New insights on the behavior of PRODAN in homogeneous media and in large unilamellar vesicles

The behavior of 6-propionyl-2-dimethylaminonaphthalene (PRODAN) was studied in homogeneous media and in large unilamellar vesicles (LUVs) of the phospholipid 1,2-di-oleoyl-sn-glycero-3-phosphatidylcholine (DOPC), using absorption, emission, depolarization, and time-resolved spectroscopies. In homogeneous media, the Kamlet and Taft solvatochromic comparison method quantified solute-solvent interactions from the absorption and emission PRODAN bands. These studies demonstrate that the absorption band is sensitive to the polarity-polarizability (pi) and the hydrogen bond donor ability (alpha) parameters of the media. PRODAN in the excited state is even more sensitive to these parameters and to the hydrogen bond acceptor ability (beta) of the media. The transition energy (expressed in kcal/mol) for both absorption and emission bands gives a linear correlation with the well-known polarity parameter E(T30). The results from the absorption and emission bands also reveal that PRODAN aggregates in water. The monomer has two fluorescence lifetimes, 2.27 and 0.65 ns, while the aggregate has a lifetime of 14.6 ns. Using steady-state anisotropy measurements, the calculated volumes of the aggregate and the monomer are 5590 and 222 mL mol(-1), respectively. In DOPC LUVs, PRODAN undergoes a partition process between the water bulk and the DOPC bilayer. We show that the partition constant (K(p)) value is large enough that only at [DOPC] below 0.15 mg/mL PRODAN in water can be detected. PRODAN dissolved in LUVs at [DOPC] > 1 mg/mL exists completely incorporated in its monomer form and senses two different microenvironments within the bilayer: a polar region in the interface near the water and a less polar and also less viscous environment, between the phospholipid tails. These environments were characterized by their fluorescence lifetimes (tau), showing that PRODAN in the polar microenvironment has a tau value of approximately 4 ns while in the less polar region gives a value of 1.2 ns. Moreover, this probe also senses the micropolarity of these two different regions of the bilayer and yields values similar to that of methanol and tetrahydrofuran.     

A PHOTOPHYSICAL STUDY OF SOLVATOCHROMIC PROBE 6-PROPIONYL-Z(N,N-DIMETHYLAMINO)NAPHTHALENE (PRODAN) IN SOLUTION

A photophysical study of 6-propionyl-2-( N,N -dimethylamino)naphthalene (PRODAN) in room-temperature solutions under various conditions is reported. The results show no unusual photophysical properties, except for an extremely large solvatochromic shift of PRODAN fluorescence spectrum. The previously reported extra blue emission band for PRODAN in an aqueous solution is identified to be due entirely to trace water-soluble impurities in the sample. The excited-state dipole moment of PRODAN is determined based on solvatochromic results using a known twisted intramolecular charge transfer (TICT) molecule p -( N,N -diethylamino)ethylbenzoate (DEAEB) as a reference to probe specific solute-solvent interactions. The possibility of TICT state formation in PRODAN is discussed, and a solvation equilibrium mechanism is proposed to account for the photophysical behavior of PRODAN.     

Dual fluorescence of diphenyl carbazide and benzanilide: effect of solvents and pH on electronic spectra

The absorption and fluorescence spectra of benzanilide (BA) and diphenyl carbazide (DPC) in solvents of different polarities and pH have been analysed. The spectral characteristics of DPC and BA are compared with diphenyl amine molecule. In water and methanol, a dual fluorescence is observed for both DPC and BA molecules. The normal stokes shifted emission originates from a locally excited pi* electronic state and the large stokes shifted band is due to emission from a twisted intramolecular charge transfer (TICT) state. pH studies show that both monocations and monoanions are non-fluorescent. The excited state acidity constants determined by fluorimetric titration and F?rster cycle methods, have been reported and discussed.     

