Twisted intramolecular charge transfer states: rotationally resolved fluorescence excitation spectra of 4,4'-dimethylaminobenzonitrile in a molecular beam

We report the observation at high resolution of seven vibronic bands that appear within approximately 200 cm(-1) of the electronic origin in the S(1)-S(0) fluorescence excitation spectrum of 4,4'-dimethylaminobenzonitrile (DMABN) in a molecular beam. Surprisingly, each band is found to be split into two or more components by a (coordinated) methyl group tunneling motion which significantly complicates the analysis. Despite this fact, high quality [(Observed-Calculated)< or =30 MHz] fits of each of the bands have been obtained, from which the rotational constants, inertial defects, torsion-rotation interaction constants, methyl group torsional barriers, and transition moment orientations of DMABN in both electronic states have been determined. The data show that DMABN is a slightly pyramidalized (approximately 1 degree) but otherwise (heavy-atom) planar molecule in its ground S(0) state, and that its electronically excited S(1) state has both a more pyramidalized (approximately 3 degrees) and twisted (approximately 25 degrees) dimethylamino group. Large reductions in the methyl group torsional barriers also show that the S(1)<--S(0) electronic transition is accompanied by significant charge transfer from the nitrogen atom to the pi* orbitals of the aromatic ring. Thereby established is the participation of all three vibrational coordinates in the dynamics leading to the "anomalous" emissive behavior of DMABN in the condensed phase.     

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.     

Fluorescence and ultraviolet absorption spectra, and the structure and vibrations of 1,2,3,4-tetrahydronaphthalene in its S1(pi,pi*) state

The ultraviolet absorption spectra in the static vapor phase and the laser induced fluorescence spectra (both fluorescence excitation and single vibronic level fluorescence spectra) of jet-cooled 1,2,3,4-tetrahydronaphthalene have been used along with theoretical calculations to assign many of the vibronic levels in the S1(pi,pi*) state. These have been compared to the corresponding vibrational levels for the S0 ground state. Analysis of the upper states of the ring-twisting vibration nu(31) and three other low-frequency modes has allowed us to construct an energy map of the lowest vibrational quantum states for both S0 and S1. The molecule is highly twisted in both electronic states with high barriers to planarity, which are calculated to be 4811 cm(-1) for S0 and 5100 cm(-1) for S1. However, the experimental data show that the barrier should be lower in the S1 state.     

Paracyclophanes as model compounds for strongly interacting π-systems. Part 2: mono-hydroxy[2.2]paracyclophane

The structure of the electronic ground- and first excited state of mono-hydroxy [2.2]paracyclophane (MHPC) and the S(1)← S(0) electronic transition have been investigated by resonance-enhanced multiphoton ionisation (REMPI) and by quantum chemical spin-component-scaled-approximate coupled cluster second order (SCS-CC2) computations. The origin of the S(1)← S(0) transition was located at 30,772 cm(-1) (3.815 eV) in the REMPI spectrum. The value has to be compared with a computed excitation energy of 3.79 eV. The vibrational structure of the spectrum confirms a significant geometry change upon excitation along the coordinates corresponding to twist- and shift-motions in the molecule. It gives rise to an experimentally observed progression with a fundamental of +30 cm(-1) and an inverse anharmonicity. From the experimental data a shallow potential along the twist coordinate was derived for the S(1) state. For the shift vibration a wavenumber of +91 cm(-1) was observed, while +85 cm(-1) was computed. The ionisation energy of MHPC was determined to be 7.63 ± 0.05 eV using synchrotron radiation. When compared to earlier results on the parent compound [2.2]paracyclophane and pseudo-ortho-dihydroxy[2.2]paracyclophane it can be seen that already small variations in the substitution pattern have a significant impact on the shapes of the involved potential energy surfaces leading to strong variations in ground and excited state geometries and opto-electronic properties governing the exciton transfer processes.     

