Photoinduced intramolecular charge transfer to meta position of benzene ring in 6-aminophthalides

Substitution of non-fluorescent phthalide (Pd) with amino group at meta (6) position in relation to the electron-accepting part of the lactone ring completely changes Pd photophysics: a new long-wavelength absorption band arises and the molecule becomes highly fluorescent. The experimental data and the analysis of vertical electronic transitions with TDDFT method indicate that the first absorption band in 6-aminophthalides (6-APds) comprises a single CT transition to the S1 state. Almost equal absorption and emission transition dipole moments indicate that S0 <--> S1 transition in all 6-APds is not affected by any mixing with other electronic states, the excited-state vibrational relaxation is not accompanied by significant conformational changes and the Stokes shifts reflect mainly solvation energetics of these molecules. Excited state dipole moments obtained from solvatochromic plots and from CASSCF calculations confirm large charge displacement from amino group towards the meta position of the benzene ring upon excitation of 6-APds to S1 state. Long fluorescence lifetimes and high fluorescence quantum yields demonstrate efficient and stable excited state charge separation in 6-APds. Taken together with sensitivity of 6-APds to polarity and proticity of the environment these properties make them good candidates for fluorescent probes of long-time scale molecular dynamics.     

Highly sensitive detection of discrete absorption and B-type delayed fluorescence of dibenzanthracene in PMMA

High resolution S0 --> Sn and T1 --> Tn electronic absorptions and B-type delayed fluorescence of 1,2,7,8-dibenzanthracene in polymethylmethacrylate (PMMA) were experimentally observed by flash and laser flash photolysis technique. Dibenzanthracene molecules were excited in a two-step process. In the first step, an excited singlet is created, which undergoes intersystem crossing to triplet state, then T-T absorption creates an excited triplet dibenzanthracene molecule, which returns to the first excited singlet level by intersystem crossing. The re-created first excited singlet of dibenzanthracene decays back to the ground state by emitting B-type of delayed fluorescence, which was observed at the same emission band of prompt (normal) fluorescence, and R-, E-, P-types of delayed fluorescences. For normal fluorescence, S1 state is decaying to S0 ground state. For E- and P-type of delayed fluorescences, T1 state is decaying to S0 via S1 state, and for B-type of delayed fluorescence, T2 state is decaying to S0 via S1 state.     

S2 fluorescence and ultrafast relaxation dynamics of the S2 and S1 states of a ketocyanine dye

Dynamics of the excited singlet (both the S2 and S1) states of a ketocyanine dye, namely, 2,5-bis[(2,3-dihydroindolyl)-propylene]-cyclopentanone (KCD), have been investigated in different kinds of media using steady-state absorption and emission as well as femtosecond transient absorption spectroscopic techniques. Steady-state fluorescence measurements, following photoexcitation of KCD to its second excited singlet state, reveal dual fluorescence (emission from both the S2 and S1 states) behavior. Although the intensity of the S2 --> S0 fluorescence is weaker than that of the S1 --> S0 fluorescence in solutions at room temperature (298 K), the former becomes as much as or more intense than the latter in rigid matrixes at 77 K. The lifetime of the S2 state is short and varies between 0.2 and 0.6 ps in different solvents. After its creation, the S2 state undergoes two simultaneous processes, namely, S2 --> S0 fluorescence and S2 --> S1 internal conversion. Time-resolved measurements reveal the presence of an ultrafast component in the decay dynamics of the S1 state. A good correlation between the lifetime of this component and the longitudinal relaxation times (tauL) of the solvents suggests that this component arises due to solvation in polar solvents. More significant evolution of the spectroscopic properties of the S1 state in alcoholic solvents in the ultrafast time domain has been explained by the occurrence of the repositioning of the hydrogen bonds around the carbonyl group in the excited state of KCD. In 2,2,2-trifluoroethanol, a strongly hydrogen bond donating solvent, it has even been possible to establish the existence of two distinct forms of the S1 state, namely, the non-hydrogen-bonded (or free) molecule and the hydrogen-bonded complex.     