Synthesis and photophysical properties of models for twisted PRODAN and dimethylaminonaphthonitrile

The synthesis and photophysical properties of 7-cyano-3,4-dihydro-2H-1,4-ethano-benzo[g]quinoline and 3,4-dihydro-2H-1,4-ethano-7-propionyl-benzo[g]quinoline are reported. These compounds possess a quinuclidine substructure that locks the tertiary amino group perpendicular to the naphthalene ring. Their excited states are models for the twisted excited states of 2-(dimethylamino)-6-naphthonitrile (DMANN) and 6-propionyl-2-(dimethylamino)naphthlene (PRODAN). In contrast to DMANN and PRODAN, the fluorescence of these twisted derivatives is strongly deactivated in polar solvents. Neither DMANN nor PRODAN likely emit from TICT excited states.     

An example of how to use AOT reverse micelle interfaces to control a photoinduced intramolecular charge-transfer process

6-Propionyl-2-(N,N-dimethyl)aminonaphtahalene, PRODAN, is widely used as a fluorescent molecular probe due to its significant Stokes shift in polar solvents. It is an aromatic compound with intramolecular charge-transfer (ICT) states which can be particularly useful as sensors. In this work, we performed absorption, steady-state, time-resolved fluorescence (TRES), and time-resolved area normalized emission (TRANES) spectroscopies on PRODAN dissolved in nonaqueous reverse micelles. The reverse micelles are composed of polar solvents/sodium 1,4-bis-2-ethylhexylsulfosuccinate (AOT)/n-heptane. Sequestered polar solvents included ethylene glycol (EG), propylene glycol (PG), glycerol (GY), formamide (FA), dimethylformamide (DMF), and dimethylacetamide (DMA). The experiments were performed with varying surfactant concentrations at a fixed molar ratio W(S) = [polar solvent]/[AOT]. In every reverse micelle studied, the results show that PRODAN undergoes a partition process between the external solvent and the reverse micelle interface. The partition constants, K(p), are quantified from the changes in the PRODAN emission and/or absorption spectra with the surfactant concentration. The K(p) values depend strongly on the encapsulated polar solvent and correlate quite well with the AOT reverse micelle interface's zones where PRODAN can exist and emits. Thus, the partition toward the reverse micelle interface is strongly favored in DMF and DMA containing micelles where the PRODAN emission comes only from an ICT state. For GY/AOT reverse micelles, the K(p) value is the lowest and only emission from the local excited (LE) state is observed. On the other hand, for EG/AOT, PG/AOT, and water/AOT reverse micelles, the K(p) values are practically the same and emission from both states (LE and ICT) is simultaneously detected. We show here that it is possible to control the PRODAN state emission by simply changing the properties of the AOT reverse micelle interfaces by choosing the appropriate polar solvent to make the reverse micelle media. Indeed, we present experimental evidence with the answer to the long time question about from which state does PRODAN emit, a process that can be controlled using the unique reverse micelle interfaces properties.     

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 () 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.     

The crystal structure and vibrational spectra of two molecules emitting dual fluorescence: 4-(1H-Pyrrol-1-yl)benzonitrile (PBN) and 5-cyano-2-(1pyrrolyl)-pyridine (CPP)

Crystal structures and vibrational spectra are reported for the two title molecules which exhibit dual fluorescence due to the existence of a low lying charge transfer excited state. The data show that in the ground state PBN is twisted whereas CPP is planar, and the crystal structures are quite different. The experimental spectra are in very good agreement with quantum mechanical calculations, which also predict considerable differences between the vibrational spectra of CPP in the ground state and in the charge transfer excited state.     

Theoretical Study of the Electronic Excited States and Fluorescence Spectra of Squaraine in Solution

Bis[4-(dimethylamino)phenyl]squaraine (SQ-DMA) has been used as a long wavelength fluorescence dye. In spite of various experimental and theoretical studies, its excited state properties and the relaxation mechanism have not been elucidated. In this work, we tried to clarify these points from a theoretical point of view. The heats of reaction from the planar to possible twisted conformers in the first excited state S₁ in solvents were calculated to be significantly endothermic, thus the twisted structures turn out to be less important, which contradicts earlier proposals made in experimental studies. This behavior is in a sharp contrast with that of the related molecule 4-(N,N-dimethylamino)benzonitrile, and is explained by the difference in their electronic characters of their relevant excited states; the S₁ state of SQ-DMA is a simple HOMO-to-LUMO excited state with a delocalized character. Furthermore, the theoretically simulated absorption and fluorescence spectra with the planar structure of SQ-DMA are in good agreement with the corresponding experimental results. These results suggest that the responsible S₁ state is the lowest ¹B_1u state with a planar D_2h structure.     

