。以乙撑基联接的非共轭型苄叉亚胺奥化合物4,5,6未观察到基态时的电荷转移现象,但在激发条件下可明显的发生分子内的电荷转移而使荧光强度大大减弱。MMA等对1的荧光有明显的猝灭作用,表明其间发生了电荷转移。丙烯酸则因会引起西佛碱的水解,而使荧光减弱。     

A combined experimental and theoretical study on photoinduced intramolecular charge transfer in trans-ethyl p-(dimethylamino)cinamate

Intramolecular charge transfer (ICT) behavior of trans-ethyl p-(dimethylamino)cinamate (EDAC) in various solvents has been studied by steady-state absorption and emission, picosecond time-resolved fluorescence spectroscopy and femtosecond transient absorption experiments as well as time-dependent density functional theory (TDDFT). Large fluorescence spectral shift in more polar solvents indicates an efficient charge transfer from the donor site to the acceptor moiety in the excited state compared to the ground state. The energy for 0,0 transition (ν_0,0) for EDAC shows very good linear correlation with static solvent dielectric property. The relaxation dynamics of EDAC in the excited state can be effectively described by a “three state” model where, the locally excited (LE) state converts into the ICT state within 350 ± 100 fs. A combination of solvent reorganization and intramolecular vibrational relaxation within 0.5–6 ps populates the relaxed ICT state which undergoes fluorescence decay within few tens to hundreds of picoseconds.     

A novel carbazole-based fluorescent probe:3,6-Bis-[(N-ethylcarbazole-3-y1)-propene-1-keto]-N-ethylcarbazole

<正>A novel carbazole-based compound 5,3,6-bis[(N-ethylcarbazole-3-yl)-propene-1-keto]-N-ethylcarbazole has been designed, synthesized and characterized.The absorption and fluorescence spectra in solvents of different polarities prove that the compound has a distinct intramolecular charge transfer character.Compound 5 can be used as a new class of fluorescent probe or biosensor due to its sensitivity to the local microenvironment such as solvent polarity.     

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].     

Excited State Intramolecular Proton Transfer of New Diphenylethylene Derivatives Bearing Imino Group: A Combination of Experimental and Theoretical Investigation

In this paper, we described the synthesis and characterization of new diphenylethylene bearing imino group. We concentrated particularly on the investigation of the possibility of the excited state intramolecular charge transfer (ESIPT) of the new dyes experimentally and theoretically. The absorption and fluorescence spectroscopy of the dyes were determined in various solvents. The results showed that the maximal absorption wavelength of 2‐[(4′‐N,N‐dimethylamino‐diphenylethylene‐4‐ylimino)methyl]phenol ( C1 ) and 4‐[(4′‐N,N‐dimethylamino‐diphenylethylene‐4‐ylimino)methyl]phenol ( C2 ) exhibited almost independence on the solvent polarity. While as contrast, the maximal fluorescence wavelength of the dyes showed somewhat dependence on the solvent polarity. In particular, C1 displayed well‐separated dual fluorescence spectroscopy. The second fluorescence peak was characterized with an "abnormal" fluorescence emission wavelength in aprotic solvents with large Stokes shift (ca. 140 nm in THF), which was much more than normal Stokes shift (ca. 30 nm in THF). This emission spectroscopy could be assigned to ESIPT emission. On the other hand, the ESIPT fluorescence of C1 was much reduced or lost in the protic solvents. While, only normal fluorescence emission was detected in various solvents. Although the absorption maxima of C1 exhibited about 10 nm red‐shift with respect to those of C2 , the normal fluorescence maxima of C1 and C2 were almost identical in various solvents. These results suggested that C1 could undergo ESIPT, but C2 was not able to proceed ESIPT. The molecular geometry optimization of phototautomers in the ground electronic state (S₀) was carried out with HF method (Hartree‐Fock) and at DFT level (Density Functional Theory) using B3LYP both, while the CIS was employed to optimize the geometries of the first singlet excited state (S₁) of the phototautomers of C1 and C2 respectively. The properties of the ground state and the excited state of the phototautomers of C1 and C2 , including the geometrical parameter, the energy, the frontier orbits, the Mulliken charge and the dipole moment change were performed and compared completely. The data were analyzed further based on our experimental results. Furthermore, the absorption and fluorescence spectra were calculated in theory and compared with the measured ones. The rate constant of internal proton transfer (9.831×10¹¹ s^?1) of C1 was much lower than that of salicylidene methylamine ( C3 , 2.045×10¹⁵ s^?1), which was a typical Schiff base compound and was well demonstrated to undergo ESIPT easily under photoexcitation.     

Dipolar 3-methoxychromones as bright and highly solvatochromic fluorescent dyes

Herein, three environment-sensitive (solvatochromic) fluorescent dyes presenting a strong electron acceptor 3-methoxychromone unit and varied electron donor 2-aryl were developed. All three dyes showed remarkable polarity-dependent shifts of the emission maximum, which increase with extension of the dye conjugation. For the 3-methoxychromone bearing a 7-(diethylamino)-9,9-dimethylfluoren-2-yl donor group the difference between the excited and the ground state dipole moments, estimated from the Lippert-Mataga expression, reached 20 D, which is among the largest reported for neutral dipolar fluorophores. Moreover, the new dyes are characterized by significant two-photon absorption cross-section (up to 450 GM) and large fluorescence quantum yields. The strong decrease in the fluorescence quantum yields of the dyes in polar protic solvents was observed together with the increase in the non-radiative deactivation rates, which can originate from twisted intramolecular charge transfer and intermolecular proton transfer phenomena. In comparison to the parent 3-hydroxychromone derivatives, the new dyes presented significantly improved photostability, which confirms that photodegradation of 3-hydroxychromones occurs from a product of the excited-state intramolecular proton transfer (phototautomer). Finally, an application of the new dyes for probing local binding site polarity of serum albumin was shown. This new class of fluorescent dyes may serve as attractive building blocks for future molecular sensors utilizing environment-sensitive fluorophores.     

Photophysical Properties of Intramolecular Charge Transfer in a Tribranched Donor–π–Acceptor Chromophore

The photophysical properties of intramolecular charge transfer (ICT) in a novel tribranched donor–π–acceptor chromophore, triphenoxazine‐2,4,6‐triphenyl‐1,3,5‐triazine (tri‐PXZ‐TRZ), with thermally activated delayed fluorescence character was investigated in different aprotic solvents by steady‐state spectroscopy and femtosecond and nanosecond transient absorption spectroscopy measurements. Increasing the solvent polarity led to a significant increase in the Stokes shift. The large Stokes shift in highly polar solvents was attributed to ICT properties upon excitation; this resulted in a strong interaction between the tri‐PXZ‐TRZ molecule and the surrounding solvent, which led to a strong solvation process. Quantum‐chemical calculations and changes in the dipole moment showed that this compound has a large degree of ICT. Furthermore, an apolar environment helped to preserve the symmetry of tri‐PXZ‐TRZ and to enhance its emission efficiency. The femtosecond and nanosecond transient absorption spectroscopy results indicated that the excited‐state dynamics of this push–pull molecule were strongly influenced by solvent polarity through the formation of a solvent‐stabilized ICT state. The excited‐state relaxation mechanism of tri‐PXZ‐TRZ was proposed by performing target model analysis on the femtosecond transient absorption spectra. In addition, the delayed fluorescence of tri‐PXZ‐TRZ was significantly modulated by a potential competition between solvation and intersystem crossing processes.     

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.