Synthesis and Photophysical Properties of Models for Twisted PRODAN and Dimethylaminonaphthonitrile

Solvent-Dependent Excited-State Evolution of Prodan Dyes

Excited-state character and dynamics of two 6-(dimethylamino)-2-acylnaphthalene dyes (Prodan and Badan-SCH₂CH₂OH) were studied by picosecond time-resolved IR spectroscopy (TRIR) in solvents of different polarity and relaxation times: hexane, CD₃OD, and glycerol-d₈. In all these solvents, near-UV excitation initially produced the same S₁(ππ*) excited state characterized by a broad TRIR signal. A very fast decay (3, ∼100 ps) followed in hexane, whereas conversion to a distinct IR spectrum with a ν(C═O) band downshifted by 76 cm^-1 occurred in polar/H-bonding solvents, slowing down on going from CD₃OD (1, 23 ps) to glycerol-d₈ (5.5, 51, 330 ps). The final relaxed excited state was assigned as planar Me₂N → C═O intramolecular charge transfer S₁(ICT) by comparing experimental and TDDFT-calculated spectra. TRIR conversion kinetics are comparable to those of early stages of multiexponential fluorescence decay and dynamic fluorescence red-shift. This work presents a strong evidence that Prodan-type dyes undergo solvation-driven charge separation in their S₁ state, which is responsible for the dynamic fluorescence Stokes shift observed in polar/H-bonding solvents. The time evolution of the optically prepared S₁(ππ*) state to the S₁(ICT) final state reflects environment relaxation and solvation dynamics. This finding rationalizes the widespread use of Prodan-type dyes as probes of environment dynamics and polarity.

Photophysical Properties of BADAN Revealed in the Study of GGBP Structural Transitions

The fluorescent dye BADAN (6-bromoacetyl-2-dimetylaminonaphtalene) is widely used in various fields of life sciences, however, the photophysical properties of BADAN are not fully understood. The study of the spectral properties of BADAN attached to a number of mutant forms of GGBP, as well as changes in its spectral characteristics during structural changes in proteins, allowed to shed light on the photophysical properties of BADAN. It was shown that spectral properties of BADAN are determined by at least one non-fluorescent and two fluorescent isomers with overlapping absorbing bands. It was found that BADAN fluorescence is determined by the unsolvated "PICT" (planar intramolecular charge transfer state) and solvated "TICT" (twisted intramolecular charge transfer state) excited states. While "TICT" state can be formed both as a result of the "PICT" state solvation and as a result of light absorption by the solvated ground state of the dye. BADAN fluorescence linked to GGBP/H152C apoform is quenched by Trp 183, but this effect is inhibited by glucose intercalation. New details of the changes in the spectral characteristics of BADAN during the unfolding of the protein apo and holoforms have been obtained.

A new interpretation of the absorption and the dual fluorescence of Prodan in solution

Remarkable interest is associated with the interpretation of the Prodan fluorescent spectrum. A sequential hybrid Quantum Mechanics/Molecular Mechanics method was used to establish that the fluorescent emission occurs from two different excited states, resulting in a broad asymmetric emission spectrum. The absorption spectra in several solvents were measured and calculated using different theoretical models presenting excellent agreement. All theoretical models [semiempirical, time dependent density functional theory and and second-order multiconfigurational perturbation theory] agree that the first observed band at the absorption spectrum in solution is composed of three electronic excitations very close in energy. Then, the electronic excitation around 340 nm-360 nm may populate the first three excited states (π-π^*L_b, n-π^*, and π-π^*L_a). The ground state S₀ and the first three excited states were analyzed using multi-configurational calculations. The corresponding equilibrium geometries are all planar in vacuum. Considering the solvent effects in the electronic structure of the solute and in the solvent relaxation around the solute, it was identified that these three excited states can change the relative order depending on the solvent polarity, and following the minimum path energy, internal conversions may occur. A consistent explanation of the experimental data is obtained with the conclusive interpretation that the two bands observed in the fluorescent spectrum of Prodan, in several solvents, are due to the emission from two independent states. Our results indicate that these are the n-π^* S₂ state with a small dipole moment at a lower emission energy and the π-π^*L_b S₁ state with large dipole moment at a higher emission energy.

