Structural and Electronic Factors Controlling the Efficiency and Rate of Intersystem Crossing to the Triplet State in Thiophene Polycyclic Derivatives

• Published in: Chemistry : a European journal 2024-12, Vol.30 (68), p.e202402721-n/a
• Main Authors: Griffrith, Cameron, Krul, Sarah E., Hoehn, Sean J., Phan, Tram, Crespo‐Hernández, Carlos E.
• Format: Article
• Language: English
• Subjects: Absorption spectroscopy; Acetonitrile; Atomic energy levels; Benzene; Density functional theory; Dynamic structural analysis; Electronic structure; Emission spectroscopy; Environmental pollutants; Excitation; Intersystem crossing; Light emitting diodes; Mathematical analysis; Photodegradation; Photovoltaic cells; Photovoltaics; Polycyclic aromatic sulfur heterocycles; Sulfides; Thiophene derivatives; Triplet state; π-conjugated systems
• ISSN: 0947-6539; 1521-3765; 1521-3765
• DOI: 10.1002/chem.202402721

Thiophene polycyclic derivatives are widely used in organic light‐emitting diodes, photovoltaics, and medicinal chemistry applications. Understanding the electronic and structural factors controlling their intersystem crossing rates is paramount for these applications to be successful. This study investigates the photophysical, electronic structure, and excited state dynamics of 1,2‐benzodiphenylene sulfide, benzo[b]naphtho[1,2‐d]thiophene, and benzo[b]naphtho[2,3‐d]thiophene in polar aprotic and non‐polar solvents. Steady‐state absorption and emission spectroscopy, femtosecond transient absorption spectroscopy, and DFT and TD‐DFT calculations are employed. Low fluorescence quantum yields of 1.2 to 2.7 % are measured in acetonitrile and cyclohexene, evidencing that the primary relaxation pathways in these thiophene derivatives are nonradiative. Linear interpolation of internal coordinates calculations predict that an S−C bond elongation reaction coordinate facilitates the efficient intersystem crossing to the T1 state. Excitation of 1,2‐benzodiphenylene sulfide and benzo[b]naphtho[1,2‐d]thiophene at 350 nm or benzo[b]naphtho[2,3‐d]thiophene at 365 nm, populates the lowest‐energy 1ππ* state, which relaxes to the 1ππ* minimum in tens of picoseconds or intersystem crosses to the triplet manifold in ca. 500 ps to 1.1 ns depending on the position at which the benzene rings are added. Excitation at 266 nm does not affect the intersystem crossing rates. Laser photodegradation experiments demonstrate that the thiophene polycyclic derivatives are highly photostable. This study establishes the electronic factors that control the intersystem crossing to the triplet state in six thiophene polycyclic derivatives in acetonitrile and cyclohexane solutions. It further underscores the significance of the C−S bond elongation coordinate and structural modifications on the nonradiative intersystem crossing to the triplet manifold in these thiophene polycyclic derivatives.