Spectral image adaptation and visual experience of DBA/PMMA

High resolution S0-->Sn and T1-->Tn electronic absorptions and B-type delayed fluorescence of 1,2,7,8-dibenzanthracene in polymethylmethacrylate (PMMA) were experimentally observed by flash and laser flash photolysis technique. Dibenzanthracene (hereafter DBA) molecules were excited in a two-step process. In the first step, an excited singlet is created, which undergoes intersystem crossing to triplet state, then T-T absorption creates an excited triplet dibenzanthracene molecule, which returns to the first excited singlet level by intersystem crossing. The re-created first excited singlet of dibenzanthracene decays back to the ground state by emitting B-type of delayed fluorescence, which was observed at the same emission band of prompt (normal) fluorescence, and R-, E-, P-types of delayed fluorescences. For normal fluorescence, S1 state is decaying to S0 ground state. For E- and P-type of delayed fluorescences, T1 state is decaying to S0 via S1 state, and for B-type of delayed fluorescence, T2 state is decaying to S0 via S1 state. The spectrum image showing the absorption/emission bands mentioned was also examined by image processing techniques in order to improve the visual experience of each band by localizing to a specific region of interest (ROI). Experimental results illustrate how the exact location of emission/absorption bands was clearly extracted from the spectral image and further improvements in the visual detection of absorption/emission bands.     

Intermediate state representation approach to physical properties of electronically excited molecules

Propagator methods provide a direct approach to energies and transition moments for (generalized) electronic excitations from the ground state, but they do not usually allow one to determine excited state wave functions and properties. Using a specific intermediate state representation (ISR) concept, we here show how this restriction can be overcome in the case of the algebraic-diagrammatic construction (ADC) propagator approach. In the ISR reformulation of the theory the basic ADC secular matrix is written as a representation of the Hamiltonian (or the shifted Hamiltonian) in terms of explicitly constructable states, referred to as intermediate (or ADC) states. Similar intermediate state representations can be derived for operators other than the Hamiltonian. Together with the ADC eigenvectors, the intermediate states give rise to an explicit formulation of the excited wave functions and allow one to calculate physical properties of excited states as well as transition moments for transitions between different excited states. As for the ground-state excitation energies and transition moments, the ADC excited state properties are size consistent so that the theory is suitable for applications to large systems. The established hierarchy of higher-order [ADC(n)] approximations, corresponding to systematic truncations of the IS configuration space and the perturbation-theoretical expansions of the ISR matrix elements, can readily be extended to the excited state properties. Explicit ISR matrix elements for arbitrary one-particle operators have been derived and coded at the second-order [ADC(2)] level of theory. As a first computational test of the method we have carried out ADC(2) calculations for singlet and triplet excited state dipole moments in H(2)O and HF, where comparison to full CI results can be made. The potential of the ADC(2) method is further demonstrated in an exploratory study of the excitation energies and dipole moments of the low-lying excited states of paranitroaniline. We find that four triplet states, T1-T4, and two singlet states, S1 and S2, lie (vertically) below the prominent charge transfer (CT) excitation, S3. The dipole moment of the S3 state (17.0D) is distinctly larger than that of the corresponding T3 triplet state (11.7D).     

Computational study of thioflavin T torsional relaxation in the excited state

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

Harvesting highly electronically excited energy to triplet manifolds: state-dependent intersystem crossing rate in Os(II) and Ag(I) complexes

A series of newly synthesized Os(II) and Ag(I) complexes exhibit remarkable ratiometric changes of intensity for phosphorescence versus fluorescence that are excitation wavelength dependent. This phenomenon is in stark contrast to what is commonly observed in condensed phase photophysics. While the singlet to triplet intersystem crossing (ISC) for the titled complexes is anomalously slow, approaching several hundred picoseconds in the lowest electronic excited state (S(1) → T(1)), higher electronic excitation leads to a much accelerated rate of ISC (10(11)-10(12) s(-1)), which is competitive with internal conversion and/or vibrational relaxation, as commonly observed in heavy transition metal complexes. The mechanism is rationalized by negligible metal d orbital contribution in the S(1) state for the titled complexes. Conversely, significant ligand-to-metal charge transfer character in higher-lying excited states greatly enhances spin-orbit coupling and hence the ISC rate. The net result is to harvest high electronically excited energy toward triplet states, enhancing the phosphorescence.     

Electronically excited states of tryptamine and its microhydrated complex

The lowest electronically excited singlet states of tryptamine and the tryptamine (H2O)1 cluster have been studied, using time dependent density functional theory for determination of the geometries and multireference configuration interaction for the vertical and adiabatic excitation energies, the permanent dipole moments, and the transition dipole moment orientations. All molecular properties of the seven experimentally observed conformers of tryptamine could be reproduced with high accuracy. A strong solvent reorientation has been found upon electronic excitation of the 1:1 water cluster of tryptamine to the L(a) and L(b) states. The adiabatically lowest excited singlet state in case of the tryptamine monomer is the L(b) state, while for the 1:1 water complex, the L(a) is calculated below the L(b) state.     

