The lowest excited triplet (T1) energies of p-methoxybenzaldehyde and p-cyanobenzaldehyde vapors estimated from the temperature dependence of the T2(n,pi*) phosphorescence and the S1(n,pi*) fluorescence spectra

Invisible energy levels of the T1(pi, pi*) state of p-methoxybenzaldehyde (anisaldehyde) and p-cyanobenzaldehyde vapors have been estimated through the temperature dependence of the T2(n, pi*) --> S0 phosphorescence and the S1(n, pi*) --> S0 delayed fluorescence spectra. It is shown that the T1(pi, pi*) levels are located at 900 +/- 100 and 300 +/- 100 cm(-1) below the T2(n, pi*) levels, respectively, for p-methoxybenzaldehyde and p-cyanobenzaldehyde vapors. The estimated T1 energy levels are in good agreement with the phosphorescence origins in rigid glass at 77 K.     

Photophysical processes in quinoxaline vapor at low pressure

Excitation and pressure dependence of fluorescence and phosphorescence quantum yields has been reinvestigated in detail for quinoxaline in the static vapor phase at pressure range from 10(-3) to 10(-1) Torr. It is shown that the ratio of the nonradiative rate from T(1)(pi, pi*) to the rate of the S(1)(n, pi*) approximately -->T(1)(pi, pi*) intersystem crossing decreases with increasing the excitation energy in the S(0)-->S(1) excitation region. The phosphorescence quantum yield measured as a function of the excitation energy at low pressure shows an abrupt decrease on going the excitation from S(0)-->S(1) to S(0)-->S(2), indicating the slow vibrational energy redistribution between the S(1) levels optically populated and those populated through the internal conversion from S(2) to S(1).     

The benzophenone S1(n,pi*) --> T1(n,pi*) states intersystem crossing reinvestigated by ultrafast absorption spectroscopy and multivariate curve resolution

The well-known benzophenone intersystem crossing from S(1)(n,pi*) to T(1)(n,pi*) states, for which direct transition is forbidden by El-Sayed rules, is reinvestigated by subpicosecond time-resolved absorption spectroscopy and effective data analysis for various excitation wavelengths and solvents. Multivariate curve resolution alternating least-squares analysis is used to perform bilinear decomposition of the time-resolved spectra into pure spectra of overlapping transient species and their associated time-dependent concentrations. The results suggest the implication of an intermediate (IS) in the relaxation process of the S(1) state. Therefore, a two step kinetic model, S(1) --> IS --> T(1), is successfully implemented as an additional constraint in the soft-modeling algorithm. Although this intermediate, which has a spectrum similar to the one of T(1)(n,pi*) state, could be artificially induced by vibrational relaxation, it is tentatively assigned to a hot T(1)(n,pi*) triplet state. Two characteristic times are reported for the transition S(1) --> IS and IS --> T(1), approximately 6.5 ps and approximately 10 ps respectively, without any influence of the solvent. Moreover, an excitation wavelength effect is discovered suggesting the participation of unrelaxed singlet states in the overall process. To go further discussing the spectroscopic relevancy of IS and to rationalize the expected involvement of the T(2)(pi,pi*) state, we also investigate 4-methoxybenzophenone. For this neighboring molecule, triplet energy level is tunable through solvent polarity and a clear correlation is established between the intermediate resolved by multivariate data analysis and the presence of a T(2)(pi,pi*) above the T(1)(n,pi*) triplet. It is therefore proposed that the benzophenone intermediate species is a T(1)(n,pi*) high vibrational level in interaction with T(2)(pi,pi*) state.     

