Zerner's Intermediate Neglect Of Differential Overlap Research Articles

Advances, challenges and perspectives of quantum chemical approaches in molecular spectroscopy of the condensed phase.

The purpose of this review is to demonstrate advances, challenges and perspectives of quantum chemical approaches in molecular spectroscopy of the condensed phase. Molecular spectroscopy, particularly vibrational spectroscopy and electronic spectroscopy, has been used extensively for a wide range of areas of chemical sciences and materials science as well as nano- and biosciences because it provides valuable information about structure, functions, and reactions of molecules. In the meantime, quantum chemical approaches play crucial roles in the spectral analysis. They also yield important knowledge about molecular and electronic structures as well as electronic transitions. The combination of spectroscopic approaches and quantum chemical calculations is a powerful tool for science, in general. Thus, our article, which treats various spectroscopy and quantum chemical approaches, should have strong implications in the wider scientific community. This review covers a wide area of molecular spectroscopy from far-ultraviolet (FUV, 120-200 nm) to far-infrared (FIR, 400-10 cm-1)/terahertz and Raman spectroscopy. As quantum chemical approaches, we introduce several anharmonic approaches such as vibrational self-consistent field (VSCF) and the combination of periodic harmonic calculations with anharmonic corrections based on finite models, grid-based techniques like the Numerov approach, the Cartesian coordinate tensor transfer (CCT) method, Symmetry-Adapted Cluster Configuration-Interaction (SAC-CI), and the ZINDO (Semi-empirical calculations at Zerner's Intermediate Neglect of Differential Overlap). One can use anharmonic approaches and grid-based approaches for both infrared (IR) and near-infrared (NIR) spectroscopy, while CCT methods are employed for Raman, Raman optical activity (ROA), FIR/terahertz and low-frequency Raman spectroscopy. Therefore, this review overviews cross relations between molecular spectroscopy and quantum chemical approaches, and provides various kinds of close-reality advanced spectral simulation for condensed phases.

Structural, electrochemical and optical properties of 4′-n-alkyl-4-cyanobiphenyl (nCB) – A computational approach

ABSTRACTThe structures of nCB (n = 6 & 7 where n is the number of carbon atoms in the alkyl chain) have been optimized using the Becke3-Lee-Yang-Parr (B3LYP) hybrid functional with 6–31G+(d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been computed using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the DFT method. The features of electronic transitions and excited states have been calculated via configuration interaction singles (CIS) with the semiempirical Hamiltonian Zerner intermediate neglect of differential overlap (ZINDO). The photo sensitivity of liquid crystalline alkyl cyanobiphenyl has been presented on ultraviolet (UV) absorption based approach through Density functional theory (DFT) calculations. The structural and electrochemical properties such as HOMO (H), LUMO (L), and energy gap (Eg = EL – EH) have been investigated. A comparison of dimers during the different modes of interactions suggests an absorption maxima at longer wavelength for 7CB, indicating the high photo sensitivity. Further, the 6CB dimers exhibit a lower band gap; hence its conductivity is high in comparison with the 7CB dimers.

Effect of Molecular Interactions and Homologue Number on UV Absorption Spectra of Liquid Crystalline Alkyl Cyanobiphenyl Dimers: DFT Calculations

The ultraviolet (UV) absorption spectra of the liquid crystalline dimer complexes of 4′-n-alkyl-4-cyanobiphenyls (nCB: n = 3, 4, 5 where n is the number of carbon atoms in the alkyl chain) have been presented. The nCB structures have been optimized using the density functional Becke3-Lee-Yang-Parr (B3LYP) hybrid functional with 6-31+G (d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been computed using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the density functional theory (DFT). The features of electronic transitions and excited states have been calculated via configuration interaction singles (CIS) with the semiempirical Hamiltonian Zerner intermediate neglect of differential overlap (ZINDO). The various modes of molecular interactions and the homologue number are found to be structural parameters affecting the formation of mesophases, UV absorption spectral characteristics, and photo stability of the compounds. These results offer a hint for the calculations involving the different modes of molecular interactions, and separations between dimers, to model photo stability or in tuning the absorbing chromophore to match the wavelength of desired application.

