Chapter 15 Molecular Luminescence Spectrometry

A very sensitive and selective instrumental technique with some of the lowest LOD's for molecules that luminesce (not all do!) - down to single-molecule detection. diamagnetic paramagnetic isoelectronic in external magnetic field isoelectronicsplits into 3 energy levels short (ns) lifetimes; "allowed" longer (  s or longer) lifetimes; "forbidden" large  small 

Note: fluorescence, phosphorescence, and non-radiative decay can all occur simultaneously depending on the "quantum yields". ps (Jablonski Diagrams)

The quantum yield for a given process is the ratio of the number of molecules that undergo a particular process to the total number of excited states. SoSo S1S1 T1T1

Kasha's Rule: fluorescence in fluid solution always occurs from the lowest vibrational level of the 1 st excited singlet state. S o (  ) collisions C = O H H. : ** e-e C  O H H : : e-v C = O H H : : ~ ~ solvent molecules S 1 (  *) S o (  ) S 1 (  *) e-t

SoSo S3S3 S2S2 S1S1 Favored by overlapping vibrational levels. SoSo S1S1 T1T1 Change in spin state from a singlet to a triplet and vica-versa. Heavy atom effect – atoms and small molecules such as Hg, Br 2, I 2, and O 2 have large “spin-orbit coupling” that helps flip spin.

excited state population C → time → CoCo  unique and useful for identification inversely proportional to  C o = initial population of the excited state k = 1 st order rate constant (units s -1 )

Most intense from molecules containing multiple aromatic rings,  →  * transitions, and structural rigidity. no fluorescence fluorescence  f = 1.0  f = 0.2

excitation spectra – measure luminescence at a fixed wavelength while scanning the excitation wavelengths. emission spectra - excite at a fixed wavelength while scanning the luminescence. excitation spectra – measure luminescence at a fixed wavelength while scanning the excitation wavelengths. emission spectra - excite at a fixed wavelength while scanning the luminescence. overlap at the 0-0 band.

The absorbance compensating cell contains the dye molecule Rhodamine B which absorbs all wavelengths equally and emits in a pattern that mimics the variation in the lamp output. The corrected excitation spectra are produced by dividing the raw spectrum with the Rhodamine B spectrum.

The ability to distinguish and identify different molecules based on differences in luminescence spectra. Frequently enhanced by cooling the sample.

PoPo PtPt PfPf P f =  f P a =  f (P o – P t ) =  f P o (1 – P t /P o ) from Beer’s Law: PtPaPtPa = 10 -  bc so P f =  f P o (1 – 10 -  bc ) Converting to base e and expanding e -  bc in a MacLaurin Series* – P f =  f P o (1 – 2.303e -  bc ) P f =  f P o [2.303  bc – (2.303  bc) 2 /2! + (2.303  bc) 3 /3! +….. P f =  f P o  bc *

A = -log P/P o Why fluorescence is inherently more sensitive.

anthracene (MW = 178)

1.Fluorimetric Determination of Inorganic Species 2.Fluorimetric Determination of Organic Species 3.Forensics (latent blood stains - luminol chemiluminescence) 4.Fluorescence Imaging Methods

Polynuclear Aromatic Hydrocarbons (PAH's) in the environment. perylene 1,2-benzanthracene anthracene fluorescence LOD phosphorescence LOD (parts per trillion) (parts per billion)