Jet-cooled phosphorescence excitation spectrum of the T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one

The T1(n,pi*) <-- S0 transition of 2-cyclopenten-1-one (2CP) was investigated by using phosphorescence excitation (PE) spectroscopy in a free-jet expansion. The origin band, near 385 nm, is the most intense feature in the T1(n,pi*) <-- S0 PE spectrum. A short progression in the ring-bending mode (nu'(30)) is also observed. The effective vibrational temperature in the jet is estimated at 50 K. The spectral simplification arising from jet cooling helps confirm assignments made previously in the room-temperature cavity ringdown (CRD) absorption spectrum, which is congested by vibrational hot bands. In addition to the origin and nu'(30) assignments, the jet-cooled PE spectrum also confirms the 28(0)(1) (C=O out-of-plane wag), 29(0)(1) (C=C twist), and 19(0)(1) (C=O in-plane wag) band assignments that were made in the T1(n,pi*) <-- S0 room-temperature CRD spectrum. The temporal decay of the T1 state of 2CP was investigated as a function of vibronic excitation. Phosphorescence from the v' = 0 level persists the entire time the molecules traverse the emission detection zone. Thus the phosphorescence lifetime of the v' = 0 level is significantly longer than the 2 micros transit time through the viewing zone. Higher vibrational levels in the T1 state have shorter phosphorescence lifetimes, on the order of 2 micros or less. The concomitant reduction in emission quantum yield causes the higher vibronic bands (above 200 cm(-1)) in the PE spectrum to be weak. It is proposed that intersystem crossing to highly vibrationally excited levels of the ground state is responsible for the faster decay and diminished quantum yield. The jet cooling affords partial rotational resolution in the T1(n,pi*) <-- S0 spectrum of 2CP. The rotational structure of the origin band was simulated by using inertial constants available from a previously reported density functional (DFT) calculation of the T1(n,pi*) state, along with spin constants obtained via a fitting procedure. Intensity parameters were also systematically varied. The optimized intensity factors support a model that identifies the S2(pi,pi*) <-- S0 transition in 2CP as the sole source of oscillator strength for the T1(n,pi*) <-- S0 transition.     

Effects of benzoannulation and alpha-octabutoxy substitution on the photophysical behavior of nickel phthalocyanines: a combined experimental and DFT/TDDFT study

The photophysical properties of a group of Ni(II)-centered tetrapyrroles have been investigated by ultrafast transient absorption spectrometry and DFT/TDDFT methods in order to characterize the impacts of alpha-octabutoxy substitution and benzoannulation on the deactivation pathways of the S1(pi,pi*) state. The compounds examined were NiPc, NiNc, NiPc(OBu)8, and NiNc(OBu)8, where Pc = phthalocyanine and Nc = naphthalocyanine. It was found that the S1(pi,pi*) state of NiNc(OBu)8 deactivated within the time resolution of the instrument (200 fs) to a vibrationally hot T1(pi,pi*) state. The quasidegeneracy of the S1(pi,pi*) and 3(dz2,dx2-y2) states allowed for fast intersystem crossing (ISC) to occur. After vibrational relaxation (ca. 2.5 ps), the T1(pi,pi*) converted rapidly (ca. 19 ps lifetime) and reversibly into the 3LMCT(pi,dx2-y2) state. The equilibrium state, so generated, decayed to the ground state with a lifetime of ca. 500 ps. Peripheral substitution of the Pc ring significantly modified the photodeactivation mechanism of the S1(pi,pi*) by inducing substantial changes in the relative energies of the S1(pi,pi*), 3(dpi,dx2-y2), 3(dz2,dx2-y2), T1(pi,pi*), and 1,3LMCT(pi,dx2-y2) excited states. The location of the Gouterman LUMOs and the unoccupied metal level (dx2-y2) with respect to the HOMO is crucial for the actual position of these states. In NiPc, the S1(pi,pi*) state underwent ultrafast (200 fs) ISC into a hot (d,d) state. Vibrational cooling (ca. 20 ps lifetime) resulted in a cold (dz2,dx2-y2) state, which repopulated the ground state with a 300 ps lifetime. In NiPc(OBu)8, the S1(pi,pi*) state deactivated through the 3(dz2,dx2-y2), which in turn converted to the 3LMCT(pi,dx2-y2) state, which finally repopulated the ground state with a lifetime of 640 ps. Insufficient solubility of NiNc in noncoordinating solvents prevented transient absorption data from being obtained for this compound. However, the TDDFT calculations were used to make speculations about the photoproperties.     