Ultrafast dynamics and excited state spectra of open-chain carotenoids at room and low temperatures

Many of the spectroscopic features and photophysical properties of carotenoids are explained using a three-state model in which the strong visible absorption of the molecules is associated with an S0 (1(1)Ag-) --> S2 (1(1)Bu+) transition, and the lowest lying singlet state, S1 (2(1)Ag-), is a state into which absorption from the ground state is forbidden by symmetry. However, semiempirical and ab initio quantum calculations have suggested additional excited singlet states may lie either between or in the vicinity of S1 (2(1)Ag-) and S2 (1(1)Bu+), and some ultrafast spectroscopic studies have reported evidence for these states. One such state, denoted S*, has been implicated as an intermediate in the depopulation of S2 (1(1)Bu+) and as a pathway for the formation of carotenoid triplet states in light-harvesting complexes. In this work, we present the results of an ultrafast, time-resolved spectroscopic investigation of a series of open-chain carotenoids derived from photosynthetic bacteria and systematically increasing in their number of pi-electron carbon-carbon double bonds (n). The molecules are neurosporene (n = 9), spheroidene (n = 10), rhodopin glucoside (n = 11), rhodovibrin (n = 12), and spirilloxanthin (n = 13). The molecules were studied in acetone and CS2 solvents at room temperature. These experiments explore the effect of solvent polarity and polarizability on the spectroscopic and kinetic behavior of the molecules. The molecules were also studied in ether/isopentane/ethanol (EPA) glasses at 77 K, in which the spectral resolution is greatly enhanced. Analysis of the data using global fitting techniques has revealed the ultrafast dynamics of the excited states and spectral changes associated with their decay, including spectroscopic features not previously reported. The data are consistent with S* being identified with a twisted conformational structure, the yield of which is increased in molecules having longer pi-electron conjugations. In particular, for the longest molecule in the series, spirilloxanthin, the experiments and a detailed quantum computational analysis reveal the presence of two S* states associated with relaxed S1 (2(1)Ag-) conformations involving nearly planar 6-s-cis and 6-s-trans geometries. We propose that in polar solvents, the ground state of spirilloxanthin takes on a corkscrew conformation that generates a net solute dipole moment while decreasing the cavity formation energy. Upon excitation and relaxation into the S1 (2(1)Ag-) state, the polyene unravels and flattens into a more planar geometry with comparable populations of 6-s-trans and 6-s-cis conformations.     

Hydrogen bond formation between 4-(dimethylamino)pyridine and aliphatic alcohols

The photophysics of 4-(dimethylamino)pyridine (DMAP) has been investigated in different solvents in the presence of aliphatic and fluorinated aliphatic alcohols, respectively. For most systems, consecutive two-step hydrogen-bonded complex formation is observed in the presence of alcohols. Equilibrium constants are determined from UV spectroscopic results for the formation of singly and doubly complexed species. The resolved absorption and fluorescence spectra for the singly and doubly complexed DMAP are derived by means of the equilibrium constants. Exceptionally large hydrogen bond basicity values are found for the ground and singlet excited DMAP molecules. In n-hexane, as a consequence of complex formation, the intramolecular charge transfer (ICT) emission becomes dominant over of the locally excited fluorescence; the fluorescence and triplet yields increase considerably with complexation. In polar solvents, both the fluorescence and triplet yields of the complex are much smaller than that of the uncomplexed DMAP. The dipole moments derived for the singly complexed species from the Lippert-Mataga analysis are much larger than those of the uncomplexed molecules. However, for the relaxed ICT excited-state one obtains different dipole moments in apolar and polar solvents. This may be explained by a conformational change of the molecule in the ICT excited state from planar geometry in apolar solvent to the perpendicular structure (characterized with bigger dipole moment) in polar solvent.     

The influence of molecular symmetry and topological factors on the internal heavy atom effect in aromatic and heteroaromatic compounds.

