E. C. Lim

Quantitative Study of Luminescence in Aromatic and Heteroaromatic Molecules

Quantum yields of fluorescence and phosphorescence were determined for about 20 organic compounds (and their deuterated analogs) using fluorescence yield of 9, 10‐diphenylanthracene as a reference standard. The emission yields so obtained are combined with appropriate phosphorescence lifetime data to deduce conclusions related to radiative and radiationless transitions in these molecules. Evidence is presented which indicates that the dominant singlet→triplet intersystem crossing is S1→ T1 in benzene but S1→ Tj(j>1) in larger aromatic hydrocarbons. The results also indicate that deuteration of polycyclic hydrocarbons significantly increases the radiative lifetime of the lowest state, contrary to previously held views. Solvent and temperature dependence of fluorescence in nitrogen‐heterocyclic compounds and phosphorescence characteristics of aromatic carbonyl compounds are discussed in terms of vibronic interactions between nearby ππ* and nπ* states.

’’Proximity effect’’ in radiationless transitions

The quantum statistical approach to relaxation phenomena was employed in a numerical investigation of the effect of vibronic interaction between the first and second excited states on the radiationless decay rate of the first excited state. It is shown that the vibronically active out‐of‐plane modes may be the dominant accepting modes for the radiationless transitions of N‐heterocyclics with close‐lying nπ* and ππ* excited states. The decay rate was found to vary with the strength of nπ*–ππ* vibronic interaction, the nπ* and ππ* separation, the energy gap between the initial and final electronic states of the radiationless process, and isotopic substitution, in a manner which is consistent with experimental observation.

Radiationless singlet deactivation in isolated large molecules. I Naphthalene, naphthol, and naphthylamine

Fluorescence lifetimes of naphthalene, β‐naphthol, and β‐naphthylamine have been measured under collision‐free conditions in the vapor. The lifetimes were then combined with the measured quantum yield (for naphthalene), or with the estimated radiative decay rate (for β‐naphthylamine), to deduce the nonradiative decay rate of the molecule as a function of excitation energy. The results indicate that (1) the nonradiative decay probability increases, first slowly then very rapidly, as the excess vibrational energy in the lowest excited singlet state (S1) is varied over a range of nearly 20 000 cm−1, and (2) a plot of nonradiative decay rate versus excitation energy exhibits a break at photon energy corresponding to the second excited singlet state of a molecule. Observation (1) is interpreted in terms of two different pathways of radiationless transition that compete in the singlet deactivation of the molecules. It is argued that, in the region of low excess energies, the intersystem crossing to the triplet ...

Quantum beats in the fluorescence of jet‐cooled diazabenzenes

We report here the observation of intramolecular quantum beats in the fluorescence of jet‐cooled pyrimidine and pyrazine, which represents only the second example of interference effects exhibited by closely‐spaced singlet–triplet molecular eigenstates of polyatomic molecules. (AIP)

Rotational modulation of fluorescence decay and quantum beats in expansion‐cooled azabenzenes: Small molecule behavior of rotationally cold s‐triazine in the lower vibronic levels of the à 1E″ state

The measurements of the fluorescence decay and quantum beats as a function of photoselected rovibronic levels in the 1E″(nπ*) state of jet‐cooled s‐triazine indicate that the intramolecular dynamics of the molecules excited to low rotational states of the 610 level, and still lower rotational states of the 620 level, fall into the small molecule limit. For the 620 (E″) level, changes from the small molecule decay behavior to the intermediate case decay behavior can be induced by rotational excitation of the molecule. Comparison of the fluorescence decay characteristics of s‐triazine with those of pyrimidine and pyrazine indicates that s‐triazine has much stronger singlet–triplet coupling and much smaller triplet level density as compared to the other azabenzenes. The strong singlet–triplet coupling in s‐triazine can be attributed to the efficient spin‐orbit coupling between the 1E″(nπ*) and 3A′(ππ*) states, while the small density of triplet levels is consistent with its small singlet–triplet energy gap, ...