The different photoisomerization efficiency of azobenzene in the lowest n pi* and pi pi* singlets: the role of a phantom state

Azobenzene E<==>Z photoisomerization, following excitation to the bright S(pi pi*) state, is investigated by means of ab initio CASSCF optimizations and perturbative CASPT2 corrections. Specifically, by elucidating the S(pi pi*) deactivation paths, we explain the mechanism responsible for azobenzene photoisomerization, the lower isomerization quantum yields observed for the S(pi pi*) excitation than for the S1(n pi*) excitation in the isolated molecule, and the recovery of the Kasha rule observed in sterically hindered azobenzenes. We find that a doubly excited state is a photoreaction intermediate that plays a very important role in the decay of the bright S(pi pi*). We show that this doubly excited state, which is immediately populated by molecules excited to S(pi pi*), drives the photoisomerization along the torsion path and also induces a fast internal conversion to the S1(n pi*) at a variety of geometries, thus shaping (all the most important features of) the S(pi pi*) decay pathway and photoreactivity. We reach this conclusion by determining the critical structures, the minimum energy paths originating on the bright S(pi pi*) state and on other relevant excited states including S1(n pi*), and by characterizing the conical intersection seams that are important in deciding the photochemical outcome. The model is consistent with the most recent time-resolved spectroscopic and photochemical data.     

Phosphorescence excitation spectrum of the T(1)(n,pi*)<--S(0) transition of 4H-pyran-4-one

The phosphorescence excitation (PE) spectrum of 4H-pyran-4-one (4PN) vapor at 40-50 degrees C was recorded near 366 nm. The most intense vibronic feature in this region of the spectrum is the T(1)(n,pi*)<--S(0) origin band. The value of nu(0) for the 0(0)(0) transition was determined to be 27 291.5 cm(-1) by comparing the observed spectrum to a simulation in the T(1)<--S(0) origin-band region. Attached to the origin band in the PE spectrum are several Deltav=0 sequence bands involving low-frequency ring modes. From the positions of these bands, together with the known ground-state combination differences, fundamental frequencies for nu(18') (ring bending), nu(13') (ring twisting), and nu(10') (in-plane ring deformation) in the T(1)(n,pi*) excited state were determined to be 126, 269, and 288 cm(-1), respectively. These values represent drops of 15%, 32%, and 43%, compared to the respective fundamental frequencies in the S(0) state. The changes in these ring frequencies indicate that the effects of T(1)(n,pi*)<--S(0) excitation extend beyond the nominal carbonyl chromophore and involve the conjugated ring atoms as well. The delocalization may be more extensive for T(1)(n,pi*) than for S(1)(n,pi*) excitation.     

Determination of the ground state,excited state and change in dipole moments of magnesium and zinc chlorin

High-resolution Stark effect measurements on the S1 <-- S0 (pi pi*) origin of magnesium chlorin (MgCh) and zinc chlorin (ZnCh) in single crystals of n-octane at 4.2 K are reported. The corresponding change in dipole moment (absolute value(delta mu(ge))) associated with each transition was estimated to be 0.23 +/- 0.04 and 0.27 +/- 0.05 debye, respectively. Each molecule's orientation in the n-octane crystal was also determined. The change in dipole moment of MgCh was also found using solvatochromic shift data (absolute value(delta mu(ge))) = 0.33 +/- 0.08 debye). The ground state dipole moment (mu(g)) of MgCh was determined by dielectric constant measurement of MgCh/benzene solutions (mu(g) = 2.26 +/- 0.08 debye). These were combined to calculate the average excited state dipole moment of MgCh (mu(e) = 2.51 +/- 0.08 debye). The ground state dipole moment of ZnCh was also determined using solvatochromic shift data (mu(g) = 3.17 +/- 0.08 debye). This was combined with its measured absolute value(delta mu(ge)) to calculate the excited state dipole moment of ZnCh (mu(e) = 3.44 +/- 0.08 debye); the S1 <-- S0 (pi pi*) origin band of both complexes was red-shifted at room temperature as the polarity of the solvents was increased, which implies that delta mu(ge) is positive.     