The absorption and fluorescence properties of 26 specially selected aromatic and heteroaromatic compounds, from different classes, are studied quantum chemically and experimentally at room temperature (293 K). Seven of these compounds have not been studied before. The compounds are arranged in seven groups, which illustrate different cases of the internal heavy atom effect. The quantum yield of fluorescence, gamma and fluorescence decay time, tau(f) of deaerated and non-deaerated cyclohexane or ethanol solutions are measured. The oscillator strength, f(e), fluorescence rate constant, k(f), natural lifetime, tau(0)t, and intersystem crossing rate constant, kST, were calculated for each compound. The orbital nature of the lowest excited singlet state and direction of polarization of the S0 --> S1 transitions are determined using the PPP-Cl method for each molecule. The investigation shows that substitution of a heavy atom(s) (Cl, S, Br, I etc.) into an aromatic or heteroaromatic molecule may produce different changes in all the fluorescence parameters (sometimes dramatically) and not necessarily lead to the quenching of fluorescence. Substitution of a heavy atom(s) may increase the value of the spin-orbit operator, Hso, if the S0 --> S1 excitation is localized to some extent on a carbon atom bonded to a heavy atom(s) or on the heavy atom itself (O or S). Such substitution may change the symmetry of a molecule and hence the values of the [psiS1/Hso/psi'T1] matrix elements would change (in molecules of higher symmetry groups not all Ti states are able to mix with the perturbing S1 state). Such substitution may change the arrangement of Ti states below the S1, state and hence, the Franck-Condon factors would change. Such substitution may also change the value of the [psiS0/Mj/psiS1] matrix element and, consequently, the oscillator strength of the S0 --> S1 transition would change. A combination of all these possible changes determines the value of k(f) and kST and, consequently, determines the value of gamma and tau(f). It is observed that in many cases, the value of the spin-orbit operator is related to the dipole moment operator, e.g. if the introduction of a heavy atom increases kST then, as a rule, it decreases f(e)(1A --> 1La).     

Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field∕∕configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T(2) electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T(1)→T(3) and T(1)→T(5) transitions, supporting that the intermediate triplet state (T(2)) decays by internal conversion to T(1).     

Distinct photophysics of the isomers of B18H22 explained

The photophysics of the two isomers of octadecaborane(22), anti- and syn-B(18)H(22), have been studied by UV-vis spectroscopic techniques and theoretical computational methods. In air-saturated hexane, anti-B(18)H(22) shows fluorescence with a high quantum yield, Φ(F) = 0.97, and singlet oxygen O(2)((1)Δ(g)) production (Φ(Δ) ～ 0.008). Conversely, isomer syn-B(18)H(22) shows no measurable fluorescence, instead displaying much faster, picosecond nonradiative decay of excited singlet states. Computed potential energy hypersurfaces (PEHs) for both isomers rationalize these data, pointing to a deep S(1) minimum for anti-B(18)H(22) and a conical intersection (CI) between its S(0) and S(1) states that lies 0.51 eV higher in energy. Such an energy barrier to nonradiative relaxation is not present in the PEH of syn-B(18)H(22), and the system therefore has sufficient initial energy on excitation to reach the (S(0)/S(1)) CI and to then decay to the ground state without fluorescence. The computational analysis of the geometries at stationary points along the PEH of both isomers shows that the determining factor for the dissimilar photophysics of anti- and syn-B(18)H(22) may be due to the significant differences in the geometrical rearrangements at their respective conical intersections. Thus, the syn isomer shows one very large, B-B elongation of 1.2 ? from 1.8 ? in the ground state to 3.0 ? at the CI, whereas the anti isomer shows smaller elongations (below 1 ?) in several B-B connectivities at its (S(0)/S(1))(CI). The absorbed energy in S(1) for the anti-B(18)H(22) is therefore redistributed vibrationally into several regions of the molecule rather than almost completely into a single vibrational mode as in the case for the syn isomer. The consequent prolonged S(1) lifetime for the anti isomer allows for relaxation via fluorescence.     

Excited state dipole moments of fluoroanilines from solvatochromic shift measurements

The dipole moments of fluorinated anilines in the first excited singlet state (¹L_b) have been determined from the solvent shifts of absorption and fluorescence spectra. It is concluded that in the monofluoro isomers as well as in aniline itself this dipole moment must be of the order of 5 debye, whereas the gas phase dipole moment is estimated to be some 2 debye only from Stark effect measurements. Ortho-substituted difluoro- and trifluoroanilines show anomalous Stokes shifts of the absorption and fluorescence spectra which are indicative of substantial reorganization of their nuclear framework in the excited state; in these cases no excited state dipole moment could be determined.     

Structural changes accompanying excited-state electron transfer in meta- and para-dialkylaminopyridines

The electronic structure of the lowest excited singlet states and molecular geometries of a series of dialkylaminopyridines (DAAPs) representing electron donor–acceptor systems were studied by photostationary and time-resolved UV–vis spectroscopic methods and quantum chemical calculations. The comparative studies allow us to rationalize dual luminescence of 4-DAAPs in terms of the TICT state model—the analysis of the electronic transition dipole moments indicates a nearly orthogonal conformation of the fluorescent ICT states. Introduction of the amino group at meta position as in 3-diisopropylaminopyridine completely changes photophysics of these pyridine derivatives: (i) the Franck-Condon excited state initially reached upon excitation and the solvent equilibrated fluorescent state are most probably of the same nature (both excited states do not correspond to a full separation of charges) and (ii) the electronic structure and geometry of the fluorescent CT states of m-DIAP are solvent dependent.     