Rotationally resolved electronic spectra of 1,2-dimethoxybenzene and the 1,2-dimethoxybenzene-water complex

Rotationally resolved S(1) <-- S(0) electronic spectra of 1,2-dimethoxybenzene (DMB) and its water complex have been observed and assigned. The derived values of the rotational constants show that the bare molecule has a planar heavy-atom structure with trans-disposed methoxy groups in its ground and excited electronic states. The transition of DMB is polarized along the b-axis bisecting the methoxy groups, demonstrating that its S(1) state is an (1)L(b) state. Higher energy bands of DMB are also polarized along the b-axis and have been tentatively assigned to different vibrational modes of the (1)L(b) state. The water complex origin appears 127 cm(-1) to the blue of the bare molecule origin. Analyses of the high resolution spectra of DMB/H(2)O and DMB/D(2)O suggest that the water molecule is attached via two O-H...O hydrogen bonds to the methoxy groups in both electronic states. A tunneling motion of the attached water molecule is revealed by a splitting of these spectra into two subbands. Potential barriers to this motion have been determined.     

The intramolecular vibrational energy redistribution threshold in S(1) deuterated p-difluorobenzene

The intramolecular vibrational energy redistribution (IVR) in S(1) deuterated p-difluorobenzene (pDFB-d(4) or -d(4)) has been studied to determine the IVR threshold. For this, the S(1) <-- S(0) fluorescence excitation (FE) spectrum of jet-cooled d(4) was investigated in the 2000-3250 cm(-1) vibronic energy range of the S(1) electronic state, and single vibronic level fluorescence (SVLF) spectra have been acquired by exciting selected levels lying between 750 and 2850 cm(-1) in vibrational energy in the S(1) excited state. Congestion of the dispersed fluorescence in this molecule first appears as the vibrational level energy climbs above 2000 cm(-1). By comparing the SVLF spectra of pDFB-d(4) with those of p-difluorobenzene (pDFB or -h(4)), it is obvious that IVR threshold in -d(4) is localized with a few hundreds cm(-1) lower than that in pDFB. This decrease is entirely due to the increase in vibrational state density due to deuteration.     

Laser induced fluorescence and resonant two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone

We carried out laser induced fluorescence and resonance enhanced two-color two-photon ionization spectroscopy of jet-cooled 1-hydroxy-9,10-anthraquinone (1-HAQ). The 0-0 band transition to the lowest electronically excited state was found to be at 461.98 nm (21,646 cm(-1)). A well-resolved vibronic structure was observed up to 1100 cm(-1) above the 0-0 band, followed by a rather broad absorption band in the higher frequency region. Dispersed fluorescence spectra were also obtained. Single vibronic level emissions from the 0-0 band showed Stokes-shifted emission spectra. The peak at 2940 cm(-1) to the red of the origin in the emission spectra was assigned as the OH stretching vibration in the ground state, whose combination bands with the C=O bending and stretching vibrations were also seen in the emission spectra. In contrast to the excitation spectrum, no significant vibronic activity was found for low frequency fundamental vibrations of the ground state in the emission spectrum. The spectral features of the fluorescence excitation and emission spectra indicate that a significant change takes place in the intramolecular hydrogen bonding structure upon transition to the excited state, such as often seen in the excited state proton (or hydrogen) transfer. We suggest that the electronically excited state of interest has a double minimum potential of the 9,10-quinone and the 1,10-quinone forms, the latter of which, the proton-transferred form of 1-HAQ, is lower in energy. On the other hand, ab initio calculations at the B3LYP/6-31G(d,p) level predicted that the electronic ground state has a single minimum potential distorted along the reaction coordinate of tautomerization. The 9,10-quinone form of 1-HAQ is the lowest energy structure in the ground state, with the 1,10-quinone form lying approximately 5000 cm(-1) above it. The intramolecular hydrogen bond of the 9,10-quinone was found to be unusually strong, with an estimated bond energy of approximately 13 kcal/mol (approximately 4500 cm(-1)), probably due to the resonance-assisted nature of the hydrogen bonding involved.     