Theoretical study of solvent influence on the electronic absorption and emission spectra of kynurenine

Neutral/zwitterionic form equilibrium, excited state wave functions, absorption and emission spectra of kynurenine (KN) in various solvents (water, methanol, ethanol, and dimethylsulfoxide) have been studied theoretically. The ground electronic state geometries have been optimized by density functional theory methods; the geometries of the first two singlets excited electronic states have been optimized using the CASSCF technique. The influence of the solvent was taken into account by the calculation of the solvation free energies using the Polarizable Continuum Model (PCM). The spectra of electronic absorption and fluorescence emission have been calculated by the CS‐INDO S‐CI and SDT‐CI methods [Momicchioli, Baraldi, and Bruni, Chem Phys, 1983, 82, 229]. The calculated data reproduce the experimental positions of maxima and the solvent‐induced shifts of the absorption and emission bands well. The energy gap between the two lowest excited states of KN increases from aprotic to protic solvents. This fact suggests that the “proximity effect” cannot be responsible for the ultrafast decay of KN fluorescence in protic solvents. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Excited state intramolecular charge transfer in N,N-heterocyclic-4-aminobenzonitriles: a DFT study

The excited state intramolecular charge transfer (ICT) reaction in a series of N,N-heterocyclic 4-aminobenzonitriles is investigated theoretically by a combination of density functional theory and multi-reference configuration interaction (DFT/MRCI). Experimentally, increasing ICT emission is observed with increasing ring size. Formation of both a planar and twisted ICT (PICT and TICT) state are energetically unfavorable in the small systems due to high inversion barriers. With increasing ring size, the TICT state is more stabilized than the PICT state. A good agreement of the computed TICT state dipole moment is found with experimental values. The red-shifted fluorescence of all systems is explained by the TICT model due to both arguments.     

How the environment controls absorption and fluorescence spectra of PRODAN: a quantum-mechanical study in homogeneous and heterogeneous media

The spectroscopic behavior of 6-propionyl-2-(N,N-dimethyl)aminonaphthalene (PRODAN) is investigated in different environments, ranging from homogeneous solutions of different polarities to diffuse interfaces mimicking membranes. The variety of experimental data as well as computational results present in the literature still do not clarify the nature of the emission process; in particular, it is not well-established whether fluorescence in such a molecule occurs from a planar or from a twisted intramolecular charge transfer state. The first part of the work is thus devoted to better understand how the electronic transition processes occur in homogeneous solvents. The effect of the medium polarity as well as the hydrogen bond formation are studied. In the second part of the paper, a first attempt to interpret the experimental results of PRODAN in unilamellar vesicles is carried out. The complexity of the still-open questions about the photophysics of PRODAN has prompted us to base the study on quantum-mechanical calculations performed at various levels of theory, namely, DFT, TDDFT, CIS, and SAC-CI, and to include the effects of the environment in a self-consistent way. This is achieved by using the integral equation formalism version of the polarizable continuum model (IEFPCM). IEFPCM is a quite versatile approach, being able to treat equilibrium and nonequilibrium solvation in both homogeneous and heterogeneous media.     

The equilibria and conversions between three excited states: the LE state and two charge transfer states, in twisted pyrene-substituted tridurylboranes

Excited-state conversions were observed from a series of twisted pyrene-substituted tridurylboranes, corresponding to a locally excited (LE) state, a more planar charge transfer (CT) state with a higher fluorescence quantum efficiency, and a more twisted CT state.     