Photodynamics of the excited states of trimethylamine in a supersonic beam

The photophysics of trimethylamine (TMA) and rare gas-TMA van der Waals molecules has been studied under supersonic beam conditions. Dual exponential fluorescence decays observed for excitation of the second excited singlet state (S₂) are attributed to a novel S₂-S₁ relaxation induced by the vibrational predissociation of van der Waals molecules.     

Ultrafast time-resolved spectroscopy of xanthophylls at low temperature

Many of the spectroscopic features and photophysical properties of xanthophylls and their role in energy transfer to chlorophyll can be accounted for on the basis of a three-state model. The characteristically strong visible absorption of xanthophylls is associated with a transition from the ground state S0 (1(1)Ag-) to the S2 (1(1)Bu+) excited state. The lowest lying singlet state denoted S1 (2(1)Ag-), is a state into which absorption from the ground state is symmetry forbidden. Ultrafast optical spectroscopic studies and quantum computations have suggested the presence of additional excited singlet states in the vicinity of S1 (2(1)Ag-) and S2 (1(1)Bu+). One of these is denoted S* and has been suggested in previous work to be associated with a twisted molecular conformation of the molecule in the S1 (2(1)Ag-) state. In this work, we present the results of a spectroscopic investigation of three major xanthophylls from higher plants: violaxanthin, lutein, and zeaxanthin. These molecules have systematically increasing extents of pi-electron conjugation from nine to eleven conjugated carbon-carbon double bonds. All-trans isomers of the molecules were purified by high-performance liquid chromatography (HPLC) and studied by steady-state and ultrafast time-resolved optical spectroscopy at 77 K. Analysis of the data using global fitting techniques has revealed the inherent spectral properties and ultrafast dynamics of the excited singlet states of each of the molecules. Five different global fitting models were tested, and it was found that the data are best explained using a kinetic model whereby photoexcitation results in the promotion of the molecule into the S2 (1(1)Bu+) state that subsequently undergoes decay to a vibrationally hot S1 (1(1)Ag-) state and with the exception of violaxanthin also to the S* state. The vibrationally hot S1 (1(1)Ag-) state then cools to a vibrationally relaxed S1 (2(1)Ag-) state in less than a picosecond. It was also found that a portion of the S* population is converted into S1 (2(1)Ag-) during deactivation, but this process and the relative yield of S* was found to depend on temperature, consistent with it being associated with a twisted conformation of the xanthophyll. The results of the global fitting suggest that subpopulations of twisted conformers of xanthophylls already exist in the ground state prior to photoexcitation.     

Negatively charged pi-(C60-)2 dimer with biradical state at room temperature

A negatively charged pi-(C60-)2 dimer bonded by two single bonds was found in the ionic multicomponent complex {(MDABCO+).CoIITMPP}2.(C60-)2.(C6H4Cl2)2.5.(C6H5CN)1.5 (1). In contrast to the previously described diamagnetic sigma-(C60-)2 dimer, the negatively charged pi-dimer has a biradical state at room temperature: (C60*-)2 (S = 1). The behavior of spins in this dimer can be described by a model with a singlet ground state (S = 0) and a close lying excited triplet (S = 1) state with the energy gap of 2|JAF| = 70 +/- 2 cm-1. On the whole, complex 1 shows a strong antiferromagnetic interaction of spins with a Weiss constant of -34 K.     

Photophysics of aromatic molecules with low-lying pi sigma* states: fluorinated benzenes

Unlike fluorinated benzenes with four or less fluorine atoms, pentafluorobenzene (PFB) and hexafluorobenzene (HFB) exhibit very small fluorescence yields and short fluorescence lifetimes. These emission anomalies suggest that the nature of the first excited singlet (S(1)) state may be different for the two classes of fluorobenzenes. Consistent with this conjecture, the time-dependent density-functional theory calculations yield S(1) state of pi pi(*) character for fluorinated benzenes with four or less F atoms, and S(1) state of pi sigma(*) character for PFB and HFB. The pi sigma(*) character of the S(1) state of PFB and HFB has been confirmed by laser-induced fluorescence, which reveal the presence of a new electronic transition to the red of the (1)pi pi(*) (L(b))<--S(0) transition, which can be identified with the predicted low-energy (1)pi sigma(*)<--S(0) absorption. The low fluorescence yields and the short fluorescence lifetimes of PFB and HFB are consistent with the small radiative decay rate of the (1)pi sigma(*) state and efficient S(1) (pi sigma(*))-->S(0) internal conversion between two electronic states of very different geometries.