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.     

Excited state electron transfer precedes proton transfer following irradiation of the hydrogen-bonded single water complex of 7-azaindole with UV light

High resolution electronic spectra of the single water complex of 7-azaindole (7AIW) and of a deuterated analog (7AIW-d(3)) have been recorded in a molecular beam, both in the absence and presence of an applied electric field. The obtained data include the rotational constants of both complexes in their ground (S(0)) and first excited (S(1)) electronic states, their S(1)-S(0) electronic transition moment and axis-tilting angles, and their permanent electric dipole moments (EDM's) in both electronic states. Analyses of these data show that the water molecule forms two hydrogen bonds with 7AI, a donor O-H···N(7) bond and an acceptor O···H-N(1) bond. The resulting structure has a small EDM in the S(0) state (μ = 0.54 D) but a greatly enhanced EDM in the S(1) state (μ = 3.97 D). We deduce from the EDM's of the component parts that 0.281 e(-) of charge is transferred from the acidic N(1)-H site to the basic N(7) site upon UV excitation of 7AIW, but that water-assisted proton transfer from N(1) to N(7) does not occur. A model of the resulting electrostatic interactions in the solute-solvent pair predicts a solvent-induced red-shift of 1260 cm(-1) which compares favorably to the experimental value of 1290 cm(-1).     

Mössbauer, electron paramagnetic resonance, and theoretical study of a high-spin, four-coordinate Fe(II) diketiminate complex

The iron(II) complex LFeCl 2Li(THF) 2 (L = beta-diketiminate), 1, has been studied with variable-temperature, variable-field Mossbauer spectroscopy and parallel mode electron paramagnetic resonance (EPR) spectroscopy in both solution and the solid state. In zero applied field the 4.2 K Mossbauer spectrum exhibits an isomer shift delta = 0.90 mm/s and quadrupole splitting Delta E Q = 2.4 mm/s, values that are typical for the high-spin ( S = 2) state anticipated for the iron in 1. Spectra recorded in applied magnetic fields yield an anisotropic magnetic hyperfine tensor with A x = +2.3 (+ 1.0) T, A y = A z = -21.5 T ( solution) and a nearly axial zero-field splitting of the spin quintet with D = D x approximately -14 cm (-1) and rhombicity E/ D approximately 0.1. The small, positive value for A x results from the presence of residual orbital angular momentum along x. The EPR analysis gives g x approximately 2.4 (and g y approximately g z approximately 2.0) and reveals a split " M S = +/- 2" ground doublet with a gap distributed around Delta = 0.42 cm (-1). The Mossbauer spectra of 1 show unusual features that arise from the presence of orientation-dependent relaxation and a distribution in the magnetic hyperfine field along x. The origin of the distribution has been analyzed using crystal field theory. The analysis indicates that the distribution in the magnetic hyperfine field originates from a narrow distribution, sigma phi approximately 0.5 degrees , in torsion angle phi between the FeN 2 and FeCl 2 planes, arising from minute inhomogeneities in the molecular environments.     

Laser-induced fluorescence and single vibronic level emission spectroscopy of chiral (R)-1-aminoindan and some of its clusters in a supersonic jet

Two low energy conformers of the chiral (R)-1-aminoindan molecule are identified in supersonic jet and their ground and excited states vibrational spectroscopy has been investigated by laser-induced fluorescence (LIF) excitation and single vibronic level (SVL) emission spectroscopy. Ab initio calculations confirm the existence of two lowest-energy structures, where the amino group is in equatorial position with its lone pair directed opposite to the aromatic electron cloud. Harmonic frequencies have been calculated for these two conformers at the DFT level with B3LYP functional. A low-frequency progression of 118 cm(-1) and 114 cm(-1), respectively, appears in the fluorescence excitation spectrum of the two conformers, with its ground state counterpart at approximately 147 cm(-1). It has been assigned to the puckering motion coupled with the ring flapping mode. The other calculated low-frequency mode corresponds to the puckering motion coupled with the ring twisting mode and its ground state frequency has been observed at 119 cm(-1) and 111 cm(-1) from SVL spectra. Both conformers form similar 1 : 1 water clusters, whose 0-0 transitions are shifted to the blue by 41 cm(-1) and 44 cm(-1), respectively, and whose SVL spectra are similar. Interestingly, one of the conformers seems to preferentially make complexes with (S)-methyllactate, while the other one shows selective complexation to (R)-methyllactate.     