1,5-Prodan Emits from a Planar Intramolecular Charge-Transfer Excited State

1-Propionyl-5-dimethylaminonaphthalene (8, 1,5-Prodan) and two derivatives where the amino group is constrained in a seven-membered (9) and five-membered (10) ring are prepared. All three exhibit strong fluorescence and similar degrees of solvatochromism. Their fluorescence is strongly quenched in alcohol solvents. Because the amino group in 9 and especially 10 is forced to be coplanar with the naphthalene ring, the similar photophysical behavior of all three suggests that emission arises from a planar excited state (planar intramolecular charge transfer).

Hydrogen bond strengthening induces fluorescence quenching of PRODAN derivative by turning on twisted intramolecular charge transfer

Researchers have proposed different effective mechanisms of hydrogen bonding (HB) on the fluorescence of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) and its derivatives. Herein, excited state transition and dynamics analysis confirm that the fluorescence of PD (a derivative of PRODAN with ethyl replaced by 3-hydroxy-2,2-dimethylpropan) emits from the planar intramolecular charge transfer (PICT) state rather than twist ICT (TICT) state, because the fluorescence emission and surface hopping from the TICT state to the twist ground (T-S₀) state is energy forbidden. Nevertheless, the strengthening of intramolecular-HB (intra-HB) and intermolecular-HB (inter-HB) of PD-(methanol)₂ smooth the pathway of surface hopping from TICT to T-S₀ state and the external conversion going to planar ground state by decreasing the energy difference of the two states. This smoothing changes the fluorescence state of PD-(methanol)₂ to the TICT state in which fluorescence emission does not occur but surface hopping, leading to the partial fluorescence quenching of PD in methanol solvent. This conclusion is different from previous related reports. Moreover, the inter-HB strengthening of PD-methanol in PICT state induces the cleavage of intra-HB and a fluorescence red-shift of 54nm compared to PD. This red-shift increases to 66nm for PD-(methanol)₂ for the strengthening of the one intra-HB and two inter-HBs. The dipole moments of PD-methanol and PD-(methanol)₂ respectively increase about 10.3D and 8.1D in PICT state compared to PD. The synergistic effect of intra-HB and inter-HB induces partial quenching of PD in methanol solvent by turning on the TICT state and fluorescence red-shift. This work gives a reasonable description on the fluorescence red-shift and partial quenching of PD in methanol solvent, which will bring insight into the study of spectroscopic properties of molecules owning better spectral characteristics.

Base-Promoted Selective Synthesis of 2H-Pyranones and Tetrahydronaphthalenes via Domino Reactions

A highly efficient domino protocol has been developed for the synthesis of 6-aryl-4-(methylthio/amine-1-yl)-2-oxo-2H-pyran-3-carbonitriles and 4-aryl-2-(amine-1-yl)-5,6,7,8-tetrahydronaphthalene-1-carbonitriles from simple and readily available α-aroylketene dithioacetals, malononitrile, secondary amines, and cyclohexanone. This elegant domino process involved consecutive addition-elimination, intramolecular cyclization, and ring opening and closing sequences. Notably, in situ generated 2-imino-4-(methylthio/amine-1-yl)-6-aryl-2H-pyran-3-carbonitrile plays multiple roles in the construction of various novel polyaromatic hydrocarbons.

