An investigation of binding energies of triplet excimers in carbazolyl-containing polymers

Abstract

The temperature-induced interchange between nonexcimeric and excimeric phosphorescence of poly [TV-[ (vinyloxy)carbonyl]carbazole] (PFCZ) and poly [N-[ (allyloxy)carbonyl] carbazole] (PACZ) solid films has been investigated from 20 to 240 K. In addition, polystyrene doped with either IV-ethylcarbazole (NEC) or !V-(carboethoxy)carbazole has been studied. Activation energies for trapping of triplet excitons at excimer-forming sites in the matrix are all small, between 1.1 and 2.6 kJ/mol, and nearly the same for all of these carbazole species. On the other hand, activation energies for detrapping are much larger for the doped polymer films and for PACZ than for PFCZ or for poly(lV-vinylcarbazole) which had been investigated earlier. The rate-determining step in triplet excimer formation apparently involves atom and electron rearrangement associated with bond formation as opposed to the translation of the mobile triplet exciton or internal rotations associated with the polymer chain. It is concluded that steric freedom of the reacting chromophore pair leads to excimers in a relaxed configuration whereas more restricted pairs produce excimers having a higher degree of steric strain.

Photophysics of N-ethylcarbazole in fluid solution: evidence for solvent dependence and triplet excimer formation

“…for certain derivatives of poly(TV-vinylcarbazole). 23 Of particular relevance to the present work is the fact that even small molecules such as /V-ethylcarbazole and its related compounds as well as triphenylamine and its derivatives, when present as dopants in polystyrene or other inert polymers, exhibit a temperaturedependent ratio of excimeric to monomeric phosphorescence. 23•24 This behavior shows that there is a finite activation barrier which must be overcome to achieve the bonding necessary for excimer formation.…”

Photophysical and Photochemical Processes of Excited Singlet and Triplet [3n]Cyclophanes (n = 2–6) Studied by Emission Measurements and Steady-State and Laser Flash Photolyses

“…Recently, a number of reports have appeared on inter-and intramolecular triplet excimers of carbazolyl chromophores and dinaphthyl compounds containing heteroatoms in fluid and rigid media. [19][20][21][22][23][24] It seems that the triplet excimer problem is still far from being settled. Recently, triplet exciplexes and excimers including charge transfer interactions are of great importance in current industrial and academic developments toward organic photovoltaic materials.…”

“…It seems that the structural requirements and dynamics of singlet excimers have been established. Conversely, in these four decades, the presence of triplet excimer in intramolecular and intermolecular systems has been alleged. − Lim et al have published a review on theoretical and experimental works to characterize triplet excimer of naphthalene derivatives in solution . One of their main conclusions on triplet excimer is that the geometry of the naphthalene triplet excimer is completely different from that of the corresponding singlet excimer where the two naphthalene moieties are arranged in parallel to each other.…”

“…Nickel and Rodrigez Prieto, however, could not find any excimeric phosphorescence of naphthalene and 1, n -di-α-naphthalenes in solution. , Negative results on triplet excimer formation in solutions of 1-bromonaphthalene and naphthalene were successively reported. Recently, a number of reports have appeared on inter- and intramolecular triplet excimers of carbazolyl chromophores and dinaphthyl compounds containing heteroatoms in fluid and rigid media. − It seems that the triplet excimer problem is still far from being settled. Recently, triplet exciplexes and excimers including charge transfer interactions are of great importance in current industrial and academic developments toward organic photovoltaic materials. , For instance, perylene–bisimide dimers have been put under attention in relation to the excimer research fields. − …”

“…In a rigid matrix at low temperature, detecting triplet excimer emission from such molecular systems is substantially less expected due to rigidity of media that does not allow for aromatic moieties to set for the triplet excimer interaction. In some cases, transient absorption measurements by laser photolysis techniques at room temperature have been carried out to characterize triplet excimeric states − ,− because the absorption is not affected by the presence of small quantities of strongly luminescent impurities. The transient absorption spectra of excimer triplets have been assigned by subtracting that of a monomeric triplet from the whole spectrum observed.…”