Luminescence of Biphenyl and Geometry of the Molecule in Excited Electronic States

Fluorescence and phosphorescence spectra of biphenyl, biphenyl‐d10, and related compounds were studied in n‐heptane at 77°K. The fluorescence spectrum of each of these compounds displays a long progression involving odd quanta of a low‐frequency vibration, while the phosphorescence spectrum exhibits a short progression involving even quanta of the same vibrational mode. The fundamental frequencies in these progressions are 635 cm−1 for biphenyl, 595 cm−1 for biphenyl‐d10, and 626 cm−1 for 4,4′‐difluorobiphenyl. From their appearance in Raman spectra (but not in infrared), the ratio of the frequencies in the isotopic species, their dependence on the position of substitutions, and their absence in the fluorescence spectrum of crystalline biphenyl, these frequencies are concluded to be the torsional frequencies in the ground state. The prominence of odd quanta of the torsional mode in fluorescence and even quanta in phosphorescence is shown to be a strong indication that the molecule of biphenyl is planar in...

Vibrational analysis of the S2→S1 fluorescence of azulene in a naphthalene mixed crystal at 4.2 K

The S2→S1 fluorescence of azulene in a naphthalene mixed crystal has been measured from 740 to 1000 nm at 4.2 K. Of the 25 in‐plane fundamentals (exclusive of C–H stretches) of the S1 state, 20 are readily identifiable in emission. A number of other bands are present in the spectrum and these are tentatively identified as S1 out‐of‐plane vibrations induced by vibronic coupling to σπ* and/or πσ* excited electronic states. An emission previously assigned as T1→S0 phosphorescence is shown to be a combination band of S2→S1 fluorescence. The branching ratio of S2 →S0 fluorescence to S2→S1 fluorescence is approximately 7500 for xenon arc lamp excitation; this is an order of magnitude larger than the value reported for short pulse laser excitation. A summary of photophysical parameters for the low‐lying electronic states of azulene, based on the most recent and reliable experimental data, is also presented.

Excess energy dependence of radiationless transitions in naphthalene vapor: competition between internal conversion and intersystem crossing

Experimental and theoretical evidence for effective internal conversion to the ground state from highly vibrationally excited states is presented for naphthalene‐h8 and ‐d8. The evidence is based upon both the variation of the deuteron effect and strong exponential increase of the nonradiative decay rate as observed in the excess energy region between 10 000 and 18 000 cm−1. The calculations are only consistent with the experiments if it is assumed that the excess energy does not redistribute completely according to the statistical weighting among the normal modes but rather stays in the class of strongly accepting modes.

Electronic relaxation of acridine as studied by picosecond spectroscopy

Internal conversion (IC) and intersystem crossing (ISC) rates of the lowest excited singlet state (S1) of acridine have been measured in n‐hexane at room temperature by picosecond spectroscopy. The rates deduced from the buildup of triplet–triplet absorption and the quantum yield of intersystem crossing were found to be 3×1010 sec−1 for the internal conversion and 3×1010 sec−1 for the intersystem crossing. Corresponding rates for perdeuterated acridine are found to be similar. These rates, when combined with the upper limit of the unobserved fluorescence, lead to the nπ* assignment of the lowest excited singlet state. The kinetics of repopulation of the ground state following a pulsed excitation of the sample give strong indication of the occurrence of S1→S0 internal conversion.

Radiationless transitions in aromatic molecules with nonbonding electrons: Role of vibronically active out‐of‐plane modes

By virtue of their large frequency change and equilibrium displacement, the vibronically active out‐of‐plane bending modes are argued to be important accepting modes for radiationless transitions of aromatic molecules with nonbonding electrons. The conclusion is shown to be consistent with the position dependence of the deuterium isotope effect in T1(ππ*) → S0 radiationless transitions of nitrogen‐heterocyclic and aromatic carbonyl compounds.