Competition between alpha-cleavage and energy transfer in alpha-azidoacetophenones

Molecular modeling demonstrates that the first excited state of the triplet ketone (T1K) in azide 1b has a (pi,pi*) configuration with an energy that is 66 kcal/mol above its ground state and its second excited state (T2K) is 10 kcal/mol higher in energy and has a (n,pi*) configuration. In comparison, T1K and T2K of azide 1a are almost degenerate at 74 and 77 kcal/mol above the ground state with a (n,pi*) and (pi,pi*) configuration, respectively. Laser flash photolysis (308 nm) of azide 1b in methanol yields a transient absorption (lambdamax=450 nm) due to formation of T1K, which decays with a rate of 2.1 x 105 s-1 to form triplet alkylnitrene 2b (lambdamax=320 nm). The lifetime of nitrene 2b was measured to be 16 ms. In contrast, laser flash photolysis (308 nm) of azide 1a produced transient absorption spectra due to formation of nitrene 2a (lambdamax=320 nm) and benzoyl radical 3a (lambdamax=370 nm). The decay of 3a is 2 x 105 s-1 in methanol, whereas nitrene 2a decays with a rate of approximately 91 s-1. Thus, T1K (pi,pi*) in azide 1b leads to energy transfer to form nitrene 2b; however, alpha-cleavage is not observed since the energy of T2K (n,pi*) is 10 kcal/mol higher in energy than T1K, and therefore, T2K is not populated. In azide 1a both alpha-cleavage and energy transfer are observed from T1K (n,pi*) and T2K (pi,pi*), respectively, since these triplet states are almost degenerate. Photolysis of azide 1a yields mainly product 4, which must arise from recombination of benzoyl radicals 3a with nitrenes 2a. However, products studies for azide 1b also yield 4b as the major product, even though laser flash photolysis of azide 1b does not indicate formation of benzoyl radical 3b. Thus, we hypothesize that benzoyl radicals 3 can also be formed from nitrenes 2. More specifically, nitrene 2 does undergo alpha-photocleavage to form benzoyl radicals and iminyl radicals. The secondary photolysis of nitrenes 2 is further supported with molecular modeling and product studies.     

Excitation-energy dependence of the phosphorescence quantum yields of pyridinecarboxaldehyde vapors

Emission and excitation spectra of 3- and 4-pyridinecarboxaldehyde vapors have been measured at different pressures down to 10(-2)Torr. The phosphorescence quantum yield measured at low pressure as a function of excitation energy is nearly constant in the range of excitation energy corresponding to the S1(n, pi*) state, but it decreases abruptly at the S2(pi, pi*) threshold. The onset of the abrupt decrease of the yield corresponds to the location of the S2 absorption origin of each molecule, indicating that the nonradiative pathway depends on the type of the excited singlet state to which the molecule is initially excited. The relaxation processes are discussed based on the pressure and excitation-energy dependence of the phosphorescence quantum yield.     

Photophysical properties of 2,5-diphenyl-1,6,6a-trithiapentalene revealed by time-resolved spectroscopy

The fluorescence decay from S2(pi, pi*) state of 2,5-diphenyl-1,6,6a-trithiapentalene (DP-TTP) in cyclohexane, tetrahydrofuran and acetonitrile solutions of a quantum yield of approximately 0.02-0.04 were measured. The results indicate that, the dominant process of radiationless deactivation of the S2 state, is internal conversion to the S1 state. Upon laser pulse excitation (lambda(ex) = 532 nm) from the S1(pi, pi*) state, DP-TTP in deoxygenated benzonitrile, acetonitrile, ethanol and tetrahydrofuran solutions give rise to transient triplet triplet absorption (lambdaTmax = 700-720 nm). Kinetic data are presented for intrinsic triplet lifetimes, self-quenching and quenching by oxygen.     