Solvatochromic fluorene-linked nucleoside and DNA as color-changing fluorescent probes for sensing interactions

A nucleoside bearing a solvatochromic push-pull fluorene fluorophore (dCFL ) was designed and synthesized by the Sonogashira coupling of alkyne-linked fluorene 8 with 5-iodo-2'-deoxycytidine. The fluorene building block 8 and labeled nucleoside dCFL exerted bright fluorescence with significant solvatochromic effect providing emission maxima ranging from 421 to 544 nm and high quantum yields even in highly polar solvents, including water. The solvatochromism of 8 was studied by DFT and ADC(2) calculations to show that, depending on the polarity of the solvent, emission either from the planar or the twisted conformation of the excited state can occur. The nucleoside was converted to its triphosphate variant dCFLTP which was found to be a good substrate for DNA polymerases suitable for the enzymatic synthesis of oligonucleotide or DNA probes by primer extension or PCR. The fluorene-linked DNA can be used as fluorescent probes for DNA-protein (p53) or DNA-lipid interactions, exerting significant color changes visible even to the naked eye. They also appear to be suitable for time-dependent fluorescence shift studies on DNA, yielding information on DNA hydration and dynamics.

Cyclopenta[b]naphthalene cyanoacrylate dyes: synthesis and evaluation as fluorescent molecular rotors

We describe the design, synthesis and fluorescent profile of a family of environment-sensitive dyes in which a dimethylamino (donor) group is conjugated to a cyanoacrylate (acceptor) unit via a cyclopenta[b]naphthalene ring system. This assembly satisfies the typical D-π-A motif of a fluorescent molecular rotor and exhibits solvatochromic and viscosity-sensitive fluorescence emission. The central naphthalene ring system of these dyes was synthesized via a novel intramolecular dehydrogenative dehydro-Diels-Alder (IDDDA) reaction that permits incorporation of the donor and acceptor groups in variable positions around the aromatic core. A bathochromic shift of excitation and emission peaks was observed with increasing solvent polarity but the dyes exhibited a complex emission pattern with a second red emission band when dissolved in nonpolar solvents. Consistent with other known molecular rotors, the emission intensity increased with increasing viscosity. Interestingly, closer spatial proximity between the donor and the acceptor groups led to decreased viscosity sensitivity combined with an increased quantum yield. This observation indicates that structural hindrance of intramolecular rotation dominates when the donor and acceptor groups are in close proximity. The examined compounds give insight into how excited state intramolecular rotation can be influenced by both the solvent and the chemical structure.

Steric hindrance inhibits excited-state relaxation and lowers the extent of intramolecular charge transfer in two-photon absorbing dyes

Steric hindrance inhibits excited-state relaxation and intramolecular charge-transfer, which significantly alters the photophysical properties.

Attachable solvatochromic fluorophores and bioconjugation studies

The synthesis and utility of attachable cyclopenta[b]naphthalene solvatochromic fluorophores related to Prodan are described. Two fluorophores were selected for functionalization and bioconjugation studies. The skeletons were chemically modified to include reactive functional groups and showed minimal alteration of the optical properties when compared to the parent dyes. The functionalized fluorophores were covalently attached to the carboxyl group of a fatty acid, and azido- and thiol-containing amino acids, demonstrating their potential for labeling biomolecules.

Local solvent acidities in β-cyclodextrin complexes with PRODAN derivatives

The local solvent acidities (SA scale) of six 6-carbonyl-2-aminonaphthalene derivatives as β-cyclodextrin complexes in water are determined through fluorescence quenching. The local polarities (E(T)(N) scale) are determined through the shift of the emission center-of-mass. The apparent SA values reflect the solvent structure surrounding the guest’s carbonyl group, whereas the apparent E(T)(N) values reveal the net polarity of the entire guest molecule. Comparison of these values affords greater insight into the structures of the host–guest complexes. Derivatives 1 and 5 show unusually large acidities, indicative of highly exposed carbonyl groups. The remaining compounds give emission intensities pointing to shielded carbonyl groups. In this study, PRODAN and its derivatives are functioning as dual channel sensors of their local environment.