Electronic spectroscopy and photoisomerization of trans-urocanic acid in a supersonic jet.

trans-Urocanic acid (trans-UA), a component of the epidermal layer of skin, exhibits wavelength-dependent photochemistry. The quantum efficiency of isomerization to cis-UA is greatest when the molecule is excited on the long wavelength tail of its absorption profile in solution (300-320 nm). However, exciting the molecule where it absorbs UV light most efficiently (260-285 nm) causes almost no isomerization. We have used fluorescence excitation and dispersed emission methods in a supersonic jet to investigate the electronic states involved in this complex and interesting photochemistry. Three distinct regions are present in the excitation spectrum. Region I, which is below the isomerization barrier, contains sharp, well-resolved peaks that upon excitation emit from the S(1) state of trans-UA. Region II exhibits peaks that increase in broadness and decrease in intensity with increasing excitation energy. Upon excitation these peaks produce dual emission from the S(1) states of both trans- and cis-UA. The trans to cis isomerization barrier is estimated to be 1400 cm(-1). Region III exhibits excitation to the S(2) electronic state and has a broad structure that spans 3000 cm(-1) and occurs 4000 cm(-1) above S(1). S(2) excitation results in essentially no trans to cis isomerization.     

Electronic state spectroscopy of c-C5F8 explored by photoabsorption, electron impact, photoelectron spectroscopies and ab initio calculations

The electronic transitions and resonance-enhanced vibrational excitations of octafluorocyclopentene (c-C5F8) have been investigated using high-resolution photoabsorption spectroscopy in the energy range 6-11 eV. In addition, the high-resolution electron energy loss spectrum (HREELS) was recorded under the electric dipolar excitation conditions (100 eV incident energy, approximately 0 degrees scattering angle) over the 5-14 eV energy loss range. A He(I) photoelectron spectrum (PES) has also been recorded between 11 and 20 eV, allowing us to derive a more precise value of (11.288 +/- 0.002) eV for the ground neutral state adiabatic ionization energy. All spectra presented in this paper represent the first and highest resolution data yet reported for octafluorocyclopentene. Ab initio calculations have been performed for helping in the assignment of the spectral bands for both neutral excited states and ionic states.     

Fine structure of the (S(1)<--S(0)) band origins of phthalocyanine molecules in helium droplets

The laser-induced fluorescence (LIF) excitation spectra of free base phthalocyanine (Pc), Mg-Pc, and Zn-Pc molecules in superfluid helium droplets at T=0.38 K have been studied. The spectra reveal the rich vibronic structure of the S(1)<--S(0) electronic transitions. The band origins of the transitions consist of zero phonon lines accompanied by phonon wings, which originate from simultaneous electronic excitation of the molecule and excitation of the collective modes of the helium surrounding it. The phonon wings have discrete structures suggesting localization of some helium atoms in the neighborhood of the molecules. Zero phonon lines of Mg-Pc and Zn-Pc molecules are split into three components, which are separated by 0.2-0.4 cm(-1). Possible mechanism of splitting involves static or dynamic Jahn-Teller interaction of metal-phthalocyanine molecules in the twofold degenerate S(1)((1)E(u)) state with the helium shell.     