Time-resolved spectroscopy of sulfur- and carboxy-substituted N-alkylphthalimides

The photophysical and photochemical properties of N-phthaloyl-methionine (1), S-methyl-N-phthaloyl-cysteine methyl ester (2) and N-phthaloyltranexamic acid (3) were studied by time-resolved UV/Vis spectroscopy, using laser pulses at 248 or 308 nm. The quantum yield of fluorescence is low (phi(f)< 10(-2)) for 1-3 in fluid and glassy media, whereas that of phosphorescence is large (0.3-0.5) in ethanol at - 196 degrees C. The triplet properties were examined in several solvents, at room temperature and below. The spectra and decay kinetics are similar, but the population of the pi(pi*) triplet state, as measured by T-T absorption, is much lower for 1 and 2 than for 3 or N-methyltrimellitimide (5') at ambient temperatures. The quantum yield (phi(delta)) of singlet molecular oxygen O2(1deltag) formation is substantial for 3 and 5' in several air- or oxygen-saturated solvents at room temperature, but small for 2 and 1. The quantum yield of decomposition is substantial (0.2-0.5) for 3 and small (<0.05) for 2 and 1. It is postulated that photoinduced charge separation in the spectroscopically undetectable 3n,pi* state may account for the cyclization products of 1 and 2. In aqueous solution, this also applies for 3, whereas in organic solvents cyclization involves mainly the lower lying 3pi,(pi*) state. Triplet acetone, acetophenone and xanthone are quenched by 1-3 in acetonitrile; the rate constant is close to the diffusion-controlled limit, but smaller for benzophenone. While the energy transfer from the triplet ketone occurs for 3, a major contribution of electron transfer to the N-phthalimide derivative is suggested for 1 and 2, where the radical anion of benzophenone or 4-carboxybenzophenone is observed in alkaline aqueous solution.     

Intra- and intermolecular electronic relaxation of the second excited singlet and the lowest excited triplet states of 1,3-dimethyl-4-thiouracil in solution

Intramolecular processes of deactivation of 1,3-dimethyl-4-thiouracil (DMTU) from the second excited singlet (S2) (pi, pi*) and the lowest excited triplet (T1) (pi, pi*) states have been studied using perfluoro-1,3-dimethylcyclohexane (PFDMCH) as a solvent. The spectral and photophysical (PP) properties of DMTU in CCl4, hexane and water have also been described. For the first time, the fluorescence from S2 state DMTU has been observed. The picosecond lifetime of DMTU in the S2 state (tau(S2)) in PFDMCH has been proposed to be determined by a very fast intramolecular reversible process of hydrogen abstraction from the ortho methyl group by the thiocarbonyl group. The shortening of tau(S2) in CCl4 is interpreted to be caused by the intermolecular interactions between DMTU (S2) and the solvent. Results of the phosphorescence decay as a function of DMTU concentration were analyzed using the Stern-Volmer formalism, which enabled determination of the intrinsic lifetime of the T1 state (tau0(T1)) and rate constants of self-quenching (k(sq)). The lifetimes, tau0(T1), of DMTU in PFDMCH and CCl4 are much longer than the values hitherto obtained in more reactive solvents. The PP properties of DMTU both in the S2 and T1 states have been shown to be determined by the thiocarbonyl group.     