Carbonyl-twisted 6-acyl-2-dialkylaminonaphthalenes as solvent acidity sensors

Derivatives of 2-propionyl-6-dimethylaminonaphthalene (PRODAN) with twisted carbonyl groups were investigated as highly responsive sensors of H-bond donating ability. The PRODAN derivative bearing a pivaloyl group (4) was prepared. The torsion angle between the carbonyl and naphthalene is 26° in the crystal. It shows solvatochromism that is similar to five other PRODAN derivatives (1-3, 5, 6). Twisted-carbonyl derivatives 3, 4, and 6 show strong fluorescence quenching in protic solvents. The order of magnitude of the quenching is linearly related to the H-bond donating ability of the solvent (SA) but not to other solvent properties. Binary mixtures of protic solvents show specific interaction effects with respect to quenching and solvatochromism. Aggregation in water is an issue with the pivaloyl derivatives.

Synthesis and Properties of Two PRODAN-based Fluorescent Models of Cholesterol

The syntheses and photophysical properties of 1-(5-methylhexyl)-2,3,7,8-tetrahydro-1H-naphtho[2,1-e]indol-9(6H)-one (7a) and 1-(5-methylhexyl)-2,3,8,9-tetrahydro-1H-naphtho[2,1-e]indol-6(7H)-one (7b) are reported. They are prepared in eight steps from the corresponding bromonaphthylamines. These fluorescent compounds have PRODAN-like cores, and they are structurally similar to cholesterol. Compound 7a is the first reported PRODAN derivative where both the amino and carbonyl groups are constrained to be coplanar with the naphthalene core. Comparing the photophysical behavior of these compounds with related compounds indicates that locking the amino group in a five-membered ring enhances their desirable properties as solvent polarity sensors.

Absorption and fluorescence of PRODAN in phospholipid bilayers: a combined quantum mechanics and classical molecular dynamics study

Absorption and fluorescence spectra of PRODAN (6-propionyl-2-dimethylaminonaphthalene) were studied by means of the time-dependent density functional theory and the algebraic diagrammatic construction method. The influence of environment, a phosphatidylcholine lipid bilayer and water, was taken into account employing a combination of quantum chemical calculations with empirical force-field molecular dynamics simulations. Additionally, experimental absorption and emission spectra of PRODAN were measured in cyclohexane, water, and lipid vesicles. Both planar and twisted configurations of the first excited state of PRODAN were taken into account. The twisted structure is stabilized in both water and a lipid bilayer, and should be considered as an emitting state in polar environments. Orientation of the excited dye in the lipid bilayer significantly depends on configuration. In the bilayer, the fluorescence spectrum can be regarded as a combination of emission from both planar and twisted structures.

Polarity-sensitive fluorescent probes in lipid bilayers: bridging spectroscopic behavior and microenvironment properties

We have studied the emission features of the fluorescent polarity-sensitive probes known as Prodan and Laurdan in a liquid-crystalline DPPC bilayer. To this purpose, we have combined high-level quantum mechanical electronic structure calculations with a molecular field theory for the positional-orientational-conformational distribution of the probes, in their ground and excited states, inside of the lipid bilayer, taking into account at both levels the nonuniformity and anisotropy of the environment. Thus, we can interpret the features of the fluorescence spectra of Prodan and Laurdan in relation to the position and orientation of their chromophore in the bilayer. We have found that the environment polarity is not sufficient to explain the large red shifts experimentally observed and that specific effects due to hydrogen bonding must be considered. We show that the orientation of the probe is important in determining the accessibility to water of the H-bond-acceptor group; in the case of Laurdan interesting conformational effects are highlighted.

What is solvatochromism?

Solvatochromism is commonly used in many fields of chemical and biological research to study bulk and local polarity in macrosystems (membranes, etc.), or even the conformation and binding of proteins. Despite its wide use, solvatochromism still remains a largely unknown phenomenon due to the extremely complex coupling of many different interactions and dynamical processes which characterize it. In this study we analyze the influence of different solvents on the photophysical properties of selected charge-transfer probes (4-AP, PRODAN, and FR0). The purpose is to achieve a microscopic understanding of the intermolecular effects which govern the absorption and fluorescence properties of solvated molecular probes, such as solvent-induced structural modifications, polarization effects, solubility, solute-solvent hydrogen-bonding interactions, and solute aggregation. To this aim we have exploited a time dependent density functional theory (TDDFT) approach coupled to complementary solvation approaches (continuum, discrete and mixed discrete and continuum). Such an integration has allowed us to clearly disentangle the complex interplay between specific and nonspecific interactions of the solvent with the probes and show that strong H-bonding effects not only can lead to large solvatochromic shifts but also can affect the nature of the emitting species with resulting reduction of the quantum yield.

