Excited state deactivation processes are crucial in photochemistry. Internal conversion and intersystem crossing allow molecules to release energy without emitting light, influencing everything from photosynthesis to OLEDs.

Various factors affect these processes, including energy gaps, molecular structure, and environment. Understanding deactivation mechanisms is key for applications in fields like photodynamic therapy, atmospheric chemistry, and materials science.

Excited State Deactivation Processes

Internal conversion in excited states

• Internal conversion (IC) facilitates non-radiative transitions between electronic states with same spin multiplicity occurs between higher excited states or from lowest excited state to ground state
• IC rapidly converts electronic energy to vibrational energy on femtosecond to picosecond timescale competes with fluorescence
• Energy conservation during IC dissipates excess energy as heat to surrounding molecules (solvent molecules)
• Franck-Condon principle governs IC probability depends on overlap of vibrational wavefunctions (potential energy surfaces)

Intersystem crossing in photochemistry

• Intersystem crossing (ISC) enables non-radiative transitions between electronic states of different spin multiplicity usually from singlet to triplet state or vice versa
• ISC populates triplet states leads to phosphorescence and triplet-state reactivity (photosensitization)
• Spin-orbit coupling mechanism allows ISC to occur by mixing spin and orbital angular momentum
• ISC generally occurs slower than internal conversion typically on nanosecond to microsecond timescale

Factors affecting deactivation rates

• Energy gap law dictates smaller energy gaps between states lead to faster non-radiative transitions
• Vibronic coupling strength increases rate of internal conversion
• Heavy atom effect enhances spin-orbit coupling increasing ISC rate (iodine, bromine)
• Molecular structure influences deactivation rates:
  • Rigid structures often have slower IC rates
  • Flexible structures facilitate faster IC (aromatic hydrocarbons)
• Higher temperatures increase rates of both IC and ISC
• Solvent polarity and viscosity influence deactivation rates (polar solvents)
• Excited state geometry affects energy gaps and transition rates through conformational changes

Applications of deactivation processes

• Photosynthesis utilizes:
  1. IC in light-harvesting complexes to funnel energy
  2. ISC in reaction centers to generate long-lived charge-separated states
• Photodynamic therapy employs ISC in photosensitizers to produce reactive oxygen species (singlet oxygen)
• Organic light-emitting diodes (OLEDs) use ISC to harvest both singlet and triplet excitons for improved efficiency
• Photochromic materials control color-changing mechanisms through IC and ISC (sunglasses, smart windows)
• Atmospheric chemistry involves IC and ISC in ozone formation and depletion processes
• DNA photodamage occurs through ISC in thymine dimers formation under UV exposure
• Fluorescence quenching uses IC and ISC as mechanisms for reducing fluorescence intensity in sensors (molecular beacons)