Spectroscopy and photophysics of 1,4-bis(phenylethynyl)benzene: effects of ring torsion and dark pi sigma* state

A combination of supersonic-jet laser spectroscopy and quantum chemistry calculation was applied to 1,4-bis(phenylethynyl)benzene, BPEB, to study the role of the dark pisigma* state on electronic relaxation and the effect of ring torsion on electronic spectra. The result provides evidence for fluorescence break-off in supersonic jet at high S1(pi pi*) <-- S0 excitation energies, which can be attributed to the pi pi*-pi sigma* intersection. The threshold energy for the fluorescence break-off is much larger in BPEB (approximately 4000 cm(-1)) than in diphenylacetylene (approximately 500 cm(-1)). The high-energy barrier in BPEB accounts for the very large fluorescence quantum yield of the compound (in solution) relative to diphenylacetylene. The comparison between the experimentally derived torsional barrier and frequency with those from the computation shows overall good agreement and demonstrates that the low-energy torsional motion involves the twisting of the end ring in BPEB. The torsional barrier is almost an order of magnitude greater in the pi pi* excited state than in the ground state. The finding that the twisting of the end ring in BPEB is relatively free in the ground state, but strongly hindered in the excited state, provides rationale for the well-known temperature dependence of the spectral shape of absorption and the lack of mirror symmetry relationship between the absorption and fluorescence at elevated temperatures.     

An optical-optical double resonance probe of the lowest triplet state of jet-cooled thiophosgene: rovibronic structures and electronic relaxation

The vibrational structure, rotational structure, and electronic relaxation of the "dark" T1 3A2(n,pi*) state of jet-cooled thiophosgene have been investigated by two-color S2<--T1<--S0 optical-optical double resonance (OODR) spectroscopy, which monitors the S2-->S0 fluorescence generated by S2<--T1 excitation. This method is capable of isolating the T1 vibrational structure into a1, b1, and b2 symmetry blocks. The fluorescence-detected vibrational structure of the Tz spin state of T1 shows that the CS stretching frequency as well as the barrier height for pyramidal deformation are significantly greater in the 3A2(n,pi*) state than in the corresponding 1A2(n,pi*) state. The differing vibrational parameters of the T1 thiophosgene relative to the S1 thiophosgene can be attributed to the motions of unpaired electrons that are better correlated when they are in the excited singlet state than when they are in the triplet state of same electron configuration. A set of T1 structural parameters and the information concerning the T1 spin states have been obtained from least-square fittings of the rotationally resolved T1<--S0 excitation spectrum. The nearly degenerate mid R:x and mid R:y spin states are well removed from mid R:z spin component, indicating that T1 thiophosgene is a good example of case (ab) coupling. The decay of the mid R:z spin state of T1 thiophosgene, obtained from time-resolved S2<--T1<--S0 OODR experiment, is characteristic of strong-coupling intermediate-case decay in which an initial rapid decay is followed by recurrences and/or a long-lived quasiexponential decay.     

Photoinduced C-N bond cleavage in 2-azido-1,3-diphenyl-propan-1-one derivatives: photorelease of hydrazoic acid

Photolysis of 3-azido-1,3-diphenyl-propan-1-one (1a) in toluene yields 1,3-diphenyl-propen-1-one (2), whereas irradiation of 3-azido-2,2-dimethyl-1,3-diphenyl-propan-1-one (1b) results in the formation of mainly 2,2-dimethyl-1,3-diphenyl-propan-1-one. Laser flash photolysis (308 nm) of 1a,b in acetonitrile reveals a transient absorption (lambda max = approximately 310 nm) due to the formation of radicals 4a and 4b, respectively, which have lifetimes of approximately 14 micros at ambient temperature. TD-DFT calculations (B3LYP/6-31+G(d)) reveal that the first and second excited states of the triplet ketone (T1K (n,pi*) and T2K (pi,pi*)) in azide 1a are almost degenerate, at approximately 74 and 76 kcal/mol above the ground state (S0), respectively. Similarly, azide 1b has T1K and T2K 75 and 82 kcal/mol above S0, respectively. The calculated transition state for cleaving the C-N bond is located 71 and 74 kcal/mol above S0 in azides 1a and 1b, respectively. The calculated bond dissociation energies for breaking the C-N bond are 55 and 58 kcal/mol for azides 1a and 1b, respectively, making C-N bond breakage accessible from T1K in azides 1 at ambient temperature. In comparison, the irradiation of azides 1 in argon matrices at 14 K lead to the formation of the corresponding triplet alkyl nitrenes (1-n), via intramolecular energy transfer from T2K. The characterization of 1-n was supported by isotope labeling, IR spectroscopy, and molecular modeling.     