Effect of molecular interactions and end chain length on ultraviolet absorption behavior and photo stability of alkoxycinnamic acids: Theoretical models of liquid crystal

The ultraviolet (UV) absorption spectra of the liquid crystalline dimer complexes of 4-n-alkoxycinnamic acids (nOCNA: n=4, 6, 8, where n is the number of carbon atoms in the alkoxy chain) have been presented. The nOCNA structures have been optimized using the density functional Becke3–Lee–Yang–Parr (B3LYP) hybrid functional with 6-31+G (d) basis set using the crystallographic geometry as input. The electronic structures of the dimer molecules have been evaluated using the optimized geometries. The spectra of the dimer molecules have been calculated by employing the density functional theory (DFT), semiempirical complete neglect of differential overlap/spectroscopy (CNDO/S), and intermediate neglect of differential overlap/spectroscopy (INDO/S) parameterizations. The features of electronic transitions and excited states have been calculated via configuration interaction singles (CIS) with the semiempirical Hamiltonian Zerner intermediate neglect of differential overlap (ZINDO). The effect of molecular interactions and the end chain length may be reasonably useful for the progress and efforts in developing the photo-addressed color displays.

Theoretical study of two-photon absorption properties of a series of platinum (II) acetylide complexes

Transition metal acetylide compounds have attracted enormous attention because of the extraordinary photophysical properties arising from the electronic interaction between the transition metal and the organic molecular fragment. In this study, the geometrical structure, electronic structure, one-photon absorption (OPA) and two-photon absorption (TPA) properties of a series of D–π′–A–π–[Pt]–π–A–π′–D type and A′–π′–A–π–[Pt]–π–A–π′–A′ type platinum(II) acetylides were studied theoretically by using density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods for getting TPA materials possessing large TPA cross-section. Our analysis suggests that intramolecular charge transfer between center metal and π-conjugated organic fragment dominates in TPA transitions, in which metal center increases conjugation length in the direction of long ligand. This contribution provides detailed theoretical analysis of one- and two-photon absorption property of Platinum (II) acetylide compounds and an effective way for designing of platinum (II)/nickel (II) acetylide compounds possessing large TPA cross-section.

Understanding the electronic energy transfer pathways in the trimeric and hexameric aggregation state of cyanobacteria phycocyanin within the framework of Förster theory

In the present study, the electronic energy transfer pathways in trimeric and hexameric aggregation state of cyanobacteria C-phycocyanin (C-PC) were investigated in term of the Förster theory. The corresponding excited states and transition dipole moments of phycocyanobilins (PCBs) located into C-PC were examined by model chemistry in gas phase at time-dependent density functional theory (TDDFT), configuration interaction-singles (CIS), and Zerner's intermediate neglect of differential overlap (ZINDO) levels, respectively. Then, the long-range pigment-protein interactions were approximately taken into account by using polarizable continuum model (PCM) at TDDFT level to estimate the influence of protein environment on the preceding calculated physical quantities. The influence of the short-range interaction caused by aspartate residue nearby PCBs was examined as well. Only when the protonation of PCBs and its long- and short-range interactions were properly taken into account, the calculated energy transfer rates (1/K) in the framework of Förster model at TDDFT/B3LYP/6-31+G* level were in good agreement with the experimental results of C-PC monomer and trimer. Furthermore, the present calculated results suggested that the energy transfer pathway in C-PC monomer is predominant from β-155 to β-84 (1/K = 13.4 ps), however, from α-84 of one monomer to β-84 (1/K = 0.3-0.4 ps) in a neighbor monomer in C-PC trimer. In C-PC hexamer, an additional energy flow was predicted to be from β-155 (or α-84) in top trimer to adjacent β-155 (or α-84) (1/K = 0.5-2.7 ps) in bottom trimer.