Does PRODAN possess an O-TICT excited state? Synthesis and properties of two constrained derivatives

The synthesis and photophysical properties of 7-(dimethylamino)-3,4-dihydrophenanthren-1(2H)-one (7) and 3-(dimethylamino)-8,9,10,11-tetrahydro-7H-cyclohepta[a]naphthalen-7-one (8) are reported. These compounds possess a cycloalkanone substructure that controls the extent of twisting of the carbonyl group. The six-membered ring in 7 forces the carbonyl group to be coplanar with the naphthalene ring, whereas the seven-membered ring in 8 induces a significant twist. Both have the substructure of PRODAN (6-propionyl-2-(dimethylamino)naphthalene, 1). Comparing the photophysical behavior of these compounds with that of PRODAN and 2,2-dimethyl-1-(4-methyl-1,2,3,4-tetrahydrobenzo[f]quinolin-8-yl)propan-1-one (3) indicates that PRODAN likely emits from a PICT excited state rather than from an O-TICT excited state.

Genetic incorporation of a small, environmentally sensitive, fluorescent probe into proteins in Saccharomyces cerevisiae

Here, we report that the fluorescent amino acid, 3-(6-acetylnaphthalen-2-ylamino)-2-aminopropanoic acid (Anap), can be genetically incorporated into proteins in yeast with excellent selectivity and efficiency by means of an orthogonal tRNA/aminoacyl-tRNA synthetase pair. This small, environmentally sensitive fluorophore was site-specifically incorporated into Escherichia coli glutamine binding protein and used to directly probe local structural changes caused by ligand binding. The small size of Anap and the ability to introduce it by simple mutagenesis at defined sites in the proteome make it a useful local probe of protein structure, molecular interactions, protein folding, and localization.

Spectral heterogeneity of PRODAN fluorescence in isotropic solvents revealed by multivariate photokinetic analysis

This paper describes a multivariate analysis of the fluorescence emission of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) in a series of isotropic solvents of differing polarity and hydrogen-bonding ability. Multivariate methods distill the essential features from spectral data matrices so that the structural details that are embedded within the data are revealed to the analyst. In the aprotic solvents investigated, the analysis reveals a pair of emission components that have emission maxima that scale with the orientational polarizability. In the alcohols, short-lived, polarity-independent blue bands tentatively attributed to neutral hydrogen-bonded solute-solvent complexes form and relax prior to emission from paired bands that have Stokes shifts that scale with the solvent hydrogen-bonding ability rather than the polarity. In water, the short-lived blue bands were not observed, but the shift in the paired bands did scale with the solvent hydrogen-bonding ability.

Solvent induced shifts of electronic spectra IV. Computational study on PRODAN fluorescence and implications to the excited state structure

Vertical S(1)-S(0) electronic transitions of the highly solvent-sensitive fluorescence label 2-propionyl-6-dimethylamino naphthalene (PRODAN) are modeled by semiempirical CISD AM1 and TD DFT calculations in a large number of solvents of different polarity and hydrogen donating ability. Calculations correctly reproduce the observed solvent induced shifts of the emission maxima. The fluorescence Frank-Condon transition energies in solvent can be predicted quantitatively at the AM1 SM5.42 OPEN(2,2) C.I.=5 CISD level. For the planar PRODAN emitting state at the latter level we obtain a regression with practically unit slope and zero intercept for aprotic solvents. The respective relationship for the O-twisted S(1) state has a slope of 0.59 and intercept of 9100 cm(-1). These results support the concept that no geometry twist in the S(1) state of PRODAN is necessary to explain the observed solvent effects on fluorescence.