Impact of intramolecular twisting and exciton migration on emission efficiency of multifunctional fluorene-benzothiadiazole-carbazole compounds

Novel donor-acceptor compounds consisting of singly bonded fluorene (Fl), benzothiadiazole (BT), and carbazole (Cz) functional units in the same molecule were investigated. Analysis of the optical spectra and fluorescence transients of the compounds revealed the domination of intramolecular charge transfer (ICT) states with high fluorescence quantum yield (72%-85%). A similar Cz-Fl-Cz compound exhibiting 100% fluorescence quantum yield and no ICT character was also studied as a reference to reveal the impact of electron-accepting BT groups. Thorough examination of the optical properties of the compounds in different media, i.e., dilute solution and polymer matrix, indicated their twisted conformations due to steric hindrance in the ground state and flattened geometry in the excited state for both reference and ICT compounds. Remarkable fluorescence efficiency losses (amounting to 70%) observed upon casting the molecular solutions into neat films were determined to originate from the low-fluorescent twisted conformers and migration-facilitated exciton quenching. The majority of emission efficiency losses (over 70%) were caused by the twisted conformers, whereas only less than 30% by exciton-migration-induced nonradiative deactivation.     

Excited-state dynamics of phenol-pyridinium biaryl

The excited-state dynamics of a donor-acceptor phenol-pyridinium biaryl cation was investigated in various solvents by femtosecond transient absorption spectroscopy and temperature dependent steady-state emission measurements. After excitation to a near-planar Franck-Condon delocalized excited S(1)(DE) state with mesomeric character, three fast relaxation processes are well resolved: solvation, intramolecular rearrangement leading to a twisted charge-shift (CSh) S(1) state with localized character, and excited-state proton transfer (ESPT) to the solvent leading to the phenoxide-pyridinium zwitterion. The proton transfer kinetics depends on the proton accepting character of the solvent whereas the interring torsional kinetics depends on the solvent polarity and viscosity. In nitriles, ESPT does not occur and interring twisting arises with no significant intrinsic barrier, but still slower than solvation. The CSh state is notably fluorescent. In alcohols and water, ESPT is faster than the solvation and DE → CSh relaxation processes and yields the zwitterion hot ground state, which strongly quenches the fluorescence. In THF, solvation and interring twisting occur first, leading to the fully relaxed, weakly fluorescent CSh state, followed by slow ESPT towards the zwitterion. At low temperature (77 K), the large viscous barrier of the solvent inhibits the torsional relaxation but ESPT still arises to some extent. Strong emission from the DE geometry and planar zwitterion is thus observed. Finally, quantum chemical calculations were performed on the ground and excited state of model phenol-pyridinium and phenoxide-pyridinium compounds. Strong S(1) state energy stabilization is predicted upon twisting in both cases, consistent with a fast relaxation towards the perpendicular geometry. A substantial S(0)-S(1) energy gap is still present for the twisted cationic species, which can explain the long-lived emission of the CSh state in nitriles. A quite different situation arises with the zwitterion for which the S(0)-S(1) energy gap predicted at the twisted geometry is very small. This suggests a close-lying conical intersection and can account for the strong fluorescence quenching observed in solvents where the zwitterion is produced by ESPT.     

Solvent effects on the absorption and fluorescence spectra of coumarins 6 and 7 molecules: determination of ground and excited state dipole moment

Absorption and fluorescence emission spectra of coumarins 6 and 7 were recorded in solvents with different solvent parameters, viz., dielectric constant epsilon and refractive index n. The fluorescence lifetime of these dyes were measured in butanol at higher values of viscosity over temperature. Experimental ground and excited state dipole moments are determined by means of solvatochromic shift method and also the excited state dipole moments are determined in combination with ground state dipole moments. It was determined that dipole moments of the excited state were higher than those of the ground state in both the molecules.     

