We present quantum-mechanical calculations for the vibrational states of anthracene.He-3(N) and anthracene.He-4(N) (N = 1, 2) clusters in the ground (S-0) and first excited singlet state (S-1) of the anthracene molecule. The anthracene-He potential in the S0 state was described in terms of a sum of Lennard-Jones atom-atom potentials, while the potential in the S-1 state also included changes in the dispersive energy and in the repulsive interactions. Variational calculations were carried out for anthracene.He-1. For anthracene.He-2, configuration interaction calculations were performed, accounting for the boson and fermion permutation symmetry. For both helium isotopes of the N = 1 cluster, tunneling splitting is negligible (<0.01 cm(-1)), as an appreciable interaction of the densities was only found for highly excited states above the potential-energy barrier of side crossing (for energy eigenvalues &GE; -22 cm(-1) below the dissociation limit). The two-boson anthracene &BULL;He-4(2) system assumes a singlet (1)A(1) ground state due to zero spin of the He-4 isotope. Because of the dominance of the two-particle over the one-particle interactions, the two-fermion anthracene &BULL;He-3(2) system has a triplet (B-3(2)) vibrational ground state. The singlet-triplet (1 B-3(2)-1 B-1(2)) splitting between the two lowest states of the same spatial symmetry of anthracene&BULL;He-3(2) was calculated to be 10.5 cm(-1). Mass and permutation symmetry effects on the vibrational level structure of anthracene&BULL;He-1 and anthracene&BULL;He-2 were explored for anthracene&BULL;He-4(1), anthracene&BULL;He-3(1), the two-boson system anthracene&BULL;He-4(2), the two-fermion system anthracene&BULL;He-3(2) and for the hypothetical fermion system of mass 4. While the isotope effect on the zero-point energies ε(0) in the S-0 state is &UDelta;ε((1))(0)/epsilon(0)((1)) = [epsilon(0)(anthracene.He-3(1))-epsilon(0)(anthracene.He-4(1))]/epsilon(0)(anthracene.He-4(1)) = 12%, in accord with the mass effect in the harmonic approximation, the zero-point energy difference between the ground states of the two-fermion anthracene.He-3(2) and the two-boson anthracene.He-4(2) system is Deltaepsilon(0)((2))/epsilon(0)((2)) = [epsilon(0)(anthracene.He-3(2)) - epsilon(0)(anthracene.He-4(2))]/epsilon(0)(anthracene.He-4(2)) = 10%, manifesting a cancellation of mass and permutation symmetry effects. The isotope effect on the red spectral shift delta of the electronic origin for the S-0-->S-1 transition of anthracene.He-1 is Deltadelta((1)) = delta(anthracene.He-4(1))-delta(anthracene.He-3(1)) = 0.28 cm(-1), while Deltadelta((2)) = delta(anthracene.He-4(2))-delta(anthracene.He-3(2)) = -0.50 cm(-1), being of the opposite sign than Deltadelta((1)). These features of the spectral shifts as well as the small isotope effects on the energetics and Franck-Condon factors for the S-0-->S-1 vibronic spectra exhibit a delicate balance between differences in mass effects, He-He repulsion, and permutational symmetry of the boson and fermion systems. (C) 2003 American Institute of Physics.