Which factor increases emission bandwidth?
Emission bandwidth reflects spread of microenvironments and vibrational coupling during S1 lifetime. In highly polar solvent, fluorophores experience heterogeneous solvation shells with slightly different orientation and stabilization energies, causing inhomogeneous broadening. Dynamic solvent relaxation during nanosecond window creates distribution of S1 energies, broadening spectrum according to Franck-Condon envelope. Lowering temperature decreases collisional and vibrational heterogeneity, narrowing band significantly. Energy transfer mainly reduces intensity or shifts position but does not broaden inherently. High polarity solvent thus increases energetic heterogeneity, widening emission profile observed for biological probes and extrinsic dyes.
Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.