Theoretical investigation of the two-photon absorption properties of 3,6-bis(4-vinylpyridinium) carbazole derivatives—new biological fluorescent probes

The present study focuses on a series of carbazole derivatives, which are being investigated as potential two-photon fluorescent probes (TFP) for DNA detection and two-photon bioimaging. The geometric structures, electronic structures, and the one-photon (OPA) and two-photon (TPA) absorption properties of 3,6-bis(1-methyl-4-vinylpyridinium) carbazole (BMVC) derivatives, as well as their dications and diiodized derivatives, were studied using density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) method. The results showed that the TPA spectra of the diiodized BMVC derivatives and their dications are all found in the near-infrared region (NIR). At the same time, the diiodized BMVC derivatives presented larger TPA cross-sections than the neutral BMVC derivatives and the dications. These theoretically derived values were also in good agreement with their corresponding experimentally observed values, and they indicated that the diiodized BMVC may be the form of this TFP that combines with DNA. The diiodized BMVC derivatives and the dications have the potential to be excellent TPA materials, especially when used as TFPs in nucleic acid imaging applications.

Theoretical Study on the Two-Photon Absorption Properties of Pyrazole Aluminum Compounds

An extensive series of pyrazole aluminum compounds containing an Al_2N_4 center as a pseudo-conjugated system were theoretically investigated for their one-photon absorption and two-photon absorption(OPA and TPA) properties by density functional theory(DFT) and Zerner's intermediate neglect of differential overlap(ZINDO) methods.The results indicate that pyrazole aluminum compounds are good TPA materials and that the TPA maximal absorption cross-section(δ_(max)) can reach 2860.1 GM(1 GM=10~(-50) cm~4·s·photon~(-1)).By incorporating electron-acceptors in the central core,aπ-conjugated bridge and terminal groups,the OPA and TPA properties can be modulated.This research provides strategies for the enhancement of molecular TPA in the target region.The origin of the largeδ_(max) of some of the studied molecules was determined using a three-level energy model.We conclude that an increase in the intramolecular charge transfer can enhanceδ_(max).Moreover,the pyrazole aluminum compounds behave in a similar manner to pyrazaboie chromophores in terms of linear optical,and TPA properties and they possess an increasedδ_(max) to some extent.

A comparative study of one- and two-photon absorption properties of pyrene and perylene diimide derivatives

Two important classes of organic molecules, perylene diimide (PDI) and pyrene derivatives have been found to possess relatively excellent photophysical and photochemical properties and especially high two-photon absorption cross sections (δ(T)(max)). Herein, one-photon absorption (OPA) and two-photon absorption (TPA) properties of some novel PDI and pyrene derivatives were comparatively investigated by the density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The calculated results indicate that with respect to PDI derivatives, the maximum TPA cross-sections for pyrene compounds increase obviously, the maximum peaks of OPA and TPA spectra are blue-shifted, the ΔE(H-L) (energy gaps between the highest occupied orbital and the lowest unoccupied orbital) increase. The different π-conjugated bridges (fluorene and pyrene) and terminal groups have slight effect on the OPA properties. Nevertheless, the molecules bearing 1,6-disubstituted pyrene as the π-conjugated bridge display the largest δ(T)(max) in both series of compounds 3 and 4. Moreover, the δ(T)(max) values of molecules with benzothiazole-substituted terminal groups are larger than those of the molecules with diphenylamine, which is attributed to benzothiazole groups stabilizing the planarity of the branch parts, extending the conjugated length and increasing the π-electron delocalized extent. Furthermore, the molecular size has marked effect on OPA and TPA properties. It is worthy to mention that cruciform 8 displays the largest δ(T)(max) among all the studied molecules in the range of 600-1100 nm. This research could provide a better understanding for the origin of the linear and nonlinear optical properties, and it would be helpful to gain more information about designing two-photon absorption materials with large δ(T)(max).

Star‐shaped Organic Molecules That Comprise a 1,3,5‐Trisubstituted Benzene Core and Three Oligoaryleneethynylene Arms as Light‐emitting Materials

AbstractStar‐shaped rigid molecules that comprise a 1,3,5‐trisubstitued benzene core and three oligoaryleneethynylene arms have great potential application in organic light‐emitting devices (OLEDs). Their optical and electronic properties are tuned by the star‐shaped molecular size. To reveal the relationship between the properties and structures, we perform a systemic investigation for these organic molecules. The ground and excited state molecules are studied using density functional theory (DFT), the ab initio HF, and the single excitation configuration interaction (CIS), respectively. And the electronic absorption and emission spectra are investigated with time‐dependent density functional theory (TDDFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials (IP), electron affinities (EA), absorption and emission spectra are controlled by the star‐shaped molecular size, which favor the hole and electron injection into OLEDs. With increasing the molecular conjugated length, the absorption and emission spectra exhibit red shifts to some extent and are in good agreement with the experimental ones. Also, the calculated emission spectra range from 330 to 440 nm. All the calculated show that the star‐shaped molecules are promising as blue light emitting materials