Photokinetic analysis of PRODAN and LAURDAN in large unilamellar vesicles from multivariate frequency-domain fluorescence

This paper describes a multivariate photokinetic analysis of the membrane phase dependence of PRODAN and LAURDAN photokinetics in DMPC vesicles. Decay data, arranged in the form of Fourier transformed emission-decay matrices (FT-EDMs), were collected as a function of temperature around the gel phase transition temperature. Each matrix was partitioned into the emission spectra and decay profiles of the underlying emission components using methods based on principal components analysis. The analysis revealed that both probes typically emit at least three spectral components, which vary in intensity as the membrane undergoes gel to liquid-crystalline phase transitions: a locally excited species (lambda max approximately 415 nm), a charge-transfer species (lambda max approximately 435 nm), and a solvent relaxed species (lambda max approximately 490 nm). In contrast to previous reports, the most red-shifted species is not photoexcited, but evolves from the locally excited species and does not exhibit the dynamic Stokes' shifts associated with conventional solvent relaxation. The primary difference in the emission of the two probes is the prominence of the charge-transfer species in the LAURDAN emission.

Long-wavelength analogue of PRODAN: synthesis and properties of Anthradan, a fluorophore with a 2,6-donor-acceptor anthracene structure

We have synthesized the environment-sensitive fluorophores 2-cyano-6-dihexylaminoanthracene and 2-propionyl-6-dihexylaminoanthracene (Anthradan) starting from 2,6-diaminoanthraquinone. Anthradan is the benzologue of the well-known family of naphthalene 2-propionyl-6-dimethylaminonaphthalene (PRODAN) fluorophores. The additional spectral red shift of the anthracene avoids the autofluorescence of many biological systems and provides for more favorable excitation wavelengths for fluorescence applications. Furthermore, Anthradan exhibits polarity-sensitive emission comparable to that of PRODAN and displays high quantum yields in a range of solvents. Single molecules of these anthracene-containing fluorophores have been imaged in polymer hosts as a proof-of-principle.

New Insights on the Photophysical Behavior of PRODAN in Anionic and Cationic Reverse Micelles: From Which State or States Does It Emit?

6-propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe because of its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer states (ICT) that can be particularly useful as a sensor. The nature of the emissive states has not yet been established despite the detailed experimental and theoretical investigations done on this fluorophore. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES) and time-resolved area normalized emission (TRANES) spectroscopies on the molecular probe PRODAN in the anionic water/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane and the cationic water/benzyl-n-hexadecyl dimethylammonium chloride (BHDC)/benzene reverse micelles (RMs). The experiments were done by varying the surfactant concentrations at a fixed molar ratio (W = [H2O]/[Surfactant]) and changing the water content at a constant surfactant concentration. The results obtained varying the surfactant concentration at W = 0 show a bathochromic shift and an increase in the intensity of the PRODAN emission band due to the PRODAN partition process between the external solvent and the RMs interface. The partition constants, Kp, are quantified from the changes in the PRODAN emission spectra and the steady-state anisotropy (<r>) with the surfactant concentration in both RMs. The Kp value is larger in the BHDC than the AOT RMs, probably due to the interaction between the cationic polar head of the surfactant and the aromatic ring of PRODAN. The partition process is confirmed with the TRES experiments, where the data fit to a continuous model, and with the time-resolved area normalized emission spectroscopy (TRANES) spectra, where only one isoemissive point is detected. On the other hand, the emission spectra at W = 10 and 20 show a dual fluorescence with a new band that emerges in the low-energy region of the spectra, a band that was previously assigned to the PRODAN emission from the water pool of RMs. Our studies demonstrate that this band is due to the emission from an ICT state of the molecular probe PRODAN located at the interface of the RMs. These results are also confirmed by the lifetime measurements, the TRES experiments where the results fit to a two-state model, and the time-resolved area normalized emission spectroscopy (TRANES) spectra where three or two isoemissive points are detected in the AOT and BHDC RMs, respectively. In the AOT RMs, Kp values obtained at W = 10 and 20 are almost independent of the water content; the values are higher for the BHDC RMs due to the higher micropolarity of this interface.

