The Luminescence of Complex Molecules in Relation to the Internal Conversion of Excitation Energy. Part I

Abstract

The relative fluorescence yields of a variety of double molecules have been m e a s~~r e d as a function of the \vavelength of exciting light. I n some cases the yield from the second excited state is lower than from the first. This effect is attributed to either photodecomposition or to the enhancement of intramolec~~lar energy transfer processes which can compete with the internal conversion between the excited singlet states. The experiments on bianthrone have suggestecl a method of elucidating the mechanism of the photochromic iso~nerization of this compound. INTRODUCTIONThe fluorescelice of tlie vast majority of aromatic compounds arises from the radiative transition -first excited singlet to ground (singlet) state. On excitation to higher electronic states this same luminescence is observed and usually with unchanged yield. This implies that the rate of internal co~lversio~i to the radiating level is fast co~npared to the rate of intersystem crossing between upper excited states of different n~ultiplicity. I t has been suggested (3) that the fall-off in fluorescence yield of dibromoanthrace~ie excited with light below 3000 A is due to heavy atom-induced i~ltersystem crossing to the second triplet level. This indicates comparable rates for these co~npeting processes. Preliminary studies have indicated that a variety of molecules wliich are apparently quite stable photochemically have luminescence efficiencies wliich are anolllolous in ternis of the above rule.In this work the relative luminescence yields of a series of double molecules have been measured as a function of the exciting wavelength. A double molecule is one co~isisting of two identical parts coupled through a conjugating liillctge. E X P E R I M E N T A LThe fluorescence measurements were liiade by illuminating the samples contained in a Vycor tube or silica cell with the bean1 from a hydrogen lamp and Beck~lian D.U. monochromater. The luminescence was detected over a 20" region centering a t 70" to the main beam by liieans of an end-on-type pllotolnultiplier with S11 response. The incident radiation was filtered out by 1.5 c~i i of a saturated solution of sodium nitrite in water. By using suitable Corning filters, the cutoff could be extended to longer wavelengths. This combination provided good cutoff a t 3950 A. The relative quanta1 output of the hydrogen lamp was l;no\vrn a t each wavelength from 2500-3800 A in ternis of the luminescence of pure anthracene, u~hich is lcno~v~i to be collstallt over this region (3,4). T h e range was extended to 4100 -4 by calibrating tlie lanlp output against the luminescence intensity from a totally (99y0) absorbing solutioli of tetracene. The fluorescelice yield of tetracene is independent of the exciting light wavelength in this region of its absorption.The hydrogen lamp becornes less conti~iuous above 3700 A but good reproducibility could be obtained by opening up the mollochrolnater slit for this range of excitation. All the solutions urhich were studied absorbed a t least 99% of the incident li...

Photochemistry of Uracil. Intersystem Crossing and Dimerization in Aqueous Solution

“…A more indirect experiment by Mokeeva and Sveshnikov [38] on the temperature dependence of phosphorescence to fluorescence ratios with temperature also can be interpreted as increased ISC from vibrational levels. Hochstrasser [39] has also observed a number of compounds in which fluorescence and phosphorescence quantum yields are dependent upon the energy of the exciting light.…”

On the excitation wavelength dependence and Arrhenius behavior of stilbene isomerization rates in solution

“…The focus of this work is on trans-stilbene in solution, especially on the excitation wavelength (k exc ) dependence [1,2] of isomerization kinetics, and on how the kinetics can be modeled with theoretical Arrhenius rates [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] k ¼ A expðÀE b =k B TÞ ð 1Þ…”

“…In jets and gases at collisionless conditions the rates k strongly depend on k exc [3][4][5][6][7][8] while in solutions the dependence is absent or nearly absent [1,2,8,10,11]. This is usually ascribed to a rapid solute-solvent energy transfer.…”

The Luminescence of Complex Molecules in Relation to the Internal Conversion of Excitation Energy: Part Ii. N-Heteroaromatics

“…In general, the fluorescence and phosphorescence quantum yields of arorllatic molecules clo not depend on which electronic state of the molecule is initially excited (6,8,9,13,25). I n Part I of this series (9) seine exceptional cases were reported and it was shown that this is not ilecessarily true if the excited illolecule is free to isomerize.…”

“…I n Part I of this series (9) seine exceptional cases were reported and it was shown that this is not ilecessarily true if the excited illolecule is free to isomerize. In these cases it was found that the quantum yield of fluoresceilce was considerably lower when the higher electronic states of the ~nolecule were excited.…”