Resolved fluorescence emission spectra of PRODAN in ethanol/buffer solvents

The fluorescence steady-state emission spectra of lipophilic fluorescence probe PRODAN in ethanol/buffer solvents of different concentrations (0.3, 0.9, 3 mol L(-1) ethanol) were extensively studied and analytically described. The complex experimental spectra, corrected for background effects, were fitted by two Gaussian curves. The energy separation of two maxima, (0.147+/-0.002) eV at 37 degrees C and (0.143+/-0.003) eV at 25 degrees C, was independent of ethanol concentration. The blue shifts observed for both maxima were linearly dependent on solvent polarity. The linear dependences of fluorescence's intensities on PRODAN concentration in all ethanol/buffer solvents indicate that no PRODAN self-quenching takes place even at the highest measured PRODAN concentrations.     

Color-tuning mechanism in firefly luminescence: theoretical studies on fluorescence of oxyluciferin in aqueous solution using time dependent density functional theory

The first singlet excited state geometries of various isomers and tautomers of firefly oxyluciferin (OxyLH2), as well as their fluorescence spectra in aqueous solution, were studied using time dependent density functional theory (TDDFT). With changing pH in aqueous solution, three fluorescence peaks, blue (450 nm), yellow-green(560 nm), and red (620 nm) correspond to neutral keto and enolic forms, the monoanionic enolic form,and the monocationic keto form respectively. A counterion, Na+, was predicted to cause a blue shift in the fluorescence of anionic OxyLH2. The contributions of a charge transfer (CT) state upon electronic excitation of the planar and twisted structures were predicted. CT was large for the twisted structures but small for the planar ones. The differences between pK and pK* of various oxyluciferin species were predicted using a Forster cycle. A new possible light emitter, namely, the monocation keto form (keto+1), was considered.     

Phase transition affects energy transfer efficiency in phospholipid vesicles

The fluorescence quenching of 6-propionyl-2-dimethylaminonaphtalene (PRODAN) and 6-dodecanoyl-2-dimethylaminonaphtalene (LAURDAN) by octadecyl rhodamine B (ORB) in a model system of small unilamellar vesicles (SUV) of dipalmitoylphosphatidyl-choline (DPPC) was investigated. Non-linear Stern-Volmer behaviour was observed in both systems in the gel phase (25 degrees C) and in the fluid phase (50 degrees C), resulting from association processes and from static quenching. The relative quenching efficiencies of both dyes depend on the phase state of the bilayer and indicate a deeper incorporation of PRODAN and LAURDAN into the membrane in its fluid phase than in its gel phase.     

Low-lying excited singlet states and S2→S0 luminescence of dipyrido[3,4-b: 2,3-d]phenazine

The absorption, fluorescence and excitation fluorescence spectra dipyrido[3,4-b:2,3-d]-phenazine (DPPZ1) have been measured in non-polar and polar matrices at room temperature, and were taken into account to explain the origin of the relatively weak emission of this molecule in both type of environment. The electronic structure of DPPZ1 was calculated using a modified INDO CI method. The geometry optimization has been performed using the MNDO method. According to the spectra and the results of calculations, the lowest excited singlet state S₁ of DPPZ1 molecule is of n,π^*-type and the next one, S₂ state, is of π,π^*-type. The energy gap ΔE_calc is equal 4770 cm⁻¹. The low efficiency of the emission observed in the hydroxylic solvent can be interpreted in terms of thermal quenching of the π,π^*-type fluorescence. However, experimental results obtained suggest that in nonpolar solvents the emission of the molecule examined is an anomalous S₂→S₀ fluorescence.     

Synthesis and fluorescence characterization of MEHPPV oligomers

Trimer, tetramer, and pentamer oligomers based on the polymer backbone structure of poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) have been synthesized by Horner-Wadsworth-Emmons reactions. The fluorescence spectra, emission quantum yields, and lifetimes of the oligomers have been characterized in dilute chloroform solutions. The oligomers exhibit a sequential increase in absorption and emission wavelength maxima and a decrease in fluorescence lifetime as the π conjugation length is increased. The shortening in excited state lifetime is shown to be due to an increase in the rates of both radiative and nonradiative processes. The absence of a mirror-image relationship for the absorption and fluorescence spectra of the oligomers is attributed to the photoexcitation of a range of torsional configurations followed by relaxation to a more planar arrangement that then emits.