Ultrafast relaxation and coherent oscillations in aminobenzonitriles in the gas phase probed by intense-field ionization

4-Aminobenzonitrile derivatives have two excited states of similar energy: besides the benzene-like L(b) state (also termed "locally excited" or LE state) one with charge-transfer (CT) character that is slightly higher in the isolated molecules. The CT state can be lowered by solvents of suitable polarity, so that dual fluorescence can be observed in them. It is controversial along which coordinate this state is displaced, although the amino-group twist is a wide-spread assumption. We investigated a number of such compounds by transient ionization in the gas phase, initially exciting the higher-lying L(a) state (S(2)). Here we briefly review the previous results on 4-(dimethylamino)benzonitrile (the prototype of this class of molecules), 4-piperidino-, pyrrolidino- and pyrrolyl-benzonitrile and compare them with new results on 4-aminobenzonitrile and on the bridged derivative N-methyl-6-cyano-1,2,3,4-tetrahydroquinoline (NMC6). Although in the latter two molecules the CT state has never been detected before, we find the same relaxation path for all compounds: From S(2), the wave packet passes through a conical intersection (CI); from there part of it reaches the S(1) (L(b)) state directly, whereas another part temporarily populates the CT state (also in NMC6), from where it goes around the CI also to the L(b) well. The wave packet directly reaching the L(b) well oscillates there along coordinates involving amino-group twist and wagging or molecular arching and a quinoidal distortion. These coordinates must be components of the CI displacement vector. A vibration involving bond-length alternation of the benzene ring is ascribed to a momentum caused by the electronic symmetry change in the CI, i.e., to the nonadiabatic coupling vector. Also the CT state involves amino-group twist, as to conclude from the anisotropy of the corresponding signal. The six-membered aliphatic ring in NMC6 hinders the twist and raises the CT state to an energy that is, however, still below the L(a) state, so that it can be temporarily populated in a barrierless process. Also in aminobenzonitrile the CT state is between L(a) and L(b) and is reached from L(a) without a barrier. The twist is rationalized by vibronic interaction with a higher state that is pi-antibonding between the amino group and the aromatic ring.

Extreme Fluorescence Sensitivity of Some Aniline Derivatives to Aqueous and Nonaqueous Environments: Mechanistic Study and Its Implication as a Fluorescent Probe

Effects of solvent water on the photophysical properties of a series of meta- and para-substituted anilines have been investigated by means of time-resolved fluorescence, transient absorption, and photoacoustic measurements. Some aniline derivatives exhibit extremely short fluorescence lifetime (tau(f)) and small quantum yield (Phi(f)) in water (e.g., tau(f) = 45 ps and Phi(f) = 0.0019 for m-cyanoaniline (m-ANCN) in H(2)O), which is in marked contrast with their much larger values in nonaqueous solvents (tau(f) = 7.3 ns and Phi(f) = 0.14 for m-ANCN in acetonitrile). Photoacoustic and transient absorption measurements show that the remarkable fluorescence quenching of m-ANCN in water is attributed almost exclusively to fast internal conversion. The lifetime measurements of m-ANCN in H(2)O/acetonitrile binary solvent mixtures reveal that the quenching is related to variation of hydrogen-bonding interactions between the amino group and water molecules and the conformational change of the amino group upon electronic excitation. Similar fluorescence quenching due to solvent water is also found for N-alkylated m-ANCNs. The drastic differences in the fluorescence intensity and lifetime of m-ANCNs under hydrophobic and hydrophilic environments and also the large solvent polarity dependence of the fluorescence band position suggest the possibility that they can be utilized as fluorescent probes for investigating the microenvironment of biological systems. In suspensions of human serum albumin (HSA) in water, remarkable enhancement of the fluorescence intensity and lifetime is observed for m-ANCN and its N-alkylated derivatives, demonstrating that m-ANCNs can be a candidate for novel fluorescent probe with small molecular size.