Optical control of excited-state vibrational coherences of a molecule in solution: The influence of the excitation pulse spectrum and phase in LD690

Spectral and phase shaping of femtosecond laser pulses is used to selectively excite vibrational wave packets on the ground (S0) and excited (S1) electronic states in the laser dye LD690. The transient absorption signals observed following excitation near the peak of the ground-state absorption spectrum are characterized by a dominant 586 cm(-1) vibrational mode. This vibration is assigned to a wave packet on the S0 potential energy surface. When the excitation pulse is tuned to the blue wing of the absorption spectrum, a lower frequency 568 cm(-1) vibration dominates the response. This lower frequency mode is assigned to a vibrational wave packet on the S1 electronic state. The spectrum and phase of the excitation pulse also influence both the dephasing of the vibrational wave packet and the amplitude profiles of the oscillations as a function of probe wavelength. Excitation by blue-tuned, positively chirped pulses slows the apparent dephasing of the vibrational coherences compared with a transform-limited pulse having the same spectrum. Blue-tuned negatively chirped excitation pulses suppress the observation of coherent oscillations in the ground state.     

Surface-enhanced resonance Raman scattering and density functional calculations of hemicyanine adsorbed on colloidal silver surface

Resonance Raman (RR) and surface-enhanced resonance Raman scattering (SERRS) of 4'-(N,N'-dimethylaminostyryl)-4-propylpyridinium bromide (hemicyanine, HC dye) in acetonitrile solution and on a colloidal silver surface have been investigated. The structure of the dye in the ground (S0) and excited (S1) electronic states was optimized using density functional calculations along with the B3LYP and the configuration interaction with the singlet excitation (CIS) methods, respectively, using the 6-31G basis set. The vibrational frequencies of the molecule were computed at the optimized geometry and compared with the observed Raman bands. A complete normal-mode analysis has been carried out because it is essential for the accurate assignment of the vibrational spectra. From the observed enhancement along various in-plane and out-of-plane vibrations in the SERRS spectrum and from theoretical calculations, it has been inferred that the interaction with the silver surface occurs via the nitrogen lone pair of the pyridyl or the dimethylamino group of the molecule with a tilted orientation. The observed red-shifts in the SERRS spectrum along various vibrations indicate strong interaction (chemisorption) of the HC dye with the silver surface. This is also supported by the presence of a Ag-N stretching vibration at 241 cm(-1). The effect of the dye concentration on the orientation of the molecule is also discussed.     

High-resolution spectroscopy of jet-cooled 1,1'-diphenylethylene: electronically excited and ionic states of a prototypical cross-conjugated system

The photophysics of a prototypical cross-conjugated π-system, 1,1'-diphenylethylene, have been studied using high-resolution resonance enhanced multiphoton ionization excitation spectroscopy and zero kinetic energy photoelectron spectroscopy, in combination with advanced ab initio calculations. We find that the excitation spectrum of S(1) displays extensive vibrational progressions that we identify to arise from large changes in the torsional angles of the phenyl rings upon electronic excitation. The extensive activity of the antisymmetric inter-ring torsional vibration provides conclusive evidence for a loss of symmetry upon excitation, leading to an inequivalence of the two phenyl rings. Nonresonant zero kinetic energy photoelectron spectroscopy from the ground state of the neutral molecule to the ground state of the radical cation, on the other hand, demonstrates that upon ionization symmetry is retained, and that the geometry changes are considerably smaller. Apart from elucidating how removal of an electron affects the structure of the molecule, these measurements provide an accurate value for the adiabatic ionization energy (65274 ± 1 cm(-1) (8.093 eV)). Zero kinetic energy photoelectron spectra obtained after excitation of vibronic levels in S(1) confirm these conclusions and provide us with an extensive atlas of ionic vibronic energy levels. For higher excitation energies the excitation spectrum of S(1) becomes quite congested and shows unexpected large intensities. Ab initio calculations strongly suggest that this is caused by a conical intersection between S(1) and S(2).     