Electrical conductivity of π-conjugated polymers with nitro substituents

π-Conjugated polymers bearing nitro substituent(s), e.g., poly(aryleneethynylene) (PAE) type polymers and poly(4,8-dinitroanthraquinone-1,5-diyl) P(4,8-NO₂-1,5-AQ), show semiconducting properties with electrical conductivities of an order of 10⁻⁷ to 10⁻⁶ S · cm⁻¹ at room temperature without special oxidation and reduction of the polymer. P(4,8-NO₂-1,5-AQ) shows a large shift of phase in alternating current (ac) measurements and a unique magnetism at low temperature.     

Towards a photophysical model for 5-hydroxyflavone

The photophysics of the 5-hydroxyflavone (5HF) molecule has been revised. Conversely to what has been hitherto reported, the proton-transfer fluorescence of 5HF has been recorded under xenon lamp excitation in cyclohexane, hexane, ethanol, ethyl ether, 2-methyl-2-propanol, and dimethylsulfoxide at room temperature. The 5HF fluorescence spectra only exhibit one emission band centered at ca. 700 nm. A small photoreaction quantum yield of 10(-5)-10(-6) denotes the great photostability exemplified by 5HF in hydrocarbon solvent, ethanol, and dimethylsulfoxide. This great photostability is predominantly explained owing to an internal conversion process from the first excited singlet state 1(pi,pi*)1 (S1), which has a repulsive (proton-transfer) potential energy curve with respect to the stretching of the OH bond and only one energy minimum for the proton-transfer tautomer. The S1'-S0' energy gap proves to be small because of important modifications found in the molecular geometry of 5HF upon photoexcitation. A computational strategy, based upon theoretical calculations at the B3LYP density functional theory (DFT) and time-dependent DFT levels, supports the experimental spectroscopic evidence. Also an abnormal singlet-triplet splitting for a pi,pi* configuration has been found in 5HF.     

Fluorescence and ultraviolet absorption spectra and structure of coumaran and its ring-puckering potential energy function in the S1(pi,pi*) excited state

The fluorescence excitation (jet cooled), single vibrational level fluorescence, and the ultraviolet absorption spectra of coumaran associated with its S1(pi,pi*) electronic excited state have been recorded and analyzed. The assignment of more than 70 transitions has allowed a detailed energy map of both the S0 and S1 states of the ring-puckering (nu45) vibration to be determined in the excited states of nine other vibrations, including the ring-flapping (nu43) and ring-twisting (nu44) vibrations. Despite some interaction with nu43 and nu44, a one-dimensional potential energy function for the ring puckering very nicely predicts the experimentally determined energy level spacings. In the S1(pi,pi*) state coumaran is quasiplanar with a barrier to planarity of 34 cm(-1) and with energy minima at puckering angles of +/-14 degrees. The corresponding ground state (S0) values are 154 cm(-1) and +/-25 degrees . As is the case with the related molecules indan, phthalan, and 1,3-benzodioxole, the angle strain in the five-membered ring increases upon the pi-->pi* transition within the benzene ring and this increases the rigidity of the attached ring. Theoretical calculations predict the expected increases of the carbon-carbon bond lengths of the benzene ring in S1, and they predict a barrier of 21 cm(-1) for this state. The bond length increases at the bridgehead carbon-carbon bond upon electron excitation to the S1(pi,pi*) state give rise to angle changes which result in greater angle strain and a nearly planar molecule.