TORSIONAL POTENTIALS AND FULL-DIMENSIONAL SIMULATION OF ELECTRONIC ABSORPTION SPECTRA OF para-PHENYLENEVINYLENE OLIGOMERS USING SEMIEMPIRICAL HAMILTONIANS

A systematic study of torsional potential curves in electronic ground state based on second-order Møller–Plesset energy (MP2), density functional theory (DFT), and Austin mode 1 (AM1) methods is presented for para-phenylenevinylene oligomers constructed from two to four aromatic rings. The semiempirical AM1 approach gives the correct location of potential energy minima in comparison with the reference MP2 calculations and literature data. However, the semiempirical AM1 energy barriers at perpendicular orientation are ca. 30% smaller than the MP2 ones. The DFT calculations indicate optimal planar structures and the barriers are three times higher than MP2 values. Excited-state potential energy curves evaluated from vertical excitations at the time-dependent density functional theory (TD-DFT) and ab initio CI levels exhibit much steeper increase of values in the vicinity of perpendicular orientation than in the semiempirical Zerner's intermediate neglect of differential overlap (ZINDO) and ab initio RI-CC2 cases. The effects of vibrational motion of phenylene rings on the torsional broadening of absorption spectra were estimated from semi-classical molecular dynamics simulations and harmonic oscillator sampling. The simulated spectra agree well with the experiment and allow estimating the conformer distribution of the molecules.

Ultraviolet—Visible Spectral Analysis for the Reaction of Hydrogen Peroxide with a Titanium(IV)-Porphyrin Reagent

A Ti-TPyP reagent, i.e., an acidic aqueous solution of oxo[5,10,15,20-tetra(4-pyridyl)porphyrinato]titanium(IV) complex, TiO(tpyp), was developed as a highly sensitive and selective spectrophotometric reagent for the determination of traces of hydrogen peroxide. The reagent exhibited a single absorption peak at 432 nm, and the addition of hydrogen peroxide to the reagent gave rise to a new peak at 450 nm, with height being proportional to the added hydrogen peroxide concentration. Recently we succeeded in clarifying the reaction specificity of the TiO(tpyp) complex to hydrogen peroxide from the viewpoint of the reaction mechanisms and molecular orbitals based on ab initio calculations. In the present study, we performed ultraviolet-visible (UV-Vis) spectral simulations for individual species in the proposed reaction mechanisms based on the theoretical calculations carried out using the Zerner's intermediate neglect of differential overlap (ZINDO) method, since the assignment of the observed absorption peaks to the respective reaction species is important to confirm the reliability of the analysis of hydrogen peroxide using the Ti-TPyP reagent. The absorption peaks at 432 nm and 450 nm were assigned to a H(2)O-adduct structure of the protonated TiO(tpyp) complex and the important structure corresponding to the monoperoxo TiO(tpyp) complex, respectively, under acidic conditions. Particularly, it should be noted that the addition of water molecules to the protonated TiO(tpyp) complex affects the maximum absorption wavelength appreciably. Taking the addition of water molecules to the complex into consideration, the reaction mechanism proposed previously was revised in this study. The results should contribute to providing a new way to evaluate analytical reagents.

Ground, Excited Structures and Photoelectronic Properties of Poly(p-phenylenevinylene) Oligomers with Biphenyl Bridge

Ground and excited state structures of several poly(p-phenylenevinylene) (PPV) oligomers that contain a biphenyl bridge were fully optimized by density functional theory (DFT) and the configuration interaction singles (CIS) method, respectively. The most rational geometric structures for the ground state, the excited state and frontier molecular orbitals were obtained. The absorption and emission properties of the oligomers were then calculated using the semi-empirical Zerner's intermediate neglect of differential overlap (ZINDO) and time-dependent density functional theory (TD-DFT). We analyzed the effect of an increase in the biphenyl chain length on frontier molecular orbitals and the energy gap. Results indicate that absorption and emission properties hardly change as the length of the biphenyl chain increases. A cross configuration between neighboring PPV oligomer chains and the degree of intrachain twist becomes increasingly obvious. Molecular symmetry is reduced and the π-π stacking effect is weakened among these types of conjugated molecules in a solid. This is perhaps the most important factor for an increase in the fluorescence quantum efficiency of light-emitting diodes.

