Study Of The Photodissociation Dynamics Of Pentaatomic Heterocycles

There has been interest for many years in excited state dynamics of thiophene, furan and other pentaatomic heterocycles because of their role in the pharmaceutical synthesis, resins and organic synthesis, etc. With the photoelectric materials research and development, thiophene oligomers have used extensively in light-emitting diodes, lasers and field-effect transistors because of their excellent electron and energy transfer properties through theÏ€-conjugation. From then on, how to optically control such molecular wires of thiophene oligomers by external stimuli becomes an important theme. As is now generally recognized, the S₁ state lifetimes of thiophene oligomers increases with the aggregate length. conical intersection of electronic potential energy surfaces(PES) play an central role in ultrafast internal conversion, radiationless decay etc. The information about conical intersection gained from examination of the excited-state PES helps to investigation of the excited-state dynamics.In this paper, the short-time photodissociation dynamics of thiophene, furan, selenophene and thiazole in cyclohexane and methanol solution have been investigated by the Resonance Raman spectra in combination with DFT and CASSCF calculation. We also discussed the photodissociation process in the photochemical reactions. Main contributions of the present work are summarized as follows,(1) We obtained the absorption spectrum of thiophene in cyclohexane solution which has a charge-transfer(CT) band near 240nm. Resonance Raman spectrums were obtained for thiophene in cyclohexane solution with 239.5nm and 266nm excitation wavelength in resonance with the CT-band absorption spectrum. The results indicate that the Franck-Condon region short-time photodissociation dynamics of thiophene have multidimensional characters with motion predominantly along the nominal C₂=C₃-C₄=C₅ in-plane symmetric stretch modesν₃ (A₁) at 1394 cm₁ andν₄ (A₁) at 1344 cm₁, the nominal C-H in-plane wag modeν₅ (A₁) at 1074 cm₁, the nominal C₃-C₄ stretch +C=C-C bend modeν₆ (A₁) at 1028 cm₁, the nominal C-S in-plane symmetric stretch+C-C=C bend modeν-7 (A₁) at 835 cm₁, the nominal C-S anti-symmetry stretch+C-C=C bend modeν₂1 (B₂) at 754 cm₁. The nominal C-S anti-symmetric stretch + C-C=C bend modeν₂1 (B₂) at 754 cm₁ possesses much more Raman intensity than the nominal C-S in-plane symmetric stretch + C-C=C bend modeν₇ (A₁) at 835 cm₁.This indicates that thiophene molecule in the S₁ state undergoes large bond length changes along the two C-S bond lengths with one C-S bond becoming much longer while the other becomes somewhat shorter. This kind of short-time dynamics suggests that a ring opening reaction may occur in or nearby the Franck-Condon region due to a curve crossing between S₁ and Sn state.The electronic transition energies, the excited state structures and the conical intersection points CI(S-3/S₁) and CI(S₂/S₁) between the S₁ and the S₂ or S₃ potential energy surfaces of thiophene were determined by using complete active space self-consistent field theory computations. These results were correlated to the Franck–Condon region photodissociation dynamics of thiophene in its S₁ state. The ring opening photodissociation reaction pathway through cleavage of one of the C-S bonds and via a conical intersection point CI(S₃/S₁) was revealed to be the predominant ultrafast reaction channel for thiophene in the S₁ state. The internal conversion pathway via a conical intersection point CI(S₂/S-1) was also found to be another reaction channel for thiophene.(2) We obtained the absorption spectrum of selenophene in cyclohexane solution which has a charge-transfer(CT) band near 250nm. Resonance Raman spectrums were also obtained for selenophene in cyclohexane solution with 239.5nm and 252.7nm excitation wavelength in resonance with the CT-band absorption spectrum. The results indicate that the Franck-Condon region short-time photodissociation dynamics of selenophene have multidimensional characters with motion predominantly along the nominal nominal C₂=C₃-C₄=C₅ in-plane symmetric stretch modesν₃ (A₁) at 1416 cm₁ andν₄ (A₁) at 1345 cm₁, the nominal C-H in-plane wag modeν₅ (A₁) at 1073 cm₁, the nominal C₃-C₄ stretch +C=C-C bend modeν₆ (A₁) at 1012 cm₁, the nominal C-Se in-plane symmetric stretch+C-C=C bend modeν₇ (A₁) at 755 cm₁, the nominal C-Se anti-symmetry stretch+C-C=C bend modeν₂1 (B-2) at 624 cm₁. FT-Raman spectroscopy, Resonance Raman spectroscopy and spectral assignments of thiophene is similar to selenophene. so we conclude that the Frank-Condon region short-time photodissociation dynamics of selenophene almost completely seems the thiophene.(3) We obtained the absorption spectrum of furan in cyclohexane solution which has a charge-transfer band near 209nm. Resonance Raman spectrums were obtained for furan in cyclohexane solution with 217.8nm and 208.8nm excitation wavelength in resonance with CT-band absorption spectrum and Resonance Raman analysis was done. The results indicate that the Franck- Condon region short-time photodissociation dynamics of furan have multidim- ensional characters with the reaction coordinates with the nominal C3-C4 stretch+C=C?C bendν₄ (Δ=1.01,λ=702 cm₁), the nominal C₂=C₃/C₄=C₅ stretch + C-O-C bendν₃ (Δ=0.98,λ=712 cm₁). The asymmetric mode is not appear on the Resonance Raman spectra of furan and spectral assignments of furan is very different from thiophene, for these reason we conclude that the Frank-Condon region short-time photodissociation dynamics of furan is quite different from thiophene.(4) We obtained the absorption spectra of thiazole in cyclohexane and methanol solution which has a charge-transfer band near 230nm. Resonance Raman spectrums were obtained for furan in cyclohexane solution with 228.7nm and 239.5nm and in methanol solution with 239.5nm excitation wavelength in resonance with CT-band absorption spectrum and Resonance Raman analysis was done. The results indicate that the Franck- Condon region short-time photodissociation dynamics of thiazole have multidimensional characters with the reaction coordinates or displacements occurring with the nominal CH rockν₁5 (Δ=1.20,λ=572 cm₁), the nominal C₂=N₃-C₄=C₅ in-plane symmetric stretchν₅ (Δ=1.12,λ=864 cm₁), the nominal C-S anti-symmetry stretch+C-C=C bendν₁2 (Δ=0.95,λ=341 cm₁). As the replacement of nitrogen atoms, the nominal C-S anti-symmetric stretch possesses much more Raman intensity than the nominal C-S in-plane symmetric stretch, similarly the nominal C-S anti-symmetric stretch of thiazole possesses much more Raman intensity than thiophene. We note that the substitution of the C atom by the N atom fewly modulates the Frank-Condon region photodissociation dynamics. Which indicated thiazole and thiophene have the similar photodissociation process in the Frank-Condon region.