Excited electronic states of thiophene: high resolution photoabsorption Fourier transform spectroscopy and ab initio calculations

Abstract

The recently introduced synchrotron radiation-based Fourier transform spectroscopy has been employed to study the excited electronic states of thiophene. A highly resolved photoabsorption spectrum has been measured between ∼5 and 12.5 eV, providing a wealth of new data. High-level ab initio computations have been performed using the second-order algebraic-diagrammatic construction (ADC(2)) polarization propagator approach, and the equation-of-motion coupled-cluster (EOM-CC) method at the CCSD and CC3 levels, to guide the assignment of the spectrum. The adiabatic energy corrections have been evaluated, thereby extending the theoretical study beyond the vertical excitation picture and leading to a significantly improved understanding of the spectrum. The low-lying π→π* and π→σ* transitions result in prominent broad absorption bands. Two strong Rydberg series converging onto the X(~)(2)A2 state limit have been assigned to the 1a2→npb1(1)B2 and the 1a2→nda2(1)A1 transitions. A second, and much weaker, d-type series has been assigned to the 1a2→ndb1(1)B2 transitions. Excitation into some of the Rydberg states belonging to the two strong series gives rise to vibrational structure, most of which has been interpreted in terms of excitations of the totally symmetric ν4 and ν8 modes. One Rydberg series, assigned to the 3b1→nsa1(1)B1 transitions, has been identified converging onto the Ã(2)B1 state limit, and at higher energies Rydberg states converging onto the B(~)(2)A1 state limit could be identified. The present spectra reveal highly irregular vibrational structure in certain low energy absorption bands, and thus provide a new source of information for the rapidly developing studies of excited state non-adiabatic dynamics and photochemistry.

Aiming at an accurate prediction of vibrational and electronic spectra for medium‐to‐large molecules: An overview

“…Figure compares the TD AH FC spectra computed using different DFT functionals, at 298K, with the experimental photoionization spectra of thiophene . The good match between the theoretical envelope and experimental band at about 9 eV suggests that indeed the latter is related to the transition as postulated in earlier works . Moreover, in the current case, all functionals predict very similar band positions (within ± 0.1 eV), all only slightly (0.1–0.3 eV) red‐shifted with respect to the experimental band.…”

“…For this reason, a more reliable validation can only be achieved by simulating the band‐shape from the convolution of vibronic transitions, including, where necessary, solvent and temperature effects . Figure compares the TD AH FC spectra computed using different DFT functionals, at 298K, with the experimental photoionization spectra of thiophene . The good match between the theoretical envelope and experimental band at about 9 eV suggests that indeed the latter is related to the transition as postulated in earlier works .…”

“…The simulation of the vibrational and vibrationally resolved electronic spectra of thiophene (T) and 2,2′‐bithiophene (2T) will cover cases ranging from semirigid systems and small PES changes on electronic transition up to floppy systems with a well‐defined large‐amplitude degree of freedom. Those molecules have been extensively studied experimentally and used as benchmarks to test theoretical models . The importance of a direct comparison between theoretical and experimental spectra have been recently demonstrated from a joint study on the phosphorescence of 2,2′‐bithiophene .…”

“…In this respect, there are still experimental spectra of T and 2T, which require further theoretical investigations. For instance, the recently recorded high‐resolution photo‐absorption Fourier transform spectrum of thiophene calls for a more detailed assignment of its vibrational structure. Thiophene and its oligomers and derivatives are receiving also significant attention as important “building blocks” molecules, being the simplest hetero‐aromatic cycles of interest in astrochemistry or prototypes of π ‐conjugated systems for technological applications .…”

Excited state dynamics of thiophene and bithiophene: new insights into theoretically challenging systems

“…ADC (2) 5.82(0.093) 6.23(0.112) 6.45(0.011) 6.60(0.0) 6.77(0.0) MS-CASPT2 21 5.85(0.067) 6.14(0.109) 6.57(0.0) 6.65(0.0) -EOM-CCSD 59 5.78(0.081) 6.13(0.084) 6.33(0.013) 6.37(-) 6.19(-) DFT-MRCI 19 5 59 and no minimum was found at the CASPT2 level, 21 implying that the B 2 state is most likely unbound. Our ADC(2) computations support this view as no B 2 minimum was located.…”