Theoretical study of solvent effect on one- and two-photon absorption properties of starburst DCM derivatives

The equilibrium geometries, electronic structures, one-photon absorption (OPA) and two-photon absorption (TPA) properties of starburst 4-(dicyanomethylene)-2-methyl-6-[p-(dimethylamino)styryl]-4H-pyran (DCM) derivatives have been studied by using density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) program. Results showed that increasing the number of branches in DCM molecules and introducing stronger electron withdrawing groups are efficient ways to enhance the TPA cross sections. For the two- and three-branched molecules of the DCM derivatives, the solvent effect on the OPA and TPA properties was also considered by adapting the self-consistent reaction field (SCRF) theory with the polarizable continuum model (PCM) to model the interaction with the solvent in ZINDO program. The calculated molecular structure, transition dipole moment and intramolecular charge transfer (ICT) in different solvents have been analyzed to approximate how the solvent affects the TPA cross section. Results indicated that the TPA cross section increases as the polarity of the solvents increases, and it reaches a maximum when the dielectric constant epsilon equals 20.7.

Theoretical Investigation of One- and Two-Photon Absorption Properties of Platinum Acetylide Chromophores

In this paper, we have theoretically investigated bis((4-phenylethynyl)phenyl) ethynyl)bis(trimethylphosphine)platinum(II) (PE2) and its analogs three platinum acetylide complexes (1-3) that feature highly pi-conjugated ligands (alkynyl-dimethylfluorene substituted with electron-donating or -withdrawing moieties). The geometrical and electronic structures are calculated at the ECP60MWB//6-31G*(H, C, P, N, S) basis set level by the density functional theory (DFT) method; one-photon absorption properties have been calculated by using time-dependent DFT (TDDFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods, and two-photon absorption (TPA) properties are obtained with the ZINDO/sum-over-states method. The values of beta(sp) and beta(d) for Pt are adjusted to -1 eV and -28.5 eV, respectively, to make one-photon absorption spectra calculated by ZINDO closest to the experimental data and TDDFT results. The calculated results indicate that all molecules in this work (involving cis isomers of molecules 1-3) take on two TPA peaks in the 600-800 nm region. The peak at 700-750 nm should not be simply attributed to the appearance of noncentrosymmetric cis isomers in solution, although trans and cis isomers adhere to a different selection rule. Every TPA peak results from its transition character. Molecules 1-3 show greater two-photon absorption strength compared with PE2 and retain good transparency.

Theoretical study of one-photon and two-photon absorption properties of perylene tetracarboxylic derivatives

The geometrical structure, electronic structure, one-photon absorption (OPA) and two-photon absorption (TPA) properties of the perylene tetracarboxylic derivatives (PTCDs) were studied theoretically by using density functional theory (DFT) and Zerner's intermediate neglect of differential overlap (ZINDO) methods. The results revealed that increasing the number of naphthalene nucleus, extending the conjugated length on long axis, increasing the strength of donor group on lateral side, decreasing the DeltaE(H-L) (energy gap between the highest occupied orbital and the lowest unoccupied orbital) and keeping the conjugation effect and inductive effect along the same molecular axis are the efficient ways to enlarge TPA cross section of PTCDs compounds. The results that PTCDs compounds exhibited extremely large TPA cross section of around 800-1100 nm (near infrared region) shed light into the significance of the PTCDs compounds for applications in TPA labeling materials in vivo.

