| Summary: | The photophysical relaxation mechanisms of 1-cyclohexyluracil, in vacuum and water, were investigated by employing the Multi-State CASPT2 (MS-CASPT2, Multi-State Complete Active-Space Second-Order Perturbation Theory) quantum chemical method and Dunning’s cc-pVDZ basis sets. In both environments, our results suggest that the primary photophysical event is the population of the <inline-formula><math display="inline"><semantics><msub><mo>S</mo><mn>1</mn></msub></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mn>1</mn></msup><mrow><mo>(</mo><mi>π</mi><msup><mrow><mi>π</mi></mrow><mo>*</mo></msup><mo>)</mo></mrow></mrow></semantics></math></inline-formula> bright state. Afterwards, two likely deactivation pathways can take place, which is sustained by linear interpolation in internal coordinates defined via Z-Matrix scans connecting the most important characteristic points. The first one (Route 1) is the same relaxation mechanism observed for uracil, its canonical analogue, i.e., internal conversion to the ground state through an ethylenic-like conical intersection. The other route (Route 2) is the direct population transfer from the <inline-formula><math display="inline"><semantics><msub><mo>S</mo><mn>1</mn></msub></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mn>1</mn></msup><mrow><mo>(</mo><mi>π</mi><msup><mrow><mi>π</mi></mrow><mo>*</mo></msup><mo>)</mo></mrow></mrow></semantics></math></inline-formula> bright state to the <inline-formula><math display="inline"><semantics><msub><mi mathvariant="normal">T</mi><mn>2</mn></msub></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mn>3</mn></msup><mrow><mo>(</mo><mi>n</mi><msup><mrow><mi>π</mi></mrow><mo>*</mo></msup><mo>)</mo></mrow></mrow></semantics></math></inline-formula> triplet state via an intersystem crossing process involving the (<inline-formula><math display="inline"><semantics><msub><mo>S</mo><mn>1</mn></msub></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mn>1</mn></msup><mrow><mo>(</mo><mi>π</mi><msup><mrow><mi>π</mi></mrow><mo>*</mo></msup><mo>)</mo></mrow></mrow></semantics></math></inline-formula>/<inline-formula><math display="inline"><semantics><msub><mi mathvariant="normal">T</mi><mn>2</mn></msub></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mn>3</mn></msup><mrow><mo>(</mo><mi>n</mi><msup><mrow><mi>π</mi></mrow><mo>*</mo></msup><mo>)</mo></mrow></mrow></semantics></math></inline-formula>)<inline-formula><math display="inline"><semantics><msub><mrow></mrow><mrow><mi>S</mi><mi>T</mi><mi>C</mi><mi>P</mi></mrow></msub></semantics></math></inline-formula> singlet-triplet crossing point. As the spin-orbit coupling is not too large in either environment, we propose that most of the electronic population initially on the <inline-formula><math display="inline"><semantics><msub><mo>S</mo><mn>1</mn></msub></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mn>1</mn></msup><mrow><mo>(</mo><mi>π</mi><msup><mrow><mi>π</mi></mrow><mo>*</mo></msup><mo>)</mo></mrow></mrow></semantics></math></inline-formula> state returns to the ground following the same ultrafast deactivation mechanism observed in uracil (Route 1), while a smaller percentage goes to the triplet manifold. The presence of a minimum on the <inline-formula><math display="inline"><semantics><msub><mo>S</mo><mn>1</mn></msub></semantics></math></inline-formula><inline-formula><math display="inline"><semantics><mrow><msup><mrow></mrow><mn>1</mn></msup><mrow><mo>(</mo><mi>π</mi><msup><mrow><mi>π</mi></mrow><mo>*</mo></msup><mo>)</mo></mrow></mrow></semantics></math></inline-formula> potential energy hypersurface in water can help to understand why experimentally it is noticed suppression of the triplet states population in polar protic solvent.
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