Laser-induced fluorescence spectra, structure, and the ring-twisting and ring-bending vibrations of 1,4-benzodioxan in its S0 and S1(pi,pi*) states

The laser-induced fluorescence (LIF) spectra, both the fluorescence excitation spectra (FES) and single vibrational level fluorescence spectra (SVLF) from several different vibronic states, along with the ultraviolet (UV) absorption spectra of 1,4-benzodioxan have been recorded and analyzed. A detailed energy map has been constructed for four low-frequency vibrations and their combinations for both the S(0) and S(1)(pi,pi) electronic states. These are nu(48) (ring-bending), nu(25) (ring-twisting), nu(47) (ring-flapping), and nu(24) (skeletal-twisting). Both the experimental and ab initio calculations show the molecule to be twisted in both the S(0) and S(1)(pi,pi) states with high barriers to planarity. The experimentally determined ring-twisting quantum states, which are confined to the lower regions of the potential energy surface, were used to calculate one-dimensional potential functions in terms of the twisting coordinates, and the extrapolated barriers were estimated to be 5700 and 4200 cm(-1) for the S(0) and S(1) states, respectively. Two-dimensional calculations, which included the interactions with the bending modes, gave values of 3906 and 1744 cm(-1), respectively. The S(0) value compares favorably with the ab initio value of 4095 cm(-1).     

Interplanar torsion in the S1<--S0 electronic spectrum of jet cooled 1-phenylimidazole

The S(1)<--S(0) transition of 1-phenylimidazole (1PI) has been studied in a supersonic jet expansion by resonant two-photon ionization. The origin band at 36 075 cm(-1) is accompanied by a low frequency progression associated with torsion about the bond connecting phenyl and imidazole groups. Torsional potentials have been determined for both states. In S(0), phi(min)=37.2+/-0.5 degrees and the planar barrier is 339+/-20 cm(-1), while in S(1), phi(min)=17.6+/-0.5 degrees and the planar barrier is 57+/-2 cm(-1). The transition moment alignment is observed to be consistent with an excited state of L(b) character, in spite of the "off-axis" conjugation provided by the imidazole ring. These results are compared with ab initio calculations on both states, performed using Hartree-Fock, M?ller-Plesset second-order perturbation, density functional theory with the Becke3-Lee-Yang-Parr functional, time-dependent density functional theory, configuration interaction singles, and complete active space self-consistent field methods. Solution-phase UV spectra of neutral and protonated 1PI are also reported.     

The excited states and vibronic spectroscopy of diphenyldiacetylene and diphenylvinylacetylene

Laser induced fluorescence (LIF) excitation scans and dispersed fluorescence (DFL) spectra have been recorded for two four-carbon α,ω-diphenyl systems, diphenyldiacetylene (DPDA, φ-C≡C-C≡C-φ) and trans-diphenylvinylacetylene (DPVA, φ-CH≡CH-C≡C-φ) as isolated molecules cooled in a supersonic expansion. While these molecules have similar conjugation length, they exhibit strikingly different vibronic spectroscopy and photophysics. The near-UV LIF excitation spectrum of diphenyldiacetylene has its electronic origin at 32,158 cm(-1), and a strong progression in the C≡C stretch (2156 cm(-1)). All transitions are inherently broad, with widths of ~30 cm(-1) fwhm or greater. The S(1) origin DFL spectrum is composed of sharp transitions with Franck-Condon activity mirroring that in the excitation spectrum, and broad emission shifted well to the red ascribable to phosphorescence on the μs timescale. Using ab initio calculations, it is possible to show that DPDA exists as a single, planar conformer with D(2h) symmetry. In contrast, trans-diphenylvinylacetylene shows intense sharp transitions in both LIF and DFL spectra with an S(0)-S(1) origin of 31,183.2 cm(-1) and long progressions involving the in-plane fundamentals ν(53) (bridge-phenyl bending) and ν(51) (bridge-phenyl stretch). A sharp reduction in fluorescence yield in DPVA occurs within 300 cm(-1) of the S(1) origin. Possible causes for the photophysical processes occurring in the two molecules are discussed.