Quantum chemistry of adsorption and hydrogenation of DBT and carbazole on NiMoS using ZINDO/I method

The purpose of this study was to develop a fundamental understanding of the adsorption and first-step hydrogenation mechanisms of sulfur and nitrogen compounds over molybdenum disulfide (MoS2). To do so, molecular simulation of the dibenzothiophene (DBT) and carbazole over NiMoS micro-crystal active surface was performed using Zerner's intermediate neglect of differential overlap (ZINDO) program with Hyperchem software. This study discusses the adsorbed structural parameters of DBT and carbazole molecules and proposes possible reaction pathways of hydrogenation on the NiMoS catalyst surface. The most stable configuration of adsorbed DBT is with the molecular plane perpendicular to the catalyst surface, while carbazole preferably adsorbed with the molecular plane parallel to the surface. Reaction enthalpies of DBT and carbazole adsorption processes on the catalyst surface were simulated based on the different configuration structures. The conformation heats and the activation energies of first-step hydrogenation of DBT and carbazole molecules over different configuration structures were also obtained by quantum chemistry simulation. The simulations on different catalyst surface structures indicated that the less vacant sites on the surface would result in the relatively low activation energies for DBT molecule. However, the hydrodenitrogenation (HDN) of carbazole increased with more vacant sites on the catalyst surface.

Semiempirical (ZINDO-PCM) Approach to Predict the Radiative and Nonradiative Decay Rates of a Molecule Close to Metal Particles

In this work, we present an extension of a model previously proposed (Andreussi et al. J. Chem. Phys. 2004, 121, 10190) to treat the effect of a metal particle on the optical properties of a molecule in solution (close to such a particle) in the framework of the polarizable continuum model (PCM). This extension concerns the combination of such a model with the semiempirical method Zerner's intermediate neglect of differential overlap (ZINDO), which allows us to treat large size molecular systems, as the ones normally used in the experiments. A refinement of the model is also introduced to take into account the effect of the metal specimen on the absorption process of the molecular system, which affects the probability that a molecule reaches the excited state. Numerical tests are presented to validate the reliability of the ZINDO results with respect to quantum-mechanical DFT methods. Comparisons with experimental results on two different large molecular systems are reported, and the effect of the metal on the absorption process is discussed.

Study of electronic and spectroscopic properties on a newly synthesized red fluorescent material

The ground state (S(0)) and the lowest singlet excited state (S(1)) of a newly synthesized red fluorescent material, 2-[3-(2-{4-[(2-Hydroxy-ethyl)-methyl-amino]-phenyl}-vinyl)-5,5-dimethyl-cyclohex-2-enylidene]-malononitrile (A31), are investigated. The S(0) and S(1) geometries are optimized at the ab initio Hartree-Fock and the singles configuration interaction (CIS) levels of theory, respectively. The CIS and semiempirical Zerner's Intermediate Neglect of Differential Overlap (ZINDO) methods provide the results for the absorption (S(0)-->S(1)) and emission (S(1)-->S(0)) transition energies. The Stokes shifts calculated at the CIS and ZINDO levels of theory are obtained. The absorption spectra in various solvents are calculated using the time-dependent density-functional theory method in combination with the polarized continuum model, which are in very good agreement with our experimental measurements. The solvent effects are discussed.

Absorption and Fluorescence of 2,5-Diarylidenecyclopentanones in Acidic Media: Evidence for Excited-State Proton Transfer

Spectroscopic properties for a series of 2,5-diarylidenecyclopentanones in weak and strong acid environments are reported. Electronic absorption and fluorescence spectra have been measured for the all-E configurations of 2,5-dibenzylidenecyclopentanone (1), 2,5-bis(3-phenylallylidene)cyclopentanone (2), and 2,5-bis(5-phenylpenta-2,4-dienylidene)cyclopentanone (3) in acetic acid and sulfuric acid solutions. The spectroscopic evidence indicates that in 96% sulfuric acid 1, 2, and 3 are protonated both in the ground state and on the S1 potential energy surface. This assignment is supported by Zerner's intermediate neglect of differential overlap (ZINDO) and time-dependent density functional theory (TD-DFT) calculations. In glacial acetic acid, 1, 2, and 3 are unprotonated in the ground state. The absence of observable fluorescence from 1 in glacial acetic acid indicates that S1 is npi, whereas the observation of fluorescence from 2 and 3 in acetic acid is consistent with S1 being pipi. A combination of spectroscopic data, molecular orbital calculations, and fluorescence lifetime measurements indicate that 2 and 3 undergo intermolecular excited-state proton transfer in glacial acetic acid and diluted sulfuric acid solutions. Photochemical studies reveal that, unlike its behavior in organic solvents, 1 does not undergo efficient E,E --> E,Z photoisomerization in 96% sulfuric acid.