![]() ![]() One approach is by emissions from higher excited electronic states (e.g., S 3 to S 0, S 2 to S 1). Exceptions that break Kasha’s rule exist. Even if an electron is excited into higher electronic states, it rapidly relaxes back to S 1 through nonradiative decay, followed by fluorescence emission and resulting in an identical emission spectrum. The microscopic explanation of Kasha’s rule is that for most fluorophores, the emission only involves the transition from the lowest excited electronic state (S 1) to the ground state (S 0). Kasha’s rule states that the same fluorescence emission spectrum is generally observed irrespective of the excitation wavelength, meaning that typically the fluorescence emission spectrum of a fluorophore remains the same upon a change in the excitation wavelength. The intrinsic band-shifting behavior is a counterintuitive luminescent phenomenon that violates Kasha’s rule and has gained more and more attention due to the great potential in various optical applications. Furthermore, we coined the term “intrinsic band-shifting” referring to environmental-factor-independent and excitation-wavelength-dependent emission. Here, we suggest adopting “band-shifting” and “band shift” to describe the phenomenon that the photoluminescence emission wavelength change with excitation wavelength and certain environmental factors, such as temperature and solvent, which can better unify the same phenomenon in different fields and eliminate ambiguity caused by existing naming confusion. For example, not all the so-called environmental-factor-dependent fluorescence/emission literature are related to band shift since some of them only show environmental-factor-dependent intensity change (i.e., only the emission intensity varies with the change of certain factors) and do not change the emission wavelength or spectral line shape. In the organic photoluminescent materials community, “environmental factor-dependent fluorescence/emission” is the most commonly used description, probably due to the clear indication of the influencing factor of the emission band, but often times, this is unclear and confusing because the nature of the fluorescence change is not clearly stated (intensity, wavelength, phase or polarization, etc.). Different material research communities lack a unified term for this emission shift phenomenon. Besides temperature, many other factors, such as pH, solvent/polarity, molecular weight, pressure/mechanical force, and viscosity, can also cause band shift. Temperature, for example, is one of the common factors to cause band shift and has been harnessed for fluorometric temperature sensing or luminescence thermometry applications. From a photoluminescent materials perspective, CA-based fluorophores and polymers represent an emerging class of optical materials, thereof some possessing high quantum yields (>70%), while others exhibiting intriguing photophysical properties, such as band-shifting behaviors.īand-shifting behavior in photoluminescent materialsĪny environmental factor that influences the emission band of a material can result in band shift. In recent years, CA-based polymers and small molecules have emerged as a novel family of materials for a large number of applications in different fields, including tissue engineering and regenerative medicine, biosensing, imaging, and antimicrobial treatment. Since it has 3 carboxyl groups and 1 hydroxyl group, the abundance of functional groups renders CA great flexibility to react with diverse chemicals and produce various types of chemical products such as ester, imide, urethane, anhydride, and ether. Besides its crucial biological functions, CA can act as a versatile building block in the design of functional molecules and materials. Citric acid or citrate (CA) is an essential intermediate in the tricarboxylic acid cycle (aka citric acid cycle or Krebs cycle), a central metabolic pathway for most aerobic species, and participates in a variety of substance and energy metabolism activities. ![]()
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